Return to Io by the Galileo spacecraft:

Transcription

1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 105, NO. All, PAGES 25,363-25,378, NOVEMBER 1, 2000 Return to o by the Galileo spacecraft: Plasma observations L. A. Frank and W. R. Paterson Department of Physics and Astronomy, University of owa, owa City Abstract. On October 11, 1999, a series of high-resolution plasma measurements were obtained during the second close flyby of o of the Galileo mission. The closest approach to o occurred at an altitude of 617 km along the flank of Jupiter's torus plasma flows past this moon. Energy/charge (E/Q) and mass/charge (M/Q) measurements with the plasma analyzer were used to identify the primary ions. At closest approach the ion number densities increased to their maximum values of 1200 cm -a as calculated from the plasma moments. These ions were dominated by two thermal populations of torus ions with M/Q = 16, one with density and temperature of 800 cm -a and kt = 40 ev and the other with 200 cm -a and 160 ev. On the basis of the previous Voyager 1 remote measurements with a spectrometer, these two thermal ion plasmas are identified as O + and S ++, respectively. A substantial population of pickup ions was also observed, with densities of about 100 cm -a each for S + and SO +. The plasma bulk flows were strongly deflected around the body of o, and the speed of the bulk flow increased to a factor of 1.3 greater than that for rigid corotational flow. The torus ion densities just outside of o's orbit were about 800 cm -a. These densities at the immediate position of o are smaller by a factor of about 4 relative to those during the first flyby on December 7, 1995, but should not be taken as indicative of a similar decrease in the bulk of the plasma torus. Two major plasma regimes were encountered before the closest approach to o. The first was a region of hot ions located inside of o's orbit which was previously identified as the "ribbon" of enhanced plasma densities with Voyager 1 plasma measurements and with ground-based imagery. The ion composition in the ribbon included O + and S ++. The second region, closer to o, exhibited a hot torus plasma which was mixed with pickup hydrogen ions. These hydrogen ions were detected at distances beginning at 7.8 R o from this moon, which were generally inside the Hill (Lagrange) "sphere" for o's atmosphere. 1. ntroduction o is the heart of the magnetosphere of Jupiter. ts remarkable volcanic activity provides a variable supply of neutral gases which are subsequently ionized. These resulting plasmas are a major source for the enormous volume of space which is threaded by the magnetic fields of this giant planet. The rapid rotation' rate of the planetary magnetic field, together with this unique primary source of plasmas, give o a special place in the studies of plasmas in our solar system. These studies began with the finding that there was a correlation of 22-MHz radio emissions with the position of o in its orbit around Jupiter [Bigg, 1964]. t was suggested by Piddington and Drake [1968] that this modulation of the radio emissions was associated with the electrodynamic response of o to Copyright 2000 by the American Geophysical Union. Paper number 1999JA /00/1999JA $ ,363 the motion of the Jovian magnetic fields across this moon. Goldreich and Lynden-Bell [1969] placed this suggestion into a quantitative framework in terms of a current system which was driven by the electromotive force associated with this motion of magnetic fields. The currents driven by the voltage across o were expected to close with magnetically field aligned currents directed into and out of the Jovian ionosphere. Later the theoretical analysis of the o interaction found that a standing wave should be present as the disturbance propagates with the local Alfv n speed away from o [Neubauer, 1980]. Thus the current is not magnetically field aligned but is directed along socalled "Alfvan wings" because the magnetospheric flow speed is substantial relative to the Alfv n speed. The direct detection of a field aligned current due to the interaction of o with the Jovian magnetic field was acquired with the magnetometer on board the Voyager spacecraft during its flyby of Jupiter [Ness et al., 1979]. The magnetic signature was best fit with a pair of field aligned currents,

2 25,364 FRANK AND PATERSON: RETURN TO O BY GALLEO each with 5 x 106 A and aligned parallel and antiparallel, respectively, to the Jovian magnetic field. The simultaneous measurements of the ion plasmas also supported the existence of an Alfv n wave propagating away from o [Belcher et al., 1981]. Further evidence of large currents into the Jovian ionosphere was provided by remote images of bright emissions at the foot of the magnetic field lines passing through o [Connerney et al., 1993]. These images of H3 + emissions were taken with the NASA nfrared Telescope located in Hawaii. These images of the bright spot in the Jovian ionosphere were soon followed by those of emissions at far-ultraviolet wavelengths with the Hubble Space Telescope [Clarke et al., 1996; Prang et al., 1996]. On December 7, 1995, the Galileo spacecraft passed within 890 km of o's surface on the wake side of the flow of plasmas corotating with Jupiter's magnetic field. The measurements of magnetic fields, charged particles, and plasma waves revealed a very dynamic environment. A large decrease of about 40% in the Jovian magnetic field intensity was observed and interpreted in terms of an internal dipole moment of o [Kivelson et al., 1996a,b]. Magnetically field aligned beams of energetic electrons were encountered during the closest approach [Williams et al., 1996, 1999]. These electron beams were also observed with the plasma instrument with energy fluxes sufficiently large to account for the far-ultraviolet emissions previously reported at the foot of the o flux tube in the Jovian ionosphere [Frank and Paterson, 1999b]. At closest approach to o a cool, dense thermal plasma was found to be at rest with respect to this Galilean satellite and interpreted in terms of its ionosphere [Frank et al., 1996]. The ion temperature was about 105 K, and the maximum number densities were about 2 x 104 cm -3. Similar maximum densities as inferred from the cutoff frequency of plasma waves were reported by Gurnett et al. [1996]. ndeed, o is an exciting locale for studies of space plasmas. On October 11, 1999, the Galileo spacecraft again passed by o at close distances. This time the passage began in the torus plasma flow upstream from o and was subsequently located at the flank of o during closest approach at an altitude of 617 km. We report the analyses and results of the thermal ion measurements. Among these results are the findings of large densities of heavy ions which are picked up from o's atmosphere into the torus flows. 2. nstrumentation 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. 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]. Several features of this instrumentation which are most relevant to the present study are noted here. The fan-shaped field of view of the electrostatic analyzers, full-width 157 ø, is divided into seven segments with multiple sensors and is oriented such that it is in a plane parallel to the spin axis of the spacecraft. The center of this field of view is directed perpendicular to the spin axis. Electronic sectoring of the responses of the sensors according to the phase of the spacecraft rotation allows the three-dimensional determination of the velocity distributions of the positive ion and electron plasmas, i.e., coverage of 80% of the 4 -sr solid angle for arrival directions of charged particles at the analyzers. For the presently reported measurements with the electrostatic analyzers, eight rotation sectors were sampled for each sensor. Thus a total of 8 x 7-56 angular segments are used to view each of the velocity distributions of positive ions and electrons. The rotation period of the spacecraft was s. During a single spacecraft rotation, 12 E/Q passbands were sampled during each of eight sectors. Every fourth of the full set of passbands in the E/Q range of 8 V to 52 kv is sampled during this single spacecraft rotation. During the next spacecraft rotation this sampling sequence is repeated such that every second passband is acquired during the total of two rotation periods. This two-rotation period is the typical sampling period of the ion velocity distributions reported here. Exceptions are noted as required. The third rotation is devoted alternatively to telemetering the responses of the lowest passbands of the electrostatic analyzers in the E/Q range of V or the responses of the mass spectrometers. Electron and positive ion channels are identified as E and P, respectively. The equatorial sensors (E4 and P4) and both sets of the polar sensors (El and P1, and E7 and P7) are sampled in the low-energy mode in 16 passbands and in four equispaced rotation sectors. Each passband is sampled during 0.17 s, corresponding to an azi- muthal rotation of the fields of view by 3.2 ø. This low-energy scan is acquired once per six spin cycles, or every 121 s. Because the spacecraft telemetry clock is asynchronous relative to the angular sectoring which is locked onto the celestial sphere, the acquisition time for the above sampling sequences varies slightly and is longer than the spin period. This situation occurs because the sampling must begin coincident with the occurrence of a sector boundary. The average time for this spin cycle is about 20.2 s. The elapsed time during the three rotations is 3 x 20.2 s= 60.6 s, or 1min. Thus during the first two spacecraft rotations a total of eight rotation sectors x seven sensors x 24 E/Q passbands samples is acquired for each of the electron and positive ion distributions in the E/Q range of 8 V to 52 kv. Higher E/Q resolution can be obtained by including the 24 nested E/Q passbands sampled

3 FRANK AND PATERSON: RETURN TO O BY GALLEO 25,365-20,000 r--i-- -15,000 - GALLEO PLS OCTOBER 1999 COROTATON 0550'02 UT 0400:09-0, ' :02 JUPTER > :15 TO OOO 0 62 KM/S T PERPENDCULAR TO O ORBT -2OOO,,,,,,,,,,,,,,,, ,000 Y, KM Figure 1. The trajectory of the Galileo spacecraft during its close flyby of o on October 11, The Cartesian coordinates are chosen with Y toward Jupiter, Z perpendicular to o's orbit, and X nearly parallel to the direction of rigid corotational flow of Jupiter's plasmas. The radius of o is 1815 km. Closest approach occurred at 0433:03 UT at an altitude of 617 km. The measured ion bulk flows along the spacecraftrajectory are also shown as compo- nents in the X-Y and Y-Z planes. during the subsequent fourth and fifth spin cycles. gle of 0.62 ø. This sampling sequence is not slaved Only one of the three mass spectrometers can be to the spacecraft rotation phase angle. Thus, given sampled at a time. The mass spectrometer is se- a sufficient number of these spin cycles, sampling lected by ground command. For the present series of the ions at a given E/Q and M/Q and arrival diof measurements the mass spectrometer 2 was em- rection is achieved. There are two channels in the ployed. ts field of view is directed perpendicular spectrometer: an integral channel, which records to the spacecraft spin axis, and its width in the the ion flux which is not swept out from the sensor plane of the fan angle of the electrostatic analyzers by the magnetic field, and the differential channel is 6 ø. During every sixth instrument cycle the elec- sensor, which receives that portion of the ions detromagnet in the spectrometer is stepped through a flected by a fixed angle. The responses of the difsequence of 24 current steps for a given E/Q pass- ferential channel are used in the present study. band of the electrostatic analyzer. This sequence of current steps is repeated for 20 E/Q passbands 3. Observations logarithmically spaced over the range of 3 V to 52 kv during one spacecraft spin. During an accu- The trajectory of the Galileo spacecraft in a coormulation interval of 33 ms for each sample of sen- dinate system at rest with respect to o is shown in sor counting rates the spacecraft rotates by an an- Figure 1. The Y axis is directed toward Jupiter's

4 25,366 FRANK AND PATERSON: RETURN TO O BY GALLEO Table 1. Spacecraft Coordinates During the o Flyby on October 11, UT 0400 UT 0420 UT 0440 UT Jovicentric radial distance, Rj Jovicentric ø ø -O.01ø 0.00 ø latitude System 58.5 ø 66.7 ø 75.0 ø 83.3 west longitude, kn Magnetic -7.5ø -6.6 ø -5.6 ø -4.4ø latitude Local time Jovicentric latitude of 4.8 ø 4.2 ø 3.5 ø 2.7 ø centrifugal equator Distance to centrifugal equator, Rj center, the Z axis is aligned along the normal to o's orbital plane and is positive northward, and X is chosen to complete the right-handed Cartesian coordinate system. The X axis is approximately in the direction of corotation of the torus plasmas. The projections of the ion bulk flow velocities in the X-Y and Y-Z planes are also shown in Figure 1. The bulk velocities are computed as moments of the three-dimensional velocity distributions for M/Q = 16 [Krall and Trivelpiece, 1973]. The speeds are approximately those expected for rigid corotation, 57 km s -1, with the major exception of the closest approach to o, where the flows are diverted in direction and accelerated. Closest approach was at an altitude of 617 km at 0433:03 UT. Additional coordinates for the Galileo spacecraft are given in Table 1. A description of coordinates is given by Dessler [1983]. Magnetic latitude was computed from a magnetic field model [Connerney et al., 1981; Connerney, 1993]. The responses of five of the seven ion sensors in the electrostatic analyzers, which view in directions nearest the bulk flow velocity, are shown in Plate 1. The responses of these sensors, designated as P1 through P5, are color-coded according to the color bar at the lower right-hand side of the plate. These responses are plotted as functions of energy/charge E/Q and universal time for each of the sensors. For a given sensor and given value of E/Q it is the maximum response which is recorded in Plate 1. The gap in the data records at about 0441 UT is due to the loss of telemetry during the transmission of the spacecraft tape-recorded data. The plasma analyzer is in an intense energetic charged-particle environment. The background responses for the sensors are determined at E/Q > 10 kv, for which there are no measurable responses to the thermal plasmas. These responses ranged from several thousands to about 10,000 counts per second. For reference purposes the E/Q values for rigidly corotating thermal ions are shown in Plate 1 as horizontal lines for mass/charge M/Q in units of amu, equal to 16 and 32. For Jupiter's magnetosphere the major ions at M/Q = 16 are O + and S ++, and at 32 the major ions are anticipated to be S + and SO2 ++. The maximum energies for pickup ions with M/Q = 32 are also shown. As noted for the detailed analysis of the measured ion distributions reported later in this section, the bulk flow sometimes differs significantly from that for rigid corotational flow. With neglect of the velocity of the spacecraft the cyclic motion of a pickup ion yields E/Q values in the range of 0 V to 4 times its coro- tational energy. ts net speed during this cyclic motion will be that of corotation. Reference to the trajectory shown in Figure finds that the spacecraft is moving outward from Jupiter. The spacecraft is located at about 0.15 Rj (Jupiter radius) inside of o's orbit at the beginning of the measure- ments. The record of sensor responses in Plate 1 shows that there is a substantial decrease in ion fluxes centered at about 0405 UT. The intensities of torus ions then recover, and at about 0430 UT there is a dramatic increase in intensities and in ion energies as the torus flows are diverted around o and large densities of pickup ions are present. The pickup ions are produced by the ionization of o's neutral gases from impact of the torus ions and electrons. These ions are subsequently "picked up"

5 FRANK AND PATERSON: RETURN TO O BY GALLEO 25,367 i01 P2 M/Q - 32, PCKUP M/Q - 32, THERMAL M/Q - 16, THERMAL 01 P EE bj z bj P4 COUNTS/S P5 io io 2-2 io 3 io UT X KM Y Z R Plate 1. Energy-time (E-t) spectrograms for the responses of the five ion sensors, P1 through P5, of the plasma instrumentation (PLS) which view in directions closesto the corotational velocity of the torus plasmas during the flyby of o by the Galileo spacecraft on October 11, The responses in units of counts per second are color coded according to the color bar at the bottom right-hand side and plotted as functions of energy/charge (E/Q) and universal time (UT). These responses are the maximum responses at a given E/Q value during a rotation period of the spacecraft. The E/Q values for ions with mass/charge (M/Q) = 16 and 32 are also shown as horizontal dashed lines for rigid corotational speeds of 57 km s -1 with respect to o. These E/Q values are shown for thermal plasmas and the maximum energy for pickup ions with M/Q- 32. Relevant species include O + and S ++ for M/Q- 16 and S + for 32. The coordinates for the Galileo spacecraft relative to the center of o are shown along the bottom abscissa.

6 25,368 FRANK AND PATERSON: RETURN TO O BY GALLEO OCTOBER ' :05 UT M/Q=16, THERMAL 250/CM 3 50 ev M/Q = 16, THERMAL 50/CM 3, OeV SUM P2. SEC''OR 2. A-G P3 SECTOR 2 (b 107 ii o 106 (c) 2 o 10,5! E/Q, ENERGY/CHARGE, VOLTS Figure 2. Measurements of the directional, differential intensities of ions, dj/d(e/q), in units o ' 1on s cm- 2 s- sr- -1 V-,1 or three directions in the velocit distribution unction during the period 08 0: 0-08 :0 UT (solid dots). The three directions correspond to those or ion sensor? in rotation sector, 78 in sector, and 74 in sector 8. The angle between the field o view o the plasma analyzer at E/ - 00 V and the direction or rigid corotational flow is 8. The ion spectra are fitted with two MaxwellJan distributions which are convecting with the corotational velociw as viewed in the rest s stem o the spacecraft. These two distributions have the same M/ value o 16 but different densities and temperatures, as noted in the upper le t-hand corner o the figure. t is useful to place this measurement in the overall context o the spectrograms shown in Plate 1. by the electric fields in the torus plasmas. The ions component at M/Q = 16 is characterized with a at E/Q values >1 kv in sensors P1 and P2 as density and temperature of 50 cm -3 and 10 ev, reshown in Plate are mainly the signatures of these pickup ions. n order to determine the primary ions in the multispecies plasmas, detailed analyses of their velocity distributions are presented here for four time intervals, 0350, 0411, 0420, and 0437 UT, spectively. The relative contributions of the two components and their sum are compared with the measured values in Figure 2. The angle between the direction of rigid corotational flow and the analyzer field of view at E/Q = 500 V is. As expected, the ion intensities are largest for smaller which corresponded to the major plasma domains values of a. The mismatch of measurements for shown in Plate 1. The first velocity distribution is fitted with two thermal distributions with M/Q - 16, as shown in Figure 2. The three panels show E/Q < 200 V for P2, sector 2 is due to its large field of view relative to the angular variations in ion fluxes. An improved fit can be gained with segthe differential, directional fluxes in units of cm -2 mentation of this field of view and subsequent secs- sr- V - as functions of E/Q for three directions ond-order fit to the observed intensities. The stain the ion velocity distribution. The density tistical uncertainty in the measured values is genand temperature of the primary component are 250 erally equal to or less than the solid circles. With cm -3 and kt = 50 ev, respectively. The secondary considerations of the finite solid angles of the fields

7 -- FRANK AND PATERSON: RETURN TO 0 BY GALLEO 25,369 of view of the plasma analyzers with respect to the relatively narrow angular distributions of the ions, the fits are good. The ion velocity distribution at 0411 UT is fitted with two thermal distributions with M/Q- 16 as shown in Figure 3. The primary component is again that with the higher temperature. The total ion density is about 500 cm -3. The corotational flow speed, as computed from the classical plasma moments [Krall and Trivelpiece, 1973] for M/Q = 16, has slowed from rigid corotational values of 57 km s - to 52 km s -. The total ion densities are slowly increasing as the spacecraft approaches o. At 0420 UT the dominant components of the ion plasmas are two thermal components, each with M/Q = 16. The fits to these velocity distributions are displayed in Figure 4. Now the dominant distribution is that with the lower temperature. The total density is about 600 cm -3. By now the reader will note that the temperatures for all of the velocity distributions discussed above are in the range of ev. This range of ion temperatures is similar to those reported for Voyager measurements [Belcher, 1983]. t is also important to note that the M/Q values are identified by assuming that the plasmas are nearly corotating. This is a critical factor in the analyses of the velocity distri- butions. For example, consider the fluxes in the tail of the distribution at kv for sensor P4 in sector 3 of Figure 4. Without further information it is possible that the M/Q of these ions is 32, rather than 16. The responses of the mass spectrometers can be employed in order to independently validate this identification of the M/Q value of the dominant ion. n Figure 5 are shown the responses of the differential channel of mass spectrometer 2, which views into the flow of torus ions during the rotation of its field of view perpendicular to the spacecraft spin axis. Mass spectrometer 2 views in the same directions as ion sensor P4 of the electrostatic analyzers. For the M/Q spectrum shown in Figure 5 the spectrometer field of view is at 16 ø with respect to the flow velocity of the torus ions. The responses of this spectrometer channel are above background for M/Q - 16 at E/Q = 960 V. At this energy the M/Q of 32 is beyond the range of the mass spectrometer. However, the observed responses of OCTOBER : :18 UT M/Q = 16, THERMAL 500/C M 5, 40 ev M/Q=16, THERMAL 200/CM 3, 0 ev SUM P5 SECTOR 2 (o) -- =16" P4 SECTOR (b) = P5 SECTOR 2 (c) 54 ø i 106 i0 5 i! O io 2 io E/Q, ENERGY/CHARGE, VOLTS Figure 3. Continuation of Figure 2 for the period 0410: '18 UT. The measurements are fitted with the sum of two velocity distributions with M/Q - 16.

8 25,370 FRANK AND PATERSON: RETURN TO O BY GALLEO OCTOBER ' '27 UT M/Q=16, THERMAL 400/CM 3 30 ev M/Q=16, THERMAL 200/CM 3, OOeV SUM (b) P4SECT R 3 (c) >l " 24 ø J) 106 %1 10,5 iii ' E/Q, ENERGY/CHARGE, VOLTS Figure 4. Continuation of Figure 2 for the period 0419: '27 UT. The measurements are also fitted with two thermal velocity distributions with M/Q- 16. about 2000 counts per second in the M/Q - 16 channel account for the fluxes shown at 960 V in the Figure 4c. The background responses are determined at 66 V and for directions away from the flow direction for which there are no significant responses to the torus ions. Thus the dominant M/Q = 16 at these energies. Without these measurements with the mass spectrometer it was not possible to determine whether the observed distribution should be fit with a hotter species with M/Q- 16 or with the additional presence of a species with M/Q- 32 with a density of about 200 cm -3. The mass spectrometer responses show that the M/Q- 32 contributions are at least a factor of 3 smaller relative to those from the lighter ion. At the closest approach altitudes for the o flyby the ion velocity distributions are considerably more complex relative to those described above for the torus. These velocity distributions for 0437 UT are shown for four directions of the analyzer field of view in Figure 6. The two main effects are the diversion of the direction of flow due to the conducting biddy of o and the presence of substantial fluxes of pickup ions. The reader is referred to Figure 1 for the diversion of the ion flows at o and to the spectrograms of Plate for P1, P2, and P5 at E/Q > kv for the presence of pickup ions. Again it is very important to establish the principal ion species for the calculations of the bulk flow velocities, in particular, and the ion densities and temperatures. The ion composition as measured with mass spectrometer 2 for E/Q V is shown in Figure 7. The angle of the field of view of mass spectrometer 2 with respect to the direction of bulk flow is 22 ø. The coverage of the M/Q spectrum includes M/Q The M/Q of the primary ion is 16, with responses smaller by factors of >2 for the other ions. This determination of M/Q is fundamental for computing the speed and direction of the torus plasmas in the vicinity of. o because these plasmas are not rigidly corotating with Jupi- ter's magnetic field. The components of the bulk flow velocity at 0437 UT are (Vx, Vy, Vz) = (71,-17, -8 km s-1). The fitted velocity distribution for these torus ions with M/Q - 16 is shown in Figures 6b and 6c for 0437 UT. The ion number density is

9 FRANK AND PATERSON: RETURN TO O BY GALLEO 25, cm -3 with a temperature of 40 ev with an ad- of ions in velocity space is required to account for ditional contribution of 200 cm -3 with a tempera- this difference in the character of the velocity disture of about 160 ev. The presence of heavy pickup ions at 0437 UT is prominent in the cut through the ion velocity distribution shown in Figure 6a. The identification of these ion distributions at higher energies relative to those of the thermal torus ions is not intuitively obvious. There are in fact 56 such cuts through the velocity distribution, of which only four are displayed in Figure 6. Figures 6a-6d have been selected in order to show the primary characteristics of the combined thermal torus and ion pickup distributions. The presence of significant pickup ion fluxes is noted by the fact that the fluxes of ions at 2 kv for P2 in sector 2 are about a factor of 100 higher than those for P4 in sector 3, even though the peak fluxes of torus thermal ions at 500 V are tributions at higher energy. The plane of this ring distribution is perpendicular to the magnetic field vector, and the center of its cycloid is positioned at the bulk velocity components of the thermal torus ions as given in the previous paragraph. The direction of the magnetic field has been taken from a Jovian global model [Connerney et al., 1981; Connerney, 1993]. A perpendicular temperature Tñ = 100 ev is chosen for the ring ion distribution. t is stressed that this Tñ is the thermal temperature perpen dicular to the magnetic field in an infinitesimal angular segment of the ring distribution. The responses of the ion analyzer are then computed for all 56 angular sectors, four of which are shown in Figure 6. ons in the ring distribution are not detected for P3, sector 2 and P4, sector 3. The very similar. f the higher-energy fluxes were due individual contributions from the two thermal toto a thermal distribution, the fluxes in P3, sector 2 and P4, sector 3 would be similar to those large rus distributions and the three ion pickup distributions are shown in Figures 6a-6d, together with fluxes detected in P2, sector 2. A ring distribution the sum from all five ion distributions. ndeed the GALLEO PLS MASS SPECTROMETER 2 DFFERENTAL CHANNEL TO 24 OCTOBER UT M/Q= 2 4 0, E/Q = 960 V VEWNG NTO TORUS FLOW ---o, E/Q =66V BACKGROUND, VEWNG AWAY FROM F LOW / \ / \ o i i i \ / \ // w 6000 ß ---- ',. / M/Q STEP NUMBER Figure 5. Measurements of the M/Q spectrum in the range of 1-16 for the torus ions at 0421 UT at E/Q = 960 V with mass spectrometer 2. The M/Q of the primary ion is 16.

10 25,372 FRANK AND PATERSON: RETURN TO O BY GALLEO OCTOBER : :.'58 UT... M/Q=16, THERMAL 800/CM 3, 40 ev M/Q =16, THERMAL 200/CM 3 160eV... M/Q=i6, PCKUP O0/CM 3, T.L = 00 ev M/Q = 32, PCKUP O0/CM 3, T.L= 00 ev M/Q= 48, PCKUP O0/CM 3, Ti = 00 ev SUM P2 SECTOR 8=20 ø ' 0 ß 0 :- ' ' l -- p3 SEoCTOR 2./% (b) 05 ß P4 SECTOR 3 (c) 28 ø = = : P5 SECTOR (d) - 50 ø Pl litf i02 i03 i E/Q, ENERGY/CHARGE, VOLTS Figure 6. Continuation of Figure 2 for the period 0436: :38 UT near the closest approach to o. The ion velocity distributions are shown for four directions. The measurements are modeled with velocity distributions which are convecting at a speed which is a factor of 1.3 greater than the corotational speed because the plasmas are accelerated as they are deflected by o. The five distributions are two thermal components, each with M/Q = 16, and three pickup ion distributions with M/Q = 16, 32, and 48, respectively. The presence of H + ions can be seen in the responses of P2 in sector 2 at E/Q < 100 V. fits are sufficiently good to provide evidence for a relatively low intensity peak of M/Q- 48 ions at about 2 kv in P5, sector 2 in Figure 6d. Note that the two torus thermal distributions cannot account for these intensities in this sector. Further examination of Figure 6a for E/Q > 1 kv shows how the intensities from the three ion species at M/Q = 16, 32, and 48 combine to form the intense ring distribution function. A value of M/Q = 48 provides significant fluxes which are observed at E/Q = 4.5 kv. Specifically, the pickup ions with M/Q = 32 and 48 provide the responses in the E/Q range of about 2-

11 FRANK AND PATERSON: RETURN TO O BY GALLEO 25, kv, and the pickup ions at M/Q = 16 fill in the remaining response minimum at about 1-2 kv. On the other hand, a value of M/Q = 64 would provide measurable intensities at 6.5 kv, but such fluxes are not observed. n summary, the measured pickup velocity distributions are fitted with three species at M/Q = 16, 32, and 48, each with number densities of about 100 cm -3. The fitting of the ion velocity distributions is a nontrivial iterative task, but the scientific yield is well worth the effort. The fits to these rings are based upon a value of 0.75 of the maximum particle energy during its cyclic motion. That is, this maximum particle energy is 4 times that corresponding to that of thermal ion bulk flow. The fractional value of 0.75 is based upon the results of previous extensive analyses of the pickup ions in the water group, M/Q = 16, 17, and 18, at Europa [Paterson et al., 1999]. This fractional value can be qualitatively understood in terms of the sampling of the velocity distributions in finite-sized angular sectors. For sampling with much smaller angular sectors the maxima would be more nearly at the maximum particle energy, but for the actual sampling the sector averages substantially reduce the E/Q for the responses due to the ring distributions. t should be recalled by the reader that for a homogeneous ring distribution in velocity space due to ion pickup, the plasma analyzer is most sensitive to the ions at their maximum speed in the cycloidal motion because the sensitivity of an electrostatic analyzer varies as (E/Q) 2. The low-energy part of the cycloidal motion is at intensities which are below the instrumental threshold. Thus the pickup ion distributions are detected in the higher E/Q channels of the PLS. This ring should, and does, appear in the angular sectors P2, sector 2 and P5, sector 1. This location of the ring is perpendicular to the model magnetic field vector, as is expected for the ion pickup. H + is also observed at E/Q < 100 V. Although the directional, differential intensities are rela- GALLEO PUS i0 24 MASS SPECTROMETER 2! OCTOBER 1999 DFFERENTAL CHANNEL 0439 UT z o o 0, M/Q = =, E/Q =540V VEWNG NTO TORUS FLOW ---% E/Q =66V BACKGROUND, VEWNG AWAY FROM FLOW f -- / k A/// -, // ' o, /o- / \\ OOO M/Q STEP NUMBER Figure 7. Measurements of the M/Q spectrum in the range of 2-32 for the torus ions at 0439 UT at E/Q- 540 V with mass spectrometer 2. The plasmas are dominated by ions with M/Q- 16.

12 25,374 FRANK AND PATERSON: RETURN TO O BY GALLEO N /CM 3 T L i i i 1... p((:]) GALLEO PLS 0 3!- OCTOBER 1999,, o C/A / - V x, KM/S 6O (c) q Vy 0 i (d) Vz 0-60 (e) 6O (f) 0,,,,, UT Figure 8. Plasma bulk parameters as measured during the close flyby of o on October 11, Shown are (a) the ion densities, (b) the temperatures, (c-e) the three components of the bulk flow velocity, and (d) the flow speed. The bulk flow velocity is given in the o rest frame. The slight modulation of some of the parameters with time is due to the interleaving of ion passbands sampled with the plasma analyzer. The time for closest approach (C/A) to o is also shown. The velocity components and speed for rigid corotational flow are given by the dotted lines. tively large, the number densities are low relative to those of the heavier ions because the energies are low. These H + ion distributions are also affected by the spacecraft potential in the range of -10 V, which must be accounted for in fitting these distributions. Such an analysis was previously performed for the H + ions observed during the o flyby on December 7, 1995 [Frank and Paterson, 1999a]. We note here that there are again measurable H + densities from the ionization of hydrogen atoms in o's atmosphere and defer the detailed analysis to a later report. The ion number densities, temperatures, components of the bulk flow velocity, and bulk speed are shown in Figure 8. These plasma parameters are computed as moments of the measured three-dimensional velocity distributions [Krall and Trivelpiece, 1973]. The M/Q- 16 as established with the mass spectrometer measurements, and the E/Q range was limited to 80 V < E/Q < 1.4 kv in order to minimize the effects of hydrogen pickup ions at low energies and pickup of heavy ions at higher energies. As can be noted in Figure 1, the Galileo spacecraft is moving toward o in roughly the corotational direction. Prior to closest approach it is just inside of o's orbit and then outside of this orbit after the flyby. Examination of Figure 8 finds that the ion densities slowly increased from about 200 to 600 cm -3 before closest approach. A transient minimum of about 100 cm -3 occurred at

13 FRANK AND PATERSON: RETURN TO O BY GALLEO 25,375 about 0405 UT. Before this minimum in density the ion temperatures were about 2 x 105 K. Afterward the ion temperature monotone increased to about x 106 K at 0430 UT. Closest approach to o occurred at 0433 UT. During the above period of UT, significant fluxes of H + ions were present which were not observed at earlier times. During this time period, there were significant departures from rigid corotation, as seen in the bulk flow components in Figure 8. That is, the speed along the corotation direction decreased by several kilometers per second, and there was a gradual increase of the flow toward Jupiter from essentially 0 km s -1 at 0405 UT to about 10 km s -1 at 0427 UT. At closest approach at 0433 UT a maximum in the ion number densities was observed. This density was 1200 cm -3 with a temperature of 7 x 105 K. As shown in Figure and Figures 8c-8f, the bulk flow was strongly diverted by the presence of o. This flow was also accelerated to a maximum increase of 30% relative to the flow speed for rigid corotation. 4. Summary and Discussion On October 11, 1999, the Galileo spacecraft returned to o for a second close flyby during its mission at Jupiter. The high-resolution plasma data on the spacecraft recorder covered the period UT, with the closest approach to o occurring at an altitude of 617 km at 0433 UT. During the tape-recording the spacecraft trajectory began at a Jovian radial distance of about 0.15 Rj inside of the orbit of o and ended just outside of o's orbit. That is, the spacecraft was moving approximately in the direction of corotation of Jupiter's torus plasmas and approached o with a speed of 11 km s -1 before the close flyby. Several useful coordinates for the position of the spacecraft are given in Table 1. At o the spacecraft was positioned at system longitude, M, of 80 ø and 0.3 Rj below the centrifugal equator. For comparison, these coordinates were 270 ø and 0.2 Rj above the equator, respectively, during the previous flyby of o with the Galileo spacecraft on December 7, Thus the distance from the centrifugal equator for these two encounters is not a large factor in the differences in observed torus densities, but k and the elapsed time can be important. The present analyses of the E/Q spectra for the measurements of the three-dimensional ion velocity distributions with the electrostatic analyzers were improved upon relative to the initial results for the torus crossing of December 7, 1995, as reported by Crary et al. [1998]. This improvement was gained primarily by using the determinations of the M/Q of the primary ions with one of the three mass spectrometers in the plasma instrumentation and subsequent iteration of the E/Q spectra with ions of the appropriate species and temperatures. Outside of the interaction of the torus plasmas at closest distances to o, the E/Q spectra were fitted with two M/Q- 16 distributions with different temperatures and densities. For example, at 0411 UT the densities and temperatures of these two components were 300 cm -3 and 40 ev and 200 cm -3 and 10 ev. The combination of E/Q and M/Q measurements can identify the dominant ion as either O + or S + but cannot distinguish between O + and S ++. Our identification of the dominant ion as O +, and not S ++, is based upon the study of torus compositions which included the Voyager measurements of thermal plasmas and ultraviolet emissions [Bagenal, 1994]. Examination of Figure 8 finds that there are two principal torus regions outside of the direct interaction at o. The time intervals for these two regions are approximately UT and UT, which are separated by a deep minimum in ion densities. Although the possibility that this minimum is due to a temporal fluctuation in the topology of the torus cannot be excluded, with observations with a single spacecraft we interpret this feature in terms of a spatial structure on the basis of previous measurements. Both of these intervals are prior to the closest approach to o and occur inside of its orbit. The first interval is centered at a Jovicentric distance of about 5.75 Rj and is located at the expected position of the torus "ribbon" at a local time of about 1000 and system longitude of 60 ø according to the ground-based survey of S + emissions reported by Schneider and Trauger [1995]. A similar narrow region of maximum intensities of torus emissions at these positions inside of o's orbit was recorded in the measurements of S ++ with the ultraviolet spectrometer on board the Voyager spacecraft [Dessler and Sandel, 1992]. The detailed analysis of the in situ observations of plasma densities with the plasma analyzer on board Voyager also finds a density maximum at the position of the ribbon [Bagenal, 1994]. However, more recently, the plasma ribbon was not detected with the plasma wave instrumentation on the Galileo spacecraft during its passage into the inner torus on December 7, 1995 [Bagenal et al., 1997]. Our presently reported measurements with the Galileo plasma analyzers on October 11, 1999, find that the principal ion species are O + and S ++ in the ribbon. Representative densities and temperatures for these two ions are 250 cm -3 and 50 ev for O + and 50 cm -3 and 10 ev for S ++, where again the identification of the relative contributions of O + and S ++ must rely upon the Voyager 1 spectrometer results reported'by Bagenal [1994]. That is, these earlier results are used to determine which one of these two ions is greater in terms of number densities. An upper limit to the S + density in the ribbon is about 90 cm -3. The reason for the existence of the ribbon remains enigmatic; it could possibly be a cross-tail electric field [p and Goertz, 1983; Barbosa and Kivelson, 1983], an unidentified heating mechanism [Dessler and Sandel, 1992], or a nonlinear diffusion process coupled with a feedback mechanism [Herbert, 1996].

14 25,376 FRANK AND PATERSON: RETURN TO O BY GALLEO The second torus region prior to the o flyby was first detected at a boundary at a radial distance of about 13,500 km at 0405 UT during the spacecraft's approach to o. A steep decrease in ion densities from about 300 to 100 cm -3 occurred at this distance in the torus flows. Subsequently, there was a monotone increase in the ion temperatures until 0430 UT, which was just before closest approach. These ion temperatures increased from 2 x 105 to x 106 K during this time period. During this interval the component of bulk flow in the direction of Jupiter smoothly increased from 0 km s -1, the value for rigid corotation, to about 10 km s -1. At the same time, there was a detectable decrease in the component of flow along the corotational direction by a few kilometers per second from its rigid corotational value of 57 km s -. Thus the torus ions were being heated and slightly diverted in their flows by the presence of a neutral gas. The existence of such a gas is strongly supported by the presence of pickup H + during this period of time, but not previously at greater distances from o. The boundary at 13,500 km, or 7.4 Rio, compares favorably with the radius of the Hill (Lagrange) "sphere" for o of 5.8 Rio. This radius is the average distance from o for which Jupiter's gravitational force minus the centrifugal force is equal to o's gravity [Johnson, 1990]. t is noted here that the escape speed from o's gravitational field is 2.5 km s - which corresponds to Elk K. Thus o appears to possess an extended hydrogen atmosphere inside of its Hill sphere which is in common orbital motion with this moon. The presence of pickup hydrogen during the close flyby of o on December 7, 1995, was directly detected with the plasma instrumentation [Frank and Paterson, 1999a] and also inferred from observations of plasma waves [Chust et al., 1999]. There is some evidence of the presence of hydrogen pickup ions out to distances of ~8000 km from o on the outbound leg of the trajectory [see Frank and Paterson, 1999a, Plate 1]. At the closest approach distances to o, the torus plasma flows were strongly diverted, as expected for the presence of the conducting body of o. n addition, the flow speeds increased by a factor of 1.3 relative to those for rigid corotational flow. The maximum ion number densities were 1200 cm -3 from the moments calculation. The primary contributors to this total number density were thermal M/Q- 16 ions, one distribution with density 800 cm -3 with kt = 40 ev and the second distribution with 200 cm -3 and k T = 160 ev. From the analysis of the combined Voyager plasma analyzer and remote spectrometer measurements as reported by Bagenal [1994], we conclude that these two ions are O + and S ++, respectively. n addition, pickup ions with M/Q - 32 and 48 were detected. These ions presumably are S + and SO +, respectively. t is of interest to note that the magnetometer provided a simultaneous, independent detection of ion cyclotron waves from SO +, although determination of the number densities was not possible [Russell and Kivelson, 2000]. The estimated densities for the SO + pickup ions were significant and in the range of 100 cm -3 for each of the above two species. No detectable responses were recorded with the plasma analyzer at M/Q = 64, or SO2 +. n addition to the heavier ions, the velocity distributions indicate a pickup distribution at M/Q - 16 with a den- sity of 100 cm -3. The reader is reminded that the number densities are reported here in Figure 6. The charge densities are estimated as cm cm -3 for the singly charged ions and 200 cm -3 x cm -3 for the doubly charged ions, or a total of 1500 cm -3. Of course, ff the dominant component is doubly charged, in disagreement with the Voyager findings, then the charge density is increased to a total of 1500 cm cm -3, or 2300 cm -3. f plasma wave instrumentation detects the upper hybrid emission line during this flyby, then this issue of charge density is resolved. This emission was present during the first close flyby on December 7, 1995 [Gurnett et al., 1996]. Other species with M/Q 48 also may be present in the ion velocity distributions but are not resolved by the plasma analyzer. The observed pickup ions in o's atmosphere are a strong source for torus plasmas, as concluded from the previous modeling of the torus by Shemansky [1988]. The quantitative determination of this source strength awaits future modeling of the interaction of the torus ions with this moon's atmosphere. The maximum ion densities reported here at closest approach to o are 1200 cm -3 and are considerably smaller than those observed in the wake of o on December 7, These latter densities were 20,000 cm -3 with cool temperatures and no significant bulk flows relative to the rest system of o [Frank et al., 1996]. The presence of these ions was interpreted in terms of an o ionosphere. The present series of observations were gained along the flank of o with respect to the torus flows. The range of densities inside of o's orbit is about cm -3. These densities are smaller than those observed at similar locations on December 7, 1995, which ranged from about cm -3. The ion densities just outside of o's orbit were 800 cm -3, as reported here for October 11, 1999, and 3500 cm -3 on December 7, Although differences in local time, system longitude, and distance from the centrifugal equator may influence the measurements of density, these factors do not appear to be sufficiently significant to account for the differences by factors of 3-4 for these two o fiybys. The most direct explanation for these substantial differences is a temporal change in the torus densities at o, presumably due to o's volcanic activity. The analysis of the set of Galileo traversals through the o torus will be necessary in order to resolve the reasons for the decreased densities at o's orbit during October 11, n any case, these differ-

15 FRANK AND PATERSON: RETURN TO O BY GALLEO 25,377 ences in the density of the torus flows at o's orbit must be considered in using models to confirm or deny the existence of an internal dipole moment for o [Khurana et al., 1997, 1998; Linker et al., 1998; Neubauer, 1998; Saur et al., 1999]. The fortuitous acquisitions of the plasma observations at o during December 7, 1995, and October 11, 1999, for two dissimilar flyby trajectories provide important parameters for accurate modeling of the complex interaction of this moon with the plasma torus, which will lead to an accurate assessment of an intrinsic magnetic moment or its upper limit. Acknowledgments. We greatly benefited from discussions during a workshop on o and its plasma torus which was hosted by Fran Bagenal at the University of Colorado, Boulder during February 28-29, Participants included Michael Brown, John Clarke, Melissa McGrath, Nick Schneider, and William Smyth. 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16 25,378 FRANK AND PATERSON: RETURN TO O BY GALLEO teraction with the o plasma torus: Asymmetric plasma flow, J. Geophys. Res., 104, 25,105-25,126, Schneider, N.M., and J. T. Trauger, The structure of the o torus, Astrophys. J., 450, , Shemansky, D. E., Energy branching in the o plasma torus: The failure of neutral cloud theory, J. Geophys. Res., 93, , Williams, D. J., B. H. Mauk, R. E. McEntire, E. C. Roelof, T. P. Armstrong, B. Wilken, J. G. Roederer, S. M. Krimigis, T. A. Fritz, and L. J. Lanzerotti, Electron beams and ion composition measured at o and its torus, Science, 274, , Williams, D. J., R. M. Thorne, and B. H. Mauk, Energetic electron beams and trapped electrons at o, J. Geophys. Res., 104, 14,739-14,753, L. A. Frank and W. R. Paterson, Department of Physics and Astronomy, 212 Van Allen Hall, University of owa, owa City, A (louis-frank@ uiowa.edu) (Received December 29, 1999; revised May 4, 2000; accepted May 4, 2000.)