Plasmas observed near local noon in Jupiter s magnetosphere with the Galileo spacecraft

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2002ja009795, 2004 Plasmas observed near local noon in Jupiter s magnetosphere with the Galileo spacecraft L. A. Frank and W. R. Paterson Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA Received 25 November 2002; revised 8 June 2004; accepted 28 June 2004; published 23 November [1] Measurements of electron beams and thermal ions were achieved by the Galileo spacecraft during the later phases of its mission for three outbound passages of the local afternoon and noon sectors of Jupiter s magnetosphere. During the first two passages the spacecraft passed through the rigidly corotating plasma torus with outer boundary near 8 R J, into the transition region from torus to plasma sheet, and then into the plasma sheet proper at distances beyond 25 R J. Telemetry coverage for the third passage began in the transition region. At the outer boundary of the torus the ion densities and temperatures were 500 cm 3 and 10 6 K. In the transition region the ion densities decreased with increasing distance to 0.1 cm 3, and temperatures increased to K. In the plasma sheet the ion densities were typically 0.1 cm 3 with temperatures of 10 8 K. The azimuthal component of plasma flow slows to 70% of the corotational value in the transition region. For the first two passages, strong radial flows toward Jupiter increase with increasing radial distances. For the third passage near local noon the plasma flows are considerably more stagnant than those at local evening. The variations in the flow suggest that the solar wind influence extends to radial distances in the range of 10 R J. Electron beams parallel and antiparallel to the magnetic field were observed during the passages on field lines connected to the main auroral ring in the ionosphere. Major constraints on the heating/acceleration mechanism for the main ring are (1) the presence of the electron beams at and near the equator and (2) previous remote observations of emissions in the vicinity of the atmospheric methane layer. These constraints support a heating mechanism in this layer driven by magnetospheric convection without acceleration by field-aligned electrostatic fields. At radial distances R J near local noon the plasma densities exhibited a System III effect, increasing once per planetary rotation near a longitude 300. In addition, the finding of thermal ion beams directed parallel to the magnetic field near 20 R J resolves the problem of radial force balance previously identified with Voyager measurements. That is, the outward centrifugal force for these beams is sufficient to balance the inward radial force of the magnetic field. INDEX TERMS: 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); 2704 Magnetospheric Physics: Auroral phenomena (2407); 5737 Planetology: Fluid Planets: Magnetospheres (2756); 5780 Planetology: Fluid Planets: Tori and exospheres; KEYWORDS: Jovian auroras, Jovian plasma sheet, planetary magnetospheres Citation: Frank, L. A., and W. R. Paterson (2004), Plasmas observed near local noon in Jupiter s magnetosphere with the Galileo spacecraft, J. Geophys. Res., 109,, doi: /2002ja Introduction Copyright 2004 by the American Geophysical Union /04/2002JA009795$09.00 [2] Our investigation is directed toward measurements of thermal plasmas near the local noon sector of Jupiter s magnetosphere with the Galileo spacecraft. Specifically, a radial distance range of R J (Jovian radii) is of present interest. Previous observations have been acquired during the inbound segments of the flybys by Pioneer 10 in 1973, Pioneer 11 in 1974, Voyager 1 and 2 in 1979, and Ulysses in The latitudes of their inbound trajectories were near the equatorial plane of Jupiter with the exception of Pioneer 11, which was located at a latitude of 11 south of the equator. The Galileo spacecraft later passed through the local noon sector as it approached injection into orbital motion about Jupiter in December However, the fields-and-particles measurements were acquired only at radial distances <7.8 R J and inside the Io torus. The initial position of the Galileo apojove was located in the dawn sector of the magnetosphere, and it was not until January of 2002 that Jupiter s orbital motion about the Sun allowed the observations of thermal plasmas near local noon. It is one of the remarkable achievements of the Galileo spacecraft that it survived the extreme radiation environment of this giant planet for such a long period of time. 1of19

2 [3] Pioneer 10 and 11 provided the first direct measurements of the planetary magnetic field and the high intensities of energetic charged particles in the Jovian magnetosphere. The immense size of this magnetosphere and its rapid rotation with a 10-hour period which spun the plasmas in its outer magnetosphere into a thin disc were among the impressive features observed with Pioneer spacecraft. Overviews of the energetic particle intensities are provided by McDonald and Trainor [1976] and Van Allen [1976], and overviews of the magnetic fields are provided by Smith et al. [1974, 1976] and Acuña and Ness [1976a, 1976b]. It was not until the flybys by the Voyager spacecraft that substantial information concerning the lowenergy plasmas was acquired in the local noon sector of the Jovian magnetosphere. The two most relevant instruments were a system of Faraday cups covering the energy per charge (E/Q) range of 10 V to 5.95 kv [Bridge et al., 1979a, 1979b; Belcher, 1983] and an array of solid-state sensors covering the electron and ion energy ranges of tens of kev and above [Krimigis et al., 1979a, 1979b; Krimigis and Roelof, 1983]. [4] From these Voyager results for the local noon sector of the magnetosphere it was found that the plasma flows were approximately in rigid corotation outward to 20 R J. At radial distances beyond this to 50 R J in the plasma sheet the flows were generally in the corotational direction but with speeds usually in the range of 50 60% of the expected value for rigid corotation [McNutt et al., 1981; Belcher, 1983; Krimigis and Roelof, 1983; Mauk and Krimigis, 1987; Kane et al., 1995; Sands and McNutt, 1988]. These two spacecraft were within 1.5 hours of local noon in the dayside magnetosphere. The energetic ion sensors were used to show that the temperatures of the heavy ions such as oxygen and sulfur beyond 10 R J were in the range of kev and that the ion pressures were comparable to the magnetic pressures [Krimigis et al., 1981; Krimigis and Roelof, 1983]. Measurements of the thermal electrons near a radial distance of 17 R J showed that there were two distributions, one with temperature of 1 kev and density 0.5 cm 3, and a cold distribution with temperature of 20 ev and density of 4 cm 3 [Scudder et al., 1981; Belcher, 1983]. For the dayside plasma sheet, McNutt et al. [1981] reported that there was a local time asymmetry in the thermal ion bulk flows with a component of bulk flow directed away from the plasma sheet on the dayside and toward the sheet in the night sector of the magnetosphere. The temperature of the ions in the plasma sheet, 20 ev, was also reported to be cooler relative to plasmas at higher magnetic latitudes. In a later report, Kane et al. [1992] used detailed analyses of both the energetic particle and thermal ion data to show that there was no major disparity between these two measurements at 30 R J. [5] The balance of magnetic, centrifugal, and ion pressure forces in the dayside plasma sheet has attracted considerable attention. On the basis of Voyager measurements, Mauk and Krimigis [1987] have pointed out that there is an excess of magnetic field radial forces in the radial distance range of R J. That is, the centrifugal and pressure gradient forces which are directed outward are insufficient to balance the inward directed magnetic force. Sands and McNutt [1988] used thermal ion measurements to show that these were important for determining the centrifugal terms in the radial force balance and that the pressure gradient terms were primarily due to the heavy ions in the energy range of tens of kev. Their work also showed that there appeared to be a missing contributor to the radial force balance. Later analysis of the observations from the two Voyager spacecraft by Paranicas et al. [1991] found that the force balance was satisfied on the nightside of the magnetosphere but that the determination of the balance in the dayside sector was not secure. [6] Although the thermal ions were not measured during the dayside passage of Ulysses through the Jovian magnetosphere, measurements of the energetic ions [Staines et al., 1996; Hawkins et al., 1998] and of the thermal electrons [Phillips et al., 1993] confirmed that the plasma flows at radial distances of R J were primarily azimuthal at speeds of km s 1, significantly lesser than those for rigid corotation. The inbound trajectory was generally positioned above the magnetic equator, but there were three crossings of the current sheet at radial distances of R J. These measurements are compared by Cowley et al. [1996]. The Ulysses magnetometer clearly recorded the crossings of the equatorial plasma sheet during its inbound trajectory, but the major field-aligned currents were detected in the dusk sectors at the higher magnetic latitudes achieved by the deflection of the spacecraft orbit by Jupiter s gravity [Balogh et al., 1992; Dougherty et al., 1993, 1998]. [7] Recently, the presence of transient energetic ion and electron phenomena, qualitatively similar to the storm activity in Earth s magnetosphere, has been reported with observations from the Galileo spacecraft [Mauk et al., 1997, 1999]. The dispersion of the arrival of electrons as a function of their energy as seen at the spacecraft is clearly well defined. On the basis of their catalog of over 100 such events at a radial distance range of 9 27 R J they are distributed over all local times and System III longitudes. Related transient events with durations of several hours are remotely sensed in the Galileo auroral radio emissions which imply large fluctuations in the plasma sheet densities at radial distances of 10 R J [Louarn et al., 2000]. An extensive survey of bulk flow speeds of plasmas as calculated from the angular anisotropies of hot ions at energies greater than tens of kev with the energetic particle detector on Galileo has been published [Krupp et al., 2001a, 2001b]. These measurements are limited to radial distances beyond 10 R J in the local morning through midnight to local evening sectors, that is, to the early Galileo orbits. One of our primary interests here will be the plasma bulk flows in the local afternoon to noon sector during the later orbits. [8] We report here the densities, temperatures, and bulk flow velocities measured during the outbound segments of three fortuitous orbits during the late phase of the Galileo mission at Jupiter. The radial distance range of the presently described observations was R J. The local times and dates of the three trajectories at a midradial distance of 30 R J were 1600 LT on 23 May 2000, 1400 LT on 31 December 2000, and 1200 LT on 19 January These trajectories provided a unique opportunity to examine and compare the thermal plasma features in the local time sector extending from local afternoon to noon. In addition, the detection of electron beams on magnetic field lines which 2of19

3 Table 1. Number of Samples as a Function of Cycle Time Cycle Time, min G28 a G29 b I33 c a G28 is 0000 UT, 21 May 2000, to 2400 UT, 24 May b G29 is 0000 UT, 29 December 2000, to 2400 UT, 1 January c I33 is 0400 UT, 18 January 2002, to 1200 UT, 21 January cross the plasma sheet at radial distances of R J, and which thread the main auroral oval, is discussed. 2. Instrumentation [9] The plasma instrumentation (PLS) on board the Galileo spacecraft is composed of spherical-segment electrostatic analyzers which are capable of measurements of the positive ion and electron velocity distributions over the energy/charge (E/Q) range of 0.9 V to 52 kv in 64 passband steps. Three miniature magnetic spectrometers are also positioned at the exit apertures of the ion electrostatic analyzers in order to determine the mass/charge (M/Q) of these ions. The PLS has been previously described by Frank et al. [1992]. The fan-shaped field of view of each of the ion and electron analyzers is divided into seven segments with multiple sensors. Electronic sectoring of the responses of the sensors according to the phase of the spacecraft rotation then allows the three-dimensional determinations of the velocity distributions of the positive ion and electron plasmas, that is, coverage of 80% of the 4p steradian solid angle for arrival of charged particles at the analyzers. [10] The coverage of the outbound orbital segments at Jovian radial distances in the range of 8 50 R J which are used in the present investigation was gained with low-rate telemetry which was transmitted in real time to Earth. The telemetry mode was labeled RTS0. For the RTS0 mode, seven sensors were sampled in four azimuthal spin sectors in 14 E/Q passbands for each of the positive ion and electron velocity distributions. Every fourth passband in the passband step range was selected for an E/Q range of 7 V to 52 kv. This was accomplished in two consecutive spacecraft rotation periods. The rotation period was s. The spacecraft rotation was divided into four 90 viewing quadrants which were fixed on the celestial sphere. In each of these quadrants, seven E/Q passbands were sampled in contiguous sampling intervals of 0.5 s. During one sampling interval the spacecraft rotated by 9.5. Then, during the second spacecraft rotation, the seven highest passbands were sampled in the same manner. The total number of samples for each of the ion and electron velocity distributions was thus 392. The time resolutions for these instrument cycles depend upon the spacecraft telemetry rate and are summarized in Table 1 for the three orbital segments employed in the present study. These spacecraft orbits are labeled according to the Jovian satellite which is targeted for closest approach, that is, Io (I) and Ganymede (G), and the orbit number for the spacecraft revolution around this giant planet. The total number of instrument cycles, that is, the number of determinations of each of the ion and electron velocity distributions, is also given in Table 1. [11] The calculation of the ion plasma moments presented here as determined from the E/Q spectra with the electrostatic analyzers depends upon the M/Q of the positive ions. The determination of this M/Q for the dominant ion has been derived from measurements with the PLS mass spectrometers during the high-rate data with the onboard tape recorder and with fits to the three-dimensional E/Q spectra from the electrostatic analyzers. These determinations are extensively discussed in the published literature [Frank et al., 1996; Frank and Paterson, 1999a, 1999b, 2000a, 2000b, 2001a, 2001b, 2002a, 2002b; Frank et al., 2002]. Specifically, a value of M/Q = 16 is employed in the calculation of plasma moments from the measured three-dimensional velocity distributions of the positive ions [Krall and Trivelpiece, 1973]. The dominant distributions with M/Q = 16 were well fit with two Maxwellians of different temperatures with two accompanying distributions with lesser densities with M/Q = 8 and 32. The value of M/Q = 16 is also in agreement with the results of the detailed analysis of in situ plasma and remote ultraviolet measurements with the Voyager 1 spacecraft [Bagenal, 1994]. [12] An abbreviated guide to the various instrumental effects as given in the above references is given here for the convenience of the reader. Hydrogen ions in the tens of ev range are deflected by the spacecraft potential before arrival at the PLS aperture. This is an effect that is discussed in detail by Frank and Paterson [1999a]. The narrow electron beams which are aligned along the magnetic fields have an angular width which is often less than the acceptance angles of the electrostatic analyzers. These widths must be accounted for in the computation of the electron densities and fluxes [Frank and Paterson, 1999b, 2000a, 2002a]. Because the ion plasmas are often flowing with sufficient speeds, the widths of their angular distributions are smaller than the acceptance angles of the ion channels of the electrostatic analyzers, and these acceptance angles are segmented into contiguous sets ranging in populations of according to the ion sensor [Frank and Paterson, 2001b]. Frank and Paterson [2000a] compared the positive ion and electron densities which were acquired during the first traversal of the Io torus in December These densities were equal within the experimental errors. However, the slow accumulation of neutral sodium and oxygen gases onto the sensors during the course of the mission raised the threshold for detection of thermal electrons from the initial energy of 1 ev to 1 kev [Frank and Paterson, 2002a]. In order to clarify the responses of the ion analyzer to ring distributions of pickup ions, Frank and Paterson [2001b, 2002b] developed worm diagrams which projected the ion motion onto three planes referenced to the pointing directions of the seven ion sensors. At the low telemetry rates the numbers of E/Q passbands and the numbers of the fields of view were decimated relative to those acquired with the tape recorder at much higher telemetry rates. The corresponding errors associated with this decimation for determinations of ion densities, temperatures, and bulk flows are discussed by Frank et al. [2002]. The Galileo magnetic field measurements used in the present 3of19

4 Figure 1. Three Galileo orbits projected (a) onto the Jupiter-centered solar ecliptic X-Y plane and (b) onto the X-Z plane. Inbound and outbound segments inside a radial distance of 50 R J are identified (solid lines). Our present interest is in the outbound segments which are approximately located at local times 1600 (orbit G28), 1400 (G29) and 1200 (I33). study were obtained with the magnetometer described by Kivelson et al. [1992]. 3. Observations [13] The orbital segments which are of interest for the present paper are shown in Figure 1. These are the outbound segments from near or at perijove for orbits G28, G29, and I33 out to radial distances of 50 R J. The letters G and I identify the Jovian moon which was targeted for closest approach by the spacecraft, and 28, for example, refers to the orbit number. Figure 1a shows the projections of the orbit onto the solar ecliptic X-Y plane and Figure 1b shows the projections onto the X-Z plane. Our presentation begins chronologically with the orbital segment for G28 which is positioned near the local time sector of 1600, that is, near local late afternoon. [14] Plasma parameters for the G28 orbit during the period of May 2000 are shown in Figure 2. These plasma parameters are computed as moments of the threedimensional velocity distributions [Krall and Trivelpiece, 1973]. The M/Q of these positive ions is taken as 16. The ion densities are shown in Figure 2a; ion temperatures are shown in Figure 2b; Cartesian components of ion velocity are shown in Figures 2c, 2d, and 2e; components of this velocity in cylindrical coordinates V r and V f are shown in Figures 2f and 2g; and the scalar speed V is shown in Figure 2h. The vertical dashed lines indicate crossings of the current sheet as recorded by the magnetometer on board the spacecraft. These crossings are identified by reversals of the sign of the radial component of the magnetic field. The dash-dotted lines in the panels for V x, V y, and V f provide the components for a rigidly corotating plasma. Along the abscissa of Figure 2 is given the radial distance of the spacecraft position and its local time. [15] At radial distances out to 7.4 R J the Galileo spacecraft is in the plasma torus, and the plasmas are rigidly corotating with the planet as shown in Figure 2. The ion densities are nearly 1000 cm 3, and the temperatures are typical of the plasma torus at 10 6 K. There is a transition region between the rigidly corotating torus and the region of great departures from rigid corotation in the plasma sheet at radial distances of R J. In this transition region the plasmas are in the corotational directions but at lesser values than those anticipated for rigid corotation. Also the radial component of bulk flow, V r, is generally small relative to the corotational component. In this transition region the ion densities are decreasing from 1000 to 1 10 cm 3 with an accompanying increase in ion temperatures from 10 6 to K. [16] At radial distances beyond 22 R J in Figure 2 the plasma sheet becomes thin enough that, at the larger distances from the plasma sheet plane, the ion densities have decreased below the threshold sensitivity of the plasma analyzer. Inspection of Figure 2h finds that the speeds of the plasmas in this plasma sheet are typically 200 km s 1 out to a radial distance of 33 R J. Figure 2f shows that the radial component of plasma flow varies between values of ±100 km s 1. Ion densities and temperatures in this region of the plasma sheet are in the range of 0.1 cm 3 and K. Beyond a radial distance of 33 R J the plasma bulk speeds slow to values of 100 km s 1 or less, and the peak densities remain at the value of 0.1 cm 3. The ion temperatures rise to a plateau of 10 8 K. Throughout the radial distance range shown in Figure 2, there is no convincing evidence of a persistent System III dependence of ion parameters. That is, the crossings of the current sheet do not indicate variations of plasma parameters which alternate among the consecutive crossings. [17] The thermal ion plasma pressures (P i ) from the plasma analyzer and the magnetic field pressures (P mag ) from the magnetometer for the current sheet crossings which occurred at radial distances of R J during May are given in Table 2. The units of these pressures are pascals. Inspection of Table 2 finds that the ion plasma pressures are typically factors of less than those of the magnetic pressures. Of course, the plasma beta (b) as given in Table 2 is a direct measure of these ratios. [18] Our interest is also directed toward the electron beams which are aligned along the ambient magnetic field. The record of these electron anisotropies is shown in Figure 3. In Figure 3a are displayed the omnidirectional fluxes of electrons in units of cm 2 s 1 in the energy range 1.9 kev to 52 kev. The anisotropies of the electron fluxes at 3.7, 7.6, and 14.8 kev are displayed in Figures 3b, 3c, and 3d, respectively. These are the ratios of maximum to minimum intensities. Specifically, the anisotropy is com- 4of19

5 Figure 2. Plasma parameters as a function of time for orbit G28 during May These ion parameters are (a) density, (b) temperature, (c, d, and e) components of the bulk flow along solar ecliptic Cartesian coordinates X, Y, and Z, (f and g) components of these ion bulk flows along the cylindrical coordinates r perpendicular to Jupiter s spin axis and f parallel to the direction of rigid corotational flow, and (h) bulk speed. The values for rigid corotational flow are given (dash-dotted lines). Crossings of the current sheet are indicated (vertical dashed lines). The radial distance R and local time (LT) position of the spacecraft are given along the abscissa. Perijove occurred at 0452 UT at a radial distance of 6.75 R J. puted by averaging the two measurements at the extreme pitch angles and comparing with the average of the measurements at the intermediate pitch angles. Thus an anisotropy of less than unity means that the average for the two pitch angles at the minimum and maximum pitch angles was less than the average for all of the intermediate pitch angles. The reader should first note the information concerning the sampling of pitch angles which is given in Figures 3e and 3f. For example, the pitch angles a relative to the magnetic field are given in Figure 3e for each of the three electron energies. These are not the same for the three energies because the energies are sampled cyclically as the spacecraft rotates as a function of time. There are three electron sensors used to sample the angular distributions. These are sensors 2E, 4E, and 6E [Frank et al., 1992], because these sensors are closest in overall sensitivity and Table 2. Galileo Orbit G28, Plasma and Magnetic Field Pressures Date Time, UT R, R J P i,pa P mag, Pa Beta 22 May May May May of19

6 Figure 3. Characteristics of the electrons for orbit G28 during May These characteristics are (a) omnidirectional fluxes in units of cm 2 s 1 for the energy range 1.9 kev to 52 kev, (b) electron flux anisotropy at 3.7 kev, (c) anisotropy at 7.6 kev, (d) anisotropy at 14.8 kev, (e) the maximum pitch angle a sampled as referenced to the magnetic field during the determinations of electron anisotropies at each energy, and (f ) the minimum pitch angle a sampled. The crossings of the current sheet within the plasma sheet are identified (vertical dashed lines). thus can be used to avoid errors in the correction for substantial background rates due to penetrating energetic ions and electrons. For Figure 3f the pitch angles are given for the sampled directions which are closest to viewing particles propagating parallel to the magnetic field. Figure 3e is the counterpart for Figure 3f in that directions antiparallel to the magnetic field are sampled. [19] With the above information in mind, consider the first current sheet crossing at 0745 UT on 21 May in Figure 2. This crossing occurs in the rigidly corotating torus. Examination of the pitch angles sampled in Figures 3e and 3f finds that the pitch angles sampled were considerably away from field alignment, 15 away. The field-aligned electron beam was thus not accessible to the plasma analyzer. The next current sheet crossing at 1350 UT was excellent in that the directions parallel and antiparallel to the magnetic field were sampled. However, examination of the electron anisotropy plots in Figures 3b, 3c, and 3d finds no evidence of field-aligned electron beams at this radial distance of 10 R J. The first beam is seen at 0420 UT on 22 May between current sheet crossings. Continuing with the contents of Figure 3, as a function of radial distance, the first field-aligned beam is located outside of the current sheet at electron energy 14.8 kev during the current sheet crossing near 0600 UT on 22 May at a radial distance of 18 R J. Thus the current sheet crossing of these field lines is located at greater radial distances. This is followed by an extended period during which the magnetic field was fortuitously aligned such that field-aligned electron beams were detected at all three energies during 1200 to 2100 UT on 22 May at radial distances of R J. A brief period of observations at the current sheet crossing at 1600 UT during this interval was in directions significantly away from the fieldaligned direction and, as expected, the electron beams were 6of19

7 Figure 4. Continuation of Figure 2 for orbit G29 during 29 December 2000 to 1 January Perijove occurred at radial distance 7.5 R J at 0335 UT on 29 December not detected. Favorable alignment of the sensors also occurred during UT on 23 May at radial distance range of R J, and lesser anisotropies were present. With the exception of an intense field-aligned electron beam at 7.6 kev at 2200 UT on 23 May at 37 R J, at larger radial distances the anisotropies were less pronounced. However, the maxima of omnidirectional electron fluxes in Figure 3a were notable at the current sheet crossings during the period 1300 UT on 23 May through 0600 UT on 24 May at R J. [20] Both the cold and hot electron populations were measured with the Faraday cup on board the Voyager spacecraft [Scudder et al., 1981; Belcher, 1983]. For example, at radial distances of 20 R J the hot electron densities with E 1 kev were 5 10% of the cold electron densities. The wide angular acceptance of the Voyager Faraday cup precludes the definitive measurement of the hot electron beams detected with the Galileo plasma analyzer. [21] The second of the three trajectory segments which are presented in this paper is shown in Figures 4 and 5 in the same formats as those for the first telemetry segment. The three selected trajectory segments are chosen on the basis of continuous telemetry coverage in the afternoon through noon local time sectors. Only these three telemetry segments enjoyed this splendid coverage to date in these local time sectors. The plasma parameters recorded during the period 29 December 2000 through 1 January 2001 are shown in Figure 4. The spacecraft trajectory has advanced toward noon to local times of Again the rigidly corotating torus plasmas are seen out to a radial distance of 7.7 R J, beyond which the transition to the plasma sheet is observed out to 1700 UT on 30 December 2000 at a radial distance of 25 R J. The plasmas in this transition region are generally convecting in the corotational direction but with speeds which are less than those for rigid corotation. Entry into the plasma sheet at 1700 UT on 30 December is accompanied by large radial flows, V r in Figure 4f, directed toward Jupiter. These radial flows first appeared at 0300 UT in the transition region. The magnitudes of these radially directed flows vary in the range of km s 1 out to a radial distance of 29 R J at 0400 UT on 31 December. Beyond this radial distance the 7of19

8 Figure 5. Continuation of Figure 3 for orbit G29 during 29 December 2000 to 1 January radial flow was lesser for the limited number of plasma detections which were possible in the relatively intense background responses to high-energy charged particles. The overall radial profiles of ion densities and temperatures during this passage on 29 December through 1 January were similar to those measured during the previous passage as reported in Figure 2. Again there is a general stagnation of convection at the larger radial distances, in this case at radial distances beyond 37 R J, which was crossed at 2200 UT on 31 December. This second observation of a stagnation zone at these larger distances lends credence that this may be a persistent feature of the dayside magnetosphere. Thermal ion pressures and magnetic field pressures are tabulated in Table 3 for plasma sheet crossings during this passage. [22] The electron angular distributions during this trajectory segment on 29 December through 1 January are summarized in Figure 5. The geometry is not as favorable for viewing field-aligned electron beams relative to the previous case shown in Figure 3, but this fact does provide additional confidence that the electron beams are narrowly confined to pitch angles nearly parallel and antiparallel to the magnetic field. In Figures 5e and 5f a brief period of viewing parallel beams occurred at 1600 UT on 29 December at a radial distance of 12 R J. Examination of the anisotropies in Figures 5b, 5c, and 5d finds that the electron angular distributions are approximately isotropic, that is, no presence of beams at these radial distances. At larger radial distances, for viewing directions favoring field-aligned electrons, such beams were detected at (1) 3.7 and 7.6 kev during UT on 30 December at 18 R J ; (2) 3.7 Table 3. Galileo Orbit G29, Plasma and Magnetic Field Pressures Date Time, UT R, R J P i,pa P mag, Pa Beta 29 Dec Dec Dec Jan Jan of19

9 Figure 6. Continuation of Figure 2 for orbit I33 during January and 7.6 kev during 1400 UT on 30 December at 23 R J ;(3) 3.7, 7.6, and 14.8 kev during 0200 UT on 31 December at 28 R J ; and (4) 3.7, 7.6, and 14.8 kev during 2300 UT on 31 December at 37 R J. If a single example is not convincing to the reader, the ensemble of cases provides considerable support for the persistence of the electron beams in the above radial distance range. [23] The plasma parameters during the passage of the Galileo spacecraft near the local noon sector of Jupiter s magnetosphere are shown in Figure 6. The record begins at a more distant radial distance relative to the two previous passages reported in the present paper. This radial distance is 11 R J at 0400 UT on 18 January 2002 and is beyond the location of the rigidly corotating torus plasmas. The transition region from torus to plasma sheet is recorded at radial distances extending from 11 to 24 R J during the time interval 0400 UT on 18 January to 0800 UT on 19 January. As with the previous two passages the plasma flows are generally in the corotation direction but at slower speeds than those for rigid corotation. The exception is the decrease of ion speeds centered at 1900 UT on 18 January which occurs when the spacecraft is near its maximum position away from the current sheet. Beyond 24 R J the flow speeds largely vary and range from 0 to 250 km s 1 but still lesser than those for rigid corotation until the spacecraft reaches a radial distance of 40 R J at 1400 UT on 20 January. The exceptions are the bulk flows which are nearly equal to those for rigid corotation during the two current sheet crossings at 1200 UT on 18 January 2002 at 16 R J and at 1200 UT on 19 January 2002 at a radial distance of 29 R J. Beyond 40 R J the plasma flow is relatively stagnant at speeds in the range of km s 1 with no strongly preferred direction. Thermal ion and magnetic field pressures for current sheet crossings are given in Table 4. [24] In overview, the current sheet crossings for all three of the trajectory segments examined in the present paper are characterized generally by relatively stagnant flows at radial distances beyond R J. Inside of these radial distances there are isolated instances of stagnant flow in the current sheet for which the most prominent examples occur at 29.4 R J for 0446 UT on 31 December 2000 and at 9of19

10 Table 4. Galileo Orbit I33, Plasma and Magnetic Field Pressures Date Time, UT R, R J P i,pa P mag, Pa Beta 18 Jan Jan Jan Jan R J for 0813 UT on 19 January Of course, features that are not repetitive among the three orbits cannot be uniquely identified as spatial or temporal. [25] There is a remarkable System III effect which is seen in Figure 6a for ion densities during seven current sheet crossings for the period 1900 UT on 19 January through 0100 UT on 21 January. This effect is the higher ion densities recorded during alternate crossings of the current sheet. The ion temperatures for these crossings are 10 8 K and are typical of plasma sheet crossings at these radial distances of R J with the exception of the mystery crossing at 0400 UT on 20 January, for which the ion temperatures are typical of those in the rigidly corotating torus at radial distances inside of 8 R J. This low temperature of 10 6 K, 100 times less than the plasma sheet temperatures of 10 8 K, may be evidence of direct channeling of the torus plasmas with temperatures of 10 6 Kinto the outer magnetosphere. [26] The electron anisotropies observed during this passage near the local noon sector during January 2002 are displayed in Figure 7. Qualitatively these anisotropies exhibit similar features as those during the previous two passages, that is, the absence of field-aligned electron beams, when the viewing is favorable, at the lesser radial distance at 12 R J at 0600 UT on 18 January. On this day of 18 January a 3.7-keV beam is detected at 18 R J at 1600 UT. At the larger radial distance of 24 R J at 0300 UT on 19 January, field-aligned electron beams at 3.7 and 7.6 kev are encountered. About one hour prior to the plasma sheet crossing at 29 R J, electron beams at 7.6 and 14.8 kev are observed, but the opportunities for detections of electron beams at larger radial distances are not present because of the unfavorable sampling of pitch angles as shown in Figures 7e and 7f. [27] Examples of the electron energy spectra for the three major types of angular distributions are shown in Figure 8. The directional differential energy fluxes in units of cm 2 s 1 sr 1 ev 1 are plotted as functions of electron energy in kev. In these plots the maximum intensities during an angular scan of the electron intensities are coded as solid circles, and the minimum intensities are coded as open circles. Examples of field-aligned beams are shown in Figures 8a and 8b for 1706 UT on 22 May 2000 and 1158 UT on 19 January 2002, respectively. An example of a scattered beam, which exhibits pitch angle distributions wider than those for the narrow electron beams, is shown in Figure 8c for 1306 UT on 23 May The third type of angular distribution is that for isotropy, an example of which is shown in Figure 8d for 1501 UT on 18 January There is a fourth type of electron angular distribution not shown in Figure 8, that is, a distribution for which there is a loss cone along the magnetic field vector and hence maximum intensities perpendicular to this field line. [28] The details of the ion velocity distributions in the transition region between the near-jupiter torus and the plasma sheet are of considerable relevance to the force balance in the near-equatorial magnetosphere. Five examples of the ion velocity distributions acquired on 18 January 2002 are shown in Figure 9. Inspection of the abscissa labels for Figure 7 finds that the corresponding radial distance range is R J. The velocity distributions in Figure 9 have been computed with M/Q = 16 for the responses of the electrostatic analyzers. The coordinate plane is defined by the direction of rigid corotation V cor and the direction of the magnetic field vector B. Isodensity contours in units of s 3 cm 6 for the velocity distributions are shown. These velocity distributions have also been grey coded according to the scale in the upper left-hand corner of Figure 9 for facilitating the examination by the reader. There are two distributions of ions in each figure (Figures 9a 9e). These two distributions are the heavy ions positioned generally at speeds somewhat in excess of 200 km s 1 and another distribution at 75 km s 1. The bulk velocities of these two ion populations perpendicular to the magnetic field should be equal, and this allows identification of the lower speed ions as hydrogen ions with M/Q = 1. The ratio of these perpendicular components of the bulk speeds should be four, but the difference is most likely due to the fact that only every fourth energy bin is sampled. A further feature to note is the bidirectional distribution of the higherspeed ions, that is, with M/Q = 16 in Figures 9a and 9b. These distributions exhibit components with bulk motions parallel and antiparallel to the magnetic field. This feature may be associated with a scattering mechanism which favors the loss of ions with pitch angles perpendicular to the magnetic field, perhaps located at the current sheet. The positions of the Galileo spacecraft for the samples of the ion velocity distributions at 0922 and 1413 UT on 18 January are shown in Figure 10. These positions are located in a model of Jupiter s internal field [Connerney, 1998] and the superposition of a current sheet model [Connerney et al., 1981; Connerney, 1993]. The radius of curvature of these field lines R C in the plasma sheet plays an important role in the examination of the radial force balance in the following section 4 of this paper. [29] The number densities, perpendicular temperatures, and temperature anisotropies for the heavy ions with M/Q = 16 are displayed in Figures 11a, 11b, and 11c, respectively, for the passage through the local noon magnetosphere during January For the period 0400 UT on 18 January through 1000 UT on 19 January the perpendicular temperature T? increases with increasing radial distance which is accompanied by sporadic increases of the temperature anisotropy T k /T?. The pronounced System III effect previously identified during the period 1900 UT on 19 January to 0100 UT on 21 January appears in Figure 11a. 10 of 19

11 Figure 7. Continuation of Figure 3 for orbit I33 during January [30] On the other hand, the hydrogen ions exhibit different features relative to those for the heavy ions. The hydrogen number densities, perpendicular temperatures, and temperature anisotropies are shown in Figures 11d, 11e, and 11f, respectively. These hydrogen densities are lesser by factors of 2 to 5 relative to the heavy ions. The hydrogen temperatures perpendicular to the magnetic field, T?, are more or less constant during the period 0400 UT on 18 January through 1000 UT on 19 January in contrast to the factor of 20 increase in the heavy ion temperatures. The temperature anisotropies for the hydrogen ions are seen in Figure 11f to vary from 0.1 to 10 during the above time interval. No evidence of a System III variation such as that for the heavy ions is present in these hydrogen data, but this effect may be at densities below those for the threshold of the plasma analyzer. 4. Summary and Discussion [31] As the trajectory of the Galileo spacecraft advanced from local afternoon to noon during the later stages of its odyssey at Jupiter, a marvelous set of three passages with almost continuous coverage of the thermal plasmas was obtained by real-time reception of its telemetry signal at Earth. These three outbound trajectory segments were positioned at local times of 1600, 1400, and 1200 LT at a radial distance of 30 R J. The dates of these three passages were 23 May 2000, 31 December 2000, and 19 January 2002 at this radial distance, respectively. The orbits were designated in the same order as G28, G29, and I33. Measurements of the thermal plasmas are reported here for the radial distance range of 7 50R J, although the telemetry for I33 began at a radial distance of 11 R J. This radial distance range provides observations of the hot plasma torus, the transition region between this torus and the plasma sheet at greater radial distances, and this plasma sheet. The three-dimensional velocity distributions for an energy/charge (E/Q) range of 7 V to 52 kv are measured. The dominant ions are characterized with a mass/charge (M/Q) of 16 in previous publications. There are two populations of ions with M/Q = 16, each with its own temperature. The ions with higher Maxwellian temperature are interpreted as S ++, and those with lower 11 of 19

12 Figure 8. Directional differential fluxes as a function of electron energy for the maximum (solid circles) and minimum (open circles) fluxes detected during an instrument cycle. The three major types of angular distributions are shown: (a and b) field-aligned beams, (c) a scattered beam with wider angular distributions relative to those of the beams, and (d) an isotropic angular distribution. temperature are interpreted as O +. Other species of ions detected with the plasma analyzer are H +,O ++,O +++, and S +. The reader interested in pursuing the analyses of ion species is invited to examine relevant work [Frank et al., 1996; Frank and Paterson, 1999a, 1999b, 2000a, 2000b, 2001a, 2001b, 2002a, 2002b; Frank et al., 2002]. [32] The plasma measurements presented in section 3 are rich in their detail. Our purpose here is to provide an overview of these measurements with sufficient detail such that the major features are summarized. This summary appears in Figure 12. The radial distance and local time of the spacecraft position are shown along the abscissas of these three charts. The Galileo spacecraft passed out of the rigidly corotating torus at 8 R J during its radially outward passages of the magnetosphere for the first two traverses, Figures 12a and 12b. The dates of all three passages of present interest are identified in Figure 12, but it is more economical textwise to refer to their figure labels. The rigid torus densities were in the range of 500 cm 3, and the ion temperatures were 10 6 K. No telemetry was available in the rigid torus for the third passage. A transition zone for the ion plasmas was present for all three passages outside of the rigid torus and extended to radial distances of R J. This transition region was identified as a region of decreasing densities and increasing temperatures as a function of Jovian radial distance. The ion densities decreased from 100 to 0.1 cm 3 for the first two passages, as shown in Figures 12a and 12b. The temperature has increased to K at the outer boundary of this transition region. Telemetry became available for the third passage at a radial distance of 10 R J with a density of 10 cm 3 and a temperature of K. Thus the gradient in these two plasma parameters was steep. The spacecraft exited the above transition zone to the plasma sheet at the above cited radial distances and entered the plasma sheet proper. In this plasma sheet the ion densities were in the range of cm 3, and the temperatures were 10 8 K. At radial distances beyond the bars in the three Figures 12a, 12b, and 12c these plasma parameters were not reliably determined because of the relatively high sensor responses from energetic charged particles. From the viewpoint of ion densities and temperatures the three passages through the magnetosphere were remarkably similar. [33] On the other hand, the features of the ion bulk flows differed among the three passages summarized in Figure 12, in particular the radial bulk flow component V r. In the hot torus the plasma bulk flow was directed along the azimuthal component f within the measurement accuracy of 5%, as recorded during the two traversals shown in Figures 12a and 12b. In the transition region to the plasma sheet the azimuthal component of plasma flow slows to 70% of the rigid corotational value V cor during the passage shown in Figure 12a. In this transition region the radial flow component V r increases from instrumental threshold values of <100 to about 100 km s 1 at the outer region of this transition region. The radial component of flow is directed toward Jupiter by the time that the outer boundary is reached at 27 R J. This radial flow continues into the plasma sheet until the spacecraft reaches a radial distance of 35 R J. At these radial distances, V f is 200 km s 1 in the corotational direction. At radial distances beyond 35 R J both components of the ion bulk flows decrease to values <100 km s 1. It is convenient to compare the ion flows of this first passage with those observed during the second passage of Figure 12b. These differences are an increased radially inward flow of about 200 km s 1 in the outer regions of the transition region to the plasma sheet and a continuation of similar inward flow in the plasma sheet out to radial distances of 37 R J. The differing features among the three passages cannot be uniquely identified as spatial or temporal. Again, the components of bulk flow at greater distances, >37 R J, decreased to values <100 km s 1. [34] The third traverse which occurred near local noon is summarized in Figure 12c. The primary difference in the plasma bulk flows relative to the first two traverses was the occurrence of a stagnant flow region with both flow components <100 km s 1 in a plasma with typical density and temperature of 0.1 cm 3 and K at the interface between the plasma transition region and the plasma sheet proper at a radial distance of 27 R J. At greater distances the radial component of flow V r was also <100 km s 1, but the azimuthal component V f was 200 km s 1 out to a radial distance of 42 R J, beyond which these two components of bulk flow were <100 km s 1. The previous Voyager measurements that plasma bulk speeds at radial distances of 50 R J were typically 50 60% of rigidly corotating values, as cited in section 1 of our paper, are consistent with the present Galileo values. Overall, the variations of the bulk flows during these three passages of 12 of 19