1. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567
2. AAAS Dept., University of Colorado, Boulder, CO 80309-0391
3. APL/JHU, 1110 Johns Hopkins Rd., Laurel, MD 20723-6099
4. MPAe, Postfach 20, Katlenburg-Lindau, D-3791, Germany
5. C.E.T.P., 0-12 Avenue de I'Europe, 78140 Velizy, France
6. Atmospheric and Environmental Research Inc., 840 Memorial Drive, Cambridge, MA 02139
7. Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109
When the Galileo orbiter passed within I000 km of lo on December 7, 1995, the data collected were every bit as exciting as the Galileo team had expected, given the unique interaction between lo and the Jovian magnetosphere. The spacecraft detected intense beams of electrons propagating parallel to the magnetic field and a variety of unexpectedly strong wave phenomena surrounding lo that are associated with lo's production of new ions. There was also evidence for an intrinsic magnetic field, possibly generated by a dynamo within Io, and a cool 'ionospheric' plasma directly behind Io, removing tens of kilograms of material per second, a fraction of the nearly a ton per second, added to the torus as a whole.
lo had already called attention to itself by controlling decametric wavelength radio emissions. Previous attempts to explain this control assumed that the movement of lo, an object with high electrical conductance, through the Jovian magnetic field leads to falling electrical potential, particle acceleration, and a field-aligned current system coupling lo to the Jovian ionosphere.
The force generated by the electrodynamic interaction is thought to accelerate lo in its orbital motion and slow the rotation of the Jovian ionosphere. The magnetic flux tube that threads through lo thus plays a critical role in the coupling of lo and Jupiter. Since lo's orbital motion is not synchronous with Jupiter's spin, the lo flux tube slips with respect to Jupiter and/or lo. Such slippage may result in an electric field parallel to the Jovian magnetic field that accelerates charged particles along the field, as occurs in the auroral regions on Earth. If lo were a poor electrical conductor with no ionosphere and a nonconducting crust, a terrestrial Moonlike interaction would occur: the magnetic field would hardly be distorted because the plasma would be absorbed as it struck the moon. An unresolved question is whether electrical currents flow across lo and if so, how.
Flying within II lo radii south of lo, Voyager I detected a magnetic field distortion of about 3 x 106 A, which is consistent with the current for a perfectly conducting obstacle. The previously mentioned model became accepted, but Voyager provided no information on whether the current passed through the body of lo, its ionosphere, or the plasma surrounding lo.
Jupiter's rapid rotation and strong magnetic field carry the magnetospheric plasma past Io faster than lo orbits Jupiter, creating a wake in front of lo as it moves. Galileo made a close flyby through this wake.
|Figure 1. The near-Io trajectory of the Galileo spacecraft. Galileo moves from bottom to top right along the solid line. Two times, 1740 and 1750 UT, are indicated. Along the trajectory, ion cyclotron waves, mirror mode waves, quiet magnetic field, electron beams, and dense plasma (presumably from lo itself) are indicated. The dashed lines show the expected flow of the Io torus plasma around Io. These flow lines are parallel to the observed plasma flow where they cross the Galileo trajectory but are speculative otherwise.|
Figure 1 shows a synthesis of the Galileo measurements along the lo flyby trajectory (Science, October 18, 1996). The view depicts the trajectory of the Galileo spacecraft into the plane containing the flow of the lo torus plasma and the radial direction from Jupiter to lo. The dashed lines show hypothetical flow lines that are parallel to observed flow of plasma along the trajectory. The fact that these streamlines principally flow around lo and not through it indicates that apparently very little of the electrical potential drop in the flowing plasma is applied across lo.
Directly behind lo, in the wake region formed by the lo torus plasma that overtakes lo, but somewhat asymmetric with respect to the wake, was a cold, near-stagnant "ionosphere" with a peak density of about 4 x 104 cm-3 [Gurnett et al., 1996; Frank et al., 1996].
Io is surrounded by a corona, neutral gas cloud, and a plasma torus produced by the interaction of Jovian magnetospheric particles with lo and its atmosphere. If a neutral particle from lo's corona and extended neutral gas cloud becomes ionized in this flowing, magnetized plasma, it will immediately begin to drift with the background magnetized plasma, and gyrate around the magnetic field.
If many particles are picked up in the flow, the flow initially slows down according to momentum conservation, and the drift velocity and gyration velocity of newly created ions will be reduced. Thus on streamlines far from lo, we expect newly created ions with the greatest temperatures (270 eV and 540 eV for oxygen and sulfur ions) and the lowest concentration of picked-up particles. On Streamlines closest to lo, the picked up ions should be densest and coldest.
We expect that a small fraction of the incident magnetic flux tubes are not deflected to the sides around lo but slowly flow across the region of field lines that are connected to lo. These flux tubes should become heavily mass loaded, and when they exit the lo flux tube proper, they and the flux tubes that pass closest to lo should form a dense cold wake behind lo.
Outside this cold wake on streamlines not intersecting with lo, the ion temperature was 360 plus or minus 90 eV as expected for newly created ions accelerated by the corotating Jovian plasma. The average ion temperature in the wake is consistent with "ion pickup" at a velocity at least 6 times slower than the corotational velocity. Throughout much of the lo torus passage leading up to the encounter, Galileo measured ion densities that were about 50% greater than those observed by Voyager at the same distance [Bridge et al. 1979; Bagenal, 1994]. This increase suggests that lo was providing a greater rate of mass loading to the Jovian magnetosphere during the Galileo encounter than during the Voyager encounter. However, in the flux tube that crosses Io this rate approaches tens of kilograms per second - not a ton per second as estimated in the larger volume around Io.
Since the ions are picked up perpendicular to the magnetic field, their initial angular distribution is ring-shaped about the magnetic field. In general, this is a very unstable configuration, and ion cyclotron waves are expected to grow and scatter the particles in pitch angle so that they are more isotropically distributed about the magnetic field. Indeed, strong ion cyclotron waves were found by Galileo in the region around lo [Kivelson et al., 1996a] extending outward to 16 RIo and inside of lo's orbit to 6.5 RIo. In a multicomponent plasma such as that near lo, we do not expect growth associated with the gyrofrequencies of all the picked-up ions, but rather only the heavier ones. In the wake region, the character of the ion waves changed dramatically. As indicated by the square waves in Figure 1, mirror mode pulses were observed at the edges of the wake, while the center of the wake was quiet.
The presence of intense magnetic field-aligned bidirectional energetic (>15 keV) electron beams in the wake (denoted by asterisks in Figure 1) was unexpected [Williams et al., 1996]. The cause of these beams is still a mystery.
Finally, at near relativistic ion energies, lo acts as a sink for particles rather than a source (Garrard et al., 1996]. In absorbing these highly energetic particles, atoms and ions are sputtered from the surface and atmosphere, enhancing further the mass loading.
A controversial issue is whether lo has an intrinsic magnetic field. Certainly, lo is a good candidate for such a field. Its volcanism is symptomatic of the strong dissipation of tidal energy within that moon. Thus it is probable that within the period since its formation, lo has not cooled as much as similar-sized bodies, including the Earth's moon. Moreover, gravitational data indicate that lo has a dense core, possibly of iron, making it possible that lo has an internal dynamo. Galileo passed near lo's equatorial plane, downstream and within the corotational wake. The magnetic signature in this region was a reduction of the magnetic field strength, as would be produced by a dynamo in lo acting to amplify the Jovian magnetic field [Kivelson et al., 1996b]. Such a field theoretically might also be induced in any highly permeable material in the mantle, since the field strength reported to be associated with the possible lo magnetic moment is significantly smaller than the maximum that could be produced. However, the thickness of the layer of Ionian crust that is below the Curie point of iron, 1044o K, is small if heat conduction by mantle rock is assumed [Weinbruch and Spohn, 1995]. If heat transfer is dominated by fluid convection, then a thicker layer can remain below the Curie point [Cheng and Paranicas, 1996]. The moment could also be due to natural thermoremanent magnetism, induced by a larger magnetic field than in the present, when lo was closer to Jupiter, or when the Jovian field was larger. However, for ancient fields to be responsible, the timescale for the constancy of the direction of the magnetizing field must exceed the cooling time of the magnetized layer. The amount of material on lo that could contain natural remanent magnetization is smaller than that in the highly permeable regime because of the lower temperatures at which such magnetism is acquired in typical minerals.
Whether or not lo has an intrinsic magnetic field appears to be insignificant to the interaction of lo and the Jovian magnetosphere, because the interaction is clearly dominated by mass and momentum loading. Ions added to the existing torus flow around lo, and a cold wake region forms downstream, Waves arise to scatter these particles, and the magnetic field is twisted away from its purely Jovian orientation.
The plasma phenomena seen from Galileo were, in general, not unexpected, but their strength exceeded expectations. The plasma was denser than expected in the torus and in the wake region. The wave amplitudes were greater than expected. Field-aligned electron beams were expected to be unidirectional - not bidirectional. Since neither Galileo nor Voyager I entered the lo flux tube itself, it is possible that electron beams are even more intense than those observed.
We hope that the Galileo extended mission now being planned might provide data in this most interesting region. Finally, we note that the observed intense mass and momentum loading of the plasma may itself be sufficient to explain the field-aligned currents observed by Voyager. This is an important difference from the Voyager paradigm [Neubauer, 1980] because a mass/momentum-loaded "obstacle" can be arbitrarily large in contrast to the conducting sphere obstacle and thereby generate an arbitrarily large current. Higher latitude measurements during an extended mission phase could help to distinguish between these two possible sources.
The success of the Galileo mission is due to the hard work of individuals far too numerous to name, but this in no way diminishes our gratitude to them. They were essential to the work reported here.
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Bridge, H. S., et al., Plasma observations near Jupiter: Initial results from Voyager I encounter with Jupiter, Science, 204, 972, 1979.
Cheng, A. F., and C. Paranicas, Implications of lo's magnetic signature: Ferromagnetism? Geophys. Res. Lett., 23, 2879, 1996.
Frank, L. A., W. R. Paterson, K. L. Ackerson, V. M. Vasyliunas, F. V. Coroniti, and S. J. Belton, Plasma observations at Io with the Galileo spacecraft: Passage through the ionosphere, Science, 274, 394, 1996,
Garrard, T. L., F. C. Stone, and N. Murphy, Effects of absorption by lo on composition of energetic heavy ions, Science, 274, 393, 1996.
Gurnett, D. A., W. S. Kurth, A. Roux, S. J. Bolton, and C. F. Kennel, Galileo plasma wave observations in the Io plasma torus and near lo, Science, 274, 391, 1996.
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Williams, D. J., B. H. Mark, R. E. McEntire, E. C. Roelof, T. P. Armstrong, B. Wilken, J. G. Roederer, S. M. Krimigis, T. A. Fritz, and L. J. Lanzerotti, Electron beams and ion composition measured at lo and in its torus, Science, 274, 401, 1996.