Published with permission of Kluwer Academic Publishers, Dordrecht,Boston, London

------------------------------------------------------------------------
Space Science Reviews 60: 283-307,1992.
Copyright 1992 Kluwer Academic Publishers. Printed in Belgium.
------------------------------------------------------------------------


THE PLASMA INSTRUMENTATION FOR THE GALILEO MISSION


L. A. FRANK, K. L. ACKERSON, J. A. LEE, M. R. ENGLISH, and G. L.
PICKETT


Department of Physics and Astronomy, The University of Iowa, Iowa City, IA 52242, U.S.A[FRANKETAL1992]


Abstract. The plasma instrumentation (PLS) for the Galileo Missioncomprises a nested set of four spherical-plate electrostatic analyzersand three miniature, magnetic mass spectrometers. The three-dimensionalvelocity distributions of positive ions and electrons, separately, aredetermined for the energy-per-unit charge (E/Q) range of 0.9 V to 52 kV.
A large fraction of the 47 pi-steradian solid angle for charged particlevelocity vectors is sampled by means of the fan-shaped field-of-view of160 deg, multiple sensors, and the rotation of the spacecraft spinningsection. The fields-of-view of the three mass spectrometers arerespectively directed perpendicular and nearly parallel andanti-parallel to the spin axis of the spacecraft. These massspectrometers are used to identify the composition of the positive ionplasmas, e.g., H+, O+, Na+, and S+, in the Jovian magnetosphere. Theenergy range of these three mass spectrometers is dependent upon thespecies. The maximum temporal resolutions of the instrument fordetermining the energy (E/Q) spectra of charged particles and mass (M/Q)composition of positive ion plasmas are 0.5 s. Three-dimensionalvelocity distributions of electrons and positive ions require a minimumsampling time of 20 s, which is slightly longer than the spacecraftrotation period. The two instrument microprocessors provide thecapability of inflight implementation of operational modes byground-command that are tailored for specific plasma regimes, e.g.,magnetosheath, plasma sheet, cold and hot tori, and satellite wakes, andthat can be improved upon as acquired knowledge increases during thetour of the Jovian magnetosphere. Because the instrument isspecifically designed for measurements in the environs of Jupiter withthe advantages of previous surveys with the Voyager spacecraft, firstdeterminations of many plasma phenomena can be expected. Theseobservational objectives include field-aligned currents,three-dimensional ion bulk flows, pickup ions from the Galileansatellites, the spatial distribution of plasmas throughout most of themagnetosphere and including the magnetotail, and ion and electron flowsto and from the Jovian ionosphere.


1. Introduction

Although the first direct detection of the presence of plasmas in thevicinity of Io's orbit was reported by Frank et al. (1976) withmeasurements from the plasma analyzer on Pioneer 10, the firstdefinitive measurements of Jovian magnetospheric plasmas were acquiredduring the Voyager flybys. The Voyager plasma observations were used todefine the required capabilities for the Galileo plasma instrumentation.
Briefly we summarize here the plasma domains of the Jovianmagnetosphere. This information is largely taken from the review byBelcher (1983). For more recent work the reader is referred to furtheranalysis of the torus ions (Bagenal, 1985; Bagenal et al., 1985), thetorus electrons (Sittler and Strobel, 1987), and the middlemagnetosphere (Sands and McNutt, 1988). Measurements of medium-energycharged particles, E approx. > 30 keV, are summarized by Krimigis andRoelof (1983).

The heart of the Jovian magnetosphere is the great torus of plasmas thatencompasses the orbit of Io. This torus is fed by the ionization ofneutral gases from Io's atmosphere and may respond to the sporadicinjection of gases from this moon's volcanic activity. The compositionof the ion plasmas in this torus is rich in heavy ions, e.g.,S+, O+, S2+, O2+, and Na+. The plasma torus is divided into two regimes,cold torus inside Io's orbit and a hot torus at greater Jovicentricdistances. The maximum ion densities, ~ 3000 cm**-3, are located nearIo's orbit. The ion temperature decreases severely from ~ 40 eV at 6RJ (Jovian radii) to ~ 1 eV at 5 RJ. This temperature decrease is due toradiative cooling. Ion temperatures in the hot torus at radial distances~ 6 to 8 RJ are in the range of 40 to 100 eV. The electron temperaturescan be described in terms of a two-temperature Maxwellian distribution,i.e., a cold and hot distribution. At the inner edge of the torus theelectron temperatures decrease to ~ 0.5 eV with decreasing radialdistances whereas the cold electron temperatures beyond ~ 6 RJ aretypically ~ 10 to 100 eV. Characteristic temperatures of the hotelectron velocity distributions are ~ 1 keV and the number densities areless than those for the cold electrons. The torus plasmas corotate withthe planet. The corresponding corotational energy of an S+ ion is 960eV at equatorial radial distance 6 RJ. The deflection of plasma bulkflow near the Io flux tube is consistent with that expected forincompressible flow around a cylinder and is evidence for an Alfven waveassociated with the plasma flow past Io. The estimated current in the Ioflux tube is ~ 3 x 10**6 A, presumably carried in large part byelectrons.

At distances beyond the plasma torus, > 10 RJ, a plasma sheet extends tothe dayside magnetopause. At 15 RJ the typical thickness of the plasmasheet is ~ 2 RJ. These plasmas are observed to corotate more-or-lessrigidly with Jupiter's rotational motion to radial distances of about 20RJ. At distances of 20 to 40 RJ this azimuthal bulk speed of the plasmais less than that expected from rigid corotation by factors of 2 or more. Beyond 40 RJ the plasmas are again observed to rigidly corotate at frequent times as inferred from measurements with themedium-energy charged particle detector. The corotational energy of anS+ ion at 40 RJ is 43 keV. Whereas the density of the hot electrons isonly ~ 1% of the total density at 8 RJ, the hot electron density issimilar to that for the cold electrons at 40 RJ. Ion temperatures arealso higher in the plasma sheet relative to those in the torus, ~ 20 to40 keV at radial distances 30 to 100 RJ. Plasma densities in the plasmasheet are ~1 to 10 cm**-3 at 10 to 20 RJ and vary from ~10**-3 to 1cm**-3 at larger radial distances. Above and below the plasma sheet thedensities can be as low as 10**-5 to 10**-4 cm**-3.

Beyond radial distances of 130 R, in the dawn side of the Jovianmagnetosphere the ion bulk flows become generally anti-sunward with astrong component along directions that are radially outward from theplanet. This region was detected with the medium-energy charged particledetector and is called the magnetospheric wind.


2. Advantages of the Galileo Plasma Measurements

The Galileo Mission advantages for plasma investigations in the Jovianmagnetosphere are (1) the spinning section of the spacecraft, (2) aninstrument microprocessor to restructure the instrument operation byground command, and (3) a series of orbits that allow close flybys ofthe Galilean satellites, a survey of the Jovian magnetotail, and asubstantial local-time survey of the magnetosphere. The spinning sectionof the spacecraft provides the important capability for a suitablydesigned instrument to view the entire 47 pi-steradian solid angle forparticle velocity vectors at the spacecraft position. The instrumentmicroprocessor can be used to tailor the operation of the plasmainstrument for the most effective measurements in each of the diverseplasma regimes of the magnetosphere and its environs, e.g.,magnetosheath, plasma sheet, satellite wake or flux tube, ormagnetospheric wind. Targeted encounters with the satellites and a tourof the magnetosphere and magnetotail offer exceptional opportunities forstudies of most of the important plasma regions and their temporalresponses to variations of Iogenic and solar wind plasmas, and theinteractions of magnetospheric plasmas with the satellites.

The Galileo plasma instrumentation (PLS) is substantially more capablefor measurements of the Jovian plasmas than those of the Pioneer andVoyager spacecraft because it is specifically designed for this purpose.
The basic advantages are in the performance areas of (1) extended energyrange, (2) coverage of the angular distributions of plasmas, (3) angularresolution, (4) temporal resolution, and (5) ion composition.

The energy-per-unit charge ranges of the Pioneer and Voyager plasmainstruments are 100 to 4800 V and 10 to 5920 V, respectively. Thecorresponding range of the Galileo plasma analyzer is 0.9 to 52000 V.
This extended energy range spans the important energy gap between 5920and 30000 V in the combined performances for the Voyager plasmainstrument and medium-energy particle detectors. The 47 pi-steradiansolid angle for particle velocity vectors at the spacecraft position issampled adequately to provide determinations of the three-dimensionalvelocity distributions for positive ions and electrons. Thus suchimportant plasma parameters as field-aligned currents, cross-fieldcurrents, plasma bulk flow velocities, heat fluxes, and free energy areto be determined for the first time with the Galileo instrument. Theangular resolution is sufficient to provide definitive measurements ofthe above plasma parameters. Temporal resolutions for obtaining electronand positive ion spectra are about 200s for the Pioneer analyzer (ionsonly) and about 100s for the Voyager Faraday cups. The correspondingtemporal resolution for the Galileo plasma analyzer is about 0.5s;complete three-dimensional velocity distributions for positive ionsand electrons can be telemetered once each 20s. These improved temporalresolutions are particularly important during the brief encounters withthe satellites and the traversals of plasma boundaries such as those ofthe plasma sheet and current sheet in the middle and outermagnetospheres.

Three miniature mass spectrometers which are positioned at the exitapertures of the electrostatic analyzers in the Galileo instrumentprovide determinations of the positive ion composition. The Voyagerdeterminations of ion composition from E/Q spectra are model-dependentand are possible when the Mach number of the corotational flow isgreater than 5 or 6. This method is acceptable generally near the Ioorbit but as the Jovian radial distance increases, ion thermal speedsrapidly increase and prevent decisive identification of ion species. TheGalileo mass spectrometers provide a direct determination of ioncomposition, specifically the mass-per-unit charge.

In addition to the above performance features, the Galileo plasmaanalyzer can be operated flexibly via electronic reconfiguration byground command. The operational configuration of energy-per-unit charge(E/Q) passbands, mass-per-unit specific plasma region. The temporalresolution for a given measurement can also be selected. The Galileoplasma analyzer is equipped with sufficient onboard hardware andsoftware to implement automated beam capture modes for ion velocitydistributions and for determination of ion composition.


3. Several Anticipated Scientific Results

The capabilities of the Galileo plasma instrumentation are demonstratedhere by application to several plasma regimes in the Jovianmagnetosphere.

As the Galileo spacecraft crosses the plasma sheet in the middle andouter magnetospheres the magnitudes of field-aligned and cross-fieldcurrents are determined. Their values and location are correlated withthe position of the current sheet as found with the magnetometer. Themotions of the plasma sheet are directly determined from thethree-dimensional bulk flow vector and the azimuthal component isseparated from the radial outflow or inflow. Angular distributions andion compositions are examined in order to discern the contributions ofelectrons and ions from the ionosphere, the solar wind via themagnetosheath, and Io in the inner magnetosphere. Thus the formation anddynamics of the plasma sheet can be understood. The mechanism for theunusual heating of plasma with increasing radial distance is expected tobe identified.

The encounters with the Galilean satellites offer exciting opportunitiesfor observing plasma phenomena. Examination of the ion velocitydistributions in the wakes of these satellites is used to determine themechanism for ion loss from these bodies. The effectiveness ofion pickup by the magnetospheric plasma flow is derived from thesignatures in the velocity distributions of these ions. The massspectrometers are used to identify the major ions produced in thevicinity of the satellite. For Io these ions include O+, S+, and SO2+,and for icy satellites perhaps H+, C+, and H2O+ can be found. Suchmeasurements give the rate of mass loss from each satellite.
Perturbations of the plasma flow can be identified in terms of theconductivity of the satellite. During the closest satellite encountersit is possible that a magnetopause or ionopause is detected, thusproviding further information concerning the magnetic and atmosphericproperties of that body. If the flyby of the satellite is polar,detection of strong field-aligned currents to and away from the Jovianionosphere might be expected. Field-aligned acceleration of ions andelectrons by electrostatic double layers or anomalous resistivity ispossible. The relative contributions of the various Galilean satellitesfor providing the ions in the plasma torus and sheet are assessed duringthe encounters.

The substantial periods of time that the Galileo spacecraft is locatedin the plasma sheet offer the unique opportunity to view the responsesof the Jovian magnetosphere to the volcanic activity on Io. If specificIo volcanic eruptions can be identified with temporal fluctuations indensities, composition, and motions of the plasma sheet, remarkableadvances in our knowledge of the transport of mass and momentum in theJovian magnetosphere are envisioned.

Simultaneous observations of three-dimensional plasma velocitydistributions and of plasma waves with the Galileo spacecraft allow thefirst studies of wave-particle interactions in the wide-ranging types ofplasmas in the Jovian magnetosphere. A discussion of measurements ofplasma waves during the Voyager encounters has been given by Gurnett andScarf (1983). For example, the velocity distributions of ions can beexamined to determine whether or not resonant acceleration byion-cyclotron waves is an important mechanism for ion heating in thetorus and plasma sheet. Further the amplitudes of broadbandelectrostatic noise can be compared with plasma velocity distributionsto determine the importance of the anomalous resistivity in plasmaheating. Free-energy sources, e.g., ring distributions in the electronvelocity distributions, for the generation of electron cyclotron orupper hybrid waves may be identified and related to the wave amplitudesobserved with the plasma wave instrument. In general the directmeasurement of the plasma density and other parameters gives the growth,propagation and resonance conditions for plasma waves in wave-particleinteractions. Thus the mechanisms for providing Jupiter with intenseradio sources and particle precipitation into the auroral ionospherecan be further understood.

The existence of the magnetospheric wind at radial distances > 130 RJ inthe dawn sector of the magnetosphere offers exciting goals for the orbitinto the distant magnetotail. The origins of this wind are unknown. Itis possible that the magnetospheric wind develops near the Alfven point,where the corotational speed is equal to the Alfven speed. The actualposition must be determined from considerations of the tangentialstress balance (cf. Vasyliunas, 1983). Thus magnetic bubbles could beslung radially outwards into the magnetotail. The low pressures in themagnetotail would produce super-Alfvenic radial outflow. On the otherhand, the outflow wind might be thermally powered by the hot plasma inthe plasma sheet inside the Alfven point. A third possibility is thatthe magnetospheric wind is the signature of reconnection of magnetotaillines in a convection pattern controlled by dayside magnetic mergingrates. The response of the magnetotail to fluctuating internal plasmas,e.g., Iogenic plasmas, or to a varying solar wind are unknown. In themagnetotail characterized by spectacular, explosive activity or a merequiescent outflow of plasmas? The exploratory orbit into the magnetotailwill indeed answer many questions concerning the origins and dynamics ofthis immense and little understood plasma region.


4. Overview of the Plasma Instrument

The instrument is divided into two analyzers, A and B. Eachelectrostatic analyzer comprises three 70 deg. spherical-segment plates.
The outer and inner plates are grounded and the center plate is suppliedwith a programmed series of voltages to effect analyses of the energyspectra of electrons (E) and positive ions (P). The inner and outerchannels between the plates are the positive ion and electron analyzers,respectively. A charged particle successfully passes through the channelon the basis of its energy-per-unit charge (E/Q). Continuous-channelelectron multipliers, or Spiraltrons, are employed as sensors and arepositioned at the exit apertures of the electrostatic analyzers. Chargedparticles arrive at positions at the exit aperture according to theirdirection of arrival at the entrance aperture. The analyzers are mountedon the instrument (magnetometer) boom of the spacecraft such thatcharged particles moving perpendicular to the spacecraft spin axisarrive at sensors 4E and 4P, and particles generally moving parallel andantiparallel to the spin axis are detected with sensor pairs 7E, 7Pand 1E, 1P, respectively. Thus the fan-shaped fields-of-view aredivided into segments by the use of multiple sensors. Rotation of thespacecraft spinning section allows coverage of almost the entire unitsphere and angular distributions are obtained by electronicallysectoring the sensor responses as a function of spacecraft rotationangle. The angular sampling of electron velocity distributions issimilar. The instrument is placed at a sufficient distance out along theboom to avoid obstruction of the fields-of-view by the large dishantenna of the spacecraft.

Three miniature mass spectrometers are included in the instrument fordetermining the composition, i.e., mass-per-unit charge (M/Q), of thepositive ion plasmas. Two of these mass spectrometers are positioned atthe exit aperture of electrostatic analyzer B, the third spectrometer isin analyzer A. Each of these mass spectrometers is equipped with twoSpiraltrons as sensors and an electromagnet. One of these sensors isplaced behind the electromagnet such that it accepts ions not deflectedby the gap magnetic field. These 'integral flux' sensors are shown as1MI, 2MI, and 3MI. The second sensor in each mass spectrometer isdisplaced from the undeflected path and accepts ions with M/Q valuesthat are a function of the gap magnetic field. These 'differential flux'sensors are 1MD, 2MD, and 3MD. A programmed series of currents is fed tothe electromagnet. If the polar angle is taken as 0 deg. in thedirection of the spacecraft spin axis, then the fields-of-view are 11deg. to 38 deg., 87 deg. to 93 deg., and 142 deg. to 169 deg. forspectrometers 1, 2, and 3, respectively.

The aperture cover serves two purposes. Prior to and during launch thecover in its closed position prevents contamination of the sensors fromdust and condensable vapors. After the launch sequence, the cover isopened and is employed to tailor the fields-of-view of the sensorsviewing at small angles to the spin axis of the spacecraft. Thecorresponding obstructions are identified as shapers.


5. Design of the Instrumentation

5.1. ELECTROSTATIC ANALYZER


The spherical-segment analyzer plates are precision-machined from solidblocks of magnesium. The radii of the inner and outer surfaces,respectively, of the four electrostatic analyzers are 9.68 and 9.95,10.08 and 10.36, 11.77 and 12.10, and 12.23 and 12.57 cm. These platesare concentric. Thus the analyzer constant C ~ 18.2, where E/Q = CV andis the plate voltage. The angle from the center of the entrance apertureto the exit aperture of the analyzer is 70 deg. as referenced to thecommon center of curvature. Each of the concave surfaces within theanalyzers has been machined with 140 saw-tooth serrations. The interiorsurfaces of the analyzer plates have been also electrodeposited withplatinum black over gold electroplate. The two latter measures are takenin order to suppress the scattering of ultraviolet radiation and chargedparticles within the analyzers into the sensors. The entrance apertureis 60 deg. wide. Because the entrance aperture is wide in order toprovide good angular coverage in spacecraft latitude, the fields-of-viewshapers are used on the protective cover to limit excessive spreading inthe azimuthal direction for the polar sensors. The center plate of eachanalyzer pair is supplied with voltages ranging from 0.05 to 2880 V inorder to provide energy (E/Q) spectra of positive ion and electronintensities. The E/Q range is 0.9 eV q**-1 to 53 keV q**-1. There are 64plate voltages that cover this energy range in logarithmically equalincrements. The averaged full-width at half maximum responses (FWHM) ofthe ion and electron passbands are equal, DELTA E/E = 0.11. The rangeand sequence of plate voltages can be selected by ground command.

A total of seven sensors are used for the two electron analyzers, andseven for the two positive ion analyzers. These continuous channelmultipliers are Spiraltrons, model SEM 4211 with 1-mm diameter aperturesand model SEM 4213 with 3-mm diameter apertures, manufactured by GalileoElectro-Optics Corporation. Entrance apertures ofthese sensors are positioned at a distance 16 mm from the exit apertureof their respective electrostatic analyzers. The Spiraltrons with largerapertures are used for the two ion sensors that view closest to the spinaxis of the spacecraft, i.e., the polar sensors, in order to offset thereduced projected area of the entrance aperture. The sensors arescreened for stability by operation for ~2 x 10**9 accumulated counts ata gain > 10**8. Grounded mesh screens are mounted in front of theentrance apertures of the sensors to shield the sensor post-accelerationelectric fields for the prevention of the collection of secondarycharged particles produced in the interior of the instrument. Thepost-acceleration voltage for the ion sensors is approximately the biasvoltage, and about +150 V for the electron sensors. The nominal gain ofthe Spiraltrons is 5 x 10**7 to 3 x 10**8 in the saturated pulse countingmode. The output charge is collected by small plates and the collectionefficiency is improved by a potential difference of about 120 V for theelectron sensors and 200 V for the ion sensors. This charge is receivedby hybrid amplifiers and discriminators manufactured by AMPTEK Inc.,model A101. The threshold for these amplifiers was conservatively set at4 x 10**6 electrons. The high voltage for sensor bias is programmable byground command in 32 increments spanning the range 2200 to 3800 V inorder to maximize the operating lifetime of the sensors againstdegradation by using the minimum charge per pulse. The pulse pairresolution of the amplifier/discriminator is nominally 250 ns (4 mHz),and about 1.4 micro-second (700 kHz) after modification for use in theinstrument.


5.2. MINIATURE MASS SPECTROMETERS


Three miniature mass spectrometers are included in the plasmainstrument, one spectrometer in analyzer A and two spectrometers inanalyzer B. After passage through the electrostatic analyzer thepositive ions enter two collimating slits. The dimensions of the firstslit are 11.1 x 0.15 mm and for the second slit, 8.5 x 0.15 mm. Thesetwo slits are separated by 9.5 mm. The paths of the positive ions arethen deflected according to their M/Q by the magnetic field in the gapof a small electromagnet. The gap dimension is 3.0 mm and the length andwidth of the pole pieces are 9.9 and 4.0 mm, respectively. The magnetcore is fabricated from a material similar to HY MU 80 and wound withabout 5000 turns of 332-gauge silver wire. Overlapped sheets ofPermalloy 80 with thickness 0.010 inch are used to encase the plasmainstrument to reduce the maximum stray field to 16 nanotesla (nT) at adistance of 1 m. The mass of the electromagnet is 150 g. Theelectromagnet is supplied with a programmed series of 64 currentsranging from 0.6 to 105 mA. The sequence of current values can becontrolled by ground command. The corresponding range of gap magneticfields is 0.0014 to 0.225 T. The ions are detected with two Spiraltrons,one Spiraltron (integral) with a 1-mm aperture for undeflected ions, andone Spiraltron (differential) with a 3-mm aperture that is offset fromthe path for undeflected ions. The magnet is non-focusing and the 3-mmaperture Spiraltron is used to achieve approximately equal geometricfactors for the differential and integral channels. The sensor aperturesare positioned at a distance 20.1 mm from the exit face of theelectromagnet. A slit with width 0.76 mm is placed in front of each ofthe two Spiraltrons. The centers of these slits are separated by 3.30mm. The Spiraltrons are operated in a similar manner as previouslydescribed for the sensors for the electrostatic analyzers.

At higher mass channels (larger current) H2+ and OH+ are deflectedsufficiently to be detected with the differential sensor. The M/Q valuefor the integral sensor is taken at a fraction 0.5 of the undeflectedresponses. For a given current step of the mass spectrometer, theaveraged FWHM for the three mass spectrometers in terms of ion energy isDELTA E/E = 0.06. In general the differential channel is used for thedetection of trace fluxes of light ions and the integral channel forabundant heavy ions in the Jovian magnetosphere. The mass resolutions ofthe mass spectrometers are M/DELTA M = 4.1 at full-width at 50%responses (FWHM) for the differential sensors (MD) and M/DELTA M ~ 2 forthe integral sensors (MI). This resolution has been chosen to allowidentification of the species H+, H2+ (He++), He+, O++, O+, Na+, S+, andK+ with the MD sensors and H+, H2+ (He++), O++, O+, S+, and SO2+ withthe MI sensors. The E/Q ranges vary with the M/Q of the ion species,e.g., for the MD sensors, 0.9 V to 20 keV for H+, 0.9 V to 3 kV for O+,and 0.9 V to 800 V for S+. For the MI sensors, these ranges are 10 V to52 kV for H+, 0.9 V to 52 kV for O+, and 0.9 V to 14 kV for S+. The massspectrometers cannot distinguish between two ions with the same M/Q,e.g., O+ and S++. The mass spectrometers are designed in part with thecriterion that corotating SO2+ (M/Q = 64 amu, E/Q ~ 2 kV) can beidentified at Io's orbit.


5.3. GEOMETRIC FACTORS


A summary of the latitudinal coverage, energy resolutions, and geometricfactors of each of the twenty sensors in the plasma instrument is givenin Table I. The averaged geometric factors for the electron and positiveion sensors of the electrostatic analyzers and the positive ion sensorsof the mass spectrometers are 3.4 x 10**-5, 6.4 x 10**-5, and3.2 x 10**-6 cm**2 sr eV eV**-1 , respectively.

These values are computed by comprehensive ray tracing of trajectoriesthrough the electrostatic and magnetic analyzers and with the nominalentrance area of the sensor. In practice both the efficiency and thisarea vary with individual sensors and final values of the geometricfactors are derived from laboratory measurements and inflight responsesin an isotropic plasma such as that in the plasma sheet during Earth1encounter. These geometric factors are tailored to provide effectivemeasurements of both the dense plasmas in the torus and the sparseplasmas of the outer Jovian magnetosphere.

The maximum responses of a single sensor to several representativeplasmas are shown as functions of the plasma temperature, bulk flowspeed V, and species. The bulk speed of 100 km s**-1 has been chosen asscale-wise representative for the corotational speeds in the torus. Thedensities of all the plasmas are each assumed to be 1 cm**-3. Forexample, if the density of S+ ions is 1000 cm**-3, V is 100 km s**-1, andthe temperature kT is 100 eV, the maximum responses of the ion sensorsof the electrostatic analyzer (P) and of the ion sensors of the massspectrometers (M) are 4 x 10**6 and 2 x 10**5 counts s**-1, respectively,when viewing in the bulk flow direction. The geometric factor of the ionsensor (P) is sized such that these responses are somewhat above thesaturation values for the sensor/amplifier, ~10**6 counts s**-1. The ionsensors in the mass spectrometers are employed to extend the dynamicrange of these ion measurements to the larger ion densities by means oftheir lesser geometric factors. On the other hand, the large geometricfactor of the ion sensors for the electrostatic analyzers provides thecapability of the determining densities of hot (~ tens of keV),isotropic ions as low as 10**-3 to 10**-2 cm**-3 in the outer regions ofthe magnetosphere. Thus the combined geometric factors of theelectrostatic analyzers and mass spectrometers accommodate a large rangeof ion densities. If the electron densities in the center of the plasmatorus are 3000 cm**-3, then the maximum responses for the electronsensors are ~2 x 10**5 and 6 x 10**5 counts s**-1 for electron temperatureskT = 1 and 10 eV, respectively. For an electron temperature of 10 keV inthe outer magnetosphere, densities as low as 10**-4 to 10**-3 cm**-3 can bewell determined.


TABLE I
Galileo PLS performance parameters
------------------------------------------------------------------------
Sensor Polar angle Energy resolution, Geomagnetic factor coverage, theta DELTA E/E at FWHM cm**2 sr eV eV**-1------------------------------------------------------------------------
Electrons Energy range: 0.9 V < E/Q < 52 kV
1E 14 deg-41 deg 0.14 1.9 x 10**-52E 38 deg-62 deg 0.12 3.7 x 10**-53E 58 deg-80 deg 0.10 4.1 x 10**-54E 81 deg-102 deg 0.08 5.0 x 10**-5SE 100 deg-122 deg 0.10 4.1 x 10** 56E 121 deg-146 deg 0.12 3.6 x 10**-5 142 deg-171 deg 0.14 1.3 x 10**-5
Positive ions Energy range: 0.9 V < E/Q < 52 kV
1P^a 9 deg-41 deg 0.15 9.8 x 10**-52P 35 deg-59 deg 0.12 3.5 x 10**-53P 62 deg-84 deg 0.09 4.1 x 10**-54P 78 deg-99 deg 0.07 5.0 x 10**-55P 97 deg-119 deg 0.09 4.0 x 10**-56P 118 deg-141 deg 0.11 3.6 x 10**-57P^a 136 deg-166 deg 0.15 1.5 x 10**-5
Ion composition Energy range: species dependent
Differential (D) sensor: 0.9 V to 20 kV (H+) 0.9 V to 800 V (S+) Resolves: H+, H2, He+, O+, Na+, S+, K+ with M/DELTA M = 4.1
Integral (I) sensor: 10 V to 52 kV (H+) 0.9 V to 14 kV (S+) Resolves: H+, H2+, He+, O+, S+, SO2+ with M/DELTA M ~2.0
1MD^a, 1MI 11 deg-38 deg 0.03 2.4 x 10**-62MD^a, 2MI 87 deg-93 deg 0.03 4.7 x 10**-63MD^a, 3MI 142 deg-169 deg 0.03 2.4 x 10**-6------------------------------------------------------------------------
^a 3-mm entrance diameter, others are 1 mm. ^b Preliminary values basedupon ray tracing (see text).

Considerable attention in the design of the instrument was directedtoward minimizing the sensor responses to the intense fluxes ofenergetic electrons in the inner Jovian magnetosphere. The Spiraltronsare shielded in all directions by a minimum of 2.5 g cm**-2 equivalent ofaluminum. This corresponds to an electron range of ~5 MeV. In additionthe Spiraltrons used for ion sensors are operated at a sufficiently lowvoltage that two or more initial secondary electrons at their entranceapertures are necessary to yield an electron pulse above thediscriminator level of the amplifiers. This mode of operation reducesthe sensor efficiency for the detection of ions by 50% (+/- 10%), with acorresponding decrease in the geometric factors cited in Table I. Suchoperation of the sensors at bias voltages ~2400 V allowsdiscrimination against detection of penetrating electrons. Theomnidirectional geometric factors for detection of penetrating, > 5 MeVelectrons are ~10**-4 cm**2 for the ion sensors with 1-mm apertures, and~10**-3 cm**2 for the 3-mm ion sensors (see Table I). The correspondinggeometric factors for the Spiraltrons used in the electron analyzers are~10**-3 cm**2. At the orbit of Io the electron intensities with E > 5 MeVare ~2 x 10**7 cm**-2 s**-1 (Van Allen, 1976). Thus the backgroundcounting rates are ~2 x 10**3, 2 x 10**4, and 2 x 10**4 counts s**-1 for the1-mm ion sensors, the 3-mm ion sensors, and the 1-mm electron sensors,respectively. For comparison, the sensor responses in the direction offlow (S+, 1000 cm**-3, 50 eV, 100 km s**-1) are ~5 x 10**6 counts s**-1 forthe ion channels of the electrostatic analyzer and ~3 x 10**5 countss**-1 for the sensors in the mass spectrometer. The analyzer responses toelectrons (e-, 1000 cm**-3, 50 eV) are expected to be ~6 x 10**5 countss**-1. The corresponding S/N ratios are 2500, 150 (I), and 15 (D), and 30for the ion sensors, mass spectrometer sensors, and electron sensors,respectively.

At larger radial distances, > 20 R,, the intensities of electrons withE > 5 MeV are typically < 10**3-10**4 cm**-2 s**-1 within and near theplasma sheet (Baker and Van Allen, 1976). The corresponding maximumbackground rates are then < 1 and 10 counts s**-1 for the 1-mm positiveion and electron sensors, respectively. For these maximum rates, thedensities for which S/N = 1 for an isotropic, H+ plasma are 3 x 10**-3cm**-3 at kT = 10 keV and 5 x 10**-3 cm**-3 for electrons at 1 keV. Thecorresponding densities for the mass spectrometer sensors are ~ 0.1cm**-3 (I) and 1 cm**-3 (D). These above examples for H+ give the mostpessimistic values because we have assumed worst-case background ratesand because the ion plasmas are partially corotating. The S/N ratioswill be typically larger by factors of ~ 10 to 100.

The spacecraft potential is expected to be important at the lower energyrange of the analyzer. A quantitative assessment of anticipatedspacecraft potentials is given by the Voyager plasma measurements. Inthe outer magnetosphere, typical Voyager spacecraft potentials werepositive in the range of several volts to 10 V (Scudder et al., 1981).
Because the plasmas are generally hot, temperatures ~ keV, in theouter magnetosphere the plasma measurements should not be greatlyimpaired. On the other hand, in the highest density regions of the Iotorus, Voyager spacecraft potentials were negative with magnitudes up to25 V (Sittler and Strobel, 1987). In this region electron temperaturesare tens of eV or less and the observations of thermal electron plasmasmay be precluded if the Galileo spacecraft potential is similar. Theenergy range of the Galileo plasma instrument is sufficient to determinethis spacecraft potential. Determination of the magnitude of thepotential will have to await the in-situ observations. The potentialsalong the boom on which the plasma instrument is mounted and those ofthe spacecraft body will also affect the trajectories of low-energyparticles as viewed by the plasma analyzer. This effect will have to bemodeled in detail in order to determine the deflections of the' observedangular distributions as a function of the particle energy.


5.4. INSTRUMENT ELECTRONICS


The plasma instrument is divided into two analyzer systems A and B. Thisconfiguration of the instrument has been chosen in order to reduce thenumber of possible single-point failures that could resultin the total loss of the scientific objectives. Each analyzer isequipped with a set of electrostatic analyzer plates, at least oneminiature mass spectrometer, and a partial set of the sensors for themeasurements of the three-dimensional velocity distributions of positiveions and electrons. A dedicated plate voltage supply, magnet currentsupply, and sensor bias voltage supply are provided for each analyzer.
Each sensor is serviced by a 16-bit accumulator. The electronics forboth analyzers are controlled from the instrument bus.

The reduction of single-point failures of the instrument proved to beconsiderably more difficult for the data handling and control subsystem(DHCS). There are two separate buses, A and B, that can singly operatethe two analyzers. Similarly there are two RCA 1802 microprocessors, 1and 2 that are each equipped with 4 kbytes of read-only memory (ROM) and4 kbytes of read/write memory (RAM). Two bus adapters, alpha and beta,couple the microprocessors with the command data system (CDS) of thespacecraft. The instrument is operated with one bus adapter, onemicroprocessor, and one bus. The bus separator/selector allows the useof any combination of these electronic elements, e.g., bus adapteralpha, processor 2, and instrument bus A. This configuration for theDHCS is set via a hardware bus command (HBC) that transfers thenecessary information in the address portion of the packet header fromthe spacecraft CDS. The HBC is executed regardless of which processorand bus adapter are currently selected. If the currently selected busadapter fails, the HBC can be used to select the other bus adapter.

Each of the two microprocessors is provided with identical I/Oelectronics that include an analog-to-digital (ADC) converter (modelAD571, Analog Devices, Inc.), three digital-to-analog (DAC) converters,and a digital status input port. A 16-input multiplexor is used with theADC to monitor voltages within the instrument. The DACs provide thecontrol voltages for the programmable high voltage (plate and bias) andcurrent (electromagnet) supplies.

Two low-voltage power supplies, A and B, are included within the plasmainstrument. By means of a power distribution system, failure of asingle low voltage supply does not result in the loss of the DHCS orinstrument bus. Analyzer A or B becomes inoperable with the failure ofthe one of the low-voltage power supplies, A or B. A power switchingcircuit that is controlled by ground command is used to select theanalyzer to be operated with the functioning low-voltage power supply.
The replacement and supplemental heaters are used for thermal controlduring the mission. The latch for releasing the protective cover overthe instrument aperture is a one-shot redundant device with twoelectrically fired, black powder Unidynamics bellows actuators.


5.5. MASS, SIZE, AND POWER


The overall dimensions of the plasma instrument are 8.00 x 15.00 inch(mounting surface) and 13.68 inch (height). The total mass is 13.2 kg,of which 0.33 kg is used for magnetic shielding and 3.57 kg is investedin radiation shielding of the sensors and electronics with tantalum. Theaverage power, without heaters, is dependent upon the electronicconfiguration of the instrument and is in the range of 6.5 to 10.7 W.


6. Inflight Operation of the Instrument

The operating modes of the plasma instrument are designed to accommodatethe diverse plasmas in the Jovian magnetosphere. We provide here a briefintroduction to those capabilities. The instrument cycle time is 243seconds and is subdivided into 12 equal intervals, or instrument spinmodes. Each spin mode is a separate instrument operations and datacollection cycle. The duration of a spin mode is 20.3 s and thusslightly longer than the range of anticipated rotation periods for thespacecraft spinning section, 18.3 to 19.8 s. By ground command theplasma instrument can be configured to sample a combination of a givenset of sensors, a range of energy passbands, a range of mass channels,and a set of angular sectors as the fields-of-view rotate. Theoperations of analyzers A and B can be programmed independently.
Limitations of these analyzers are imposed by the minimum dwell time forthe energy passbands and mass channels of 8.3 ms, a service time of 1 msfor the processing of the contents of a count accumulator, and thetelemetry rate allocated to the instrument of 612 bits s**-1 (72 sensorsamples s**-1 plus overhead). Each sample of sensor responses isquasi-logarithmically compressed into an 8-bit word. Internal bufferscan allow rapid bursts of < 1500 measurements to be trickled into thetelemetry stream.

Consider the measurement cycle time of the plasma instrument if onboardsoftware were not available to improve the operational efficiency. Ifall energy passbands, mass channels, and sensors were sampled in each of16 angular sectors, then the time for this complete plasma measurement(1.3 x 10**6 samples) would be 5.1 hours. Such instrument operation isineffective and wasteful of the capabilities for obtaining plasmaparameters, e.g., individual 64-point energy or mass spectra in 0.5 s.
Thus the spin modes are each designed to obtain a specific type ofplasma measurement during one spacecraft rotation, e.g., athree-dimensional velocity distribution, high angular and energyresolutions of an ion beam, and the mass composition of an ion beam. Aspin mode is constructed of nested control loops. These loops control(1) the number of angular sectors sampled during a spacecraft rotation,(2) the number of energy pass-bands or mass channels in a sector, (3)the duration of an energy passband or mass channel, (4) the readout ofthe selected sensors, (5) the sequence of energy passbands, and (6) thesequence of mass channels. Four sequencing tables are used to determinethe operation of the instrument during a spin mode: (1) sensor, (2) masschannel, (3) energy passband, and (4) angular sector. The angularsectors are referenced to a fixed position on the celestial sphere bymeans of information from the spacecraft attitude control system.
Instrument software is available for five basic types of spin modes.
Default values for the sequence tables are also included in read-onlymemory in the instrument processor in lieu of values from groundcommands. We briefly illustrate below the capabilities of the variousspin modes.

Spin mode 1. Survey of positive ion and electron velocity distributions. All electron, ion, and integral ion sensors (spectrometers) are sampled. The number of angular sectors, the energy range, and the number of energy passbands are selected by ground command. The product of the numbers of passbands and angular sectors is 64. For example, the responses of all of the above sensors for 64 passbands sampled in a single angular sector of 45 deg can be telemetered each spacecraft spin period. Alternately 16 passbands (every fourth passband) in each of our 90 deg-sectors can be telemetered during a single rotation period in order to obtain the principal features of the three-dimensional velocitydistributions of positive ions and electrons once each 20.3 s.

Spin mode 2. Determination of the velocity distribution of a positive ion beam. Electron and ion sensors corresponding to those nearest the direction of the ion beam are selected. These sensors and the spacecraft rotation angle for the beam are determined with measurements from a preceding spin mode 1. The rapid energy scans in the direction of the beam are limited to the energy range of the beam as determined during spin mode 1. For example, during one spacecraft rotation, energy passbands 8 through 23 can be sampled with three sensors for positive ions and two or three sensors for electrons for each of five contiguous 22.5 deg. sectors in the direction of the beam. Two electron sensors are used for analyzers and B, with the exception of three for analyzer B if the beam is nearly perpendicular to the spacecraft spin axis. Again angular size of the sectors and the number of energy passbands can beselected by ground command.

Spin mode 3. Survey of ion composition. Mass spectrometers 1 and 2 are sampled for a selected range of gap magnetic fields. During one spacecraft rotation a single energy passband of the electrostatic analyzer is used and the gap magnetic fields are incremented over a selected series of values. Thus for a given energy passband and a single spacecraft rotation it is possible to sample the entire M/Qrange in 64 current steps in each of four 90 deg-sectors.

Spin mode 4. Survey of ion composition. This spin mode is identical to spin mode 3 with the exception that mass spectrometer 3 replaces1.

Spin mode 5. Determination of the composition of an ion beam. The mass spectrometer with direction of field-of-view nearest to that of the ion beam is chosen on the basis of previous measurements with spin mode 1. The energy passband and angular sectors for the ion beam are similarly identified. For example, during one spacecraft rotation in the plasma sheet or torus of the Jovian magnetosphere full coverage of the M/Q range in 64 channels can besampled in each of five contiguous 22.5 sectors.

The instrument cycles for analyzers A and B are each selected as asequence of 12 spin modes. The order of the spin modes and theiroperating parameters such as energy and mass ranges, angular resolution,etc., are controlled by the sequence tables. As an example, a sequenceof spin modes during an instrument cycle for analyzer A can be 1, 1, 2,1, 1, 5, 1, 1, 4, 1, 1, 3. Thus the various operating modes of theplasma instrument can be implemented and cycled automatically withminimal demand for command uplinks to the Galileo spacecraft. Majorcommand sequences are used to restructure the spin modes and theirsequencing for special events such as the close encounters with theGalilean satellites and the exploratory survey into the distantmagnetotail.


Acknowledgements

At The University of Iowa L. A. Frank is principal investigator and K.
L. Ackerson is a co-investigator for the plasma investigation. The otherco-investigators are F. V. Coroniti of the University of California atLos Angeles and V. M. Vasyliunas of the Max-Planck-Institut furAeronomie, Lindau, Germany. E. C. Stone of the California Institute ofTechnology is a co-investigator whose responsibility is the Heavy IonComposition (HIC) investigation which employs a separate instrument onthe Galileo Orbiter. The authors wish to express their appreciation tothe following personnel of the Jet Propulsion Laboratory for theirassistance in the implementation of the plasma instrumentation: J. R.
Casani, W. G. Fawcett, H. W. Eyerly, C. M. Yeates, M. S. Spehalski, W.
J. O'Neil, R. F. Ebbett, and T. V. Johnson. This research was supportedin part at The University of Iowa by the Jet Propulsion Laboratory undercontract 958778.


References
Bagenal, F.: 1985, 'Plasma Conditions Inside lo's Orbit: VoyagerMeasurements', J. Geophys. Res. 90, 311.

Bagenal, F., McNutt, R. L., Jr., Belcher, J. W., Bridge, H. S., and Sullivan, J. D.: 1985, 'Revised Ion Temperatures for Voyager PlasmaMeasurements in the Io Plasma Torus', J. Geophys. Res. 90, 1755.

Baker, D. N. and Van Allen, J. A.: 1976, 'Energetic Electrons in theJovian Magnetosphere', J. Geophys Res. 81, 617.

Belcher, J. W.: 1983, in A. J. Dessler (ed.), 'The Low-Energy Plasma in the Jovian Magnetosphere', Physics of the Jovian Magnetosphere,Cambridge University Press, Cambridge, p. 68.

Frank, L.A., Ackerson, K.L., Wolfe, J.H., and Mihalov, J.D.:1976,'0bservations of Plasmas in the Jovian Magnetosphere', J.
Geophys. Res. 81, 457.

Gurnett, D. A. and Scarf, F. L.: 1983, in A. J. Dessler (ed.), 'Plasma Waves in the Jovian Magnetosphere', Physics of the JovianMagnetosphere, Cambridge University Press, Cambridge, p. 285.

Krimigis, S. M. and Roelof, E. C.: 1983, in A. J. Dessler (ed.), 'Low-Energy Particle Population', Physics of the JovianMagnetosphere, Cambridge University Press, Cambridge., p. 106.

Sands, M. R. and McNutt, R. L., Jr.: 1988, 'Plasma Bulk Flow inJupiter's Dayside Middle Magnetosphere', J. Geophys. Res. 93, 8502.

Scudder, J. D., Sittler, E. C., Jr., and Bridge, H. S.: 1981, 'A Survey of the Plasma Electron Environment of Jupiter: A View fromVoyager', J. Geophys. Res. 86, 8517.

Sittler, E. C. and Strobel, D. F.: 1987, 'Io Plasma Torus Electrons:Voyager 1', J. Geophys. Res. 92, 5741.

Van Allen, J. A.: 1976, in T. Gehrels (ed.), 'High-Energy Particles in the Jovian Magnetosphere', Jupiter, University of Arizona Press,Tucson, p. 928.

Vasyliunas, V. M.: 1983, in A. J. Dessler (ed.), 'Plasma Distribution and Flow', Physics of the Jovian Magnetosphere, CambridgeUniversity Press, Cambridge, p. 395.