Ion-cyclotron wave generation by planetary ion pickup

Transcription

1 Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Ion-cyclotron wave generation by planetary ion pickup C.T. Russell a,, X. Blanco-Cano b a Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA , USA b Institute of Geophysics, UNAM, Ciudad Universitaria, Codigo 451, Coyoacan, Mexico Received 23 August 26; received in revised form 27 November 26; accepted 12 February 27 Available online 14 July 27 Abstract Ion-cyclotron waves play important roles in planetary magnetospheres and are diagnostic of the processes operating in the magnetosphere and of the composition of the plasma producing the waves. At Jupiter, Io s exosphere interacts with the corotating magnetospheric plasma. At Saturn, the neutral torus around the E ring interacts with the corotating plasma. At the unmagnetized planets, Mars and Venus, the interaction is between the solar-wind flow and the planetary exosphere. A possible analog of these processes exists in the vicinity of the Earth s polar cusp where the shocked solar-wind plasma penetrates the Earth s exosphere. r 27 Elsevier Ltd. All rights reserved. Keywords: Ion-cyclotron waves; Venus; Mars; Jupiter; Saturn 1. Introduction The widespread deployment of rapid-run magnetograms during the International Geophysical Year led to the discovery of Pc-1 pulsations with periods shorter than 15 s (Troitskaya, 1961). These were immediately realized to be associated with ion-gyro instabilities at high altitudes in the magnetosphere (e.g. Troitskaya and Gul elmi, 1967). Such waves have been long believed to play an important role in the maintenance of an upper limit to the flux of energetic ions in the Earth s magnetosphere (Cornwall, 1965; Kennel and Petschek, 1966). In this mechanism, the parallel velocity of the energetic ion leads to a Doppler shift in the wave frequency. When the particle and the wave meet head on, the particle sees the wave s electric Corresponding author. Tel.: ; fax: address: ctrussell@igpp.ucla.edu (C.T. Russell). and magnetic oscillations at its gyrofrequency, even though in the rest frame of the plasma the wave frequency may be well below the ion s gyrofrequency. In the rest frame of the wave, energy is conserved so that, if energy is transferred betweenthewaveandthe particle, one may easily calculate how the motion of the particle is altered (cf. Brice, 1964). If, in the headon interaction of ion and left-handed ion-cyclotron wave, the wave energy increases, not only does the ion energy decrease by the same amount, but also the velocity parallel to the magnetic field increases and the velocity perpendicular to the magnetic field decreases. Thus, when the ion-cyclotron waves gain energy, the pitch angle of the ion decreases. Similarly, if the phase of the interaction is such that the wave loses energy in the head-on interaction, the particle gains energy and thepitchangleincreases. The phase of the interaction of waves and the ions should be random so that some waves will increase in energy and some waves will decrease. The same is /$ - see front matter r 27 Elsevier Ltd. All rights reserved. doi:1.116/j.jastp

2 1724 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) true for the ions. If, however, there is an anisotropy in the pitch angle distribution so that (for example) there are more particles with low parallel velocities than with high parallel velocities (as might happen if particles traveling along the magnetic field were lost in the atmosphere), then there will be an imbalance in the number of particles that randomly increase and randomly decrease their parallel velocity and their energy. This anisotropy is often called the losscone anisotropy, because in a planetary magnetosphere it is maintained by particle loss in a small cone, aligned with the magnetic field direction. Here waves grow as particles diffuse toward the loss cone, i.e. to smaller pitch angles. Thus, ion-cyclotron waves can regulate a magnetosphere s content of energetic ions. In the Earth s magnetosphere, the plasma rest frame is usually determined by the cold plasma from the ionosphere that slowly rotates with the Earth in the plasmasphere and convects around the plasmasphere controlled by the solar wind at higher altitudes. At Jupiter and Saturn the plasma in the magnetosphere generally has a non-ionospheric source, e.g. the atmosphere of Io in Jupiter s case and a torus of neutral molecules in the region of the E ring in Saturn s case. These two planets are large, with planetary radii an order of magnitude greater than that of the Earth, and magnetospheres that are yet again another factor of 2 1 times larger than their terrestrial counterpart. Moreover, these two gas giants rotate almost 2.5 times faster than the Earth so that the velocity of the corotating plasma in the magnetosphere, where new plasma is being created, is significant, up to many tens of km/s. The unstable ion distributions in the magnetospheres of Jupiter and Saturn are quite different from the unstable ions discussed above for the Earth. Newly created ions (through impact with an energetic electron, by photoionization, or by charge exchange) will feel an outward electric field (in the case of Saturn and Jupiter that have magnetic fields opposite to that of the Earth). The ion is accelerated radially outward and begins to gyrate and drift with the corotating plasma. The ion velocity distribution is a ring at constant velocity in the direction perpendicular to the magnetic field with little parallel velocity. Theory predicts that, for pickup velocities greater than about 3 km/s in these magnetospheres, ion-cyclotron waves will grow with a peak growth rate just below (5 1% below) the ion gyrofrequency. The energy available to be released in the form of waves ranges up to 5% of the pickup particle energy when the wave velocity is much larger than the particle velocity. Since the pickup velocity is generally well determined, the wave energy flux created in this process can be used to monitor the mass-loading rate. Thus, ioncyclotron waves in a planetary setting can provide important diagnostics of the ion production processes, of both the composition and the mass produced. In this brief overview of ion-cyclotron waves in planetary magnetospheres, we first examine the waves at Jupiter and then at Saturn. Next we examine the waves at two unmagnetized planets, Mars and Venus, and finally return to the Earth to examine ion-cyclotron waves that may also be related to the ion-pickup process. 2. Ion-cyclotron waves in the Jovian magnetosphere Prior to the Galileo encounter with Io, the paradigm for the magnetospheric interaction with this Jovian moon was that of the interaction of a flowing magnetized conducting fluid with an electrically conducting body, the so called unipolarinductor model (Piddington and Drake, 1968; Goldreich and Lynden-Bell, 1969). In fact this model does not apply to Io because it has little ionosphere and its surface is non-conducting. Moreover, the flow is principally deflected around Io and only a small portion of the incoming flow ever becomes connected to the body via magnetic field lines (Russell and Huddleston, 2a). However, Io does have sufficient atmosphere that is ionized by impact ionization, photoionization and charge exchange to add enough mass to the magnetospheric flow, arriving via a corotation, to slow and deflect the magnetospheric plasma around Io. This mass-pickup process causes many of the phenomena attributed originally to unipolar induction, such as a strong field-aligned current system that creates radio noise bursts, modulated both by Io s location in the magnetosphere and by the lineof-sight to the observer. While it was known that Io was introducing close to 1 ton of ions per second into the Jovian magnetosphere based on the Voyager observations (Hill et al., 1983), the details of the mass-loading process were not understood until Galileo made its first close flyby in December Ion-cyclotron waves played a very critical role in determining what was happening when the corotating plasma encountered the Io atmosphere (Kivelson et al., 1996).

3 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Waves were seen at the gyrofrequency of SO 2 + ions with amplitudes approaching 1 nt peak to peak. These waves were consistent with a mass-loading rate near Io of about 3 kg/s (Huddleston et al., 1998). Dynamic spectra of the waves seen are shown in Fig. 1 (Russell et al., 23), illustrating the variety of observed signatures. On the first encounter on December 7, 1995, the inbound trajectory (top left) found ion-cyclotron waves at the SO + 2 gyrofrequency beginning about 2 Io radii outward from Io, a distance far larger than the expected size of its exosphere. On the outbound trajectory, the main ion-cyclotron waves were at the SO + gyrofrequency and the waves were again seen far from Io but not as far as on approach to Io. The switch in the unstable ion is possibly due to a compositional difference Fig. 1. Dynamic spectrum of the transverse power in the oscillations on five Io encounters. The time derivative was taken to flatten the spectrum before the Fourier transform was calculated.

4 1726 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) between the dayside and nightside atmospheres (Russell et al., 23). The reason for the great extent of the atmosphere will be discussed below. The next pass on October 11, 1999 (middle left) shows how narrow the wave spectrum can become and shows that waves can be generated by two separate ions at the same time. In fact, at about 443 UT there are waves at the S + gyrofrequency as well. The next pass on February 22, 2 (middle right) is different again, with more S + emissions and fewer SO 2 + emissions and a dominant SO + emission throughout. On August 6, 21 (lower left), the whole spectrum appears to be shifted downward. On this pass, the field lines cross into the wake region downstream from Io where the magnetic field is weak and the gyrofrequency is lower than at the spacecraft that remains above the wake. Thus we believe that the lower emission frequency simply reflects the lower gyrofrequency where the waves are being produced. Finally, on October 16, 21 (lower right), the waves appear most strongly at the SO 2 + gyrofrequency as they did inbound on the first pass. So the ion-cyclotron waves help us determine not only how much mass is being added to the magnetosphere but also the specific composition of that mass addition. Fig. 2 shows the geometry of these encounters. Two important points to learn from this figure are that the waves occur almost exclusively downstream from Io (to the right in Fig. 2) and the waves arise far beyond the region in which the flow and atmosphere should be interacting. We note that the I trajectory of December 1995 crosses both the I24 and I27 trajectories. Where they cross, the spectra are quite different on the different passes, suggesting that the Io atmosphere is quite temporally variable. Fig. 3 illustrates the mechanism that creates the unstable particle distribution that leads to wave growth. At the orbit of Io, the magnetosphere, locked into corotation with the ionosphere, is rotating around Jupiter at 74 km/s. Io orbits at 17 km/s so that the plasma is moving at 57 km/s relative to Io and its ionosphere. Any ion produced at Io will feel an outward electric field (in the reference frame of Io) corresponding to the product of the 57 km/s relative velocity and the 2 nt southward-directed magnetic field. The newly born ion begins to drift and gyrate about the field, creating a characteristic cycloidal trajectory in space. In the corotating plasma frame, the motion is circular at a fixed velocity and the ions form a ring in velocity space. If the initial neutrals are cold, the ring will be quite narrow. The ring also has very little parallel velocity with which to Doppler shift the emission away from the local gyrofrequency. Dispersion analysis shows that the peak growth rate from such a particle distribution occurs at a frequency just below the ion gyrofrequency, which depends on many factors such as the beam velocity and the plasma beta (Huddleston et al., 1998; Blanco-Cano et al., 21). The growth of waves takes energy from the perpendicular motion and adds energy to the parallel ion motion. Eventually the free energy is expended and the particles are thermalized. The energy extracted from the ions propagates away from the orbital plane of Io and is lost. The integrated Poynting vector, or energy flux, is proportional to the mass-pickup rate. The energy added to the plasma and that carried away by waves ultimately come from the rotational energy of the planet. The bulk motion of the plasma generates a strong centrifugal force that stretches the magnetic field lines (eventually) into a disk-like configuration and drives a circulation pattern that carries ions to the tail, where they are dumped by reconnection down the tail, and empty flux tubes return to the inner magnetosphere (Vasyliunas, 1983; Russell, 21). One mystery this discussion has not cleared up is why the waves are observed so far from Io. The waves are clearly produced by new ions, so new ions must be being created far from their original source, Io. Neutrals should not last long in the Jovian magnetosphere and you might expect a neutral atmosphere to be roughly symmetric around Io, not restricted to downstream of the corotational flow terminator and well to the two sides of Io. Fig. 4 shows the mechanism that explains this odd distribution (Russell et al., 21; Wang et al., 21). Ions are initially created close to Io and accelerated, but they are neutralized before they can move very far, so the charge-exchange probability is still high. The newly created neutrals retain the velocity of the ions, but are not trapped on field lines, and can move long distances across the magnetic field and downstream (but not upstream). In a uniform plasma, the translational motion of the neutral particle between its two ionized states is simply a translation of the particle to a new location, preserving the phase of the gyro motion. This mechanism confines the mass-loading region to a thin disk, but spreads the mass-loading region over a wide area. It also explains why Galileo saw

5 Y [Io Radii] Ion Cyclotron Waves Mirror Mode Closest Approach Flow Jupiter Corotation I24 I I27 Sun X [Io Radii] Flow Ion Cyclotron Waves Jupiter Sun Corotation Fig. 2. The trajectory of Galileo on the five Io encounters on which the dynamic spectra shown in Fig. 1 were obtained. The regions in which ion-cyclotron waves are seen are illustrated. The coordinate system keeps the flow velocity to the right and the electric field due to the corotation of the plasma is projected on the Y-direction outward from Jupiter according to the instantaneous model field at Io I32 I C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27)

6 1728 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Ion Pickup V B V inj = V IO - V CO = -57 km/s B Plasma Frame VIO 17 km/s "Ring"-Type Distribution V ll V = 57 km/s V CO ~ 74 km/s F(V ) Time 57 V B V E = -V x B Io/Neutral Rest Frame V Pickup Ion τ Days V Fig. 3. The ion-pickup mechanism at Io. The magnetospheric plasma corotating at 74 km/s meets Io orbiting at 17 km/s, and any ions produced accelerate in an electric field corresponding to 57 km/s. The particles follow cycloidal paths in the inertial frame and form rings in velocity space in the plasma frame. The rings give up their energy with time and become thermalized. Atmosphere Neutrals near torus Ionization pickup ions Neutralization hν fast neutral distant torus pickup ions To Jupiter Io hν hν slow neutral Sun light v Torus plasma flow Fast neutral B E Neutralization Ionization of Neutral Slow neutral Fig. 4. Mechanism for forming a thin mass-loading disk that enables the production of pickup ions far to the side of the ions but not in the upstream direction. Ions are first formed near Io in its moderately dense atmosphere, where charge exchange occurs frequently enough that freshly accelerated ions can be converted to fast neutrals. The fast neutrals then travel some distance across the magnetic field before the neutral again becomes an ion and releases its free energy as ion-cyclotron waves.

7 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Log Power Spectral Density [nt /Hz] Transverse Compressional Galileo Voyager Log Frequency [Hz] Fig. 5. Power spectra of the ion-cyclotron waves seen near Io by Galileo and in the same frequency range near Io by Voyager. The Galileo measurements are in the orbital plane of Io, whereas the Voyager data are 1 RIo below the orbital plane. ion-cyclotron waves at its Io encounter but Voyager, 1 Io radii below the moon, saw none as illustrated in Fig. 5 (Russell and Huddleston, 2b). 3. Ion-cyclotron waves in the Saturnian magnetosphere In contrast to the point source for mass loading at Io in the Jovian system, the rings of Saturn present us potentially with a distributed mass source. No moon of Saturn, nor any ring, is active volcanically like Io, and none is capable of producing as much pickup ion mass. Nonetheless, Saturn has abundant ion-cyclotron waves. In fact it exhibits a much more extensive region of ion-cyclotron wave activity than Jupiter. Fig. 6 shows a dynamic spectrum of the power spectral density of ion-cyclotron waves on a pass of the Cassini spacecraft through the Saturn magnetosphere. Lines corresponding to the water group and O 2 + gyrofrequency show that these waves are generated by water-group ions and not sulfur compounds as at Jupiter. Two features of the dynamic spectrum are worthy of note. Around the closest approach (perikron) the ion-cyclotron waves disappear. There are no ion-cyclotron waves inside about 3.8 Rs. Also, the waves weaken with distance after reaching a maximum strength near 4 5 Rs. It is instructive first to examine the conditions for wave growth at Saturn. Fig. 7 shows the expected growth rate for ion-cyclotron waves in the Saturnian magnetospheric plasma as the velocity of the ring increases (left) and as the density of the ring increases. To find the dispersion relation, we considered a plasma with ring distributions of H 2 O + and OH +. For velocities above 32 km/s the wave growth rate is low and is positive over only a limited range of frequencies. At 5 km/s the growth rate is large and extends over a wide frequency range. Similarly, the growth rate depends on the mass of the ring (right). At low ring densities (3 cm 3 ) the growth rate is small and positive over a narrow range of frequencies. When the number density is higher (15 cm 3 ), the growth rate is higher and a larger range of frequencies is unstable. While Enceladus at 3.9 Rs is a major source of water molecules for populating the E-ring neutral torus, the resulting cloud appears to extend far outward from the Enceladus orbit. We do not yet know the mechanism behind this transport. Perhaps it is due to fast neutrals as at Io, but at lower velocities. It is instructive to examine the velocities of picked-up ions as a function of radius in the equatorial plane. Fig. 8 shows the orbital or keplerian velocity, the escape velocity (41% higher than orbital), and the corotational velocity. The pickup velocity is the difference between the corotational velocity and the keplerian velocity. This velocity is zero just inside 2 Rs at Saturn s synchronous orbit. Inside this point the electric field due to corotation in the reference frame of the moon is inward because keplerian velocity is greater than the corotation velocity and the downstream direction or wake switches to the side opposite the direction of orbital motion. Outside the synchronous orbit, the electric field in the frame of the moon is outward and the pickup velocity increases with radius so that at Enceladus it reaches about 3 km/s and we expect waves to be growing, as illustrated in Fig. 7 for typical plasma and ring beam conditions in the Saturn magnetosphere. The cutoff in ion-cyclotron waves inside 3.8 Rs seen in Fig. 6 is probably due in part to the lower ring speed at these distances, but, as Fig. 7 illustrates, low ring mass also produces a lower growth rate. As we noted above, the ion-cyclotron waves weaken with radius beyond about 5 Rs. Since the pickup velocity continues to increase with radial distance, we

8 173 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Fig. 6. Dynamic spectrum of ULF waves seen on passage of Cassini through the closest approach on March 9, 25. The derivative has been calculated before determining the spectrum in order to flatten the spectrum. Local gyrofrequencies of mass 16, 19, and 32 ions are shown ω/ω H2 O γ/ω H2 O +..1 V ring = 5 km/s 4 km/s 32 km/s k (km 1 ) k (km -1 ) Fig. 7. Growth rate of ring beam distributions of varying pickup speed and varying ring density for conditions appropriate to the E-ring torus region. The WHAMP program (Ronnmark, 1982) was used in this calculation.

9 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Velocity (km/s) Equatorial Distance (Saturn radii) Maximum Velocity Corotational Velocity Pick-up Velocity Escape Velocity Keplerian Velocity Fig. 8. Characteristic velocities of the Saturnian magnetosphere. The dash dot line shows the Keplerian or orbital velocity; dotted line the escape velocity, solid line the corotational velocity; and dash dot dot line the maximum velocity of a picked up ion. interpret this falloff as also signaling a decrease in mass-loading rate at these distances. The amplitude variations of the pickup ion waves very much resemble those that earned Pc1 pearl pulsations their name in the terrestrial magnetosphere. Fig. 9 shows 3 min of detrended magnetic field records in the region of the E ring (Russell et al., 26b). These waves were observed in midafternoon, 1524 LT, but waves in the night sector are very similar as the Saturn magnetosphere at these radial distances is very rotationally symmetric and dominated by internal process and not the solar-wind interaction. The right-hand panel shows the transverse and compressional power as a function of frequency. The maximum wave growth is just below the water-group ion gyrofrequency, as expected for a low beta plasma and cold ring beam. The weak compressional peak indicates that the waves are propagating nearly parallel to the magnetic field. A second lower frequency transverse peak suggests the presence of a second heavier ion. The weak compressional peak at about 5 s period is probably due to weak mirror-mode waves. We do not expect to see mirror-mode waves in plasmas because the temperature anisotropy that feeds their growth also leads to ion-cyclotron waves and the ion-cyclotron wave is almost always more unstable. Fig. 1 shows waves further out on the same Cassini pass, and, contrary to our expectations, there are strong mirror-mode waves and no ion-cyclotron waves. One lesson from Earth for Saturn that we might wish to apply here is that mirror-mode growth can exceed that of the ioncyclotron wave if the beta is large (42) and there is a heavy ion species (45% in density) (Russell and Farris, 1995), but the closeness of the frequency of Detrended Magnetic Field [nt] B, B theta B r B phi LT 5.13 R s Power Spectral Density [nt 2 /Hz] Transverse MM Compressional Ω UT December 24, 25 ICW Universal Time December 24, Frequency [Hz] Fig. 9. Detrended one-second magnetic field measurement in Saturn s E ring region showing transverse ion-cyclotron waves due to watergroup ion pickup. Left, time series. Right, power spectrum.

10 1732 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Detrended Magnetic Field [nt] B r B phi B, B theta LT 6.87 R s Power Spectral Density [nt 2 /Hz] MM Transverse Ω UT December 24, 25 Compressional Universal Time December 24, Frequency [Hz] 1 Fig. 1. Detrended one-second magnetic field measurements in Saturn s E-ring region showing compressional mirror-mode fluctuations. Left, time series. Right, power spectrum. the peak amplitude to the water-group ion-gyrofrequency indicates that at Saturn the plasma beta is not high. Instead, what we believe has happened is that closer to Saturn in the unstable region ion-cyclotron and mirror-mode waves both grew, albeit the mirror mode more slowly. The ion-cyclotron waves propagated along the magnetic field, and once the satellite moved out of the unstable region these waves ceased as their continued presence requires continued generation. This is not true for the mirror mode. These waves do not propagate but should just be convected outward from Saturn with the plasma flow. Hence, what we see in Fig. 1 may simply be the maximum amplitude waves that grew closer to Saturn and were convected outward by the radial component of convection. If there is a lesson here, it is from Saturn to Earth. Fast-neutral production can be an effective transport and loss process, and some waves may appear even though their growth rates are low, simply because they do not propagate out of the growth region whereas other modes do. 4. Ion-cyclotron waves at Mars and Venus In the sections above, we have examined ion pickup of sulfur compounds produced by the volcanoes of Io and water-group ions produced ultimately by the water plume on Enceladus that produces the neutral torus around the E-ring. We do not have volcanoes in the Earth s magnetosphere and we have not examined the magnetospheres of Uranus and Neptune to see what they might add to our understanding of this important magnetospheric process. However, we do have other manifestations of ion pickup in the solar system. At Mars and Venus, proton-cyclotron waves have been seen (Brain et al., 22; Russell et al., 199, 26a). These waves appear in the magnetosheath and nearby solar wind because the solar wind can interact directly with the atmospheres of these planets that do not have shielding magnetospheres. It is important to monitor the strength of ion pickup at these planets to determine the rate of loss of the atmosphere through the solar-wind interaction. In particular, H loss may be a proxy for water loss from these two very dry planetary atmospheres. Mars cold, thin atmosphere does not hold much water even when it is saturated. Venus is presently very dry throughout the body and atmosphere, and any atmospheric water is probably tied up in the sulfuric acid clouds. The ions are picked up in the supersonic flow of the solar wind. This alters the energy given up to the waves as the ions diffuse along almost spherical velocity shells in the measurement frame, losing only a little energy to the wave as they scatter. Even

11 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Three Second Average Magnetic Field Mars Solar Orbital Coordinates [nt] B B z B y B x MGS Universal Time December 27, 1997 Fig. 11. Time series of proton-cyclotron waves seen in the solar wind near Mars. though the flow is supersonic, the wave energy is still produced at the proton-gyrofrequency because the newly produced planetary ions are picked up perpendicular to the magnetic field and it is the motion of the ion along the field direction in the reference frame of the plasma that leads to Doppler shift of the emitted wave frequency. Fig. 11 shows an example of proton cyclotron waves at Mars observed by the Mars Global Surveyor spacecraft (Wei and Russell, 26). As we discuss below, we have chosen an interval of strong background magnetic field, not because the waves only appear there but also because the zero level of the measurement is uncertain and the higher field strength periods can be trusted more for relative accuracy. The waves are seen at all field strengths (but not at all field directions). The waves resemble those seen at Jupiter and Saturn. Their amplitude is modulated; they propagate mainly along the magnetic field and they occur in a narrow band, here close to the proton-cyclotron frequency. Fig. 12 shows a power spectrum of these waves. As evident from the time-series plot, the waves are mainly transverse with a small compressional component. They are also left-hand circularly polarized, peaking close to the proton cyclotron frequency. Detailed analysis, not repeated here, reveals other important features of the waves. First they are seen to large distances from Mars up to 12 RM. At these Power Spectral Density (nt 2 /Hz) Compressional Power MGS December 27, 1997 Transverse Power Log Frequency (Hz) Fig. 12. Spectrum of the transverse and compressional power of the time series shown in Fig. 12, illustrating that the waves occur close to the proton-cyclotron frequency. large distances the waves appear intermittently. This intermittency is controlled by the interplanetary magnetic field direction. Fig. 13 shows that when the magnetic field is greater than 5.6 nt the waves only appear on the side of the planet where the interplanetary electric field points. We would expect this if the particle transport were controlled by the IMF direction. We note that this behavior breaks Ω p

12 1734 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) down gradually as the field strength drops, as the data have a 1 2 nt uncertainty. We assume that the physics is unchanging, but our ability to calculate the electric field direction is changing. Z (R Mars ) 1 1 B perp Cases with 5.6 nt <B total Mars 1 1 Y (R Mars ) Fig. 13. Distribution of wave occurrence in an electric magnetic coordinate system, whose interplanetary magnetic field strength is sufficiently large to neglect the zero level uncertainties. E Venus data reveal a similar, but slightly different, picture. Fig. 14 shows proton-cyclotron waves in the Venus magnetosheath as detected by Pioneer Venus at solar maximum, but it has not returned evidence for such waves in the solar wind (Russell et al., 26a). This may be because of limitations of the volume swept out by the orbit of Pioneer Venus, but it also could be that the Venus ionosphere at solar maximum prevents as strong a coupling of the solar wind to the atmosphere as we find at Mars. The Venus Express measurements at solar minimum should shed some light on this behavior. The observation of proton-cyclotron waves at Mars and Venus leads to a picture very similar to that we developed for Io. Fig. 15 shows this model. As at Io, fast neutrals are produced when newly born and accelerated ions are neutralized by charge exchange. The ex-ions become a spray of fast neutrals in the plane perpendicular to the magnetic field and principally on the side of the planet in which the electric field points. There they are ionized again at a greater distance from the planet. The IMF is not steady and when it rotates around the planet sun line the neutral particle disk (or exodisk) rotates with it and the wave source region will move away from the location of the spacecraft. Thus, this model explains both the great extent of the proton cyclotron waves and why they turn on and off. Magnetic Field in Spacecraft Coordinates [nt] B b z b y b x : : : : :54 Universal Time May 3, 1979 Fig. 14. Time series of magnetic field measurements in Venus Solar Orbital coordinates obtained by Pioneer Venus in the magnetosheath during a period of proton-cyclotron waves.

13 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) V B (Z) E (Y) Near Wake Pickup Ions hν hν Fast Neutral Distant Exodisk Pickup Ions Mars, Venus Slow Neutral hν Solar Wind Flow Fast Neutral Atmosphere Neutrals Ionization Neutralization Neutralization Ionization of Neutral Slow Neutral Fig. 15. Possible mechanism for producing an extended exodisk at Venus and Mars. Close to the planet, but in the flowing solar plasma, protons are created from the neutral hydrogen exosphere by photo, impact ionization, or charge exchange. These protons are accelerated in the solar-wind electric field and then subsequently neutralized by charge exchange. The fast neutral, so produced, crosses the magnetized solar wind to great distances until it is ionized, forming a proton, ring beam that is unstable to proton-cyclotron waves. 5. Ion-cyclotron waves in the earth s polar cusp As discussed in the introduction, ion-cyclotron waves have long been recognized as playing an important role in the Earth s magnetosphere. One region where their occurrence is a bit puzzling is in the Earth s polar cusp. They were first observed at high altitudes by the OGO-5 spacecraft (Russell et al., 1971; Scarf et al., 1972; Fredricks and Russell, 1973) and at lower altitudes by Erlandson et al. (1988). The range of frequencies is generally between the local proton and alpha particle cyclotron frequency about 4 7 Hz at the location of OGO-5. More recently these waves have been studied using Polar data near 9 RE in the region of the cusp (Le et al., 21) Fig. 16 shows 4 typical power spectra when Polar entered the polar cusp. Three of the spectra are narrow, with peak amplitudes just below the local proton-cyclotron frequency. This behavior is exactly like those of the ion-cyclotron spectra seen at Saturn and consistent with pickup of cold fresh ions from a stationary exosphere of hydrogen in a flowing plasma. The Earth does have a hydrogen exosphere that extends this high and it is possible that the magnetosheath electrons present in the cusp ionized these neutral hydrogen atoms or that the solar-wind protons charge exchanged with them, but what mechanism could produce the ring beam in velocity space that is needed to produce in turn the waves? The sighting of brief cusp encounters like these suggests that the plasma is flowing rapidly past the spacecraft. Magnetopause crossings frequently take place at tens of km per second, and so it is natural to assume that the plasma in this region is moving at similar velocities. Thus we do have a possible energy source, basically the solar wind but due to buffeting by variations in pressure, rather than having the solar-wind stream through the neutrals. One event, at 319 UT on May 24, 26, is unusual in that the shape of the power spectrum is like the others but it is significantly downshifted from its expected frequency. This downshift cannot be caused by a hot plasma because the peak is

14 1736 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) B x B y B z B (nt) B x B y B z B (nt) B x B y B z B (nt) B x B y B z B (nt) Universal Time May 24, 1996 Log Power (nt 2 /Hz) Log Power (nt 2 /Hz) Log Power (nt 2 /Hz) Log Power (nt 2 /Hz) :55-237:16 a 318:52-319:27 b fcp 354:33-355:53 c fcp fcp 435:2-436:2 d fcp Log Frequency (Hz) Fig. 16. Examples of ion-cyclotron waves seen during traversals of the polar cusp by the Polar spacecraft on May 24, The power spectra of waves seen at these times and the local proton-gyrofrequency are shown on the right-hand panels. narrow in frequency, so the Doppler-shifting velocity must have a small spread about a mean. Our experience at Mars can help us here. This beam of fast neutrals creates an exodisk of fast neutrals that become ionized well away from Mars. There is little or no Doppler shift away from the local gyrofrequency at Mars because the interplanetary magnetic field and the field in the fast-neutral source region are very similar. However, at Earth, with its own internal field, the source region for the fast neutrals and the ion-production region may have different field orientations, so the newly ionized beam may be traveling partially along the local field and not across it, thus creating a Doppler shift of a narrow spectral peak. Unfortunately, we know no way to test this hypothesis with the Polar instrumentation.

15 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Conclusions Our examination of ion-cyclotron waves at the planets reveals that ion pickup provides a ubiquitous source of free energy for their production. Io produces a localized source of sulfur-compound ring-beam ions near Io, but the waves are seen over a wide range to the scales of Io as well. This extensive mass-loading source can be explained by the creation of a massloading disk of fast neutrals through charge exchange near Io. At Saturn, the E-ring torus produces an extended source of water-group pickup ions. It seems obvious that fast-neutral production also plays an important role in maintaining the neutral torus from its origin in the plasma at Enceladus. The source region for the ion-cyclotron waves is the equator and the energy flux for the waves carries the energy away from the equator. Thus, when the source disappears, the waves disappear. This is not true for slower growing mirror-mode waves. These waves do not leave the source region, but build up in amplitude and are carried outward in the magnetosphere. At the unmagnetized planets, Mars and Venus, the solar wind can interact more directly with the atmosphere and produce pickup ions and ioncyclotron waves, but here too we find ion-cyclotron waves far from the expected ion-pickup region. Again,we see a need for a fast-neutral exodisk that spreads the mass-loading region to the side of the planet. The Venus Express data obtained at solar minimum will provide an interesting complement to the Pioneer Venus data at solar maximum when we expect the Venus atmosphere was better shielded from the solar-wind flow. Finally, this tour of planetary magnetospheres, intrinsic and induced, allows us to better understand some observations in our own magnetosphere. In particular, waves in the region of the polar cusp seem to be due to ion pickup also. We envision ionization of the terrestrial hydrogen exosphere by the polar cusp electrons and/or protons with energy supplied to the ions by the compressions and rarefactions of the magnetospheric plasma. In conclusion, we find that, in whatever planetary magnetosphere we make measurements, ion-cyclotron waves have proven themselves able to provide good diagnostics of the processes occurring therein. Acknowledgments The authors have worked with many students and colleagues on the study of these waves. CTR is particularly indebted to R.J. Strangeway, D.E. Huddleston, G. Le, M.G. Kivelson, S. Mayerberger, H. Wei, M. Cowee, and J. Leisner for their assistance with this work. CTR is also grateful for support from the National Aeronautics and Space Administration for their support for the OGO-5, Polar, Pioneer Venus, MGS, Galileo, and Cassini investigations over the years. References Blanco-Cano, X., Russell, C.T., Strangeway, R.J., 21. The Io mass loading disk: wave dispersion analysis. 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In: Physics of the Jovian Magnetosphere. Cambridge University Press, New York, pp Huddleston, D.E., Strangeway, R.J., Warnecke, J., Russell, C.T., Kivelson, M.G., Ion cyclotron waves in the Io torus: wave dispersion, free energy analysis, and SO 2 + source rate estimates. Journal of Geophysical Research 13, Kennel, C.F., Petschek, H.E., Limit on stably trapped particle fluxes. Journal of Geophysical Research 71, Kivelson, M.G., Khurana, K.K., Walker, R.J., Warnecke, J., Russell, C.T., Linker, J.A., Southwood, D.J., Polanskey, C., Io s interaction with the plasma Torus: Galileo magnetometer report. Science 274, Le, G., Blanco-Cano, X., Russell, C.T., Zhou, X.-W., Mozer, F., Trattner, K.J., Fuselier, S.A., Anderson, B.J., 21. Electromagnetic ion cyclotron waves in the high-altitude cusp: polar observations. Journal of Geophysical Research 16, Piddington, J.H., Drake, J.F., Electrodynamic effects of Jupiter s satellite Io. 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16 1738 C.T. Russell, X. Blanco-Cano / Journal of Atmospheric and Solar-Terrestrial Physics 69 (27) Russell, C.T., Farris, M.H., Ultra low frequency waves at the Earth s bow shock. Advances in Space Research 15 (8/9), Russell, C.T., Huddleston, D.E., 2a. Ion-cyclotron waves at Io. Advances in Space Research 26 (1), Russell, C.T., Huddleston, D.E., 2b. The unipolar inductor myth: mass addition or motional electric field is the source of field aligned currents at Io. Advances in Space Research 26 (1), Russell, C.T., Chappell, C.R., Montgomery, M.D., Neugebauer, M., Scarf, F.L., OGO-5 observations of the polar cusp on November 1, Journal of Geophysical Research 76 (28), Russell, C.T., Luhmann, J.G., Schwingenschuh, K., Riedler, W., Yeroshenko, Y., 199. Upstream waves at Mars: phobos observations. Geophysical Research Letters 17, Russell, C.T., Blanco-Cano, X., Strangeway, R.J., Wang, Y.L., Raeder, J., 21. A mechanism for the production of diskshaped neutral source cloud at Io. Advances in Space Research 28, Russell, C.T., Blanco-Cano, X., Wang, Y.L., Kivelson, M.G., 23. Ion cyclotron waves at Io: implications for the temporal variation of Io s atmosphere. Planetary and Space Science 51, Russell, C.T., Mayerberger, S.S., Blanco-Cano, X., 26a. Proton cyclotron waves at Mars and Venus. Advances in Space Research 38, Russell, C.T., Leisner, J.S., Arridge, C.S., Dougherty, M.K., Blanco-Cano, X., 26b. Nature of magnetic fluctuations in Saturn s middle magnetosphere. Journal of Geophysical Research 111, A1225. Scarf, F.L., Fredricks, R.W., Green, I.M., Russell, C.T., Plasma waves in the dayside polar cusp. I. Magnetospheric observations. Journal of Geophysical Research 77, Troitskaya, V.A., Pulsations of the Earth s electromagnetic field with periods of 1 15 s and their connection with phenomena in the high atmosphere. Journal of Geophysical Research 66, Troitskaya, V.A., Gul elmi, A.V., Geomagnetic pulsations and diagnostics of the magnetosphere. Space Science Reviews 7, Vasyliunas, V.M., Plasma distribution and flow. In: In: Dessler, A.J. (Ed.), Physics of the Jovian Magnetosphere. Cambridge University Press, London, pp Wang, Y.L., Russell, C.T., Raeder, J., 21. The Io mass-loading disk: model calculations. Journal of Geophysical Research 16, Wei, H.Y., Russell, C.T., 26. Proton cyclotron waves at Mars: exosphere structure and evidence for a fast neutral disk. Geophysical Research Letters 33, L2313.