Galileo spacecraft. Intense electron beams observed at Io with the

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A12, PAGES 28,657-28,669, DECEMBER 1, 1999 Intense electron beams observed at Io with the Galileo spacecraft L. A. Frank and W. R. Paterson Department of Physics and Astronomy, University of Iowa, Iowa City Abstract. On December 7, 1995, the plasma instrumentation (PLS) on board the Galileo spacecraft detected intense low-energy electron beams which were aligned along the magnetic field during the close approach of Jupiter's moon Io. The energies of these electrons were in the range of several hundred ev. An average field-aligned current density of 0.05 na/cm ' associated with the electron beams was measured during three consecutive samples of the electron velocity distributions. The total current in this beam is --1 x 10 A and in the range of the current magnitudes of 5 x 10 A previously reported at larger distances with the magnetometer on Voyager 1. Possible contributions to these currents from the thermal electrons and ions were not measured by the Galileo PLS. Estimates of the total energy influxes in the Io footprint in the Jovian atmosphere are in the range of 3 x 10 ø W and similar to those obtained from observations of the Jovian far-ultraviolet emissions with the Hubble Space Telescope. These electron beams are embedded in cool, dense thermal plasmas with k T - 4 ev, a temperature which is similar to that observed in the adjacent torus plasmas which are corotating with Jupiter. 1. Introduction The discovery of the correlation of 22-MHz radio emissions with the position of Io in its orbit around Jupiter by Bigg [1964] provided the initial impetus for studies of this moon's interaction with the planet's magnetic field which continue to the present date. Piddington and Drake [1968] thought that the modulation of the radio emission was due to the electromagnetic interaction associated with Io's motion through the Jovian magnetic field. Goldreich and Lynden-Bell [1969] quantitatively extended this suggestion in terms of a current system driven by the electromotive force from Io's slow orbital motion relative to Jupiter's rapidly corotating magnetic field. The currents driven by the voltage across Io were expected to close with magnetically field-aligned currents directed into and out of the conducting Jovian ionosphere. This model for the interaction depended upon the fact that the conductivity of Io was sufficient to significantly retard the corotational motion of the Jovian magnetic field in the body of the satellite. The conductivity of the plasma along the magnetic field lines from Io to Jupiter was assumed to be infinite. In another paper of this early period of studies, Schatten and Ness [1971] reported that most of the Copyright 1999 by the American Geophysical Union. Paper number 1999JA /99/1999JA $09.00 characteristics of radio modulation could be accounted for by an interaction near Jupiter's surface, i.e., in terms of synchrotron radiation from electrons accelerated by the potentials generated by Io's motion. Subsequent ideas for the acceleration of charged particles in terms of the formation of a plasma sheath around Io were offered by Gurnett [1972] and Shawhan [1976]. The potentials across these sheaths in the ionosphere of Io were postulated to accelerate charged particles to energies of hundreds of kev. Later, Smith and Goertz [1978] proposed that the magnetically field-aligned particle acceleration occurred across electrostatic double layers in the magnetic flux tube extending from Io to the Jovian ionosphere. Goertz [1980] followed this latter work with the suggestion that the currents generated by charge exchange of neutral gases of the Io atmosphere with the plasmas corotating with the Jovian magnetic fields could provide o r; o o field-aligned currents. More recently, these effects of mass loading of the magnetic fields in the vicinity of Io have been further considered by Cheng and Paranicas [1998]. The interaction of Io with the corotating Jovian plasmas has been shown by the theoretical analysis of Ne ba er [1980] to result in a standing Alfv n wave with respect to Io. The disturbance propagates with the local Alfv n wave speed, and thus the current is not magnetically field-aligned but is directed along so-called "Alfv n wings" be- 28,657

2 28,658 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO cause the magnetospheric flow speed is substantial relative to the Alfv n speed. Crary [1997] proposed that repeated Fermi acceleration of electrons in Io's Alfv n wing can be important in producing significant energ. influxes impacting Jupiter's atmosphere. The direct detection of a large, magnetically field-aligned current at Io was provided by magnetic field measurements with Voyager I as this spacecraft passed to the south at a distance of about 20,000 km from this moon [Ness et al., 1979]. The current in the magnetic flux tube connected to Io was about 5 x 106 A. The magnetic perturbation was best fit td a pair of field-aligned currents, i.e., parallel and antiparallel to the magnetic field when compared to that for a single line current. Considerable further impetus in the studies of Io's interaction with the corotating Jovian magnetic field was provided by the remote images of bright emissions in Jupiter's atmosphere which were located at the footprint of the Io flux tube [Con. nerney et al., 1993]. These images of the H3 + emissions were taken with the NASA Infrared Telescope Facility in Hawaii. Later observations of the far-ultraviolet emissions at the Io magnetic footprint were obtained with the Hubble Space Telescope [Clarke et al., 1996; Prang et al., 1996]. The approximate magnitude of the power radiated in these emissions was a remarkable 10 ll W. The close flyby of Io by the Galileo spacecraft on December 7, 1995, provided recent fascinating in- formation concerning the Io-Jupiter electrodynamic circuit. The spacecraft passed by at a closest distance of 890 km on the wake side of the moon. The magnetic field was found to decrease by about 40%, which indicated a strong interaction with the flowing Jovian plasmas in the torus [Kivelson et al., 1996a, b]. At closest approach a dense cool plasma at rest with respect to Io was detected [Frank et al., 1996]. The ion density and tempera- ture were 2 x 104 cm '3 and 105 K, respectively. This region of cool plasmas was surrounded by the diversion of hotter torus plasmas. An estimate of the maximum densities from the plasma wave in- strument was about 4 x 104 cm '3 [Gurnett et al., 1996]. Most relevant to our present paper were the reports of magnetically field-aligned electron beams at the closest approach [Williams et al., 1996; 1999]. Although the energy fluxes of these electrons with energies >15 kev are factors of about 100 too small to account for the emissions in the Io footprint in the Jovian atmosphere, these beam detections provided support for the possibility that adequate energy fluxes were to be found at lower energies. In this paper we report the first direct detections of intense field-aligned fluxes of low-energy electrons in the vicinity of Io which are sufficient to account for the bright emissions at its footprint in the Jovian atmosphere. These electrons were measured with the plasma instrument (PLS) on board the Galileo spacecraft in the energy range of about 100 ev to I kev. 2. Instrumentation 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 were 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 -steradian 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. The rotation period of the spacecraft was s. During a single spacecraft rotation 12 E/Q passbands were sampled during each of the 8 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 ro- tation. 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 electron velocity distributions reported here. Exceptions are noted as required. The third rotation is devoted alternately to telemetering the responses of the lowest passbands of the electrostatic analyzers in the E/Q range of 0.9 to 14 V or to the responses of the mass spectrometers. Electron and positive ion channels are identified as E and P, respectively. The equatorial sensors (E4,P4) and both sets of polar sensors (El,P1 and E7,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 a rotation of the fields of view by 3.2 ø. This low-energy scan is acquired once per 12 spin cycles, or 242 s. Because the spacecraft telemetry clock is asynchronous relative to the angular sectoring which is locked to 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

3 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO 28, ' I ' ' ' I ' ', I ' ' ' I ' ' ' I ' GALILEO PLS 7 DECEMBER '31 dupiter 4000 ""--" :46 -,J/ COROTAT ION '""- --" -' :44 UT ELECTRON BEAMS 4O00 PERPENDICULAR TO IO ORBIT -40OO Y, KM Figure 1. The trajectory of the Galileo spacecraft during its close flyby of Io on December 7, The Cartesian coordinates are chosen with Y toward Jupiter, Z perpendicular to Io's orbit, and X nearly parallel to the direction of corotational flow of Jupiter's plasmas. The radius of Io is 1815 km. Closest approach to this moon occurred at 1745:46 UT. The time period for the detection of intense, field-aligned electron beams is indicated by the solid bars s, or I min. 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 during the subsequent fourth and fifth spin cycles. This resolution is not required for the present determination of the electron velocity distributions. 3. Observations The trajectory of the Galileo spacecraft during its close flyby of Io is shown in Figure 1. This passage was positioned in the downstream wake of Jupiter's torus ion flows past Io. Io's orbital speed around Jupiter is 17.3 km/s, and rigid corotation of the torus plasmas is 74.1 km/s which results in a flow speed past Io of 56.8 km/s. At closest approach the altitude of the spacecraft was about 890 km. The intense field-aligned electron beams which are the topic of this paper were observed for about 100 s, as indicated by the thick solid bars in Figure 1. In the rest frame of Io the speed of the Galileo spacecraft was 15.0 km/s. Thus the track length through the electron beams was about 1500 km. The responses of two of the electron sensors, E5 and E7, as the spacecraft passed closely by Io are summarized in the energy-time (E-t) spectrograms shown in Plate 1. The spin-averaged responses of these sensors are color coded according to the color scale at the top right-hand corner and plotted as iunc ions of energy cnarge t.,, -, r x xn umts of volts (ordinate) and universal time (abscissa). The background responses of the sensors are determined by using the responses of the sensors in the highest-energy passbands. These responses do not vary as functions of passband energy and are due to energetic charged particles which penetrate into the sensors and also secondary electrons which are scattered along the analyzer plates into the sensors. Specifically, the value for the background responses which is subtracted from the responses at lower energies is chosen to be 4 above the aver-

4 28,660 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO GALILEO PLS,,, 7 DECEMBER 995 ELECTRONS, DETECTOR E5 i I COUNTS/S ELECTRONS, DETECTOR E7 ' I ' I ' UT X = : Y= Z= Plate 1. Energy-time (E-t) spectrograms for the responses of two of the electron sensors, E5 and E7, of the plasma instrumentation (PLS) on board the Galileo spacecraft on December 7, The responses in units of counts per second are color coded according to the color bar at the top right-hand side and plotted as functions of energy/charge (E/Q) and universal time (UT). These responses are averaged over the spin period of the spacecraft. The field of view of sensor E5 is directed at large angles to the spacecraft spin axis and views electron pitch angles which range between those nearly parallel and antiparallel to the magnetic field to those nearly perpendicular to the field. The field of view of sensor E7 is directed nearly parallel to the spacecraft spin axis and is viewing electrons with large pitch angles generally perpendicular to the magnetic field.

5 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO 28,661 GALILEO PLS ELECTRON PITCH ANGLE 7 DECEMBER 1995 DISTRIBUTIONS f ( '), S3/CM 6 i( :58 UT V 1744:42,i - 4X 104' KM/S ief 29 i(::f :5o ß i i i i I i I I : :46 ( 1746: :43, ( 1747:04 ( 1747:41,m ram,, Plate 2. Electron velocity distributions with E > 100 ev as functions of pitch angle and electron speed for the time period just prior to, during, and after the detection of the intense electron beams in the vicinity of Io on December 7, The phase space densities are color coded according to the color bar at the top left-hand side. The magnetic field is along the +Vii axis which is directed approximately southward with respecto the Jupitercentered ecliistic plane. The circled numbers designate the time order for sampling the electron beams.

6 28,662 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO age responses in the high-energy passbands where is the standard deviation of the background. The field of view of E5 covers a range of angles, 100 ø to 122 ø, with respect to the spin axis of the spacecraft. Coarsely, the spin axis is pointed toward Earth and is pointed perpendicular to the magnetic field. Thus the field of view covers a large range of pitch angles with respect to the magnetic field, i.e., nearly parallel and antiparallel to the field and perpendicular to this vector as the spacecraft rotates. On the other hand, sensor E7 views the range 142 ø to 171 ø relative to the spin axis and is limited to sampling electrons with pitch angles generally perpendicular to the magnetic field. The modest sawtooth of responses for energies <100 ev is due to interleaving of energy passbands during the instrument sampling cycle. A more detailed summary of the electron pitch angle distributions for energies >100 ev is shown in Plate 2. The times in Plate 2 correspond to the beginning of the sampling period for each velocity distribution. As the region of intense field-aligned beams is approached during 1743:58 to 1744:42 UT the electron distributions are maximum at pitch angles of 90 ø with a small component flowing antiparallel to the magnetic field along the axis, i.e., toward Jupiter's northern hemispher6. In the intense beam region during 1745:46 to 1747:04 UT the electron velocity distributions are strongly aligned in both parallel and antiparallel directions along the magnetic field. A substantial but lesser electron distribution is also present for pitch angles perpendicular to the magnetic field. Electron velocity distributions in the intense electron beam are shown in Figure 2 for directions parallel and perpendicular to the magnetic field as sampled at 1746:43 UT. The phase space densities are factors of about 20 greater at energies 100 ev in favor of directions aligned along the magnetic field. The temperature of the electrons over the energy range 300 V to I kv is about 350 ev. The similar temperatures of the electron distributions parallel and perpendicular to the magnetic field are evidence of their similar origins. The fieldaligned anisotropy of the low-energy electrons is further illustrated by the pitch angle distributions displayed in Figure 3 for three energies, i.e., 180 ev, 800 ev, and 3.1 kev. The corresponding speeds of the electrons are also given in Figure 3 in order to facilitate comparison with the velocity distributions in Plate 2, for example. Inspection of Figure 3b for 800 ev finds that the beams are bidirectional, i.e., directed both parallel and antiparallel to the local magnetic field, and that the beam intensity for the parallel beam is somewhat greater with respect to that of the antiparallel beam. For pitch angles in the range of 90 ø the phase space densities at 800 ev are factors of 5 to 10 less than those of the field-aligned populations. There is one further important feature of these electrons with velocities directed perpendicular to the local magnetic field. This feature is to be identified in the E-t spectrogram of Plate I for elec- trons with similar pitch angles as measured with sensor E7. That is, comparable intensities of these electrons are not found in the plasma torus in the immediate vicinity of Io. Thus the locally mirroring population of these 800-eV electrons most likely has its origins as electrons scattered from the field-aligned beams by Io's surface, atmosphere, and/or ionosphere. As noted above, this interpretation is also supported by the similar temperatures of these two electron distributions. Because of the interest in identifying the source of charged particle energy precipitating in the Io magnetic footprint in Jupiter's atmosphere the directional energy fluxes for E > 100 ev are given in Figure 4 as a function of time. These energy fluxes are shown for three directions, parallel, perpendicular, and antiparallel to the magnetic field. The width of the pitch angle bins is 20 ø. The maximum energy fluxes in the beam are about 2 ergs/cm 2 s sr each for the beams parallel and antiparallel to the magnetic field, respectively, and 0.2 ergs/cm 2 s sr for the fluxes perpendicular to the field. Note in Figure 3 that the phase space densities are rapidly increasing as the pitch angles approach the parallel and antiparallel directions. Because the angular dimensions of the field of view of the relevant equatorial sensors of the PLS are relatively large, about 20 ø in the polar direction and 6 ø in the spacecraft rotational direction, the measurements are consistent with narrower field-aligned beams. Our estimates of the pitch angle half width of the field-aligned electron beams are in the range of <8 ø and thus similar to the 6 ø resolution in the direction of spacecraft rotation. Williams et al. [1999] 0-26 [- 1746:45 UT c 27 C[ 28 C[ 29 c 5o DECEMBER o o,, To! T = 540 ev f '. ELECTR - - -' EZl. 550 ev., \ o I I I I I IIII I \1 I I I III E, ev Figure 2. The electron velocity distributions for pitch angles parallel (open circles) and perpendicular (solid circles) to the magnetic field in the electron beam at 1746:43 UT as observed as functions of energy with sensor E4.

7 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO 28,663 GALILEO TO FLYBY PLS i( 26 i( 27 7 DECEMBER ELECTRONS 1746:07 UT 1995 icy 28 i 29 8 X I05 KM/S ev o 90 ø 180 o i( 26 i( X 104 KM/S b 800 ev 5.5 X 104 KM/S 3 I00 ev - ic 29 ic 51 o o 90 ø 18( )o 0 o 90 ø 180 ø PITCH ANGLE PITCH ANGLE Figure 3. The electron velocity distributions as a function of pitch angle for (a) 180 ev, (b) 800 ev, and (c) 3100 ev when the spacecraft was located within the intense electron beams at 1746:07 UT. report half widths in the range of 6.4 ø to 7.4 ø for higher electron energies and a larger number of samples of the pitch angle distributions relative to the present measurements with our plasma instrument. For an assumption of this half width of the electron beam as 6 ø the corresponding solid angle is sr, and the maximum directional energy fluxes are about 60 ergs/cm 2 s sr each in the parallel and antiparallel directions. The field-aligned current densities which are directed parallel and antiparallel to the magnetic field as observed during the close flyby of Io are shown in Figure 5. The flux of electrons with pitch angles 0 ø to 90 ø is aligned parallel to the magnetic field and directed southward relative to the ecliptic plane through Jupiter's center. Because the charge of the electron is negative this electron flux corresponds to a negative current density in na/cm 2. Figure 5c gives the net current densities. The measurements of net field-aligned electron currents which are carried by electrons with E > 100 ev can be further considered by noting the time order for which the parallel and antiparallel components of the currents are measured. The time order for this sampling is designated in Plate 2 for the four relevant velocity distributions. The time resolution for determining the parallel and antiparallel components is 10 s due to the space- craft rotation. For the three consecutive velocity distributions at 1745:46, 1746:07, and 1746:43 UT the net electron flux is persistently parallel to the magnetic field, i.e., negative current densities with an average of-0.05 na/cm 2. Therefore it is unlikely that this current is due to temporal aliasing on timescales of the spacecraft rotational period of 20 s. On the other hand, the strong field- aligned current density of 0.21 na/cm 2 which is directed in the opposite direction during a single sample at 1747:04 UT may be due to time aliasing. It is important to note that the above net fieldaligned current due to the hot electron beam is only one contributor to the total current along the magnetic field line. The three other major contributors are (1) electrons in the energy range 8 ev- 30 ev in the high-speed "tail" of the thermal electron distribution which are directly observed with the plasma analyzer, (2) the large fraction of thermal electrons which are excluded from arrival at the aperture of the plasma analyzer by the spacecraft potential, and (3) the ion distribution.

8 28,664 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO,, I I ' GALILEO PLS 7 DECEMBER 1995 ELECTRONS E > I00 ev - i II TO B rf W /TO B b >- rf w z w II TO - ' c - / I I I I I I I I 1750 UT Figure 4. Directional energy fluxes for electrons with E > 100 ev at pitch angles (a) parallel to the local magnetic field, (b) perpendicular, and (c) antiparallel as functions of UT. The width of the pitch angle bucket is 20 ø. - 2XlO 9 io 9 o i i I I I i I I i I :, ' GALILEO PLS -- 7 DECEMBER 1995 ELECTRONS - E > I00 ev PITCH ANGLE RANGE ø na/cm o ø o b 0.16 x -2XlO , -180 ø 0-0 c UT Figure 5. Field-aligned current densities of electrons E > 100 ev which are directed (a) parallel and (b) antiparallel to the magnetic field during the Io flyby. (c) Net current den- Sity. The left-hand ordinate is labeled with the electron number fiux/cm 2 s and that on the right-hand side is labeled as na/cm 2.

9 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO 28,665 Table 1. Fluxes and Current Densities for the Electron Beams at Io Closest Approach Average Flux,/cm 2 s (Current Density, na/cm 2) Pitch Angle Range Energy Range 0 ø- 90 ø 90 ø- 180 ø Net 1745: :02 UT 100 ev to 52 kev 1.4 x x x 108 (-0.22) (+0.18) (-0.05) 8 ev to 30 ev 5.1 x x x 107 (-0.08) (+0.07) (-0.01) 100 ev to 52 kev 1747: :23 UT 6.7 x x x 109 (-0.11) (+0.30) (+0.21) 8 ev to 30 ev 2.0 x x x 108 (-0.03) (+0. 3) (+0. 0) Contributions (1) are summarized in Table 1, within this torus. The velocity distributions of which gives the average electron fluxes in units of these cold electrons as measured with sensor E7 (cm 2 s '1) for directions parallel and antiparallel to the magnetic field. The equivalent current densiare shown in Figure 6 for the regions inside and adjacent to the electron beam. The electron temties in units of na/cm 2 are also given in parenthe- peratures are in the range of 3 to 5 ev. The fits ses in Table 1. The fluxes and current densities provided by Maxwellian distributions of the apare given for the two energies corresponding to the electron beams and thermal electron tail, 100 ev to 52 kev and 8 ev to 30 ev, respectively. The two time intervals correspond to the net current densipropriate temperatures are shown by straight solid lines. For the sample at 1746:43 UT only one spin cycle was used because of the large gradients in electron intensities (see Plates I and 2). The electies antiparallel to the magnetic field, 1745:46- tron densities are shown for energies >0 ev which 1747:02 UT, and parallel to the field, 1747:04- are arriving at the analyzer. These densities are 1747:23 UT. These total electron fluxes are not corrected for the negative potential of the about 4 x 108 (cm 2 s -1)(-0.06 na/cm 2) and-2 x 109 spacecraft and thus do not include the ambient (cm 2 s -1)(0.31 na/cm2), respectively. Values of the densities repelled from the spacecraft by its neganet fluxes of about 108 (cm 2 s 'l) are near the tive potential. instrumental threshold due to counting statistics, The spacecraft potential is determined by evalutemporal variations, and sensor intercalibrations. ating the three significant currents to the space- The contributions to the total net current along the craft due to the ambient plasma environment. field line due to the thermal electron and ion dis- These currents are due to the torus ion fluxes imtributions, contributors 2 and 3 above, cannot be determined with the plasma analyzer because of their high densities and the low bulk drift speeds pacting the spacecraft surface, the primary electrons reaching the spacecraft, and the secondary electrons from these primary electrons. The curwhich could account for similar current densities rein; from photoelectrons due to solar illumination as above for the beam electrons. Take, for exam- is not significant at this distance from the Sun and ple, the maximum number density of thermal ions in Io's ionosphere of about 2 x 104 (cm'3)as observed with the PLS [Frank et al., 1996]. Then the for the plasma characteristics of the torus. The first two currents can be directly computed from the ion and electron velocity distributions which bulk speed of these ions corresponding to a current are measured with the plasma analyzer. Evaluadensity of 0.1 na/cm 2 is only about 0.3 km/s, which tion of the current due to the secondary electrons is less than the instrumental threshold of about 1 depends upon their energy spectrum and the yield, km/s as given by Frank et al. [1996]. The corresponding bulk speed for the thermal electrons is the same value but is orders of magnitude smaller secondary electrons per primary impacting electron [Grard et al., 1973]. The character of the secondary electron spectrum was determined with than the instrumental threshold of about 1000 measurements in the hot plasmas of Earth's km/s. This higher threshold value is due to the higher thermal speeds of the electrons. plasma sheet on December 8, 1990, when the spacecraft was positioned in our planet's shadow. A dense, cold core of electrons is detected in the At Earth's distance from the Sun and the characplasma torus and in the Io interaction region teristics of these plasmas the photoelectron fluxes

10 o 28,666 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO GALILEO PLS IO FLYBY 7 DECEMBER 1995 ELECTRONS DETECTOR E7 icf : :16 UT ''' I' ''' I '''' I '''' I' ''' t '''' FIT N = IO0/CM 3 ev, i,,,,, : :20 ''''1''''1''''1''''1''''1''' 180/CM ev 750: '23 ''''1''''1''''1''''1'''' I'''' ß ß IO0/CM5-2.8 ev 746: :03 ''''1''''1''''1''''1''''1'''' 190/CM3 - ' 4.8 ev -'. : -, 1754: :26 60/CM icf 27,,,I,,,,I,,,,I,N,,I,,,,I,,, o IO o 50 ß 1111,1 0 I E, ev E, ev Figure 6. Velocity distributions of cold electrons as functions of energy near and within the hotter electron beam as measured with sensor E7. The densities are not corrected for the spacecraft potential and thus are a record of the electrons actually arriving at the entrance aperture of the plasma analyzer. are significant in terms of a current. It is instructive to point out that the electron fluxes in Earth's shadow plasma sheet are much larger than the ion fluxes and that if only these currents are operative, then the spacecraft potential must be large and negative in order to limit the number of primary electrons reaching the spacecraft, i.e., current balance for the spacecraft must be achieved. In reality, the secondary electrons also contribute a substantial current which greatly reduces the spacecraft potential. The best fit secondary electron temperature was kt- 1.0 ev and the maximum secondary yield was 3. Current balance must be maintained for the spacecraft, i.e., at Jupiter the sum of the above three currents must be zero. The spacecraft potential is negative as displayed in Figure 7c. The electron temperatures are given in Figure 7b. The corresponding determinations of the electron densities are shown in Figure 7a. The previously reported ion densities are also shown [Frank et al., 1996]. See Crary et al. [1998] for the composition and temperatures of the ions in the torus. The electron and ion densities in the plasma torus and outside of the electron beams are in substantial agreement, within 20% to 30%, with consideration of the complexities in determining the electron and ion fluxes impacting the spacecraft. The maximum ion and electron densities of about 2 x 104 (cm'3) at closest approach to Io are also in agreement even though there are the expected differences due to rapid spatial variations due to the large gradients of plasma densities during the spin period of the spacecraft. 4. Discussion On December 7, 1995, intense, magnetically field-aligned electron beams were detected with the plasma instrumentation on board the Galileo

11 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO 28,667 N /CM 5 f ' ' ' ' I ' I I GALILEO PLS 7 DECEMBER 1995 / 10NS ELECTRONS TEMPERATURE - CORE ELECTRON b -1 SC' VOLTS -I O UT Figure 7. Densities for (a) the cold electron core and for the ions during the close flyby of Io, (b) the kt e for the cold electron core, and (c) the spacecraft potential $sc- The electron densities are corrected for the spacecraft potential. spacecraft during the close flyby of Jupiter's moon Io. The closest approach altitude was 890 km, and the electron beams were detected along a 1500-km segment of the trajectory approximately centered on this closest approach. The spacecraft trajectory was positioned in the wake of Jupiter's torus plasma flows past Io. The average electron energies in the electron beams were in the range of 200 to 300 ev and much less than the electron energies of several kev to hundreds of kev anticipated on the basis of the previously published models of Io's interaction with Jupiter's corotating plasmas [Goldreich and Lynden-Bell, 1969; Gurnett, 1972; Shawhan, 1976; Smith and Goertz, 1978; Crary, 1997]. For several of these models the electron energies were associated with the large potential difference of up to 900 kv across Io's body which is due to the motional electric fields of the torus plasma flow. Reconsideration of the mechanisms for electron acceleration by the Io interaction is desirable in consideration of the unexpectedly low energies of the accelerated electrons. There are two principal components of the electron velocity distributions, a cold thermal popula- tion and the warmer distribution which is the signature of large energy fluxes in the Jovian upper atmosphere. Cold, dense thermal electrons are observed along the entire trajectory presented here which includes the unperturbed torus plasmas and Io's wake. The kt of these cold electrons is about 4 ev and similar to the 5 ev previously reported from Voyager I [Scudder et al., 1981; Sittler and Strobel, 1987]. These cold electron densities measured with Galileo are equal to those of the ion densities within the accuracy of the measurements. The ion densities were previously reported by Frank et al. [1996]. Maximum electron densi- ties of about 2 x 104 cm '3 in Io's ionosphere at closest approach, and the lower densities in the adjacent torus are in agreement with the ion densities. There are two principal components of the warmer electron velocity distribution, an intense field-aligned component and the lesser directional intensities at pitch angles generally at large angles with respect to alignment parallel or antiparallel to the magnetic field, i.e., "trapped electrons." The energies of the bulk of both these populations are hundreds of ev. Because the fluxes of electrons with these energies are much smaller in the sur-

12 28,668 FRANK AND PATERSON: INTENSE ELECTRON BEAMS OBSERVED AT IO rounding unperturbed torus and because of the similarities of their temperatures, it is concluded that the trapped electron distribution is due to the scattering of the field-aligned electrons by Io's surface, atmosphere, and/or ionosphere upstream from the observations with the Galileo spacecraft. The total net current within the electron beams cannot be measured with the PLS because the high densities in Io's ionosphere allow significant current densities with small bulk speeds of both the thermal ion and thermal electron velocity distributions. These small bulk speeds are below the threshold for detection with the PLS. The net cur- rent due to the beam electrons can be determined for the beam which was sampled in three consecutive velocity distributions. The second beam was sampled in only one velocity distribution, and thus the net current can be the result of aliasing due to temporal and/or spatial fluctuations as the spacecraft exited the beam region. The average fieldaligned current density for the beam sampled during three consecutive distributions was 0.05 na/cm 2. An estimate of the total current in this beam can be provided by the spacecraft traversal of 1500 km during the sampling and the approximate thickness of the ionosphere of 1000 km. The total current is then about I x 106 A. An estimate of the magnetic perturbation from this current sheet is about 300 nt, a value in the range of the transverse magnetic field perturbations as observed with the magnetometer [Kivelson et al., 1996a, b]. This value for the magnetic perturbation as estimated from the PLS observations of the hot electron beams can be decreased or increased by bulk motions of the thermal ion and/or electron plasmas. If these thermal contributions are negligible, then the electron beams are providing the field-aligned currents in the Io-Jupiter electrodynamic coupling. With consideration of the uncertainties of above assumptions for total area the current of I x 106 A compares favorably with the previous determinations of currents, 5 x 106 A, as detected at 20,000 km below Io with the Voyager 1 magnetometer [Ness et al., 1979]. The simultaneous observations of ion fluxes with Voyager I were consistent with expectations of an Alfv n wave propagating southward from Io [Belcher et al., 1981]. The magnetically field-aligned beams of low-energy electrons with energies in the range of 100 ev to 1 kev as first reported here provide a significant energy flux to the Io footprint in the Jovian atmosphere in terms of optical emissions. The beams are bidirectional, i.e., one beam is arriving from the southern Jovian atmosphere and the other from the north. W[H[ams ½ al [1996, 1999] report the energy fluxes for these beams at higher energies, >15 kev, and find that these energy fluxes are approximately factors of 50 to 100 less than those for the lower-energy electrons reported here. These energy fluxes reported here are narrowly confined in pitch angle, estimated to be a cone with pitch angle half width of <8 ø aligned parallel and antiparallel to the local magnetic field. The energy flux in each of the two beams at Io is ~2 ergs/cm 2 s. For a cone half width of 6 ø the directional energy fluxes in the two beams are each about 60 ergs/cm 2 s sr. An electron with pitch angle of 6 ø at Io will mirror at an altitude above Jupiter's surface of about 0.5 Rj if there are no significant electric potential drops along the magnetic field line. In the upper Jovian atmosphere the corresponding energy fluxes over a solid angle of 2n sr are about 400 ergs/cm 2 s, respectively, in each hemisphere. With the assumption that the energy fluxes are homogeneous over the Io interaction area the corresponding total energy fluxes into the Jovian at- mosphere are estimated as 3 x 10 ø W, for a magnetic flux tube corresponding to a cross-sectional radius at Io of 2700 km (Io radius plus 890 km). These total energy fluxes are in reasonable agreement with the estimates from the far-ultraviolet emissions observed with the Hubble Space Tele- scope of about 1011 W [Clarke et al., 1996; Prangd et al., 1996]. It is of further interest to note that the electron fluxes which populate these magnetic flux tubes may provide for a trail of weaker optical emissions which are extended in longitude as the flux tube corotates away from Io. This is particularly important if the Jovian magnetic field strength at the foot of the flux tube is decreasing with a corresponding decrease of the mirror point altitudes of the electrons. 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