Flyby Of Io Research Articles

Electron densities near Io from Galileo plasma wave observations

This paper presents an overview of electron densities obtained near Io from the Galileo plasma wave instrument during the first four flybys of Io. These flybys were 10, which was a downstream wake pass that occurred on December 7, 1995; I24, which was an upstream pass that occurred on October 11, 1999; I25, which was a south polar pass that occurred on November 26, 1999; and I27, which was an upstream pass that occurred on February 22, 2000. Two methods were used to measure the electron density. The first was based on the frequency of upper hybrid resonance emissions, and the second was based on the low‐frequency cutoff of electromagnetic radiation at the electron plasma frequency. For three of the flybys, 10, I25, and I27, large density enhancements were observed near the closest approach to Io. The peak electron densities ranged from 2.1 to 6.8 × 104 cm−3. These densities are consistent with previous radio occultation measurements of Io's ionosphere. No density enhancement was observed during the I24 flyby, most likely because the spacecraft trajectory passed too far upstream to penetrate Io's ionosphere. During two of the flybys, I25 and I27, abrupt step‐like changes were observed at the outer boundaries of the region of enhanced electron density. Comparisons with magnetic field models and energetic particle measurements show that the abrupt density steps occur as the spacecraft penetrated the boundary of the Io flux tube, with the region of high plasma density on the inside of the flux tube. Most likely the enhanced electron density within the Io flux tube is associated with magnetic field lines that are frozen to Io by the high conductivity of Io's atmosphere, thereby enhancing the escape of plasma along the magnetic field lines that pass through Io's ionosphere.

Foreword [to Special Section on Plasma Interaction With Io's Atmosphere

The Jovian moon Io resides in one of the harshest space environments yet visited by spacecraft. Indeed, the energetic particles that make Jupiter's inner magnetosphere interesting to the space physicist give space engineers nightmares. After the initial orbit‐insertion passage in December 1995, the Galileo spacecraft was deliberately kept well outside the orbit of Io and the plasma torus. After 20 plus successful orbits exploring the outer 3 Galilean moons, the spacecraft's perijove was reduced to allow 3 close flybys of Io in late 1999 to early 2000.

Return to Io by the Galileo spacecraft: Plasma observations

On October 11, 1999, a series of high‐resolution plasma measurements were obtained during the second close flyby of Io of the Galileo mission. The closest approach to Io 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−3 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−3 and kT= 40 eV and the other with 200 cm−3 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−3 each for S+ and SO+. The plasma bulk flows were strongly deflected around the body of Io, 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 lo's orbit were about 800 cm−3. These densities at the immediate position of Io 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 Io. The first was a region of hot ions located inside of lo'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 Io, exhibited a hot torus plasma which was mixed with pickup hydrogen ions. These hydrogen ions were detected at distances beginning at 7.8 RIo from this moon, which were generally inside the Hill (Lagrange) “sphere” for Io's atmosphere.

Observations of plasmas in the Io torus with the Galileo spacecraft

On December 7, 1995, the Galileo spacecraft passed through the Io torus near the magnetic equatorial plane of Jupiter. High– resolution measurements of positive ions and electrons in the energy/charge (E/Q) range of 0.9 V to 52 kV were acquired over the jovicentric radial distances of 5.6 to 7.8 Rj. At radial distances beyond Io's orbit at 5.9 Rj the plasma instrument (PLS) observed intense magnetically field‐aligned beams of electrons with average energies of hundreds of eV to several keV. The directional energy fluxes at the spacecraft position ranged from ∼1.5 to 90 erg cm−2S−1sr−1 in directions parallel and antiparallel to the magnetic field. The corresponding range of energy fluxes into Jupiter's ionosphere was ∼9 to 560 erg Cm−2s−1, sufficient for the production of auroral emissions at far‐ultraviolet wavelengths with brightnesses in the range of 0.06 to 3 mega‐Rayleighs. The largest energy fluxes were observed at ∼7.8 Rj and were comparable to those observed during the close flyby of Io. The torus plasmas were observed to rigidly corotate with Jupiter out to a radial distance of ∼7 Rj and are slower at greater radial distances. Fluctuations of the bulk ion speeds and densities were found to be superposed upon the general pattern of corotation and are suggestive of interchange of magnetic flux tubes in the plasma torus. For an isolated field‐aligned electron beam, the radial component of plasma flow was directed toward Jupiter at a speed of ∼4 km s−1. The other electron beams also occurred coincidentally with the fluctuations in the radial component of flow. The observations of these low‐energy electron beams are consistent with the absence of magnetically field‐aligned acceleration of electrons into Jupiter's ionosphere. The electron acceleration appears to be due to intense heating of the ionospheric plasmas associated with large currents driven by the interchange motions in the torus plasmas.

Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program ( Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/ B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm −3), namely ≥20 cm −3. Assuming a stationary pickup process and a r − n distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10 26 protons/s are created around Io. The ultimate origin of these protons is an open issue.

Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi‐adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E−n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low‐speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field‐aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the “trapped” electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest “trapped” electron population to be detected above the background level for energies up to 200 keV.

Active Volcanism on Io as Seen by Galileo SSI

Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse.About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface.The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50–150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface SO2.A major and surprising result is that ∼30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmopshere is dominated by volcanic outgassing.Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30° of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999.

Io's plasma environment during the Galileo flyby: Global three‐dimensional MHD modeling with adaptive mesh refinement

The first results for applying a three‐dimensional multiscale ideal MHD model for the mass‐loaded flow of Jupiter's corotating magnetospheric plasma past Io are presented. The model is able to consider simultaneously physically realistic conditions for ion mass loading, ion‐neutral drag, and intrinsic magnetic field in a full global calculation without imposing artificial dissipation. Io is modeled with an extended neutral atmosphere which loads the corotating plasma torus flow with mass, momentum, and energy. The governing equations are solved using adaptive mesh refinement on an unstructured Cartesian grid using an upwind scheme for MHD. For the work described in this paper we explored a range of models without an intrinsic magnetic field for Io. We compare our results with particle and field measurements made during the December 7, 1995, flyby of Io, as published by the Galileo Orbiter experiment teams. For two extreme cases of lower boundary conditions at Io, our model can quantitatively explain the variation of density along the spacecraft trajectory and can reproduce the general appearance of the variations of magnetic field and ion pressure and temperature. The net fresh ion mass‐loading rates are in the range of ∼300–650 kg s−1, and equivalent charge exchange mass‐loading rates are in the range ∼540–1150 kg s−1 in the vicinity of Io.

A history of high‐temperature Io volcanism: February 1995 to May 1997

Ground‐based observations of Io's infrared thermal emission between February 1995 and May 1997 show several discrete brightenings for which we can constrain locations, fluxes, and durations. Several of these were brief high‐temperature events, with temperatures up to at least 1500 K, similar to but often smaller than the rare “outbursts” seen previously. Loki, Io's most powerful volcano, was relatively active before and probably during Galileo's December 1995 Io flyby, was faint during most of 1996, and began a major, long‐lived brightening between February 20 and March 12 1997. Thermal emission was not seen from Ra Patera, site of an active plume in Galileo images. Major outbursts were seen on March 2nd and September 27 1995.

The global plasma environment of Io as inferred from the Galileo plasma wave observations

We take advantage of a partial and a total occultation of the jovian radio sources observed during the flyby of Io to characterize the plasma environment of this moon. We show that it presents a strong upstream / downstream asymmetry. A dense plasma structure (n > 5 × 104 cm−3) extends far from Io (at least 1000 km) in the wake direction whereas no extended dense region is found upstream. The dense region in the wake could correspond to a the stagnation region of the flow of the corotating plasma.

Galileo at Jupiter: Changing states of the magnetosphere and first looks at Io and Ganymede

Several investigations based on data from the magnetometer on the Galileo Orbiter are described. From Galileo's initial inbound pass, we learn that the magnetosphere can experience large changes in its magnetic configuration. Between 50 and 30 R j, the radial magnetic forces on the plasma were larger in 1995 than in 1973 when Pioneer 10 passed through the same region of the magnetosphere, implying that either external or internal plasma forces were also larger. Although there are several ways to interpret the change of the magnetic configuration, we suggest that the variations are governed principally by the solar wind dynamic pressure and that the dayside magnetosphere as far in as ∼30 R j may be more strongly affected by the solar wind than has previously been recognized. Minor effects of higher mass loading may also be present. Data from the flyby of Io show a large magnetic perturbation; we argue that it is plausible that Io has an intrinsic magnetic field and that also contributions from plasma perturbations are significant. We find unexpected evidence that molecular ions are being picked up over a large spatial region in the vicinity of the moon. During the pass by Ganymede we observed a large magnetic perturbation consistent with an intrinsic dipole field. The multiple flybys of Ganymede scheduled for later portions of Galileo's mission will allow us to test our understanding of the magnetic signature.

Plasma observations at Io with the Galileo spacecraft.

Plasma measurements made during the flyby of Io on 7 December 1995 with the Galileo spacecraft plasma analyzers reveal that the spacecraft unexpectedly passed directly through the ionosphere of Io. The ionosphere is identified by a dense plasma that is at rest with respect to Io. This plasma is cool relative to those encountered outside the ionosphere. The composition of the ionospheric plasmas includes O++, O+ and S++, S+, and SO2+ ions. The plasma conditions at Io appear to account for the decrease in the magnetic field, without the need to assume that Io has a magnetized interior.

Electron beams and ion composition measured at Io and in its torus.

Intense, magnetic field-aligned, bidirectional, energetic (>15 kiloelectron volts) electron beams were discovered by the Galileo energetic particles detector during the flyby of Io. These beams can carry sufficient energy flux into Jupiter's atmosphere to produce a visible aurora at the footprint of the magnetic flux tube connecting Io to Jupiter. Composition measurements through the torus showed that the spatial distributions of protons, oxygen, and sulfur are different, with sulfur being the dominant energetic (> approximately 10 kiloelectron volts per nucleon) ion at closest approach.

Galileo on Jupiter approach

After six years of interplanetary flight, in just two months, on December 7, 1995, Galileo will arrive at Jupiter. In July, Galileo released its Atmospheric Entry Probe on the five-month solo, ballistic flight to Jupiter entry. After Probe release, the Orbiter fired its 400-N main engine for the first time to deflect the Orbiter to its Io flyby aim point, establishing the trajectory for Probe radio relay link acquisition and orbit insertion. The 400-N engine was specifically used for the deflection in order to check it out prior to orbit insertion. Last February, the Galileo Orbiter Command and Data Subsystem (CDS) and Attitude and Articulation Control Subsystem (AACS) computers were completely re-loaded to provide new software capabilities for main engine and relay link autonomous fault protection and for backup Probe data storage on the Orbiter. The execution and results of this complete inflight software change are discussed. The status of the Entry Probe prior to release is described—once released there is no communication link with the Probe until after entry. The operation and performance of the 400-N main engine at the deflection burn is explained in the context of its required use for orbit insertion. Final predictions for the Probe entry conditions, Probe-to-Orbiter radio relay link, and Orbiter insertion performance are given. Overall mission status and spacecraft health are summarized.

Io may have a bright dawn terminator

NELSON AND HAPKE1 have shown that occasional observations of post eclipse brightenings (PEB) of the planet Jupiter's satellite Io are very strongly positively correlated with the occurrence of major solar flares within 10° of the solar central meridian (as seen from Jupiter) during the 100 days before the eclipse observation. No eclipses which showed no brightening were preceded by such flares. Io is in the outer part of the jovian radiation belts. Energetic particle fluxes in the Earth's radiation belts at a roughly corresponding location are enhanced for roughly similar times (the 100 days in ref. 1) following the major geomagnetic storms caused by major flares near central meridian2,3. The 100-day time scale is also in reasonable agreement with diffusion times in recent models of the jovian magnetosphere4. Considering all this, Nelson and Hapke inferred from their flare correlation that the occurrence of a noticeable PEB effect is due to enhanced fluxes of trapped energetic particles. They suggested that there may be a process similar to thermoluminescence, which converts some fraction of the particle dose received during the eclipse, into visible light which is emitted when the surface is again sunlit, and decays with the characteristic PEB time scale of ∼10 min. Although Fanale et al.5,6 had already suggested that solid state effects in the presence of irradiation by energetic particles might play a part in the PEB phenomenon, the particular correlation with solar flares that Nelson and Hapke found points at once to an energetic particle effect because that is what would be expected to be enhanced by such flares, and at the same time apparently explains the erratic occurrence of PEB on Io and its absence on more distant satellites (because they see much lower fluxes of trapped particles, enhanced or not). In any case, Nelson and Hapke have made a novel yet plausible suggestion which can be tested by laboratory simulation experiments of the solid state process, by more eclipse observations from Earth, and by observations of jovian trapped particles by future flyby and orbiter spacecrafts, and their relation to solar flares. This note suggests another particular test of the thermoluminescence properties of Io which can be made with the unique optical observations which are programmed for the close Voyager I Io flyby in March 1979, and which does not necessarily require a previous suitable flare, as the PEB phenomenon seems to do.