Auroral radio emissions at the outer planets:

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E9, PAGES 20,159-20,194, AUGUST 30, 1998 Auroral radio emissions at the outer planets: Observations and theories Philippe Zarka Space Research Department (DESPA), Observatoire de Paris, CNRS URA-264, Meudon, France Abstract. We review both observational and theoretical aspects of the generation of auroral radio emissions at the outer planets, trying to organize the former in a coherent frame set by the latter. Important results have been obtained in the past few years on these radio emissions at the five magnetized planets, from the observations of Ulysses at Jupiter and of Wind and other Global Geospace Science spacecraft in Earth orbit, from the reanalysis of Voyager data about Saturn, Uranus, and Neptune, from ground-based high frequency-time resolution and full polarization measurements, and from pioneering multispectral observations of the Jovian and Saturnian aurorae (radio/uv/ir). In parallel, considerable progress has been made in their generation theory (Cyclotron-Maser operating in small-scale, laminar, hot-plasma-dominated radio source structures), mostly on the basis of in situ observations of terrestrial radio sources. Particle acceleration and precipitation is also better documented, thanks to in situ measurements in the Earth auroral zones and to multispectral studies of Jupiter and Saturn. Finally, the modeling of the planetary magnetic field and magnetospheric plasma at these two planets has also been considerably improved. To organize the wealth of observational results within a coherentheoretical frame, we emphasize unresolved questions (e.g., planetary radio bursts generation) and contradictions and propose ways to answer them. Our ability, already significant, to perform remote sensing of magnetoplasmas at the giant planets and, hopefully, at other distant radio sources (solar, stellar) in the near future, depends on the good understanding of the physical processes underlying the generation of auroral electromagnetic emissions. The question of the existence of exoplanetary radio emissions and the possibility to detect and study them is briefly discussed. 1. Introduction Auroral radio emissions (AREs) have been discovered and studied at the five magnetized planets: Earth, Jupiter, Saturn, Uranus, and Neptune. Being energetically negligible, they are often considered as the "poor parent" of magnetospheric physics: the radiated radio power is only a mere fraction (~5x10-6) of the input solar wind power on the magnetospheric cross section [Desch and Kaiser, 1984], which is the primary energy source for AREs, except for those of the Io-Jupiter system. However, being remotely observable, magnetospheric radio emissions, together with IR, UV, and X auroral emissions, are essential for remote sensing studies. They are also the only means to determine the rotation rate of the giant planet interiors (see, e.g., Higgins et al. [1997] for Jupiter and Lecacheux et al. [1993] for Neptune). The emissions dealt with in this paper are radio waves freely propagating on the ordinary (O) or extraordinary (X) mode, originating from the auroral, high magnetic latitude regions at the five magnetized planets, and also from Io's magnetic flux tube (IFT) at Jupiter. This definition excludes the lowfrequency "nonthermal continuum" originating from the vicinity of the dayside magnetopauses [Kurth, 1992], the narrowband kilometer (nkom) emission from the Io plasma torus [Reiner et al., 1993c], various equatorial electrostatic emissions, auroral whistler-mode hiss and chorus [Kurth and Copyright 1998 by the American Geophysical Union. Paper number 98JE /98/98JE ,159 Gurnett, 1991], as well as atmospheric lightning [Desch, 1992]. But two recently discovered peculiar emissions, possibly of auroral origin, will be briefly discussed below: Jovian quasi-periodic (QP) bursts and terrestrialow-frequency (LF) bursts. Close range observations from several spacecraft flybys are available for AREs of the five "radio planets," while only in situ observations, mainly from the Viking spacecraft, exist for the terrestrial ones. The auroral radio radiations of Earth are thus a reference for understanding the physical mechanisms (at a microscopic scale) and scenarios (at the macroscopic level) underlying ARE generation and propagation. Important theoretical advances have begun to allow use of AREs as a remote diagnostic tool of the giant planet magnetospheres, which are prime places for extending laboratory plasma physics and, hopefully, of other distant radio sources (solar, stellar) in the near future. The present paper follows many reviews on observations and theories of planetary radio emissions. Detailed synthetic accounts of radio observations at Jupiter, Saturn, Uranus, Neptune and Earth can be found in the works by Carr et al. [1983], Genova et al. [1989], and Kaiser [1993] for Jupiter; Kaiser et al. [1984] for Saturn; Desch et al. [1991c] for Uranus; Zarka et al. [1995] for Neptune; and De Fdraudy et al. [1988] for Earth. Comparative reviews include those by Kaiser and Desch [1984] for Earth, Jupiter, and Saturn; by Genova [1987] for Earth, Jupiter, Saturn, and Uranus; and by Anderson and Kurth [1989] (radio bursts and other discrete electromagnetic emissions) and Zarka [1992a] for all five planets, the latter being a good framework for section 2 below. On the theoretical side, Goldstein and Goertz [1983] review theories

2 20,160 ZARKA: AURORAL RADIO EMISSIONS AT TI-IE OUTER PLANETS of Jovian radio emission generation, while Le Qudau et al. [1983] present a critical analysis of ARE theories and conclude strongly in favor of the Cyclotron-Maser (CM) theory, further developed by Wu [1985], Lee [1989], and Louarn [1992], the latter being a well-adapted framework for section 3 below. Finally, the most recent three volumes of Planetary Radio Emissions [Rucker et al., 1988, 1992, 1997] are up-to-date collections of papers on the subject. Rather than repeating and extending previous reviews, the present paper focuses on recent observational results at the five radio planets (section 2) and their tentative organization and the fact that AREs are basically cyclotron emissions. Their high intensity, over 1 order of magnitude above the other AREs, is primarily related to the huge cross section of the Jupiter magnetosphere, resulting in a large solar wind input, and also to the relatively greater "homogeneity" of the DAM radio source, i.e., the larger ratio of the magnetic field gradient length to the wavelength (see Table 1 and Zarka [1992a]). Jovian radio emissions have been studied from the ground since 1955 (DAM) and in space with the Earth orbiters RAE 1 and 2, and the spacecraft Voyager 1 and 2, Ulysses, and within a coherent theoretical framework (section 3), Galileo. The WAVES experiment onboard Wind spacecraft emphasizing unresolved questions and important perspectives occasionally detects Jupiter from the Lagrange point L1. (section 4). Saturnian radio emissions have been studied only with Voyager 1 and 2 but are fairly regularly recognized, although 2. Observations with very faint intensity, in the data from the unified radio and plasma wave (URAP) experiment on board Ulysses. Our The average spectra of the five AREs, including Io-Jupiter emissions, are compared in Figure 1. Their observed characteristics are summarized in Table 1. Dynamic spectra are frequency-time (f-t) images of intensity or polarization variations that allow us to follow the rapid variations of emission characteristics with a good spectral resolution ( St down to <<1 s, and Sf/f <<1) over a broad spectral domain limited knowledge of Uranus and Neptune magnetospheres and radio emissions comes exclusively from Voyager 2 flybys in 1986 and Finally, spacecraft from the Global Geospace Science program (Wind, Polar, Geotail) as well as Japanese spacecraft (Exos-D/Akebono) continue, after ISEE, Dynamic Explorer A and B, and Viking, to provide observations of terrestrial radio emissions and plasma waves. As detailed in (Af/f=l). They are the main tools compensating for the Table 1 of Kaiser [1989], Ulysses, Wind (Polar, Geotail), and dramatic lack of angular resolution at decameter (DAM) to Exos-D carry radio experiments with full polarization (four kilometer (KOM) wavelengths. Figure 2 displays Stokes parameters) and two-dimensional (2-D) directionrepresentative dynamic spectra of AREs of the five radio planets, over one planetary rotation and their full frequency range, with moderate f-t resolutions. The broad spectral range of Jovian radio emissions reflects the higher amplitude of the surface magnetic field at Jupiter finding (k vector determination) capabilities. ISEE 3 had only the latter, while Viking and other Earth-orbiting spacecraft (as the Dynamic Explorers) had limited polarization and 1-D direction-finding capabilities. The Voyagers were only able to measure the intensity and circular polarization of the incoming waves (I and V Stokes parameters), therefore allowing one to identify their hemisphere of origin, while the I I I I Galileo plasma wave (PWS) experiment only measures the I (S-bursts) _ total intensity received (its orbital tour nevertheless provides I- I opportunities for interesting new measurements; see section 2.1). Jupiter is by far the most studied radio planet, partly n-lo-dam) because DAM emissions are detectable from the ground, but also due to the intrinsic complexity of their morphology, while Earth is a reference for theoretical studies. C (AKR/TKR) (NKR) URANUS (UKR) o I I I OOOO Frequency (khz) Figure 1. Comparative spectra of auroral radio emissions. Average emission ]cv½]s are sketched, no a]ized to a distance of 1 AU from the source. They arc based on 2-6 days of Voyager/PRA measurements for each spectrum, recorded from a p]aneta radii range, with LH and RH polarizations mixed together. E ors about 3 db e duc to PRA calibration. Peak flux densities are 10 times higher than the displayed levels, d Jovi S bursts up to 100 times higher. The lowfrequency limit of DAM control by Io is somewhere about 1 MHz (not accurately known). That of other planetary radio emissions is a few khz. ITKR extends the TKR range below -50 khz, down to -10 khz. The displayed cu es may vary with time d obse er location (adapted from Zarka [1992a]). The recent observations we will focus on below are mainly Ulysses/URAP observations of Jupiter during the flyby of February 1992; reanalyses of Voyager planetary radio astronomy (PRA) data from Saturn, Uranus, and Neptune; recent observations by Wind/WAVES (since 1994), Geotail (since 1992), and Exos-D/Akebono (since 1989) of the poorly studied 1-14 MHz frequency range; detailed ground-based studies of Jovian DAM (polarization, fine structures); and multispectral studies, using Wind/WAVES together with ground-based data to fully cover the 1-40 MHz range, or by correlating radio, UV, and IR observations of Jupiter (and Saturn) aurorae Observations of Jupiter Jovian AREs include three main components (Figures 1 and 2 and Table 1): (1) the broadband kilometer component (bkom) between ~10 and >300 khz (up to 1 MHz), (2) the hectometer component (HOM) from 300 khz to a few MHz (peaking about 800 khz), and (3) the decameter components, up to 40 MHz, part of which are strongly related to the

3 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,161 o +.. < o

4 20,162 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS = 0 ''"":=:... ' "'... ' '.-v.'... '"..=:: '?' :' :.-' :.:'".-:;'.".-::...-'-.'--'.-;=..-..:'.:-.. t ' Longitude ( ) from 94/11/15, 0000 VOYAGER 2 JuPite,r l '.- - '(DAM) -r- 2e t - -.t :--....i <,.: ß-; -:::':.'.-:.4'....,....:. :. '.,... -? ':;.¾/.:;, , 1 (HOM) o CML (o) from 79 7/16, 01 VOYAQER 1 tum (akr) 1 l :t: : lll::ll:? : ;: l: ll:l:::, l,?lll:lll:..i llll' '"i: :' l... l' :*ll:.l : ::..ll ll. ll ll' : "l l ll ½ :ll. :::. I ::;..'::..:l:: ::;:::l l l' lll j llllllll? 0 l I 'llll'l:l'llll:l: ll ' *l'l:: l;;l:l :l'::'?:':l ' lsl?::? { : :l '... I... :ll:l : ' SLS (ø) from 80/10/30, r-. VOYAGER 2 Uranus (UKR).. :. :. : :..,....,....:-:.... :: /.:?...- ;:;.'.-: : ;: ::..." ULS ( ) m 8 1 5, 1120 VOYAG, ER 2 Neptune (NKR) i'.'. :i,.-,,2:.::::::'i'.: ':i :"i": : :' i, i... :: ':: "11 ' 'I ";..: :.'.:-:... i:. :::'..:'..::i.i;:::.,.:.. ' :. ;'"i.'..'... t; :.'-:? : :?:'::-:::':-'-:' :-----" '"'.":'"'::'??::'.'?.:" :.'.:i'..:--'-?....:-...--?,. :-'5:".;'"'?Z ::'".i!'". :.: '::.'.?::'.:')'""-..-.'... :... -,..-'.'.'.', :'....". '"-;,:F'i..-:'.::&:'?"...';:'.'-' '.:' 0 : '-'::...'.:' '-:.::-_'-'"" E'.'..'..'...':.:7::::.. :...' ':'"..";:'. ;..1%?... O NLS (ø) from 89/08/25,: 0000 Figure 2. Typical dynamic spectra of the five AREs. One rotation is displayed for each planet, with the same time and frequency scales (except for frequencies >1.3 MHz at Jupiter). Abscissae are labeled in the corresponding longitude system (increasing westward, with rotation periods as indicated in Table 1). Increasing darkness indicates increasing intensity. Data were recorded from close range (a few to a few tens of planetary radii). Neptune emission was occulted near Voyager 2 closest approach, hence the peculiar appearance of the dynamic spectrum near 90 ø longitude. Neptune bursts, although intense, are not visible at this timescale. satellite Io (Io-DAM) and part of which are independent of it (non-io-dam, believed to be the high-frequency extent of HOM). The Io-Jupiter current system is at the origin of the Io- DAM, which can in many respects be considered as a secondary "auroral" source, at slightly lower latitudes than the non-io-dam; part of the Io-DAM consists in extremely shortlived and intense radio bursts (see section 2.1.5) HOM and bkom source locations and beaming patterns. Ulysses/URAP polarization and direction-finding measurements allowed Reiner et al.

5 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,163 [1993a,b] and Ladreiter et al. [1994] to confirm that both HOM and bkom are dominantly emitted on the X mode, with mixed weak O mode. Their sources were found to be auroral. The bkom was studied only by Ladreiter et al. [1994]. Its sources were found on open field lines at 70o-75 ø invariant latitude (L--9-15), with the emission beamed along the walls of a conical pattern symmetrical around the source magnetic field, with a half-apex angle of 30o-80 ø. Note that the emission described as "bkom" in that paper is the intense X mode emission detected in the bkom range, in that case below -400 khz, by Ulysses along its postencounter trajectory at high southern latitudes (35øS). By comparison, the bkom detected before closest approach reached higher frequencies, but was always too sporadic for direction-finding studies; the identical nature of both components remains to be demonstrated [Barrow and Lecacheux, 1995]. The case of HOM is more controversial. Using low-latitude Ulysses/URAP observations from a distance range of Rj, Reiner et al. [1993b] found average HOM sources at L--4-6 and central meridian longitude (CML)--0 ø, with a beaming pattern consistent with either a filled conical beam of 10ø-50 ø half-apex angle or with longitudinally distributed hollow cones. Using high-latitude (>30ø), close range (<14 R;) observations, Reiner et al. [1993a] derived HOM source L shells about 4-10, i.e., mostly intersecting the Io plasma torus. With a similar method and the same high-latitude, close range data, Ladreiter et al. [1994] found HOM sources on closed field lines, at 68ø-72 ø invariant magnetic latitude (L--7-11; see Figure 3), connecting to the external part of the Io plasma torus or to the inner edge of the equatorial current sheet. The sources were preferably found in a limited longitude range, CML--40ø-130 ø (<360ø!), and possibly a limited local time range LT , the emission being produced along a hollow conical beam with half-apex angle 30ø-90 ø (<40 ø for O mode). Performing ray-tracing calculations in addition to direction-finding, Menietti and Reiner [1996] turned the results of Reiner et al. [1993a] into L These various studies agree only marginally about L=7 for the HOM source location. Every approach has strengths and drawbacks linked to its underlying hypotheses. The accuracy of the direction-finding technique is in any case limited by the uncertainty in the antenna calibration, using either solar Type III bursts [Ladreiter et al., 1994] or Jovian radio emissions observed from a large distance (>50 R;)[Reiner et al., 1993a,b]. Reiner et al. [1993a,b] performed a general fitting of the observations without any a priori assumption on the linear polarization or source size, and obtained a RMS error O 740 khz --4 ø in the source direction (see, e.g., Figure 2 of both papers). ß 540 khz o 387 khz Ladreiter et al. [1994] assumed instantaneous point sources 130 Field Line' L=8 & CML=60 ø with no linear polarization (U--Q--O, consistent with Ortega- Auroral Ovals' L=8.. ', Molina and Lecacheux [ 1991 ] and Reiner et al. [ 1993a,b]) and avoided the use of Ulysses tilted axial antenna (with a spurious response), to achieve a direction-finding accuracy =lø-2ø(see!,',, HOM their Figure 5) for selected HOM events, weakly sporadic and with a high signal-to-noise ratio. Straight-line propagation was assumed in all the above studies. For high-latitude, close range observations as used by Reiner et al. [1993a] and Ladreiter et al. [1994], the radio waves did not propagate Jupiter..": /// through the Io torus, and thus propagation effects are expected... ///',',', to be unimportant. Menietti and Reiner [1996] nevertheless found that the k vector determinations of Reiner et al. [1993a] had to be modified by as much as 25 ø to obtain propagation paths intersecting the X mode cutoff (fx) surface at the observed frequency, but they attributed this inconsistency primarily to the poor description of the time-dependent and spatially inhomogeneous plasma in the Jovian magnetosphere [e.g., Gurnett et al., 1996a] by an average model such as that of Divine and Garrett [ 1983], extrapolated Azimuth / Ulysses-URAP antennas (o) from Voyager measurements. Finally, the analysis of an occultation of HOM by Ganymede, observed between 0.7 and 5.6 MHz by Galileo/PWS in 1996, has led to a HOM source Figure 3. Average positions of HOM source at three location along a field line with L> 7 (and CML--160 ø) [Kurth et frequencies, as derived from Ulysses/URAP observations in al., 1997]. direction-finding mode near closest approach from Jupiter In addition, when Ulysses traversed the Io torus (on (February 8, 1992, UT), from a 12 R j distance (longitude +23 ø, declination 30 ø, magnetic declination 21ø). February 8, 1992, about 1600 UT), VLF noise (<10 khz) was Uncertainties are <+1 ø. The L--8 field line at CML--60 ø is detected on L=7-9 field lines at a CML--100 ø [Farrell et al., plotted (dotted) together with three auroral ovals intersecting it and along which the local gyrofrequency is 740, 540 and 1993; Rdzeau et al., 1997]. Although a physical association with HOM cannot be demonstrated, both phenomen appear to 387 khz (with increasing distance from the planet; dashed originate on the same field lines and require electrons with kev portions are not visible from Ulysses). The magnetic field energy. model here is a dipole (boldface line) tilted by 9.6 ø on the Observation such as shown in Figure 3 are the first direct Jovian rotation axis (lightface). It is well adapted beyond 2-3 confirmation, at a planet other than Earth, that AREs are R; from Jupiter center. The sources at the different frequencies are well aligned along this field line when assuming emission produced close to the local electron gyrofrequency (fce) along at fce' RH polarization was observed, consistent with X mode auroral field lines. This corresponds to a 2 to 4 R; distance emission from the northern hemisphere (adapted from range for the Jovian HOM sources. Furthermore, the Ladreiter et al. [ 1994]). polarization of HOM and bkom has been found purely circular

6 20,164 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS (U--Q--O, Igl=l), with a sense consistent with dominant X mode: mostly right-hand (RH) in the northern hemisphere and left-hand (LH) in the southern. Instantaneous sources have a very small extent, but their location fluctuates with time, leading after several minutes to a dispersion larger than the ~1 ø direction-finding accuracy of Ladreiter et el. [1994]. UV observations lead to similar conclusions (see section 2.1.3). The HOM and bkom beaming is a hollow cone, with a halfapex angle possibly down to 30 ø. This value is significantly lower than the beaming angle predicted by the CM theory, at least if the source magnetoplasma is both rarefied and strongly magnetized (,fpe/fce<0.1, ft being thelectron plasma frequency; see section 3.2); s is expected in Jovian auroral regions [Divine and Garrett, 1983]. A stereoscopic study of the instantaneous, sharp HOM beaming using Wind/WAVES (in the ecliptic) and Ulysses/URAP (at high ecliptic latitudes) is in progress in order to separate temporal from spatial variations (M. L. Kaiser, personal communication, 1997) DAM polarization. Polarization is a major constraint for modeling radio emissions generation and propagation. The polarization of all kilometer-wave AREs (including Jovian HOM and bkom) has been measured 100% circular, in agreement with the expected circularization of radio radiation propagating through a plasma of decreasing density, as met by the wave propagating from the source to the outer magnetosphere and interplanetary medium [Lecacheux, 1988; Menietti and Reiner, 1996]. The polarization of Jovian DAM has been known to be highly elliptical for some 30 years [Barrow and Morrow, 1968]. It has been further studied since 1991 with the Nanqay digital spectropolarimeter and found to be 100% elliptical above ~10 MHz [Lecacheux et el., 1991; Dulk et el., 1992, 1994]. This is very rare (it is only observed for certain pulsars) and requires (1) that the emission is produced elliptically, and (2) that the polarization is frozen when exiting the source, due to strong mode coupling, which in turn, implies very low electron densities (<1-5 cm '3) in the source and its vicinity [Lecacheux, 1988; Melrose and Dulk, 1991]. In addition, the polarization of Io-DAM is very stable over the several hours' duration of a radio "storm"(figure 2) over frequency ranges of several MHz, suggesting that the corresponding radio source is extended in longitude (>70 ø- 100 ø in the Io-magnetosphere interaction region) but with only a small part visible at any given time with a constant geometry. This implies a sharp beaming, nearly perpendicular to the source magnetic field [Dulk et el., 1992]. The observed (elliptical) polarization sense is generally RH from the northern hemisphere and LH from the southern (as confirmed by analysis of DAM Faraday rotation through Io torus [Dulk et el., 1992]), consistent with dominant X mode emission. Other explanations invoked for the DAM elliptical polarization include the following. 1. There is coherent simultaneous emission of O and X mode from the same source. The recent detection of LH and RH emission coming apparently from the same source (C. A. Higgins et el., unpublished paper, 1997) brings support to this hypothesis; however, the production mechanism, which could involve X to O mode conversion in a plasma cavity (see section 3.2.9), for example in the Io flux tube, is still in an early stage of development. 2. Elliptical polarization could be produced again from circular polarization through strong mode coupling in the Io plasma torus, but the result should strongly depend on the propagation angle relative to the local magnetic field, and the observed polarization would probably not be stable for hours over a broad frequency range. For X mode propagating in a rarefied plasma, Melrose and Dulk [1991, 1993] predict a simple relation between the axial ratio T of the observed polarization ellipse, and the beaming angle 0 of the emission: cos(0)=t. DAM observations imply a beaming at 55o-85 ø from the magnetic field in the source. The lower limit of this range is low compared to predictions from the standard CM theory in a rarefied medium. Wong et el. [1982] and Wu et el. [1982] showed that the CM theory can account for such oblique beaming if fpe/fce is _>0.1, but this condition implies unrealistically high plasma densities in DAM sources. As an alternative, Melrose and Dulk [1993] invoked specific electron distributions like a spiraling beam. The above relation between beaming and polarization may explain why the polarization observed for Io-DAM sources on the eastern and western Jovian limb is different. The high degree of circular polarization of some LH emissions [Dulk et el., 1994] and of a specific, recurrent Io-controlled great "arc" in the f-t plane [Lecacheux et el., 1991] is more puzzling. It requires an emission nearly along the source magnetic field (incompatible with the CM theory) or weak mode coupling in a denser plasma (IFT vicinity?). Finally, Reiner et el. [1995] observed with Ulysses/URAP elliptical polarization for a "bursty," high-latitude HOM emission. This remains to be confirmed but would imply extremely low densities (N e <_ 0.02 cm '3) in the source region. Alternatively, this emission could have a distinct source from the usual, less bursty HOM emission Multispectral (UV/radio) correlations. A few hours after Ulysses' closest approach to Jupiter (on February 8, 1992), the Hubble Space Telescope faint object camera (HST/FOC) made the first UV observations of Jupiter aurora and detected an auroral arc at L--15 and CML--100 ø [Dols et el., 1992], made of fluctuating bright UV spots, which could well be associated to bkom. In the same period, Ulysses detected in situ, at ~20 Rj from Jupiter, field-aligned currents which were mapped to auroral field lines L [Dougherty et el., 1993; Gdrard et el., 1993]. These coordinated HST/Ulysses observations demonstrated the high interest of multispectral studies. Further UV/Radio correlation studies of the Jovian autotee followed. Prangd et el. [1993b] found correlated intensity and source longitude variations during an intense auroral event observed simultaneously in December 1990 with IUE (the International UV Explorer) and with the Nanqay and Florida decameter arrays. They showed that this event probably resulted from a large-scale magnetospheric compression (analog of a terrestrial magnetic storm, observed for the first time at an outer planet), triggered by the arrival of a coronal mass ejection at Jupiter. Subsequent statistical analysis of simultaneous IUE and Nanqay data showed a partial correlation [Prangd et el., 1993a], suggesting the coexistence of at least two distinct auroral processes, usually interpreted as diffuse autotee (due to precipitation of pitch angle scattered particles) and discrete autotee (linked to field-aligned currents). Radio emissions are only associated to the latter type of activity, like on Earth. Prangd et el. [1996] detected then for the first time, with the HST/FOC, far-uv (H 2 bands) spots at the IFT footprints. Connerney et el. [1993] previously discovered such spots in infrared (H3+ emission). Comparison of these results to DAM

7 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,165 observations of Jovian radio bursts then allowed us to build a scenario of the electron precipitations along the IFT [Zarka et al., 1996a] as well as their energy budget [Prangd et al., 1996; Zarka et al., 1997b] (see section 3.4.4) Solar wind control and other modulations, and DAM source locations. Jupiter's strong magnetic field and rapid rotation result in a strong rotational control of magnetospheric phenomena and high repeatability of radio emission patterns: the "pulsar-like" behavior [Dessler, 1983]. However, the solar wind also exerts a notable influence on bkam, HaM, and non-io-dam through its magnetic sector structure and density fluctuations, and a much weaker one, if any, on the Io-DAM [e.g., Desch and Barrow, 1984; Barrow et al., 1986; Genova et al., 1987; Zarka and Genova, 1989; Rabl, 1993, and references therein]. This has been attributed to the high latitude of the former radio sources, while the inner Jovian magnetosphere and, consequently, the Io-DAM, produced in the IFT vicinity along L--6 field lines, was thought to be "insulated" from external conditions by closed Jovian magnetic field lines. Ulysses observationsupporthese results for the HaM and bkam, generated on L>7 field lines (section 2.1.1). But a significant influence of the solar wind has also been found on the occurrence of the nkom component, for which sources are embedded in the Io plasma torus [Zarka et al., 1994]. It is thus important to check the existence and degree of solar wind control of Io-DAM in order to then address the question of its penetration in the inner magnetosphere. This is not an easy task, considering the nonpermanent nature of that component (the occurrence of which depends on Jupiter rotation period, Io orbital period, and their beatings) and the limited (Voyager) or discontinuous (ground-based) data sets available. Their common dependence on solar wind fluctuations supports the fact that HaM and non-io-dam are in fact a single radio component. This is further supported by the spectral shape of HaM, which extends in the DAM range toward a level less intense than Io-DAM (Figure 1). One may speculate that bkam is the low-frequency end of that same component and that the spectral gap between bkam and HaM (Figures 1 and 2) corresponds to the range of altitude along the source field lines from where the emission cone does not intersect a given observer direction. The natural assumption that Io-DAM originates from the IFT vicinity still raises an outstanding problem: a large downward shift of the IFT (70 ø at Io orbit, or 40 ø at Jupiter ionosphere, toward lower CML) is required to reconcile the observed Io-DAM maximum frequency and the surface gyrofrequency at the instantaneous IFT footprint [Genova and Aubier, 1985; Genova and Calvert, 1988]. About half of this shift may be attributed to the distorsion of the Jovian field by field-aligned currents or Alfv n waves in Io's magnetospheric wake and to the lag required for these perturbations to propagate from Io to Jupiter [Goldreich and Lynden-Bell, 1969; Neubauer, 1980; Hill et al., 1983], especially since Io wake densities as high as 4 x 104 cm -3 have been measured by Galileo/PWS [Gurnett et al., 1996a]. The rest constitutes a strong constraint on multipolar terms of magnetic field models, making the 06 model of Connerney [1992] still inadequate. Menietti [1995] showed that Io-DAM observations could be consistent with an 06* model, augmented by 5 MHz at the (northern) IFT footprint, without actually proposing a development in multipolar terms for this putative model. Menietti and Curran [1990a] proposed another solution in terms of second harmonic DAM emission. Wong et al. [1989] noticed that the second harmonic X mode overcomes the fundamental X mode for electron energies <1 kev and a ratio fpe/fce not too small (~0.2) in the source region, but these values are not typical of DAM radio sources (see sections 2.1.2, 3.4.3, 3.4.4, and 4.2). Rather, the low plasma density deduced above for DAM sources would imply very small growth rates for second harmonic emission on the X mode [Melrose and Dulk, 1991 ]. The 3-D modeling of DAM "arcs" in the f-t plane, in spite of being a longstanding problem [Goldstein and Goertz, 1983], is certainly a promising approach, since it uses information on the topology of a large portion (1-2 R j) of the source field line and not only the field magnitude at its footprint. Using Wind/WAVES (whose sensitivity exceeds that of Voyager/PRA except when it was <200 Rj from Jupiter) together with ground-basedata allows one to study DAM arcs over their full spectral extent, between <1 and 40 MHz [Lecacheux et al., 1998; Queinnec and Zarka, 1998]. Finally, several commonly observed DAM (and HaM) spectral features are still poorly understood [Genova et al., 1989]: 1. the so-called "modulation lanes" are patterns of quasilinear fringes with a moderate contrast, occasionally superimposed on any kind of DAM component, and slowly drifting with time toward higher or lower frequencies. Imai et al. [1997] proposed a model in terms of a diffracting plasma screen... the structure of which remains to be explained. 2. Higgins et al. [1995] identified a somewhat comparable drifting "lane" feature in HOM dynamic spectra but could not provide any clue to its origin. This feature, apparently locked in the Jovian longitude system, should not be confused with another persistent HaM dynamic spectral feature modulated at the Io torus rotation period (System III+ 2-4% period, as for nkam, due to the plasma corotation lag), interpreted by Kaiser et al. [1996a] as an effect due to HaM propagation through density inhomogeneities Io plasma torus. 3. The so-called "splitting" phenomenon [Leblanc and Rubio, 1982], which manifests itself as one or several narrow band(s) of emission "detached"(in the f-t plane) from the main DAM emission, is still unexplained Jovian S bursts. Jovian DAM emissions vary smoothly on timescales of minutes, except for the much finer structures called "millisecond" or "S" bursts (where "S" stands for "short"). Extensive studies using ground-based radio telescopes (in Nan iay, Florida, Kharkov, etc.) have identified their general characteristics [Carr et al., 1983], as follows. S bursts are the Jovian radio component with the highest flux (Table 1 and Figure 1). Their polarization [Dulk et al., 1994] and frequency range [Genova and Calvert, 1988] are consistent with X mode emissionear fce' and their occurrence is strictly controlled by Io orbital phase, strongly suggesting an origin at/near the instantaneous IFT footprints. Their detection occurrence is about 1/10th of that of Io-DAM in general, which does not imply that they are emitted only 10% of the time. This can rather be attributed to a less extended source or a narrower beaming. However, the overall energy emitted through S bursts is probably 1/10th of that of Io-DAM in general, whatever the reason (duty-cycle, source extent and/or beaming). With a typical hollow cone beaming, the S bursts' instantaneous power is thus ~108-9 W. S bursts are instantaneously narrowband (a few khz) and very sporadic, with fixed-frequency duration of a few

8 20,166 ZARKA: AURORAL RADIO EMISSIONS AT THE OIYrER PLANETS milliseconds [Ellis, 1982]. They exhibit a very complex f-t structure, mostly consisting of a fast negative drift from high to low frequencies, at tens of MHz/s (Figure 4a). On average, the drift is consistent with the adiabatic motion of ~5 kev electrons with small equatorial pitch angle (2ø-3 ø, implying v//= 2 x 104 km/s) propagating.upward from their mirror point When observed with the limited f-t resolution of Voyager/PRA (6 s x khz), S bursts appear as isolated spikes on dynamic spectra [Leblanc et al., 1980; Alexander and Desch, 1984], very similar to the fine structures of the other'ares (see below). Near Jupiter, Voyager/PRA also detected bursts of pulsed along or near the IFT [Zarka et al., 1996a]. Individual S bursts, however, have a much more complex morphology, inconsistent with this interpretation. At very high resolutions (typically <1 ms x 1 khz), S bursts radio emission at much lower frequencies (<1.5 MHz) [Evans and Snyder, 1989]. Very little is known about these bursts: they have duration between 0.15 and 1.0 s, a pulse repetition frequency of 0.3 to 3 Hz, mixed polarization, and highly are made of seemingly random spikes with >10 db amplitude, variable bandwidth. Their occurrence is minimum around revealing instantaneous source regions as small as a few kilometers (density or acceleration microstructures?), spread along a few hundred kilometers along their source magnetic field line [Cart et al., 1997]. Average S burst intensity profiles CML=200 ø, similar to HOM and DAM seen from low latitudes [Carr et al., 1983; Lecacheux et al., 1992]. Their origin is unknown, but as noted below (section 2.3), they present some similarities with some Uranian and Neptunian bursts. fall smoothly at both edges from a nearly constant value to Shoemaker-Levy 9 - Jupiter collision. zero in khz; if interpreted in terms of radio emission Little can be said about this event, spectacular at almost all beaming of sources distributed at fce along a Jovian magnetic other wavelengths, because no significant effect was detected field line, this translates into a--2 ø beam thickness [Zarka et al., 1997b]. at low-frequency radio wavelengths (KOM to DAM), contrary to tentative predictions [Farrell et al., 1994] and in spite of a NANCAY Jovian S-bursts ß ' : 24- >- 22- = ' : "' 24-0'5 1 Seconds from 95t04/14, Seconds from 95/04/14, '-' 24- a) 22- u ' U Seconds from 96/06/11, Seconds from 96/06/11, Figure 4a. Representative examples of dynamic spectra of Jovian S bursts recorded in Nanqay, with 1 s duration and 3 ms/spectrum resolution, over the whole frequency range in which emission occurred. They reveal a very complex and variable morphology. Quasi-periodic structures and burst organization in bands is well described by Ryabov et al. [1997].

9 ISEE- 1 ' ' '--. : ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,167 c..... = 510 ":'-"' --.':. -"... ',, :'"'.'.-...'., ' ' DE- 1 Earth. (TKR) ' ' ' '-':':': "'"', ':, ',.:/. :..'-1 :."... '"-:] f'.-': "'...' 'i -:'.: UT (hhmm) of 77/ EaCh... -' KR)...,,,.,, :: : : :... : :...::.::....,..? r = r-r- ; -r =, : " ; ; ; := :::::-= -.- :: :=::: ;:: =':= -- :m:::.. : :': =; : :,.:..,......, --,.., ' '- :... -.,:...:. ':! " -".-., :..- -,...,..., ::.. -':'." :.:? - = ' :, -' ": :'.- ;-':';'.. '".: -:...:'?. -.:..., - ;.,.-...' :-, '-' I "'... ' ';... " :' ". ' ,..: :.. - '-"": o. : -o- 2 UT (hhmm) o.181/10/11 VOYAGER 1 Saturn (SKR)....-,, - ' ',_::-- -; ;--.,:, :.. : :. : : : :., :-.: ,-..: ,......::. :, , ". :... ' :; :';.,...:..- j ':' ; ::, ' :.=.:j:'r '*: -'[. '--' :-;:....?.?:.'.?.---..::..:'"...' j,'? - '7"!.: ' :??? %-? ;; "::; '.':- %h :::::? ::?? A '?....-:'... :; : -, '.'--. ;;:.: '.. "...[. ';.: - : : -.:....: - ' ;:.-'.!...':'-: :- '.. '... ':':.;.: '";. - ' :-: '....'..:"=. --.-:-:...-. ['"::.- '.. ',.T.... :.?.. L...1.L... %` ` `?.....: ::.,?,,,... :: ::' :....? : :' :....-., :..'.-. =:: '"'" :.::..:...-..:-.:..:::.. : ß... : "': :... : :::.: ::: : '":: :-'-"'--::;':: ' ';... '.,:... ". :-,..,.'.'- '?::......: - :;,...: r:--:'"'.:., ':; : :--:'t? ß ::....'"- -:: : :,..,.:-., ::: - : ::::: :.:.'.':-.'-':-:'.:.:::..; :.: :: G..:...: -.. '.', - ':' -: :,: :: ::'-' : : ' ::: ::'!:'-...? ' :'.?-'-:::'? : :', "' "'"':'¾.::: '"... ::: ':.? '-:-::: - ' :'::??..: L:. "';':: ' ':': ;' :.: '-':- :' ;:' ' :?:? :?;'?' ":? :.:.;. -?: ::;:.;:" ' ': ':-': : ::';, "-:. :, : :::: : '. -.-:::... --:? ":' ".-:..... ;.:.'-. -'-:,-- :. ::. ::,..' -.,.,-. ' :::: '-: '- ::. ::::::.- '; '... ;,... -' '-':. -: : :"....: -'--., : - '. :..-.:.-"-'. "-... : :"t' = "= ::'"',- : ---, :- - ':. -: "... : r--:.. ::'.':..'.:=.: '?-:.,:"... '."=' '...-.; '/'... '.,-: -': :.... -::' : ,..: '-,: : --'-:?...;... t::::? :: '. ':: ::?J,.... :: ::: :: :? ' :.;:?:, : ;?.' ; ':" :'" ': :- ::.:... :.: -.:....' -:: -'- -:;?: ::' '"'? : : : ;?:.:- - : :::'-': : :.:::::,:.??... :..:...::.:? ,?.,.::::: ::::?:-. ;.,.....?.... `. `...`...;.... `...%.... '. ß : : - '" ' " : :' ' ',::.; :...:...,... ": " ' :': ::.....,... :i :... : '...., ' "....,-... '.... : - '... :.:..:: -'.' -. :- :.: :... : :.. "' '.:... ' ':' :'" : '-;...? : ":::". -'.. -" ' ' -... : :. : : "' - :' _ '" ' ' ' ' - : --' '. - '.: ' :- "... ' :. =.. "' ;... - ": ' t.:. :,.. ' -.'.. -:'.., :'.:=,: :':.,=... -' :,., '-' :: '--:...: ' -: -.- ' : 0" :::::: 'r ", :' :'" : ' '..-.-t ::,-:::-- :...?-.. ":'??..?...-'. '..:. "'... '":.'.':'"....." ' -.' '--" o.: ':'3o ' (hhmm) of 8'0/I' 1/17.:'- UT (hhmm) of 8:6/01/26 Figure 4b. Dynamic spectra of planetary radio emission fine structures at various timescales: discrete TKR tones, with positive and negative f-t drifts (adapted from Gurnett et al. [1979]), TKR bands and stripes (adapted from Menietti et al. [1996]), SKR bursty structure at Voyager/PRA best time resolution (30 ms), Uranus b- bursts episode (adapted from Calvert and Tsintikidis [1990]). Neptune bursts are very diluted on dynamic spectr and not easy to display [Zarka et al., 1995].

10 20,168 ZARKA: AURORAL RADIO EMISSIONS AT THE OLrI R PLANETS worldwide mobilization of ground-base decameter telescopes. Carr et al. [1995] observed from Florida two 1-min bursts of DAM radiation in the MHz range close to "Q 1" and "Q2" impacts. Maeda et al. [1995] detected from Japan a slightly more convincing LH polarized burst --30 min before "G" impact (consistent with X mode from the southern hemisphere), when the "G" fragment was crossing auroral field lines. No significant change in DAM occurrence or intensity was noticed in Nan ay, despite a sensitivity of a few kilojanskies (kjy; 1 Jy W m -2 Hz -1) over the range MHz. Even if detailed analyses reveal subtle effects, the impact of the cometary dust cloud on auroral radio emission was obviously very weak. This may be explained (1) by a kinetic energy input <1012 W, translating to a maximum radio output of 5 x 106 W (with,-,5 x 10-6 efficiency [Desch and Kaiser, 1984]) very weak compared to the usual ARE power (Table 1), and (2) by the low opacity of the cloud relative to energetic electrons, or because the latter are accelerated locally in the auroral regions, far from the cloud incidence (~44øS) Other radio auroral-like components in the Jovian system. In addition to the main auroral radio components, a weak smooth O mode emission was detected by Ulysses/URAP during quiet intervals between HOM and bkom occurrences. Reiner et al. [1994] localized its source in the same auroral regions as bkom and HOM and named it "skom" ("s" for smooth). As shown below (see section 3.2), this emission could simply be the weak O mode associated to the dominant X mode HOM/bKOM emissions. O and Z mode emissions in the range khz have also been detected from north polar regions, 4 R j away from the planet near the limit between closed and open field lines [Kaiser et al., 1993]. Besides these marginal emissions, one of the major radio discoveries of Ulysses at Jupiter was that of two families of intense quasi-periodic magnetospheric LF bursts, named QP15 and QP40 in reference to their recurrence periods about 15 and 40 min [MacDowall et al., 1993]. These bursts, negatively drifting from a few tens of khz to 1 khz, turned to be the socalled Jovian "Type III" bursts discovered by Kurth et al. [1989] in reanalyzing Voyager data. This earlier name referred to their f-t shape reminiscent of solar Type III bursts, although at much lower frequencies. But at Jupiter, the negative radio frequency drift is not due to emission at fpe or 2fp e in a plasma of decreasing density as in the solar case; it is rather consistent with the dispersion of nondrifting bursts propagating through the Jovian magnetosheath [Desch, 1994] or plasma sheet. QP15 bursts reach upper frequencies of 50 khz, and QP40 up to 700 khz, following a steep power law spectrum with variable index-2 to -7. Their substantial power (observed by Lepping et al. [1981]). The instability criterion (107 W for QP15, and 108W for QP40) requires a coherent is best fulfilled in the morningside, about LT, emission mechanism: CM is a possible candidate; others are mentionned in section 3.3. The source of QP40 bursts has been localized through direction-finding measurements in the southern auroral part of Jupiter's magnetosphere, near the where the velocity shear between the magnetospheric and solar wind plasma is maximum, and also marginally about 1800 LT. Recent UV observations of Saturn northern aurora by the HST, displayed in Figure 5b, reveal bright spots closely planet magnetic axis, a few Rj above the surface [MacDowall et al., 1993]. The observed RH polarization thus implies an emission on the O mode. Both families of bursts show a moderate rotation control (abrupt onset of QP15 near observer CML~0 ø) and solar wind control (correlation of QP40 with solar wind velocity). The LF parts of QP bursts, which contain most of the emitted power, can be broadened for hours or more through chromatic dispersion (group velocity function of frequency). They merge to form what appears to be the Jovian nonthermal continuum for remote observers [Kaiser et al., 1992]. This implies that past works on the Jovian continuum radiation have to be reexamined in the light of these conclusions. The physical origin of QP bursts is still unclear. QP40 bursts appear to be associated to kev-mev electron bursts of similar periodicity observed in the Jovian magnetosphere [MacDowall et al., 1993]. Electron and ion flux time profiles measured by Ulysses also show variations with ~40 and 10 min periods. These particles could be accelerated by magnetic (acoustic) ULF waves originating from the current sheet, with observed periods about 20, 40, and 80 min [Anagnostopoulos et al., this issue; A. N. Lachin et al., unpublished paper, 1997]. Finally, Galileo observations have revealed the magnetosphere and radio emissions of Ganymede [Gurnett et al., 1996b], thus providing evidence of a hierarchical organization of embedded magnetospheres in the solar system. Ganymede radio emissions are not auroral but, rather, are similar to the continuum radiation from planetary magnetospheres [Kurth, 1992]. The energetic particle environment of Ganymede, and the periodic reconnection of its magnetic field to that of Jupiter [Kivelson et al., 1997], raises the question of the possible control of some Jovian radio emissions by Ganymede (and/or Europa), similar to but probably less prominent than Io's [Kurth et al., 1997] Observations of Saturn Saturn's magnetosphere and radio emissions are much less intricate than Jupiter's: the magnetosphere is almost axisymmetric, no satellite plays a role equivalent to Io, and auroral radio emissions simply consist of the northern and southern kilometer X mode components (SKR), RH and LH circularly polarized, respectively (see detailed properties in Table 1). Important recent observational results includethe following. The new determination of SKR source locations through the reanalysis (in Voyager/PRA data) of SKR polarization variations along Voyager 1 and 2 trajectories [Galopeau et al., 1995]. Conjugate northern and southern sources were found (Figure 5a), of latitude _>80 ø at LT, with lower latitude extensions about 0900 LT, and marginally 1900 LT. The deduced beaming is along the walls of a hollow cone with 60o-90 ø half-apex angle. These results suggesthat electron acceleration may result from surface waves excited by a Kelvin-Helmholtz instability at the dayside magnetopause matching the above radio source location [Gdrard et al., 1995; Trauger et al., this issue]. From the reanalysis of Voyager/PRA data, a +10% variation of the maximum SKR frequency with the planetary rotation was measured, suggesting a nonaxisymmetric magnetic field anomaly about longitude 180 ø [Galopeau et al., 1991; Connerney and Desch, 1992; Galopeau and Zarka, 1992], absent from the models deduced from magnetic measurements only (e.g., the Saturn-Pioneer-Voyager model of Davis and

11 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,169 SKR NORTHERN SOURCE SKR SOUTHERN SOURCE b <......:.:.-. ;:.:...:: , ½... :.½..:,: : :... :.. :..:.?.(:...:. ::....:... <:: -:. <. ::..: : :,:: :.:: ::..;- ::; :... ::::::::::::::::::::: -: :: <: :::.... :..:... :..--::--:.. :: ½::::::: : :::::-..,.--:-.:'½ 4.: ::: :: ========================== : : :: ::: -:.?: ::.,:::::::::::::::::::::::..,...,.,.....:?½ ½ :... ::;:.. ::; :::::'. ' i½:;;".½-, :::: :,:.<..... :::: ::...::::.-..-½..-,:...: ::: : :.,:: :½. - --: - -.:: :- -:.. - :::::: <.' - --::.:%=<..:.::-..; z:.,::...:::¾......:::: :'.¾:: ::½ -., ::.... :';':: :::::::::::::::::::::::::::::::::::::::...,.. -;:::.::, : $. "-:;: : :: :4:.:.%. "-.::':::.... ½ " :": ' ;:::::..,,: '- :?.:%...,.... :::,::::,..:C'...?? '"" ':'... ============================... ::'::..;?.... :4:: ':;...,..'<::' :..4:,::.:..."::.:,...:-d::::? ':':*;': ::':: ':-:': :½: ::::::':: ': :: ½::.::: :,:.:..... ::-;<< :::,...:.., ;...:,:..:....:½...?: :4::..:.:.::½.:.: :,<::.... :::::::::::::::::::::::::::::::::::::::::::::::::::: ½½ ': : :::::: ::: -:::;.,:.,..:::::: :,.. ============================================================= ½.,,-- :::..,:.,.:.:,...::,...,...:.,,,::......,,:,,.:,.. :::::::::::::::::::::::::::::::::::::::::::: :...,...:&::. ;... :, : ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: "½ :?: :: -: -',. W :. :.-...;.:?.:::::::; :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :g;.,...-.:... :. :....: : "... =========================================================== '...::- -" ::::.., '....-? '" " -.., ;: ;::.½ : :4::.,:: :. :: :: ;:.: ::::.::: :., ; ::: ::. :,::A...: ' -- :-' :., ::: :.: :?...:.-. :** ::..;::: -: : ::::::::: ::::' ½?? :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.,, -.-:: ::... _.., ½::.: ½, :::::::::::--*½ :: ::. :-... ::::;:... :::... =================================== - :,. - ½ :::%..,½::::: Figure 5. (a) Revised SKR source locations [Galopeau et al., 1995], projected onto Saturn cloud tops. They include high latitude regions (>80 ø) about noon LT and extend toward lower latitudes on the morning sector (down to 60ø), and possibly in the evening sector. Northern and southern sources appear magnetically conjugated. (b) Hubble Space Telescope UV observations of the northern aurorae confirm these results, revealing bright spots in the morning-to-noon sector [Trauger et al., this issue].

12 20,170 ZARKA: AURORAL RADIO EMISSIONS AT THE OU'TER PLANETS Smith [1990]). A model based on the generalized inversion of magnetic field data taking into account the radio constraints is in preparation (H. P. Ladreiter et al., unpublished paper, 1994). A less recent but important result is the strong correlation of SKR with solar wind fluctuations, especially its ram pressure [Desch, 1982; Desch and Rucker, 1983], clearly demonstrated by the extinction of SKR during Saturn immersions in the distant Jovian magnetotail [Desch, 1983]. This property suggests that some knowledge on the solar wind conditions at 10 astronomical units (AU) from the Sun can be obtained from remote radio monitoring. Such a remote SKR monitoring has been performed for a few years with Ulysses/URAP, whose high sensitivity permits to detect Saturn from several AU distance, and has evidenced long-term variations of the SKR source locations, severely limiting (to ~1%) the accuracy of any measurement of its rotation period [Lecacheux et at., 1997]. Monitoring of the SKR spectrum variations, together with its theoretical modeling [Gatopeau et at., 1989], should also make it possible to perform remote measurements of the variations of electron density and energy in the Saturnian radio sources (see section 4.6). Finally, Saturn kilometric radiation also exhibits fine structures, although less prominent than in the terrestrial and Jovian cases: smooth SKR, modulated in arcs and bands, sometimes leaves place to sporadic, instantaneously narrowbanded emissions. In the example of Figure 4b, SKR consists exclusively of spikes shorter than 6 s and narrower than 20 khz, but these fine structures have never been systematically studied Observations of Uranus and Neptune In contrast with the Saturn case, the magnetospheres of Uranus and Neptune generate rich "zoos" of radio emissions, the complexity of which is at least partly due to the large tilt of Uranus' and Neptune's dipole fields on their rotation axis (Table 1). Due to the many similarities between the two systems, Uranus and Neptune have been described as magnetic or radio "twins"[zarka et al., 1995] and are discussed together here. Oblique magnetic/radio rotators are common in astrophysics (pulsars, for example), and this makes the study of Uranus and Neptune radio emissions even more interesting, although severely limited by the poor knowledge of their magnetic fields (octupole terms unresolved (for Uranus) [Ness et al., 1991] or poorly resolved (for Neptune) [Ness et al., 1995; Schulz et al., 1995]) and magnetospheric plasma Uranus. Uranus' radio zoo includes three auroral components, all on the X mode [Desch et al., 1991c]: First is a broadband, smooth (b-smooth) component, extending up to 850 khz, originating from the southern (magnetic) auroral regions. No less than seven distinct source locations, sometimes mutually exclusive, have been derived for this component by various methods, all using an offset tilted dipole model of Uranus' magnetic field. Ladreiter et al. [1993] showed that the accuracy of these source locations, projected along auroral field lines down to the planetary surface, is not better than about +20 ø in latitude and longitude, which allowed him to reconcile most of them between 10_(L_(20, along closed field lines. Consistently, Curtis et al. [1987] suggested that electron precipitations from Uranus' outer radiation belts could be the source of nightside kilometric radiation. Zarka and Lecacheux [1987] deduced for this radio component a beaming at >40 ø from the source magnetic field, and Calvert and Tsintikidis [1990] found superimposed sloping striated modulations, reminiscent of Jovian modulation lanes. Second is a broadband, sporadic (b-bursty) component, sharing its frequency range ( khz) with the b-smooth but originating from a distinct source near the south magnetic pole [Desch et al., 1991c] (confirmed by Curran et al. [1990], although via ray-tracing analysis). The higher magnetic latitude of this source (L>20) suggests its possible association with field-aligned currents. Its beaming is nearly perpendicular to the source magnetic field [Zarka and Lecacheux, 1987], and the bursts are grouped in arc-like structures, not repeatable from one rotation to the next (Figure 4b) [Calvert and Tsintikidis, 1990]. Third is a narrowband, sporadic (n-bursty) component, with a peculiar spectral behavior: bursts are instantaneously very narrowband (<5 khz), but their overall occurrence varies seemingly at random within the range khz, suggesting a small-scale, time-variable source. This behavior presents similarities with that of Jovian S bursts (section 2.1.5), although Uranian n-bursts are much less sporadic, with a typical duration <250 ms. Farrell et al. [1990, 1992] found for the n-bursts a northern cusp source, suggesting that the acceleration of the electrons responsible for the emission could be due to Alfvbn waves mode-converted from ULF magnetopause surface waves propagating in the polar cusp. This interpretation is supported by the discovery of b- smooth scintillations, with 3-6 db amplitude, due to propagation through the downstream magnetopause [Pedersen et al., 1992a]: those were attributed to surface waves excited by a Kelvin-Helmholtz instability at the magnetopause (also detected through magnetic field measurements [Lepping et al., 1987]), similar to Saturn's case. The occurrence of these traveling density enhancements (with 102 to 104 km wavelength) is variable due to Uranus' open magnetospheric shape, which results in a periodic variation (with the rotation) of the reconnection with the interplanetary magnetic field and of the energy transfer from the solar wind to the magnetosphere. They refract and focus Uranian kilometric radiations (UKR) and could thus be remotely detected by "radio sounding." Of the above three auroral radio components, only n-bursts exhibit a weak, substorm-like dependence on solar wind variations, also detected in field and particles signature [Desch et al., 1989]. An additional, dayside, component on the O mode was found to originate from the vicinity of the north magnetic pole [Menietti and Curran, 1990b]. It may be conjugated to the b- smooth and would thus be similar to the weak O mode auroral components observed at Jupiter (skom) or the Earth. Finally, a narrowband smooth component (n-smooth) was interpreted as fundamental X mode emission from a distributed equatorial source [Kaiser et al., 1989; Rabl et al., 1991], versus less convincing interpretations in terms of midlatitude sources [Sawyer et al., 1991] or second harmonic X mode [Menietti and Curran, 1990b]. This component has no equivalent in other magnetospheres except at Neptune (next section). The very repeatable morphology and stable f-t patterns of Uranus' smooth radio emissions, which permitted the developement of new source localization techniques, is reminiscent of Jupiter's "pulsar-like" radio behavior.

13 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,171 Uranus' UV aurorae were reconstructed by Herbert and Sandel [1994] as incomplete auroral ovals at L footprints about 90 ø longitude (also the magnetotail longitude). They are brighter and wider near the north magnetic pole, where the magnetic field is weaker (Q3 model of Connerhey et al. [1987]) and precipitations consequently larger. UV hot spots appear correlated to the locations of b-bursty and b-smooth sources in the south and to the dayside component in the north [Farrell, 1992; Gulkis et al., 1992]. They appear also to be associated to whistler plasma waves and kev electron fluxes. The UV output, hydrogen Lyman ct line, about 3-7xl 09 W requires an excitation power about a few 1011 W in the form of kev particle precipitations. Comparing this number to the 3 x 107 W radio power listed in Table 1, we infer a surprisingly high ratio ~10 '4 for the radio to excitation power (compared to the usual ~5 x 10-6 quoted above) Neptune. Neptune has two main (X mode) and two secondary radio components [Zarka et al., 1995]. The main smooth component (or smooth NKR, in the range khz) dominates in terms of emitted power (Table 1). It originates from northern and southern auroral sources, possibly conjugated, and extending over part of the L auroral oval. Emission is beamed around the local magnetic field along the walls of a hollow cone with 60ø-80 ø aperture [Ladreiter et al., 1991]. This source location, based on an offset tilted dipole model of Neptune's field, suffers the same inaccuracy (+20 ø) as Uranus' radio sources. Sawyer et al. [1995] used the more precise 08 model of Connerhey et al. [1991] to study radio occultations by iso-gyrofrequency surfaces and derive low-latitude sources near the quadrupole tips. This interpretation in terms of "quadrupolar" auroral sources could explain the complex NKR morphology and north magnetic polar region radiating on the O or X mode, but they warned about the lack of complete internal consistency of their results. The statistical study of Neptune's main burst characteristics may reveal distributions of occurrence, intensity, and durations similar to those of Jovian S bursts observed with Voyager/PRA f-t resolution [Alexander and Desch, 1984; Zarka, 1992b]. Neptune (and Uranus) bursts are thus possibly unresolved drifting features Remote sensing at Uranus and Neptune. From the beaming of Uranian (b-) and Neptunian (main) bursts at large angle from the source magnetic field (typically >80 ø at Uranus, >64 ø at Neptune), and assuming CM emission on the X mode, Farrell et al. [1991] derived an upper limit of cm -3 on the auroral electron densities, revealing the presence of auroral plasma cavities at these two planets, as in the case of Earth [Calvert, 1981a]. Analysis of the bursts' detailed beaming suggests densities possibly 10 times lower, even smaller than the values observed in the sources of terrestrial kilometric radiation (TKR) [Hilgers, 1992]. Note that such extremely low densities would allow kilometric radiation from Uranus or Neptune to remain elliptical (as discussed in section 2.1.2), and although Voyager/PRA had no full polarization measurement capability, ellipticity of the smooth NKR was invoked as a possible explanation for its intricate polarization [Zarka et al., 1995]. Menietti and Curran [1995] included explicitly such auroral cavities (at L--6) in Uranus and Neptune plasma models, in order to perform ray-tracing reanalyses of UKR and NKR observations. Their results are qualitatively similar to those presented above and differ only in the details of the source locations. Neptune's main smooth component, for example, polarization [Pealersen et al., 1992b], but the accuracy of the results is here limited by the nonuniqueness of the 08 field model and the complex polarization response of PRA antennas. The main bursty component presents similarities with both Uranus b- and n-bursts: like b-bursts, Neptune bursts are very sporadic (<30 ms duration) and originate in the vicinity of the southern magnetic pole, with LH circular polarization; like n- bursts, they are narrowbanded (~5 khz), occur in episodes with variable average frequency ( khz), and present some correlation with solar wind fluctuations (with the electric field resulting from merging of the interplanetary magnetic field with the magnetosphere, according to Desch et al. [1991b]). Neptune's burst source exhibits both some rotation control, with an "active" region about 260øW longitude, coinciding with the magnetic auroral zones of the 08 model, and some local time dependence, with a preferred triggering of the active region near dawn. Emission is also beamed in a hollow cone, with 75ø-80 ø aperture, and burst intensities are distributed according to a power law. Anomalous bursts were also identified at lower frequencies (<550 khz), coming from a slightly different southern source [Desch et al., 1991a]. Finally, a weak smooth high-frequency component was is found to originate from the cavities. However, the detailed results of ray-tracing studies must be taken cautiously when the underlying plasma distribution and magnetic field model are poorly known. This is especially true for multipolar terms in the near-planet magnetic field and if strong gradients are expected in the plasma, for instance, at the edges of the cavities. One important difference of Menietti and Curran [1995] with respect to previous studies concerns the bursts' source location and beaming: these authors found that burst sources in the denser regions surrounding the cavities lead to beaming angles typically <55 ø relative to the source magnetic field (down to 13 ø, i.e., corresponding to quasi-filled cones). Based on this result, they refuted the CM instability as the source of bursty emissions and proposed instead a temperature anisotropic beam instability [Wong and Goldstein, 1990; Winglee et al., 1992], although this mechanism predicts emissions at no more than 30 ø from the source magnetic field. Let us also notice that the inference by Farrell et al. [1991] of auroral plasma cavities directly relies upon the assumption of beaming angles much larger than 55 ø and on the CM theory, as explained above. It is thus in clear contradiction with the results of Menietti and Curran [1995], who place the burst source outside the cavities and derive a more parallel radio beaming. This problem is further discussed in section 4.3. observed close to the planet between 600 and 870 khz. Based on the visibility of the emission (radio horizons technique), 2.4. Observations of Earth Rabl et al. [1992] interpreted it as X mode emission from an TKR. Auroral terrestrial kilometric radiation equatorial source (possibly corresponding to a magnetic dip "anomaly"), similar to Uranus' n-smooth component. Through an analysis of observed polarization changes, Morgan andmenietti [1995] rather favored sources in the (hereafter TKR, but also called AKR by geophysicists) is by far the best studied AREs, especially through in situ radio source measurements. It is thus a reference for comparison to other planetary AREs and for theoretical developments.

14 20,172 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS TKR is generated along high latitude (.--70 ø) nightside (LT~2300) field lines connecting to Earth's magnetospheric tail, in depleted and highly magnetized cavities (,fpe<<fce) [De Kaiser et al. [1996b] discovered in Wind/WAVES data that Fdraudy et al., 1988, and references therein]. It is associated to ITKR was preceded by a nondrifting burst of 1-5 min duration, "inverted-v" electron events and field-aligned currents (see section 3.4). It also correlates with discrete UV auroral arcs (including transpolar arcs [Pedersen et al., 1992c]) produced when precipitated electrons reach the atmosphere and extending up to 500 khz. Direction-finding measurements suggested an auroral source of limited extent for this "precursor," closer than ~5 Rœ from Earth, the emission being subsequently isotropized through propagation. This emission, collisionally excite atmospheric constituents. coined "LF bursts" as a whole (burst+itkr), presents Crucial in situ observations within TKR sources, between similarities with Jovian Type III or QP bursts (section 2.1.7). ~0.5 and 2 Rœ altitude, were obtained 1986 by the Swedish It is polarized, has a moderate power (~105 to 106 W), and its satellite Viking [De F raudy et al., 1988; Bahnsen et al., 1989; Roux et al., 1993]. Roux et al. [1993] showed that the dominant X mode is accompanied by weaker O (and Z) mode from the same source, confirming earlier results of Calvert and Hashimoto [1990]. TKR was found to be generated in filamentary source structures, a few kilometers wide, perpendicularly to the magnetic field [Hilgers, 1992; Hilgers and De Fdraudy, 1992], much smaller than the large-scale auroral cavity of Calvert [1981a]. In these very depleted laminar cavities, intense X mode emission is generate down to the local gyrofrequency or even below; hot plasma becomes nonnegligible compared to cold background electrons, their total density (hot+cold) being down to ~1 cm '3 within TKR sources, i.e., 5-10 times lower than in the surrounding medium. More generally, occurrence shows no periodicity. LF bursts were detected less frequently than TKR, but the detected bursts appeared to be often, but not always, associated to TKR events. With the additional use of Geotail, Anderson et al. [1996] showed that TKR was always observed when LF bursts were detected. Some TKR emissions were probably missed by Wind simply because they are not isotropic, but preferably beamed toward the nightside (and/or refracted away from the dayside by the plasmasphere), where Geotail spends most of its time contrary to Wind at the Lagrange point L1. Finally, Desch et al. [1996] found a correlation between the occurrence of LF bursts, the solar wind speed, and the electric field resulting from the interplanetary magnetic field merging with the magnetosphere (for Neptune bursts, see section 2.3.2, and for Jupiter QP40 bursts, see section 2.1.7). ½/fc½<0.2 in periods of solar maximum, and fp½/fc½<o.14 LF bursts and ITKR appear thus as an auroral component ring solar minimum [Benson, 1995; Hilgers, 1995]. distinct from TKR. The burst precursor is detected down to Downgoing electron beams and upgoing ion beams are about twice the solar wind fp½, probably through the denser observed in the auroral regions with energy >1 kev, as well as magnetosheath and bow shock nose or flanks, while the unstable electron distributions with characteristic energies ~1-5 kev: loss-cone for upgoing electrons, hollow beams for downgoing ones [Mizera and Fennel, 1977], and quasi-trapped distributions in TKR sources [Louarn et al., 1990]. The Japanese satellite Exos-D/Akebono provided new polarization, k vector and Poynting vector measurements since March 1989, as well as observations of TKR fine f-t smooth ITKR part is detected a little later down to ~fp½, possibly after a long propagation through the magnetotail [Steinberg et al., 1988], the magnetosheath [Desch, 1994], or the interplanetary medium (J.-L. Steinberg et al., unpublished paper, 1998). Much less is known about the radiation mechanism of this component, compared to TKR or other AREs (is it CM structures [Oya et al., 1990; Morioka et al., 1990]. emission?). The question is, however, of importance, as Observations by ISEE, Dynamic Explorer 1, and Galileo already showed that TKR is mostly made of very fast and narrowband fine structures drifting positively or negatively at ITKR/LF bursts could well be the terrestrial counterpart of Jovian QP bursts, and of at least some Uranian and Neptunian bursts. a few to a few tens of khz/s in the f-t plane (Figure 4b). Fixedfrequency durations are as small as a few milliseconds, and bandwidths down to the khz or less [Gurnett, 1974; Gurnett et al., 1979; Baumback and Calvert, 1987; Menietti et al., 1996]. Comparison of these fine structures to Jovian S bursts raises several interesting questions (see sections 3 and 4) ITKR / LF bursts. Another terrestrial LF radio component, often associated to intense magnetospheric substorms, was discovered by Steinberg et al. [1988, 1990] using ISEE 1 and ISEE 3 observations from outside the magnetosphere. It is observed between the local fp½ and the TKR LF cutoff, typically in the khz range, and exhibits a negative drift in the f-t plane. Direction-finding measurements concluded to a very broad magnetotail source, the very large apparent source size (~half-space; hence the name ITKR for isotropic TKR) being attributed to scattering. Filbert and Kellogg [1989] reported observations of this component by the IMP 6 satellite within the magnetosphere, found it correlated with the TKR, and suggested an auroral source at ~3 R E altitude. Similar conclusions were reached by Louarn et al. [1994a] from Galileo/PWS observations of smooth and bursty LF radiation in the same (myriametric) frequency range, although it seems that the smooth auroral myriametric radiation addressed by these authors may be different from ITKR Hectometric radiations. Several kinds of radiation have been observed from the ground and from space in the range between <1 and 5 MHz: terrestrial hectometric radiation [Oya et al., 1985, 1990], medium frequency bursts [Weathermax et al., 1994], and auroral roar [Labelle et al., 1995]. The former is a smooth radiation which could result from conversion of UHR waves to O mode in the upper ionosphere. The latter two are impulsive and correlated to substorms and optical aurorae, but due to their low power, they probably have no influence on auroral dynamics. Little is known about their origin or even about their polarization and emission mode, but their study is far from uninteresting because it could bring insights into some of the processes involved in ARE generation. For example, the auroral roar exhibits many fine drifting structures reminiscent of planetary radio bursts [Labelle et al., 1995]. 3. Theory The constraints set by observed properties of AREs (smooth components or overall burst properties) on their generation theory can be summarized as follows: (1) very

15 ZARKA: AURORAL RADIO EMISSIONS AT TIlE OUTER PLANETS 20,173 intense radio emissions, hence nonthermal; brightness All of them also require a high level of Z mode in the source temperature generally >1015 K; (2) 100% polarized, circularly regions, which has never been observed, and moreover, or elliptically; (3) produced dominantly on the X mode, sometimes accompanied by weak O mode and possibly Z R6nnmark [1989] has shown that their efficiency is too small by -30 db. mode, from the same source region; (4) beamed at relatively Nonlinear conversion processes. large angle with respect to the local magnetic field (up to 90 ø, but the beaming angle can also be as low as 30 ø in some cases); and (5) generated near the local gyrofrequency fce, in a Barbosa [1976] studied the coupling of two electrostatic waves near fuh and found that an electromagnetic wave can be generated by the nonlinear (quadratic) currents produced in the rarefied plasma (fpe << fce)' on high-latitude field lines along plasma by the two electrostatic waves. The incoherent process which energetic electrons precipitate, as measured at Earth and has a far too low an efficiency, but phase tuning is possible in deduced at the outer planets from correlation with field-aligned currents and UV auroral emissions. the case of narrow bandwidths ( Sf << f) and allows coherent emission of the electromagnetic wave, the amplitude of which The sections below review briefly the three broad classes of generation theories proposed in the literature (section 3.1), and present in more detail the CM theory, which appears best in satisfying all of the above constraints (section 3.2). Results obtained in the past two decades, including very recent works, are presented in an attempt to build a coherent framework for organizing ARE observed properties. Intense bursts are produced together or alternately with smoother emissions. Their origin, and in particular their detailed f-t structure, deserves a specific discussion (in section 3.3). They present similarities with some solar and stellar radio spikes and with flare star emissions (RSCVN, AM Herculls, etc.) [Kuijpers, 1989; Barrow et al., 1994], hence the astrophysical interest of their study. Acceleration processes are briefly discussed in section Theories near fpe on electron density gradients. Jones [1976] removed the first stage, assuming direct Cerenkov Z mode production. Assuming intense Z mode in the auroral regions, Wu et al. [1973] and Smith [1976] showed that Z to X mode conversion was possible via a tunnel effect, while Istomin and Pokhotelov [1984] proposed it to occur near fuh in the presence of a double layer. The first two processes fail to produce dominant X mode, and are thus irrelevant for ARE generation, but they might be relevant for nonthermal continuum or myriametric radiation. is then directly related to those of the two electrostatic waves. In the weak turbulence regime, Roux and Pellat [1979] studied the coherent coupling (fuh + fuh '- X (and/or O) mode at 2 Xfuh )' Jones [1977] investigated the conversion (Z + Z -- X + O), and Goldstein et al. [1983] the process (fuh +flh '- X), with emission slightly above the X mode cutoff frequency fx, flh being the lower hybrid frequency. In the strong turbulence regime the energy of intense electrostatic waves can build up and be trapped in plasma cavitons (depleted density structures with a strong electric field), inside which strong nonlinear currents may produce electromagnetic radiation. This occurs at 2Xfp e in the case of Langmuir cavitons built on electrostatic turbulence at fpe [Galeev and Krasnosel'skikh, 1976; Maggs, 1978], or at 2xfc e if based on cyclotronic turbulence at fce [Cole and Pokhotelov, 1980]. Pottelette et al. [1992] showed that the interaction of lower hybrid solitons, generated by electron beams at density gradients in the auroral zones, with a background of upper hybrid waves (possibly loss-cone driven) may produce intense X mode waves. The very high power of AREs (Table 1) requires a highly efficient generation mechanism: up to >1% of the precipitated electrons' power can be dissipated in the form of radio waves Generation of intense X mode radiation was also claimed [e.g., Pritchett, 1986]. Only coherent processes can thus be considered. First are indirect, mode conversion processes, linked to the presence of parallel electron beams in the source regions, exciting electrostatic or Z mode waves then converted into electromagnetic O and/or X mode. Among those, one possible through interaction between electrostatic ion cyclotron waves, or between these waves and preexisting weak X mode (natural laser mechanism) [Palmadesso et al., 1976; Grabbe et al., 1980], or through resonant interaction between auroral electron beams and a modulated electric field (turbulent should distinguish linear and nonlinear conversion processes, Bremmstrahlung, e.g., in the presence of ion cyclotron the latter involving wave-wave coupling. Second are direct turbulence) [Bujarbarua and Nambu, 1983; Bujarbarua et al., generation processes, in which the kinetic free energy of an 1984]. However, several authors have shown these unstable electron population is directly converted into mechanisms to be physically irrelevant [Roux and Pellat, electromagnetic waves (near fce' i.e., coherent cyclotron 1979; Maggs and Roux, 1983; Le Qudau, 1982; Melrose and emission). The proposed theories are summarized and briefly commented on below, following Le Qudau [1982], Genova Kuijpers, 1984]. Precise wave-wave coupling mechanisms are generally not well understood. O mode is not observed as dominant as often [1987] and Louarn [1992]. They have been introduced predicted and Z mode is generally absent or weak. Several primarily to explain TKR but are relevant to all planetary AREs. mechanisms require strong electron density gradients [e.g., Jones et al., 1984]. The strong turbulence regime also raises Linear conversion processes. Oya [1971, specific problems: the corresponding mechanisms have low 1974], Scarf [1974], and Benson [1975] proposed a two-stage conversion of electrostatic waves generated near the upper hybrid frequency fuh, first into Z mode, and then into O mode efficiencies, predict mixed polarization generation, and require density structures of a few meters to emit kilometer waves (which is still unclear). Finally, in spite of the high level of electrostatic turbulence sometimes observed in the auroral regions [e.g., Gurnett and Frank, 1977; Mozer et al., 1980], indirect processes require such a fine tuning of the conversion/generation conditions that it makes them unadapted to explain the generation of AREs, which appear to be a universal manifestation of very diverse magnetized environments (planetary and, maybe, solar and stellar) Direct generation processes. H ighe r efficiencies are naturally expected from these processes, but the radiating electron population must be first "prepared" by

16 20,174 ZARKA: AURORAL RADIO EMISSIONS AT TIlE OUTER PLANETS some other external process (rapid acceleration, loss-cone, etc.) in order to possess the free energy that can be converted into electromagnetic waves. Spatial electron bunching was first proposed by Ellis and McCulloch [1963] and Ellis [1965], who only studied their incoherent cyclotron emission, while coherent cyclotron emission was studied in the case of phase-bunched electrons (in velocity space) [Fung, 1966; Melrose, 1986; Freund et al., (N--kc/o <l), thus the electron Larmor radius is small compared to the wave perpendicular wavelength (rle/) ñ --vñ/c << 1), and the electrons "see" homogeneous, time-variable wave fields E and B. Furthermore, the electric force largely dominates the magnetic one (levxbi-- evke/to < evne/c << ee), so that the X mode wave is "seen" as a rotating electric field by a gyrating electron. The CMI theory describes the interaction between these gyrating electron populations and the rotating E field of 1987; Borovsky, 1988]. an X mode wave. Since collisions are negligible OCpe << fce), a However, the origin of electron bunching (spatial or phase) kinetic treatment is required. raises a serious problem because the stochastization of Growth rate. Wu and Lee [1979] separately electron motions in natural media should lead spatially bunched or phase-bunched populations to evolve toward kinetic, statistical instabilities (involving unstable electron considered the wave propagation, supported by the dominant background cold plasma (see dispersion curves in Figure 6), and the amplification, attributed to hot (kev) electrons. The populations) [Melrose, 1973, 1976]. current distribution due to hot electrons is related to the Weibel [1959] showed that the Z mode can be unstable and amplified [see also Le Qudau, 1982], but it then requires conversion or tunnelling to X mode (cf. section and electric field E through the (linearized) Vlasov equation. A wave growth y (imaginary part of the wave pulsation to, with y to) is derived as 3.1.2), with weak efficiency [Smith, 1976]. The X mode itself can be unstable in the presence of an T 03pe2 / 032 f pñ2 3f/3pñ -1 d3p (1) electron beam with temperature anisotropy (Tñ >> T/l, the//and 2. directions referring to the local magnetic field) [Melrose, rz f l ¾//,¾ñ vñ2 3f/3vñ isto>k//v//-o ce/f) dv// dvñ 1976; Wong and Goldstein, 1990]. Such electron distributions Resonance condition. The resonance have sometimes been observed by Dynamic Explorer 1 in or condition between the electron and wave frequencies is tonear AKR source regions [Menietti et al., 1993]. k//v//-o ce/f--o, in which the Doppler term k/iv//allows us to The X mode is also destabilized by an electron population overcome the nonpropagation band between fce andfx and to presenting positive gradients in the velocity space (3f/3v>O, emit directly at fx, if the gap (~ x fce) is narrow, i.e., with f(v//,vñ) being the electrons' velocity distribution œ--fpe2/fce 2 << 1 (as observed). The Lorentz factor function), which corresponds to a natural (Cyclotron-)Maser even for weakly relativistic (kev) electrons because it [Schneider, 1959]. Melrose [1976] investigated the case transforms the resonance condition into a resonance ellipse in 3f/3v//>O, involving highly anisotropic electron beams (vii,vñ), along which the net result of the integration in (1) (Tñ> T//). Although there are many examples of intense, field- can be positive if the ellipse mainly crosses parts of the aligned electron beams (both parallel and antiparallel) in the electron function distribution where 3f/3vñ>O. This cannot be auroral regions [e.g., Lin et al., 1984; Lundin et al., 1987], a the case in the nonrelativistic condition, where the resonance large temperature anisotropy, attributed by Melrose [1976] to ellipse reduces to a straight line v//--(oj-ojce)/k// along which y the betatron effect (transfer vñ- v// along converging is always negative. Then 3f/3vñ>O corresponds to an magnetic field lines), has never been observed. Wu and Lee electron population inversion in vñ, thoughto be a loss-cone [1979], followed by many authors, studied the case 3f/3vñ >0, at Earth's ionosphere [Wu and Lee, 1979] and possibly also at which may be driven by the ionospheric loss-cone (see next section). Jupiter (after acceleration by Io [Wu and Freund, 1977]). The growth rate derived for the X mode using electrons with The latter mechanism has proved very successful in energies of a few kev (up to 10-3x 0 ce) is much larger than accounting for the observed ARE frequency ranges, that of the O mode [Wong et al., 1989], leading to a gain polarization, and mode (including O mode with a gain much ~20-30 db larger for the X mode. In a rarefied medium with lower than for X mode), as well as the required efficiencies. It is presented in more detail below The Cyclotron-Maser Instability The CM instability (CMI) has been studied extensivelysince the pioneering works of Twiss [1958], Schneider [1959], Hirshfield and Bekefi [1963], Melrose [1973], and Wu and Lee [1979]. It is now the most developed and widely accepted theory of ARE generation, at least for the smooth components. Its decisive advantages are the direct emission of X mode (and weak O mode), with high efficiency (up to 0.1-1%), at frequencies near fx, itself close to fce in the auroral regions where fpe/fce <<1. The underlying principles of the CMI theory and the most significant stages of its study are summarized below, for subsequent comparison with ARE observations (see Notations in the appendix and Louarn [1992 and references therein] for details) General remarks. In a cold, highly magnetized plasma, the O and X modes are supraluminous (03-k//v//-tOce/F) F is crucial fpe/fce<o.1, wave amplification is much stronger for directions of propagation at large angles from the magnetic field in the source [Wong et al., 1982; Le Qudau et al., 1984b; Ladreiter et al., 1991] Limitations of wave growth and source inhomogeneity. For linear wave growth (in exp{yt))in a homogeneous medium, assuming that the electron distribution is unperturbed by its interaction with the wave, the limitation of wave amplification can only result from the convection of wave energy out of the source, of perpendicular size Lñ, implying a total integrated gain G--exp{2?Lñ/Vg). In that case, Lñ must be determined from observations. A more realistic source model should include the source inhomogeneity. This was first studied through computation of the integrated gain, G=exp{l y(x)/vg(x)dx} in a medium assumed locally homogeneous [Omidi and Gurnett, 1984]. Le Qudau et al. [1985] and Zarka et al. [1986] explicitly took into account the source parallel inhomogeneity, which modifies the resonance condition as 03- k//(z)v//(z) - OJce(Z)/F = 0 (2)

17 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,175 f 2 f = kc O fx fuh fee mode 0 /,._ fpe mode Z!! fpe flh (a) I l R z R (b)!! Figure 6. (a) Dispersion curves (frequency versus wave vector) of the electromagnetic modes in a cold homogeneous plasma with f, pe << f, ce' in (tuasi-perpendicular propagation ' The X and O modes are RH and LH polarized near their cutoffs, respectively. The dashed regions are electrostatic mode domains. (b) Relative frequency ranges of the O, X, and Z modes versus the distance for representative f, pe and fee profiles over planetary auroral regions. The scales are approximate: horizontally in planetary radii, vertically corresponding to the khz to MHz range. The Z mode is trapped within a distance R<R z, while the O and X modes can propagate to infinity (from Zarka et al. [1995], adapted from Genova [1987] and Gurnett [1983]). In (2), k/l, vii and {0ce change with the altitude z in the source because of wave refraction, adiabatic motion of the electrons, and variations of the magnetic field intensity, respectively. This results in detuning the resonance over the vertical length 15z through which the phase shift (z) between the electron motion and the wave E field reaches, say, r/2. Then (z) is written (z) = (03- k//(z)v//(z)- 6)ce(Z)/I ) dz/v//(z) (3) The envelope of all 15z for the whole electron distribution forms a "natural" source height Az ( 15z) which can be associated to a perpendicular path Lñ taking into account wave propagation and refraction. It was also found that Az---> 0 when œ increases up to , i.e., the CMI is quenched for fpelfce _> Source term and TKR flux. Using the spontaneous emission of hot electrons as the source term of the transfer equation describing the X mode wave growth along its propagation through the source, Zarka et al. [1986] derived an upper limit about W m -2 Hz -] at 1 AU. This result, although twice as high as the average TKR flux (see Figure 1 and Table 1), is still 1 order of magnitude too small as compared to the most intense TKR bursts [Benson and Fainberg, 1991] and requires characteristic energies of about kev for the unstable features of the hot electron distributions (loss-cone or hollow beam; see below), instead of the observed 1-5 kev. and quenching the instability. Several authors studied the process of quasi-linear diffusion of resonant electrons, in velocity space, by the wave electric fields [Wu et al., 1981; Wagner et al., 1983, 1984; Pritchett and Strangeway, 1985; Wu, 1985]: a broadband wave spectrum ( Sf--J) and saturation levels corresponding to >1% conversion efficiency appear to be required for the quasi-linear diffusion to be able to quench CMI-driven wave growth. This saturation level would correspond to huge electric fields (up to ~1 V/m) in the source regions and maximum fluxes 10 to 100 times stronger than those observed. The alternative saturation mechanism is the resonant trapping of electrons in the wave E field [Roux, 1974; Le Qudau et al., 1984a; Le Qudau and Roux, 1987]. It is more relevant for narrowband emissions (15f << f), for which the wave coherence is greater. Such narrow tones are frequently observed; for example, 15f was measured small as 10 Hz for f= 250 khz in the case of TKR [Baumback and Calvert, 1987]. In a homogeneous medium, saturation occurs when the trapping frequency of the electron motion around the wave E field vector matches the linear growth rate (1/T -- ¾ -- 15f, with T the trapping period and 15f the wave spectrum bandwidth) [Le Qudau et al., 1984b], which also leads to very high saturated fluxes. In an inhomogeneous medium, the saturated level is obtained by balancing the trapping period T and the inhomogeneous detuning time x i. Sz/v// [Le Qudau et al., Viking studies within TKR sources Saturation processes. The CMI is expected Viking observations most significant for the theory of TKR to saturate whenever its efficiency is higher than --,10-4 [Melrose and Dulk, 1991, 1993], through nonlinear removal of the free energy of the electron distribution and creation of a generation have been described in section They are summarized below and in Figure 7a (see De Fdraudy et al. [1988], Hilgers et al. [1992], and Roux et al. [1993] for plateau in f/ vñ, thus cancelling the population inversion details). The wave signature of a TKR source crossing by 1985].

18 ... ß ß 20,176 ZARKA: AURORAL RADIO EMISSIONS AT THE OLrI R PLANETS I Waves I? 30,',,,,r??r,:,,l,: y,,,:,,:..... His I _ ß I_..,.._1 TKR source (a few km) Weamk øod e&sz fc e.s. noise time / distance along S/C trajectory I Particles I electron beams ',- -...,5 : electrons ion beams 1 j Trapped electrons oo I-'.-':'F',, -'- ', ß time / distance along S/C trajectory Enhanced Loss-cone... "'" Hollow beam 9 VII V.L UP DOWN Figure 7a. Sketch of Viking waves (E field component) and particle observations of TKR sources. Electrostatic noise is often observed on the edges of TKR sources, in association with electron beams [e.g., Pottelettet al., 1990]. Free energy sources ( f/ v L>0) usable by the CMI are indicated as arrows on electron distribution function sketches. Trapped populati6ns can be used only where hot plasma is dominant and modifies the wave dispersion to allow resonance at k//=o (see sections and 3.2.8) (adapted from Louarn et al. [1990] and Roux et al. [1993]). Viking is a funnel-shaped, dominant X mode emission, which peaks near the local gyrofrequency fce and which extends below fce' It is accompanied by weaker O and Z mode emission on the edges of the main component. At lower frequencies which a fine structure of kev electron and ion beams is superimposed. These three distributions contain population inversions (Of/Ov_L) and are thus free energy sources for the CMI. They had already been observed by the S3-3 satellite in (,f -fpe) the auroral hiss reveals the presence of a small-scale the auroral regions [Mizera and Fennel, 1977]. Viking depleted cavity (electron density as low as 1 cm-3), no more observation strongly suggesthat trapped electrons are the than a few kilometers in the latitudinal direction, i.e., perpendicular to the underlying auroral arc. TKR sources, when projected along the geomagnetic field, map hot spots in discrete UV arcs. The latter are very narrow, less than 1 km primary CMI driver for TKR emission [Louarn et al., 1990]. These observations have brought support and detail to the representation of TKR sources as acceleration structures due to magnetic-field-aligned electric fields. Viking source crossings wide at the ionosphere. Simultaneous particle measurements (sketched in Figure 7b) correspond to traversals of these confirm that the TKR highest intensities and lowest frequencies are produced in small-scale cavities, devoid of cold plasma (51 kev) and dominated by tenuous hot plasma (1-5 acceleration structures, with a.--1 kv potential drop below and above the spacecraft. The origin of these potential structures is briefly discussed in section 3.4. kev). The population of kev particles is enhanced in a region Relativistic (hot plasma) dispersion. much broader than the TKR source itself, where distributions consist mainly of loss-cone (upward moving electrons) or hollow beam (downward moving electrons). Inside the source the distribution rather consists of quasi-trapped electrons, to As Viking observations questioned the assumption of dispersion supported by the cold plasma, at least in TKR sources, where hot plasma is dominant, several authors studied the wave dispersion in a weakly relativistic plasma [see Le

19 ß ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,177 / TKR / Electric ipotentials / / / ion beams (u. ndetectable) electron beams... -/. I I I I ion beams... /... (detectable) t I! ø! / /! / / / / / rkr t! Parallel E / / / / / / / / // / / Magnetic field lines S/C trajectory! Optical aurora / '-/ ß / / to Earth " "e/'e Figure 7b. "Post-Viking" representation of a TKR source as a potential structure. Electron beams are more energetic on the edges, due to their acceleration by the full potential drop. Freja observations, at an altitude 4 times lower than Viking, actually revealed very fine structures in this potential distribution, signature of complex time-dependent acceleration processes as linked to the intense parallel electric fields associated to (solitary) kinetic Alfv n waves (see section 3.4.2) (adapted from Louarn et al. [1990]). Qudau and Louarn, 1989, and references therein]. They found that the X mode cutoff frequency fx decreases down to fce or even below, that the growth rate T is maximum strictly perpendicular to the direction of the magnetic field in the source, i.e., for k//=o, because the Doppler term k/iv// is no longer necessary to emit at or above fx from an initial frequency ~fce' Consequently, trapped electrons become the most effective free energy source. As amplification occurs very near fx, where the group velocity is close to 0 (Figure 6), a very high gain can be achieved even for perpendicular source sizes Lñ of a few kilometers. For smaller source sizes, as suggested by some Viking and Freja observations (see below), one should invoke multiple passes in the amplification region in order to achieve high enough gain Laminar source model. Both the small-scale source structure (perpendicular to the magnetic field) revealed by the observations and the possible need for "closed-loop" amplification in the source region led several authors to study the effects of perpendicular source inhomogeneity on TKR generation [Calvert, 1982; Vlasov, 1991; Louarn and Le Qudau, 1996a,b]. The most developed model is the laminar source model of Louarn and Le Qudau [1996a,b], in which propagation effects and mode conversion at the edges of the source significantly modify the predictions of standard "openloop" models. In that model (Figure 8) the radio source cavity maps an auroral arc, narrow in latitude (x direction) and elongated in longitude (y direction). Its main results are as follows. Dominant X mode is produced perpendicular to B near fx<fce in the cavity. It cannot exit the cavity at the altitude where it is produced, being unable to connect the X mode branch outside the cavity, where cold plasma dominates and fxout>fernitted =fce =fxin' After horizontal propagation across the arc curtain (approximately along the x axis), a fraction of the wave energy (about-25 db, or ~3 x 10-3) is converted into O mode and transmitted through the steep density gradient (x5) at the edge of the source. When horizontal propagation is more along the arc direction (y axis), marginal conversion to Z mode and transmission occur, most of the energy being reflected with small losses. These weak O and Z modes are those observed on the edges of the dominant X mode in Figure 7a. After upward propagation and several oblique reflections at the edges of the cavity, the emission reaches an altitude where its frequency matches the cold X mode cutoff outside the cavity

20 20,178 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS B Z Y X mod ( _.v// /,,X, / / ;Auroral arc OUT / _X mode / %,ø ***_ '... 0 mode.- v" ' -...,./ Electron I I-f-' density (cm-3)... Electron energy (kev) Figure 8. Sketch of laminar source model, displaying the model cavity parameters, the electron distributions inside and outside, the upward path of X mode waves, reflected (and partly converted into O and Z modes) at the edges. Due to this propagation, X mode exiting the cavity is mainly beamed in the longitudinal directions, along the arcs. Fine structures may be produced at auroral arc discontinuities. See section for details (adapted from Louarn and Le Qudau [1996a,b]). (fxout--femitted)' and it can thus exit the cavity there with a transmission coefficient ~1. At Earth, this occurs ~100 km above the generation level. The CMI regime is not modified in the source except that k vectors are quantified due to multiple reflections. Feedback may thus occur, and discrete modes should be produced, in agreement with the idea of laser-like amplification of radiation, first proposed by Calvert [1982, 1988] and Calvert et al. [1988]. This mode selection might explain many fine structures of TKR and possibly those of other AREs, as discussed below. spacecraft Freja detected intense lower hybrid wave turbulence in localized small-scale density depletions, at an altitude km in the auroral regions [e.g., Seyler, 1994]. This brings support to nonlinear conversion processes in the strong turbulence regime, such as Pottelette et al. [1992], which might coexist with CMI. However, the latter accounts for so many observations that it can certainly be considered as the only induced emission mechanism positively detected so far in a planetary magnetosphere Planetary Radio Bursts Finally, although the growth rate 7 is the same in both horizontal directions, the longer propagation paths along y and the weaker conversion efficiency X --) Z (as compared to X --) O conversion) result in a much higher intensity emitted along the y axis. The latitudinal beaming deduced from this model can be approximated by a hollow cone with half-apex angle > 40ø-50 ø relative to B. It is consistent with recent TKR ray-tracing performed in a model cavity, which produces a widely open filled cone of emission, but with 20 db-enhanced edges due to propagation [De Fdraudy and Schreiber, 1995]. Details of the beaming directly due to CMI seem to be smoothed out by propagation in the cavity. Strictly speaking, the laminar source model may be superimposed to any other microscopic generation mechanism As shown in section 2, all AREs are made of both smooth and bursty components, detected simultaneously or alternately. Due to their ubiquity at all the radio planets, radio bursts can no longer be considered as anecdotal (as was the case for Jovian S bursts at their discovery), and due to their similarities with solar and stellar radio spikes [Kuijpers, 1989; Barrow et al., 1994], their generation has gained the status of a general theoretical problem. High-resolution observations are available only for Jupiter (from the ground), and to a lesser extent for TKR. The limited time resolution of Voyager/PRA measurements (6 s at any given frequency) did not permit to decide whether Uranus and Neptune auroral bursts (as well as Saturn spikes) are true fast narrowband isolated spikes of radio emission or unresolved drifting features analog to Jovian S bursts (as suggested by on the X mode (at f--fx)' For example, the Swedish-German statistical studies cited in section 2.3.2).

21 ZARKA: AURORAL RADIO EMISSIONS AT THE OLYFER PLANETS 20,179 Following the mechanism proposed by Aschwanden and Benz [1988] for solar spikes, Louarn [1992] suggested that the nonlinear evolution of CMI (competition between wave growth and radiative loss of electromagnetic energy, and erosion and regeneration of free energy) could lead to pseudoperiodic solutions and thus quasi-stationary pulsations of the emission. However, planetary radio bursts are extremely intense: S bursts are the most intense Jovian radio component, and radio bursts at the other planets may exceed the intensity of smooth components by >30 db. Together with their very fine f-t structure ( Sf/f down to 10-3), they represent a challenge for the CMI [Le Qudau, 1988]. Many other theories have been proposed for their generation, mostly for Jovian S bursts or TKR fine structure. They can again be divided in three main classes Nonlinear conversion mechanisms. In the weak turbulence regime, Zaitsev et al. [1986] showed that conversion of upper hybrid to electromagnetic waves can be followed by amplification with a group velocity V g(f) which depends on the frequency, leading to f-t drifts. In the strong turbulence regime, Pottelette et al. [1992] showed that the interaction of lower hybrid solitons with a background of upper hybrid waves can produce intense bursts of X mode waves, consistent with TKR sporadicity The CMI. The bursts' discreteness may be attributed to the sporadic injection/excitation of unstable packets occur in that configuration, generating fine structures with random f-t drifts. Calvert [1982, 1988, 1995] invoked CMI in a closed-loop "lasing" regime, occurring in a narrow plasma cavity with steep edges, in order to achieve mode selection; this results in a macroscopic selective enhancement at several frequencies, and strong amplification over narrow bandwidths ( Sf<<f)at these frequencies. At Jupiter, he suggested that a variable size cavity may exist, being related to the motion of Io through the Jovian magnetosphere, which would continuously modify the conditions for mode selection and produce drifting narrow tones (the S bursts). The laminar source model of Louarn and Le Qudau [1996a,b] is a more advanced step in that direction, according to which fine structures can be produced at the moving longitudinal discontinuities of auroral arcs [Louarn, 1997]. However, at Jupiter, the production of DAM wavelengths would require an extremely narrow cavity (Lñ=100 m!). Finally, small time-variable perturbations of the Jovian magnetic field near the IFT, by Alfv n waves in Io magnetospheric wake, together with sharp radio emission beaming, could also account for some S burst characteristics (but quantitative studies remain to be done) Other direct mechanisms. Other direct mechanisms could "naturally" account for drifting bursts. The gyroemission of phase-bunched electrons (section 3.1.3) [Melrose, 1986] is still at a qualitative stage. Magneto-drift emission of highly energetic (MeV) gyrating electron beams has been proposed by Ryabov [1994] for S burst generation. Such high-energy electron flows have been observed by Ulysses in the Jovian magnetosphere [Zhang et al., 1993], but their origin, and especially their acceleration with short timescale, is unclear (see section 3.4 below). Wong and Goldstein [1990] showed that bursty emission primarily in the X mode could be generated for beam distributions with a temperature anisotropy (Tñ>>T//). The required anisotropic or gyrating electron beams could build from sporadic injections and time-of-flight effects, causing velocity filtering, between the acceleration region and the radio source [Winglee et al., 1992], which makes this mechanism a good candidate for radio burst generation. It is very complementary of CMI because it may generate X mode waves for different parameter ranges (0.4 < fpe/fce < 1), with a different beaming (filled cone with half-apex angle <30 ø around the source magnetic field), and it may reach an efficiency as high as that of CMI (>1%). Lastly, a more exotic source for discrete drifting structures electron populations (pulsed electron beams, etc.), the origin of which remains to be explained. To account for the bursts' f-t in dynamic spectra may be the effect of propagation (focusing, scintillations) of a smooth radio emission through a layered drifts, Staelin and Rosenkranz [1982] studied the low- medium (A. Lecacheux, personal communication, 1996), but frequency modulations of the radiating electron population by ion cyclotron waves. Menietti et al. [1996] interpreted, in that same way, TKR fine structures observed by Dynamic Explorer 1 and Galileo (stripes of ~1 khz x 3 s with drifts consistent with an exciter velocity ~1000 km/s, see section 2.4.1); they ruled out the explanations invoking a frequency-dependent this effect is probably limited to very specific configurations, like the rising or setting of a Jovian DAM radio source seen through low-altitude Jovian ionospheric layers. Many of the above mechanisms are consistent with the present, limited observational knowledge of planetary radio bursts. As discussed below (section 5), new observations and group velocity, in favor of fluctuations of fpe/fce or electron further in-depth analyses are required to identify the dominant pitch angle due to low-frequency ion cyclotron turbulence processes actually at work. For example, the observation of superimposed on the CMI. Pritchett and Winglee [1989] and McKean and Winglee [1991] analyzed, for the case of Earth, the CMI amplification in a source of finite extent in which the magnetic field is nonuniform: small-scale (< 100 km) amplifications of wave quantified emission frequencies and wave vectors would be a decisive argument in favor of a "lasing" source. In addition to the broad astrophysical interest for the identification of burst generation processes actually operating in solar system radio sources, the fine f-t structure of radio bursts opens new possibilities for probing the magnetoplasmas at their sources at a very small-scale, provided the knowledge of a "good" generation theory and scenario. Almost all of the above theories require the existence of sporadic electron accelerations/precipitations in the auroral regions or in the IFT vicinity. Freja and Viking spacecraft have revealed the existence of such sporadic processes in TKR sources, as discussed below Particle Acceleration A detailed discussion of this question is far beyond the scope of this paper. We restrict ourselves to a summary of the basic processes, especially those operating at Earth, as a framework for discussing the case of the outer planets (in section 4) General considerations. In order to produce the intense nonthermal AREs, electrons must be accelerated to energies much higher than the thermal or corotation values. Quasi-adiabatic processes, linked to compression, inward radial transport, and pitch angle scattering of the magnetospheric plasma (betatron and Fermi accelerations), are

22 20,180 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS known to be effective at least in the highly compressible Jovian magnetosphere [Stone et al., 1992], where they can accelerate electrons up to MeV energies [Hill et al., 1983]. However, these processes are generally slow (several planetary rotations), while AREs and, especially, bursts require fast, sporadic accelerations. Nevertheless, Farrell [1995] discussed the possibility of a Fermi acceleration process for generating TKR fine structure. Rapid processes are nonadiabatic and include magnetic reconnection, parallel electric fields, and heating by plasma waves. Reconnection occurs in Earth's magnetotail during substorms, in response to interplanetary magnetic field rotations (inverting its polarity at Earth), hence the localization of TKR sources about 2300 LT and their correlation with solar wind variations. It is thought to occur in the outer planets' magnetic tail as well, and possibly ahead of Io [Vasilyunas, 1975; Hill, 1975; Southwood et al., 1980]. The magnetic energy release (~B2/2go N per particle, with N the number density) may be in the form of particle heating and/or acceleration. However, sporadic reconnection processes are very complex and still poorly modeled. Parallel electric fields do exist in auroral regions (at least terrestrial ones) and near the IFT. They may be electrostatic or nonstationary (AlfvO, n waves) and have various origins: substorms or corotation (but the mechanisms are not understood), or interaction between Io and the Jovian magnetic field. Plasma heating may be due to electrostatic lower hybrid waves in the auroral regions of Earth [Bryant, 1992, and references therein] or to ion cyclotron or other VLF waves at Jupiter, as observed near Io torus [Farrell et al., 1993; Rdzeau et al., 1997]. However, this question also deserves further study Auroral acceleration structures at the Earth. Particle acceleration via parallel electric fields seemed to be most effective at Earth. The correlation of TKR and aurora with inverted-v electron events suggested that parallel electron acceleration results from kv electrostatic potentials distributed at high altitude (~1 RE) along highlatitude field lines. The origin of such localized kv potential drops and their role as aurora/tkr drivers were outstanding questions for a long time. The standard answer invoke0 parallel electric field buildup during substorms, within regions a few degrees wide in latitude, at the limit between open and closed field lines. The presence of these electric fields was thought to deplete auroral zones and cause kev precipitations, with inverted-v distribution. However, the typical perpendicular (latitudinal) width of inverted-v precipitation regions, when projected in the ionosphere, is a few tens of kilometers [Newell et al., 1996], much larger than discrete UV arcs. Benson and Akasofu [1984] also demonstrated that the existence of a large-scale cavity and of a bright UV arc in the underlying ionosphere are not sufficient conditions for detecting TKR emission. Viking in situ observations brought important new informations, showing that TKR is generated in small-scale filamentary or laminar cavities a few kilometers wide, much more depleted than the surrounding large-scale auroral cavity, and correlated to discrete narrow UV arcs. The presence of electron and ion beams in the sources supported their interpretation as acceleration regions corresponding to small-scale electrostatic potential structures (such as Figure 7b), UV arcs being attributed to the same precipitations that generate TKR. Following S3-3 measurements [Mizera and Fennell, 1977], Viking observations also helped to determine the shape of unstablelectron distributions (with f/ }vñ>o) near and in TKR sources: loss-cone and hollow beams were found primarily near the sources, and trapped electrons inside. The first two can be formed by electrostatic potential structures [Roux et al., 1993] (1) through collisional losses in the ionosphere, enhanced by parallel acceleration, and adiabatic mirroring of electrons for the loss-cone, where the positive gradients correspond to upward parallel velocities: V//(Z)/V2.(Z )_>[Bionosphere/B(z ) - 111/2; and (2) through localized parallel acceleration by a potential drop A V, followed by adiabatic evolution for the hollow beam, where positive gradients correspond mainly at Earth to downgoing electrons with v//= (2eAV/me)1/2. In contrast, the formation of trapped electron populations (about v// -- 0) requires a time-variable, spatially distributed electric field Ell(t), together with magnetic mirroring [Eliasson et al., 1979; Louarn et al., 1990]. Viking results thus improved our understanding and replaced the above outstanding questions with the following ones' What is the origin of small-scale acceleration structures? How does the highly conductive ambient plasma sustain kv parallel potential drops (this may be linked to the extreme rarefaction of the plasma)? What are the entry paths of the freshly injected plasma in acceleration regions? Block and Fiiltharnrnar [1990], for example, showed from Viking electric field measurements that the accelerating potential drop could be accounted for by numerous weak (~ 1 V) electrostatic double layers, and remarked that VLF electric field fluctuations may be seen as dc by electrons. High time-resolution in situ observations by Freja, at an altitude <1700 km in the auroral regions, focused the questions toward still smaller scales, revealing extremely narrow (<1 km) field-aligned precipitation peaks within inverted-v electron events [Liihr et al., 1994; Boehm et al., 1994] (confirming earlier results by Lin and Hoffman [1982]). These fine structures are the signature of complex time-dependent acceleration processes independent of electrostatic potentials, and correspond to discrete UV arcs only a few hundred meters wide in the ionosphere! According to Borovsky [1993], no theoretical explanation in terms of electrostatic fields in the high-altitude auroral zone can presently account for such narrow structures. Calvert [1997] and Calvert and Hardy [1997] reported recent measurements of the Oedipus-C rocket at an altitude ~500 km, during a substorm expansion, in support to this conclusion: these measurements reveal order-of- magnitude variations of the precipitated electron flux at a perpendicular scale ~1 km, without any correlated change of the electrons maximum energy. Calvert concluded that, while obviously associated to the TKR and aurora, electrostatic fields do not cause its (fine) structure. Calvert [1995] proposed a new substorm/aurora/tkr scenario to solve this controversy. In his model, the source regions are located on closed field lines (rather than on open ones in the "standard" model), along which spontaneous density fluctuations create local plasma depletions, where an incoming solar Type III burst triggers or enhances open-loop CMI. This provokes the scattering of electrons within the loss-cone, initially empty at the top of the auroral zone, and enhances the plasma depletion. Precipitations of electrons in the loss-cone cause diffuse aurora, while the substorm onset corresponds to that of closed-loop CMI regime (radio lasing), resulting in TKR triggering, enhanced precipitations,

23 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,181 acceleration of beams at the edges of the small-scale cavity where TKR intensity is maximum, and excitation of discrete UV arcs. In addition, particle drift might cause the longitudinal expansion of auroral arcs, and TKR may trigger the apparition and expansion of new discrete arcs. This appealing "reverse" model of the TKR/substorm relationship relies upon some speculative grounds, like the assumption of an empty losscone at the top of the auroral zone, and the efficiency of openloop CMI for enhancing plasma depletions (where do the necessary unstable electron distributions come from?). It also neglects the role of interplanetary magnetic field rotations or reversals in substorm onset triggering. Finally, the CMI triggering by Type III bursts has not be proven observationnally at Earth [Calvert, 1981b], and it would be still more problematic at the outer planets [Desch, 1988a], due to 1/R 2 decrease of the solar radio flux. Freja observations may provide an alternative solution for particle acceleration in terms of nonstatic electric fields (as already suggested by Viking measurements): very intense electromagnetic perturbations, identified as (solitary) kinetic Alfv.n waves [Louarn et al., 1994b] have been observed in strong correlation to auroral activity, together with ion acoustic waves [Wahlund et al., 1994]. Nonstatic parallel electric fields associated to the former can reach up to mv/m [Chust et al., 1998], and they provide a significant contribution to the dissipation energy budget, through plasma heating and/or acceleration [Volwerk et al., 1996]. These intense variable currents have been observed within very narrow small-scale structures (a few hundred meters across) located on the edges of larger field-aligned currents regions. The question of particle acceleration thus reduces to that of parallel electric field buildup, and at Earth the balance seems presently in favor of small-scale particle acceleration by nonstatic fields. The detailed nature and origin of these parallel electric fields is still not well understood Particle acceleration at the outer important solar wind pressure variations, and cause Fermi acceleration of particles along closed field lines, thus enhancing radio and UV auroras, as inferred at Jupiter by Prangd et al. [1993b]. 2. An alternate or additional source of particle acceleration is Kelvin-Helmholtz instability at velocity shear surfaces, i.e., at the interface between the solar wind and the magnetospheric plasma. It is strongly suspected to occur at Saturn's dayside magnetopause (see section 2.2 and Galopeau et al. [1995]) and at Uranus' magnetopause (see section and Pedersen et al. [1992a]). Parallel electric fields are expected to be associated to the surface waves resulting from the development of this instability. 3. QP bursts may also be associated to magnetic ULF waves originating from the current sheet (section 2.1.7). 4. Finally, wave-particle interactions may play a nonnegligible role in plasma heating, as suggested by the observations by Farrell et al. [1993] and Rdzeau et al. [1997] of VLF waves near Io toms, on field lines where HOM is also emitted (section 2.1.1). Rotation control is also much more pronounced than solar wind influence at Uranus and Neptune, which may be due to the large tilt of their magnetic dipoles, the precession of which makes particle dynamics and the magnetosphere shape more complex at these two planets. For example, it has been suggested that electron precipitations from Uranus' outer radiation belts could be the source of the b-smooth radio component [Curtis et al., 1987] Io-Jupiter interaction. At Jupiter the interaction of Io with the planetary magnetic field is thought to generate Alfv n waves [Neubauer, 1980], whose associated parallel electric field may accelerate electrons to kev energies or more [Crary, 1997]. As mentioned in section 2.1.3, far-uv observations of the IFT footprints allowed Prangd et al. [1996] to estimate the energy budget of IFT precipitations. They derived a precipitated power of a few x l0 TM W, planets. Contrary to the solar wind-driven terrestrial consistent with that deduced by Connerney et al. [1993] from magnetosphere, that of Jupiter is strongly controlled by the planetary magnetic field rotation and the presence of Io. Saturn is an intermediate case, with strong solar wind and rotation control of SKR, and radio sources on both open and closed field lines (see section 2.2 and Figure 5). Substorm-like activity probably exists at the giant planets, as revealed by field-aligned currents on high L shells (see, e.g., Dougherty et al. [1993] and Gdrard et al. [1993] at Jupiter) and by the solar wind control of radio components, IR observations, and with estimates of the total dissipated power ~1012 W per hemisphere. The latter is derived as -- 2 x Rio x E x I, E being the electric field across Io (E = v x B = 6 x Rj x 2j x B e / 63 = 5 x 105 V) and I the current which circulates between Io and Jupiter (-- a few 106 A as measured by Voyager [Acuga et al., 1981]). In addition, the brightness of the IR spot was found to be anticorrelated to the occurrence of DAM. This allowed Zarka et al. [1996a] to build a qualitative especially those emitted along open field lines. The Jovian scenario of the Io-Jupiter interaction leading to bkom (sections and 2.1.4), Uranus b- and n-bursts (section 2.3.1), and, probably, Neptune bursts (section 2.3.2). Some magnetic reconnection is also expected to occur in Jupiter's current sheet (and maybe in Saturn's ). The magnetic field amplitude there is of the order of 10 nt, and the density of electromagnetic emissions (UV, IR, and radio bursts). Alfv n waves produced at Io accelerate electrons which precipitate toward the planet. Some of these electrons are reflected due to the increase of magnetic field amplitude (adiabatic mirroring). Depending on this amplitude at the instantaneous IFT footprint, a variable proportion of precipitated electrons is charged particles is about 0.1 cm '3, corresponding to possible used to heat the ionosphere and produce IR radiation, while the particle acceleration up to ~2.5 kev energy [Hill et al., 1983]. rest is reflected with a loss-cone distribution and is thus able This process could be a source of kev electrons for the HOM, which originates on closed field lines possibly linked to the to produce negatively drifting S bursts (section 2.1.5). This scenario qualitatively accounts for the anticorrelation of IR inner current sheet. and radio outputs. Crary [1997] calculated that a power of some However, due to the large size and rapid rotation of the giant planets' magnetospheres, other acceleration mechanisms may occur, which are not observed at Earth. 1. Due to the compressibility of these magnetospheres (as observed by Ulysses at Jupiter [Stone et al., 1992]), largescale magnetospheric compressions may occur in response to x 10 TM W is converted from Alfv n waves into electron precipitations, over a broad range of electron energies: 1 to 500 kev. Only a fraction, ~109 W, of the precipitated power corresponds to electrons in the kev range. Those are assumed to be the primary source of radio emission, by analogy with Earth and in agreement with the energy deduced from S bursts

24 20,182 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS drift rates (section 2.1.5). Comparison with the S burst instantaneous power, ~108 to 109 W, leads to an "energy 4.2. Relevance of the CMI crisis" in the IFT, as these figures would imply an efficiency All the AREs present a high degree of similarity with TKR, >10% for the generation mechanism producing S bursts from which motivates the question: Is their generation primarily the electron kinetic energy (or worse, from their free energy) due to CMI? Prequisites for CMI are strongly magnetized [Zarka et al., 1997b]. The solution of this crisis requires either a generation mechanism different from the CMI (section 3.3), depleted regions with fpe << fce (but not necessarily and kev electron precipitations, the distribution of which more efficient, or able to use the electron energy over a contains positive gradients in perpendicular (or parallel) broader range, or a power source larger than 1012 W (magnetic velocities. reconnection ahead of Io?), or energy accumulation and release All the magnetic field and plasma observations at Jupiter with a ratio Ataccumulation / Atreleas e >>l. This question is still open, but the final kinetic-to-wave energy conversion demonstrated that fpe/fce remains typically < down to the planetary surface [Connerney, 1992; Divine and Garrett, efficiency is expected to be much higher than the usual values 1983; Bagenal, 1994]. Narrow cavities may also be present, (as derived at Uranus; see section 2.3.1). as suggested by high spatial resolution observations of the The use of S bursts for the remote sensing of the IFT at a UV aurora [Prangd et al., this issue]. At Saturn, the equatorial very fine spatial scale is discussed in section 4.6. confinement of the plasma due to the rapid rotation (as at 4. Auroral Radio Emissions at the Outer Planets' Observations Versus Theory We summarize and compare below the important observational results obtained for outer planets AREs (cf. also Table 1). We suggest possible interpretations within the theoretical framework described in section 3, and discuss some of the questions which are still to be solved. We show that direct extrapolation of the terrestrial situation to the outer planets sometimes raises contradictions Source Locations A recent and very important observational result is the first direct proof, using direction-finding and polarization measurements- that dominant auroral radio emissions at an outer planet (HOM and bkom at Jupiter) are generated near fce on the X mode (section 2.1.1). The corresponding sources are located near the limit between open and closed field lines, with HOM sources apparently located along closed field lines and bkom sources at least partly on open lines. In addition, bkom appears associated to UV auroral arcs and field-aligned currents, while HOM sources rather seem connected to the current sheet or Io external torus. We notice here that bkom appears generally more bursty than HOM emission (this remark will be used below). On the basis of its correlation with UV aurora and solar wind activity (sections and 2.1.4), non-io-dam could be interpreted as the high-frequency extent of HOM and/or bkom and thus also generated from the same field lines (L=7-15) but closer to the planetary surface. SKR sources are spread across open and closed field lines, depending on LT, and also correlated to auroral UV emission (section 2.2 and Figure 5). Uranus' b-smooth component originates from closed southern field lines, while b-bursts are generated along nearby open lines in the same hemisphere. Both sources are correlated to UV aurora, as well as n-bursts in the northern hemisphere. The latter are attributed to a cusp source and thus generated at the limit between open and closed field lines (section 2.3.1). Finally, the case of Neptune is less well-documented: the main smooth emission appears to come from sources at L--6-10, and the main bursty one from possibly higher magnetic latitudes (section 2.3.2). A given source is usually detected with a variable and limited longitude and/or LT extent. This limitation can be real or apparent, due to the nonisotropic beaming of the emission. cavities) Jupiter) depletes high-latitude regions. Galopeau et al. [1989] showed that CMI prequisites in terms of plasma depletion are fulfilled there, even in the absence of any cavity. The plasma dynamics in the magnetospheres of Uranus and Neptune is probably very different from that at Jupiter and Saturn, due to the large tilt of their dipole fields with respect to their rotation axis. The existence of auroral plasma cavities at these two planets has been suggested from radio burst beaming, but these results lead to a contradiction with the radio emission generation model, as discussed below. Finally, kev particles are present in the magnetospheres, and especially in the auroral regions, of all giant planets, as shown by direct measurements as well as whistler and UV measurements (see, e.g., Waite et al. [1988] for Jupiter, Sandel eta!. [1982] for Saturn, and Kurth and Gurnett [1991] and Herbert and Sandel [1994] for Uranus). For the IFT, many authors after Ellis [1962] attributed S burst emission to kev energy electrons, and Zarka et al. [1996a] measured 5 kev electron energies from S burst drift rates. CMI may thus operate in the auroral regions of outer planets, but in what regime does the CMI operate there? That is, is hot or cold plasma dominant, and is the source a largescale or a narrow cavity? The cold plasma paradigm has been ruled out at Earth due to the dominant presence of hot plasma in TKR sources (section 3.2.8). Quantitative studies of ARE generation by the CMI at Jupiter and Saturn assumed cold plasma propagation and a large-scale cavity (and hence openloop CMI) but have nevertheless been successful in accounting for observed properties. Saturation by homogeneous nonlinear trapping at Jupiter leads to very high saturated fluxes for the DAM: up to 10 '17 W m -2 Hz -1 as detected from a distance of 1 AU, consistent with the observations (Figure 1) [Le Qudau, 1982; Zarka et al., 1985]. This flux corresponds to a saturation electric field of several V/m in the source! Inhomogeneous nonlinear trapping at Saturn, using the relation i-- T (section 3.2.6) which does not depend on the details of the electron distribution, and simple models of Saturn's magnetic field and auroral ionosphere, allowed Galopeau et al. [1989] to derive the first theoretical model (cold, inhomogeneous, and saturated) of an ARE spectrum: that of SKR. Computed spectra are displayed in Figure 9 for different values of the plasma scale heights and of the electrons' perpendicular energy, within observed limits [Zarka, 1992c]. They match well the observed spectra.

25 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,183 Observed / Modelled SKR spectra 10 E c loo looo e! Frequency( khz ) Figur.e 9. Observed and modeled SKR spectra. Best fit of computed cold, inhomogeneous, and saturated SKR spectra (continuous lines; see section 4.2) are compared to spectra observed by Voyager 1/PRA 2-7 days after closest approach to Saturn (squares). Polarization is RH circular, i.e., corresponds to a northern source. These fits are obtained by varying the magnetospheric disk and the ionospheric plasma scale heights, and the electron perpendicular energy, within limits severely constrained by Voyager observations. They permitted to precisely match the corresponding values of these parameters (Hdisk= RS, Hionosphere km, and Eñ=l-4 kev), and to monitor their variations [Zarka, 1992c; Galapeau et al., 1989] Auroral Plasma Cavities? The existence of small-scale plasma cavities at outer planets is, however, important, as it can drastically modify the observed radio emission properties. Direction-finding studies of HOM and bkom, although they used the least sporadic data from Ulysses/URAP, nevertheless concluded that instantaneous sources are small-scale and fluctuate with time (section 2.1.1). This is supported by the high-resolution UV observations mentioned above [Prangd et al., this issue], which reveal extremely narrow features at the ionosphere. At Saturn, only indirect radio measurements (see section 4.6 below) allowed to derive very low electron densities (<1 cm -3) in SKR sources [Zarka, 1992c]. The case of Uranus. and Neptune is discussed below DAM polarization. The elliptical polarization of DAM has few counterparts in astrophysics because the polarization of radio waves propagating through a plasma where fpe/fce is not extremely small rapidly becomes circular. Jupiter observations imply electron densities lower than 1-5 cm -3 in and near the DAM radio sources, so that the polarization may remain elliptical as the waves propagate out of the source (section 2.1.2). This is much lower than the densities extrapolated from the Voyager plasma model of Divine and Garrett [1983] and suggests the possible presence of localized auroral cavities. Note that the production of coherent O and X mode from the same cavity could explain the origin of the DAM elliptical polarization (section 2.1.2) if both components were emitted in similar directions with comparable intensities. However, this seems difficult to achieve in the frame of the present generation mechanisms (section and Figure 8) The hollow cone paradigm. Many studies, mostly indirect, suggested that AREs are beamed along the narrow walls of a hollow cone of half-apex angle, typically >75 ø relative to the local source magnetic field [see Zarka, 1988, and references therein]. However, the recent observations described in section 2 demonstrate the need for revising this "hollow cone paradigm," in favor of a significantly smaller value: _)40-50 ø only for HOM and bkom (section 2.1.1), ø for DAM (section 2.1.2), down to 60 ø for the SKR (section 2.2) and Neptune main smooth component (section 2.3.2), and down to _ 50 ø for Uranus' b- smooth component (section 2.3.1). O mode emissions are even detected within filled cones of half-apex angle _ 40 ø at Jupiter. These moderate beaming angle values are difficult to reconcile with standard open-loop CMI when fpe/fce<o.1. In that case, the highest amplifications are found for quasiperpendicular propagation (section 3.2) [Wang et al., 1982; Le Qudau et al., 1984b; Ladreiter et al., 1991]. More filled emission patterns can be obtained through the CMI [Wang et

26 20,184 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS al., 1982], or invoking an anisotropic beam instability [Wong and Goldstein, 1990; Winglee et al., 1992], but they require fpe/fce_>o.1 to 0.3 and are usually associated with lower growth rates. As discussed above, such fpe/fce ratio seem significantly larger than those observed or inferred for ARE sources, but cannot be ruled out. On the other hand, the presence of a small-scale laminar source cavity wipes out the radiation diagram of the microscopic mechanism occurring in the source, in favor of a much less open beam (_> ø) governed by wave propagation in the cavity and at its edges (section 3.2.9). Other mechanisms have been invoked for solving the specific problem posed by moderately open hollow cones, as the use of a "spiralling-beam" electron distribution [Melrose and Dulk, 1993], but such a specific distribution has never been observed (at Earth) Auroral cavity "paradox". Q u a s i- perpendicular beaming seems to be the rule for most planetary radio bursts. Uranus' b-bursts are beamed at---90 ø from the magnetic field (section 2.3.1), Neptune's main bursts are beamed at _>75 ø (section 2.3.2), and S bursts at _>75 ø [Zarka et al., 1996a, 1997b]. It is thus tempting to conclude that smooth emission is more likely generated in small-scale cavities, while bursts originate from large-scale depleted regions. On the other hand, the fact that bursty emissions are often the most intense planetary radio components (section 3.3) would favor their generation through a closed-loop CMI regime, i.e., within small-scale cavities. The case of Uranus and Neptune raises a contradiction because auroral plasma cavities have been explicitly derived there in the framework of CM theory from observed large beaming angles [Farrell et al., 1991], while less open beams should be observed according to the laminar source model of Louarn and Le Qudau [1996a,b]. On the other hand, a consequence of the unusual magnetospheric configuration at Uranus and Neptune is the absence of plasma accumulation near the magnetic equator, where densities as low as <1 cm -3 have been measured [Belcher et al., 1991; Richardson et al., 1995]. Large-scale depleted regions, with fpe/fce<<l thus likely exist near the magnetic equator of these two planets (even if some cold plasma may have been "missed" by plasma detectors [Gurnett et al., 1990]). Fundamental X mode emission is thus possible there through the CMI, contrary to the case of the much denser equatorial regions of Jupiter, Saturn, and Earth. This may explain the existence of the Uranian n-smooth and Neptunian smooth high-frequency components, interpreted as equatorial X mode emissions from extended/distributed sources (sections and 2.3.2). However, as their names indicate, these components, produced in a large-scale depleted region, are smooth! Bandwidth "paradox". On the one band, propagation in a small-scale cavity should result in mode selection (lasing effect) and thus in narrow bands of emission (section 3.2.9). Observation of emissions with quantified frequencies (and wave vectors) at the outer planets would be a convincing demonstration of the existence of laminar cavities in their vicinity. On the other hand, Louarn [1992] suggested that a loss-cone distribution fills up as electrons propagate upward through the source region because its free energy is consumed through wave amplification. Free energy is thus mostly available for producing high-frequency radio waves, over a relatively small bandwidth. Conversely, the free energy associated to a trapped population could be regenerated at all altitudes by the timevarying accelerating electric field (section 3.4.2), therefore producing comparatively broadband radio emission. At Earth, trapped populations are mainly found within small-scale cavities, together with semi-filled loss-cone, while enhanced loss-cone distributions are observed outside these cavities [Louarn et al., 1990; Roux et al., 1993]. Broadband emissions should accordingly be produced in cavities and exit them after mode selection as a series of discrete bands, reminiscent of S bursts (but S burst drifts in the f-t plane require size-variable cavities). High-frequency narrowband emission should be generated outside of cavities (it could correspond to the "splitting" phenomenon; see section 2.1.4). But if the two scenarios finally result in isolated or series of narrowband emissions, where do broadband smooth emissions come from? From sections and it appears that the association between bursts and small-scale cavities on the one hand, and between smooth emissions and large-scale depleted regions on the other hand, is not straightforward. It is thus not possible to extrapolate directly the results, observational or theoretical, obtained at Earth for the TKR. Further theoretical studies are necessary (including a laminar source model with a loss-cone distribution in the cavity, which may be a realistic situation at the outer planets), as well as systematic correlation analyses of ARE sporadicity, bandwidth, frequency range, and beaming Rotation Versus Solar Wind Control A general question concerning ARE is that of the relative role of internal "control" (planetary rotation, magnetic field anomaly) versus external (solar wind variations, driving Kelvin-Helmholtz instabilities at the magnetopause, substorm activity in the magnetotail, reconnection and acceleration processes). AREs from Jupiter, Uranus, and Neptune appear strongly controlled by the planetary magnetic field rotation: detailed features of the dynamic spectra are repeatable with an accuracy of a few percent of the rotation period [Lecacheux et al., 1993]. In contrast, the variations of Earth and Saturn radio outputs are strongly driven by the solar wind pressure, density, and velocity fluctuations [Gallagher and d'angelo, 1981; Desch and Rucker, 1983] and by reconnection and substorm activity. Actually, both the rotation and the solar wind control are important at Saturn, where the SKR sources are fixed in LT but active only when a given longitude sector (coined"active") passes at noon LT. At Earth the TKR sources are fixed about 2300 LT and are independent of the rotation. Some degree of solar wind influence on Uranus' n-bursts and Neptune's main bursty component may reveal a weak substorm-like response of their magnetospheres to the solar wind, but the limited data sets at these two planets do not allow for extensive correlations. The weak substorm-like dependence of Uranus' n-burst source, localized in the cusp (section 2.3.1), suggests that the radiating electrons might have been accelerated by parallel electric fields associated to the ULF surface waves detected on Uranus' magnetopause, as in the case of Saturn (section 3.4.3). The weakness of the solar wind influence at Uranus and Neptune may be due to the large tilt of their magnetic dipoles. The interaction of the recently discovered magnetosphere of Ganymede with the Jovian field involves topological changes in the magnetic reconnection every ---5 hours [Kivelson et al., 1997] and may be a test bed,

27 ,, ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS 20,185 using Galileo's extensive measurements, for the study of the magnetospheres of Uranus and Neptune. At Jupiter the solar wind influence, although secondary to rotation, is nevertheless clearly detectable on most radio components, including the nkom, in spite of the fact that its sources are located deep within the inner magnetosphere. The Io-DAM is possibly the only exception to that solar wind control, but this must be checked (section 2.1.4). The detailed mechanisms through which the solar wind exerts its influence on radio emissions remain unclear: they may involve modifications of incident solar wind particle fluxes (density, velocity, interplanetary magnetic field?), reconnection processes in the magnetotail [Lepping et al., 1983; Lepping, 1985] or at the magnetopause [see, e.g., Sonnerup, 1985], modifications of the acceleration regions through parallel electric field buildup, or large-scale magnetospheri compressions. The latter were observed at Jupiter, in correlation with significant variations of UV and radio emissions [Prangd et al., 1993a,b]. Whatever the mechanisms involved, the solar wind appears to be an ultimate driver of all AREs, as suggested by Figure 10 and Table 1, which show that the total ARE radiated power (Pr) is proportional to the incident solar wind power on the magnetospheric cross section (P i)' Values normalized to the terrestrial ones are given in Table 1 and plotted in Figure 10 (a first version of which appeared in the work by Desch [1988b]). For details on the calculation of Pr and Pi, see Desch [1988b] and Zarka [1992a] Emission Modes, Bursts and Radio Zoos Viking observations demonstrated that dominant X mode TKR is accompanied by weaker O (and Z) mode, as already inferred by Wu and Lee [1979]. These authors suggested that O mode emission could occur directly at fce' but with a gain smaller by db than that of the X mode. As suggested in the laminar cavity model (Figure 8), wave propagation through steep density and temperature gradients (the cavity i i i... J.,/ u O. lo S Solar Wind Power / Source Homogeneity Figure 10. Radiated ARE power Pr (W) versus the input solar wind power on the magnetosphere cross section P i, (solid symbols), and versus the source homogeneity parameter A (open symbols). See sections 4.4 and 5.3, Table 1, and Zarka [1992a]. The linear correlation coefficient between Pr and P i is >99% (solid line). That between P r and A is >92% (dotted line), but it reaches >99% (dashed line) when Uranus and Neptune are excluded (the large tilt of their magnetic field may modify the homogeneity criterion). walls) can also convert a small fraction of X mode into O and Z modes. Weak auroral O mode emissions have actually been detected at the outer planets: skom at Jupiter, associated to bkom and/or HOM (auroral O and Z mode has also been detected at low frequencies in north polar regions, cf. section 2.1.7), and dayside component at Uranus, conjugated to the b-smooth component (cf. section 2.3.1). At Neptune, Voyager 2 observed very intricate circular polarization patterns, difficult to interpret. The other distinction between radio components, possibly more obvious than their emission mode (and hence observed polarization), is their smooth or bursty nature. As shown in section 4.3, it is difficult to distinguish between these two families using their beaming, bandwidth, or frequency range. But, as can be deduced from the above discussions (sections 2, 3.4.3, and 4.1), nearly all of the auroral radio bursts appear to originate on open field lines (including bkom), while smooth components originate on closed field lines (including HOM and equatorial X mode emissions). The smooth or sporadic nature of the emission could thus reflect that of energetic electron injection. This image is actually not clear at Saturn, where radio sources extend over both open and c16sed field lines and where emission structure is mixed, without any known correspondence between sporadicity and detailed source location. S bursts constitute a major exception, being generated near the IFT, but IFT field lines can there be considered as "open and connected to a generator: Io." An important question is thus the nature of the generator on open auroral field lines for the other planetary radio bursts. Radio bursts, including QP bursts or terrestrial LF bursts, may also be generated via mechanisms other than the CMI (as discussed in section 3.3 and suggested by the "energy crisis" in section 3.4.4), which would account for their bursty nature. At Jupiter, for example, acceleration by Io is expected to produce electron beams [Goldstein and Goertz, 1983], the presence of which are supported by VLF noise observations (sections and 3.4.3), and which should favor nonlinear conversion mechanisms. However, (1) very efficient mechanisms are uncommon, and (2) it should still be explained why this mechanism is more effective on open field lines. Such mechanisms could, however, be invoked for generating weaker components (e.g., auroral roar at Earth [Labelle et al., 1995]). The discrete nature of the bursts is a problem in any case. If attributed to sporadic electron injections, it displaces the problem to the origin of those pulsed injections. This is certainly a major topic to study in the near future, along with the more quantitative investigation of generation mechanisms other than CMI. It follows from this discussion that the apparently high complexity of the outer planet zoos of radio components (especially at Jupiter, Uranus, and Neptune) finds a "natural" classification in terms of (1) dominant X mode and associated O mode (plus marginal Z mode), (2) magnetic hemisphere of origin, and (3) smooth and bursty components differing by their source location and/or generation mechanism. In addition, the peculiar magnetospheric dynamics at Uranus and Neptune allows the generation of equatorial X mode emission, the presence of Io at Jupiter leads to S burst generation, and Neptune's complex magnetic field may lead to accordingly complex radio source structure (e.g., the quadrupolar sources of Sawyer et al. [1995]). One could wonder if in this case ITKR may be the smooth counterpart of the bursty TKR.

28 20,186 ZARKA: AURORAL RADIO EMISSIONS AT THE OUTER PLANETS As noted above, the knowledge of SKR, detected only around Voyager 1 and 2 flybys of Saturn, is very limited. The Radio and Plasma Wave System onboard the Cassini orbiter may reveal a new zoo of Saturnian radio components... in Remote Sensing A major interest of the good understanding of ARE generation mechanisms and scenarios is, in addition to the monitoring of outer planets' rotation rate [Higgins et al., 1997; Lecacheux et al., 1997], the ability to perform reliable radio, or better, multispectral, remote sensing of the complex and variable magnetoplasma environments of the outer planets (where in situ measurements are crually limited). The inference of auroral plasma cavities at Uranus and Neptune [Farrell et al., 1991] is a good example of the difficulties and limitations of the procedure (section 4.3.3). The Saturnian auroral magnetoplasma, very rarefied even in the absence of any cavity, allowed for the successful modeling of SKR spectra (section 4.2), with only two independent adjustable parameters (characterizing the electron density scale height and the electrons' perpendicular energy Eñ). Voyager measurements furthermore severely constrained the values of these parameters (within a factor of 2 for the scale active radio field line and the instantaneous IFT footprint [Prangd et al., 1996] is linked to the Alfvgn velocity between Io and Jupiter, and thus to the torus plasma density. All these promising techniques are not yet fully operational and require more interpretative work of the electromagnetic emissions (especially the radio emissions and, among them, S bursts). The torus integrated electron content, however, is more straightforward to derive from the Faraday effect that it produces on the DAM radiation propagating through the Io plasma torus [Dulk et al., 1992]. Using Voyager/PRA observations, not affected by the terrestrial ionosphere, Winglee [1986] also detected an enhancement in the density of energetic electrons in the torus at certain longitudes. Finally, as mentioned in sections 2.2 and 4.4, the strong correlation between solar wind fluctuations and the energy outputs of several ARE components (bkom, HOM, QP bursts, SKR, etc.) could allow us to monitor, although not very precisely, solar wind conditions simultaneously from the vicinity of the Sun (with SOHO and Global Geospace Science spacecraft in Earth orbit) to at least Jupiter (~5 AU) and Saturn (~10 AU), whose AREs are detectable from Earth's surface or orbit with sensitive instruments. Electromagnetic auroral emissions (radio, in particular) appear thus definitely as a very powerful tool for the remote diagnostic of planetary auroral magnetoplasmas. They open heights, and Eñ--l-10 kev) [Galopeau et al., 1989]. Relying upon this model, the harmonic analysis of the maximum possibilities of long-term monitoring in the near future. observed SKR frequency variations permitted to localize a magnetic field anomaly (multipolar terms) near the planetary 5. Perspectives surface and to estimate its magnitude [Galopeau et al., 1991; Galopeau and Zarka, 1992]. Fitting the observed SKR spectra 5.1. Theory and Models as a function of the above adjustable parameters (1) proved very successful in terms of spectrum shape (see Figure 9) and (2) allowed us to monitor remotely electron density and perpendicular energy variations (in the auroral regions) as a function of rotation and solar wind variations [Zarka, 1992c]. Finally, S bursts and associated UV and IR emissions are keys for understanding the Io-Jupiter interaction and performing remote sensing of the IFT. 1. S burst f-t drifts provide strong constraints on the magnetic field topology and the energy of the emitting electrons [Zarka et al., 1996a]. In addition, UV and IR observations of the instantaneous IFT footprint further constrain the magnetic topology, permitting an improved field modeling [Connerney et al., 1998], and an estimate of the total deposited energy [Prangd et ai., i996]. 2. The S burst microstructure [Carr et al., 1997; Zarka et al., 1997b] should give access to the small-scale source structure at a very fine spatial scale, depending on the available f-t resolution, to the radio emission beaming, and to information on the perturbations of the ambient field by Alfv n waves (amplitude, wavelength). 3. Modeling DAM "arcs" in the f-t plane over their full spectral extent (<1 to 40 MHz) is a complementary approach which uses information on the topology of a large portion (1- Without making an exhaustive list of the numerous theoretical needs listed in section 4, let us recall here that progress is needed in the study of nonlinear saturation mechanisms, of radio emission generation by the CMI in small-scale cavities with realistic electron distributions (e.g., loss-cone), and by other mechanisms as described in section 3.1, and in section 3.3 for bursts. The explanation of observed fine structures is particularly important: positive and negative drifts of TKR bursts, interpretation of detailed S burst shapes (Figure 4a). It may require an understanding of the origin of pulsed electron beams. Comparison with solar/stellar radio spike radiation processes may prove rewarding. Sound interpretations of the observations also require the best possible models for planetary magnetic fields (internal and external sources) [Hoime and Bioxham, 1996] and plasma distributions at the four giant planets. This is especially true for sensitive analyses as ray-tracing, which becomes less reliable when the medium is not well-known, especially if strong gradients are present (for instance, at cavity edges), or near the planet where the poorly constrained multipolar terms of the magnetic field dominate. Jovian magnetic field models have known a remarkable series of improvements, from the offset tilted dipole to the 04, 06, Ulysses, and VIP4 model [Connerney et al., 1981' Connerney et al., 1998] (including 2 Rs) of the source field line and not only the field magnitude contributions from the current sheet). The VIP4 model, which at its footprint (section 2.1.4). It should further constrain the topology of the magnetic field, as well as Alfv n waves propagation along/across it. 4. The multiplicity or nonmultiplicity of UV and/or IR spots at the ionosphere in Io wake gives information about the reflection of Alfv n waves at the ionosphere, and the variable lag (about 0o-20 ø) between the UV/IR spot or the reduces by a factor of 2 the discrepancy between the observed Io-DAM maximum frequencies and the surface gyrofrequencies at the instantaneous IFT footprint [Genova and Aubier, 1985; Genova and Calvert, 1988], does not yet provide a fully satisfactory frame for interpreting radio observations [Queinnec and Zarka, 1998]. Saturnian magnetic field models have followed a similar way (temporarily ending with the SPV

29 ZARKA: AURORAL RADIO EMISSIONS AT THE O PLANETS 20,187 model of Davis and Smith [1990] modified by H. P. Ladreiter et al. (unpublished paper, 1994) to also take into account radio constraints; see section 2.2). The situation is not so good at Uranus and Neptune, and plasma models are poor everywhere except at Jupiter, where Ulysses and Galileo measurements permitted to build better models of the ionosphere and of the Io toms [Hinson et al., 1997; Moncuquet, 1997] Observations Numerous questions directly related to observations are still unanswered. Many of them, concerning morphological problems, require statistical analyses. They include the identity of bkom observed before and after the Ulysses- Jupiter encounter; the LT dependence of HOM, bkom, non-io- DAM; the location, and nature, of the active SKR sector; and the origin of modulation lanes, striations, and "splitting" in Jovian decameter dynamic spectra (section 2.1.4), and that of the organization of S bursts within specific frequency bands [Ryabov et al., 1997]. This list is not exhaustive. The need for synoptic monitoring is particularly crucial for periodic or quasi-periodic phenomena with long periods: the search for the control of various radio components (for instance, the non-io-dam) by Ganymede or Europa; that of the Io-DAM by the solar wind; the determination of the low-frequency limit of Io-DAM and S bursts and their variations with longitude; etc. As a consequence, large databases, systematic archiving of ground-based and spacecraft radio data (and also of multispectral and in situ data), and development of computer algorithms for automated recognition, classification and even analyses of data become increasingly relevant and useful. For example, the magnetic field anomaly of Saturn was deduced from the analysis of a database of 3 months of SKR data around each Voyager-Saturn encounter, free of interference and with true (corrected) polarization [Galopeau et al., 1991]. De Lassus and Lecacheux [1997] have developed an algorithm for the automatic recognition of LF planetary radio emissions in the f-t plane, which helped to study the solar wind control of nkom (section 2.1.4). The S burst emission scenario (sections and 3.4.4) was built on the basis of the automated recognition and analysis (drifts, fixed-frequency duration and instantaneous bandwidth) of tens of thousands bursts [Farges, 1994]. All these automated methods are still in their infancy, and Voyager, Ulysses, and ground-based (e.g., Nan :ay) data sets are far from being exhausted. They will constitute an invaluable source of new studies with improved tools in the future. program) with HST observation campaigns in UV, and groundbased IR and decameter observations. Cassini, which will orbit Saturn between 2004 and 2007, carries the most sophisticated radio experiment ever flown to an outer planet, with high resolutions, full polarization, and quasiinstantaneous direction-finding capabilities. If activated during the cruise or near Jupiter, it will provide invaluable observations in the 1-15 MHz range, very poorly studied except by Wind/WAVES (this range was coined the "radioignorosphere" by M.L. Kaiser). Multispectral projects, as small UV and radio satellites devoted to the global monitoring of outer planets from Earth's orbit [e.g., Zarka et al., 1996b], are under consideration in several space agencies (NASA, CNES, ESA). They would provide complementary information on the objects studied, in terms of spatial, spectral, and temporal resolution, as well as in terms of energy ranges, and they might be much more frequenthan large missions of in situ planetary exploration. Observations of the last unexplored magnetosphere, that of Mercury, could reveal lowfrequency radio emissions blocked by the high solar wind plasma frequency. Finally, numerous ambitious perspectives are under study for the longer term, to reexplore in detail Uranus and Neptune systems, orbit the polar regions of Jupiter, or install a low-frequency radio telescope on the farside of the Moon. The next century promises to be interesting! 5.3. Auroral Radio Emissions at the Very Outer Planets? One very interesting perspective for the near future is the attempt to detect radio emissions from exoplanetary magnetospheres. In our solar system, the low-frequency radio range is dominated by nonthermal emissions from the planets and from the Sun. Contrary to any other frequency range (visible, IR, UV, etc.), planetary radio emissions can be as intense as solar radio emissions in the kilometer-to-decameter wavelength range. As seen in section 4.4, the solar wind appears to be an ultimate driver of all AREs, whatever the mechanism for its influence, with a conversion efficiency about 10-6 to In addition, Zarka [1992a] showed that the radiated ARE power also increases together with the "source homogeneity" A=LB/X., where L B is the planetary magnetic field gradient length and X. the radio wavelength (see Table 1 and Figure 10). It is interesting to extrapolate Figure 10 to the most favorable conditions: large, strongly magnetized planet, very close to its parent star, with the latter expeling a dense Ground-based observations allow higher f-t resolutions, but and/orapid wind. One can thus predict radio emissions up to statistical studies using them, e.g., the question of solar wind >105 times more intense than Jovian DAM [Zarka et al., control of Io-DAM, generally require very large data sets because they are discontinuous (a few hours per day). Spacecraft radio data are continuous and not limited by the Earth ionosphericutoff (<5-10 MHz), but they are limited by generally low data rates. Both kinds of data are thus very complementary and, increasingly often used together (see, e.g., the studies of DAM arcs with Wind/WAVES and the Nan :ay decameter array over the whole 0-40 MHz range by Lecacheux et al. [ 1998] and Queinnec and Zarka [ 1998]). For being really fruitful, future observations need to use very sophisticated technologies but should also be 1997a]. Similarly, extrapolating the Io-Jupiter interaction may lead to sporadic radio emissions ~105 times more intense than S bursts, especially in the case of "hot Jupiters" like 51 Peg B [Mayor and Queloz, 1995] if the planet is weakly magnetized. In that case, the planet may play the role of Io and be magnetically coupled to its parent star, accelerating electrons and driving intense radio emissions in the stellar corona. With such flux estimates, Zarka et al. [1997a] conclude that detecting exoplanetary radio emissions above the fluctuations of the galactic background is possible up to a distance of ~25 parsecs with a radiotelescope of effective area comparative, coordinated, and if possible, multispectral. ~105 m 2, which results in several tens of candidates. The only Galileo/PWS observations, limited to <5.6 MHz, are adapted instrument is in Kharkov (Ukraine), and a coordinated (in the frame of the International Jupiter Watch French/Ukrainian/Austrian team began the observations in

30 20,188 ZARKA: AURORAL RADIO EMISSIONS AT THE OtYI'ER PLANETS The most difficult challenge of this search is the elimination of man-made interference, which prevents highsensitivity observations [see Zarka et al., 1997a]. 6. Concluding Remarks Because they are remotely observable, auroral radio emissions have been one of the best studied magnetospheric phenomena in the past two decades. Very significant advances have been achieved, concerning both observations and theoretical modeling, especially in the past 4-5 years (since the COSPAR reviews of Louarn [1992] and Zarka [1992a]). Many of these results have been obtained by European scientists (even if this review is unvoluntarily biased), which is unfortunately not reflected by the citations in many U.S.- authored papers. In preparing this review, dealing with both observations and theories, it appeared that one can now learn much from the comparative and critical analysis of the huge number of published studies. Appendix ß Acronyms, Abbreviations, Notations, and Definitions AI. ARE AU bkom CMI CML DAM HOM Io-DAM 1TKR KOM LH (RH) nkom non-io-dam PRA PWS Q3 R s Acronyms and Abbreviations auroral radio emission astronomical unit broadband kilometer emission (Jovian) Cyclotron Maser instability central meridian longitude (Jovian longitude System III) decameter-wavelength emissions (Jovian) frequency-time (plane) hectometer-wavelength emissions (Jovian) Io flux tube Io-dependent (or Io-controlled) DAM isotropic terrestrial kilometric radiation kilometer-wavelength emissions (all five "radio-planets") McIlwain "shell" parameter (distance of a dipolar field line at the equator, in planetary radii) low frequency left-hand (right-hand) polarization ocm time Jovian narrowband kilometer emission Neptunian kilometric radiation Io-independent DAM Octupole model (octupole part of 6th degree and order model) of Jupiter internal magnetic field Octupole model (octupole part of 8th degree and order model) of Neptune internal magnetic field Planetary Radio Astronomy experiment (onboard Voyagers 1 and 2) Plasma Wave Subsystem (onboard Galileo) Quadrupole model (quadrupole part of 3rd degree and order model) of Uranus internal magnetic field quasi-periodic (Jovian bursts) Jovian radius -- 71,400 km Saturnian radius = 60,330 km Saturnian kilometric radiation terrestrial kilometric radiation Uranian kilometric radiation ultralow-frequency Unified Radio and Plasma Wave Experiment (onboard Ulysses) very low frequency A.2 Notations and Definitions fce, fci fpe' f,,i fuh, J lh fo, fx,fz œ electron, ion gyrofrequency (cyclotron frequency) electron, ion plasma frequency upper and lower hybrid resonances O, X, Z mode low-frequency cutoff = (fpe/fc e )2 = rne/mi, electron to ion mass ratio One has the following useful relations: fci --!l Xfce fpi = g l/2 X fpe fpe=œ1/2xfc e Characteristic frequencies of a magnetized plasma (with œ<< 1 and O mode cutoff: fo --fpe X mode cutoff: fx --[((fce-fci)/2 )2 + fpe2ll/2 + (fce_fci)/2 --fce(1 + œ- --fce(1 + œ) Z mode cutoff: fz - [((fce-fci)/2 )2 + fpe 2] 1/2 _(fce_fci)/2 =fce X œ Upper hybrid resonance: fuh-- [fce 2 + fpe 211/2 -- fce (1 + rd2) (= upper (n+ 1/2)fc e band) Lower hybrid resonance: flh - [fcofcix0epe 2 + fcofci)/0epe 2 + fce 2)11/2 --fce X [[l(œ+!l)] 1/2 --fce X Stokes parameters (characterizing wave polarization): I total intensity linear polarization ratios V circular polarization ratio Q2+U2+V2_-I for a completely polarized wave (Q2+U2)I/2-p linear polarization degree V--2T/(I+T 2) with T -- axial ratio of polarization ellipse Acknowledgements. The author thanks F. Bagenal for having trapped him with an invitation to give this review (although in a less advanced state) to the Magnetospheres of the Outer Planets conference, in Boulder (March 1997), R. Prang6 for the fruitful cooperation carried with her on these topics for now a few years, J.-L. Steinberg for his numerous suggestions to improve the manuscript, and the Editor for having stretched the deadline beyond his patience (or vice versa). References Acufia, M. H., F. M. Neubauer, and N. F. Ness, Standing Alfvb, n wave current system at Io: Voyager 1 observations, J. Geophys. Res., 86, , Alexander, J. K., and M.D. Desch, Voyager observation of Jovian millisecond radio bursts, J. Geophys. Res., 89, , Anagnostopoulos, G. C., P. K. Marhavilas, E. T. Sarris, I. Karanikola, and A. Balogh, Energetic ion populations and periodicities near Jupiter, J. Geophys. Res., this issue. Anderson, R. R., and W. S. Kurth, Discrete electromagnetic emissions in planetary magnetospheres, in Plasma Waves and Instabilities at Comets and in Magnetospheres, Geophys. Monogr. Ser., vol. 53, edited by B. T. Tsurutani and H. Oya, pp , AGU, Washington, D.C., 1989.

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