Several Anticipated Scientific Results

The capabilities of the Galileo plasma instrumentation are demonstrated here by application to several plasma regimes in the Jovian magnetosphere.

As the Galileo spacecraft crosses the plasma sheet in the middle and outer magnetospheres the magnitudes of field-aligned and cross-field currents are determined. Their values and location are correlated with the position of the current sheet as found with the magnetometer. The motions of the plasma sheet are directly determined from the three-dimensional bulk flow vector and the azimuthal component is separated from the radial outflow or inflow. Angular distributions and ion compositions are examined in order to discern the contributions of electrons and ions from the ionosphere, the solar wind via the magnetosheath, and Io in the inner magnetosphere. Thus the formation and dynamics of the plasma sheet can be understood. The mechanism for the unusual heating of plasma with increasing radial distance is expected to be identified.

The encounters with the Galilean satellites offer exciting opportunities for observing plasma phenomena. Examination of the ion velocity distributions in the wakes of these satellites is used to determine the mechanism for ion loss from these bodies. The effectiveness of ion pickup by the magnetospheric plasma flow is derived from the signatures in the velocity distributions of these ions. The mass spectrometers are used to identify the major ions produced in the vicinity of the satellite. For Io these ions include O+, S+, and SO2+, and for icy satellites perhaps H+, C+, and H2O+ can be found. Such measurements give the rate of mass loss from each satellite. Perturbations of the plasma flow can be identified in terms of the conductivity of the satellite. During the closest satellite encounters it is possible that a magnetopause or ionopause is detected, thus providing further information concerning the magnetic and atmospheric properties of that body. If the flyby of the satellite is polar, detection of strong field-aligned currents to and away from the Jovian ionosphere might be expected. Field-aligned acceleration of ions and electrons by electrostatic double layers or anomalous resistivity is possible. The relative contributions of the various Galilean satellites for providing the ions in the plasma torus and sheet are assessed during the encounters.

The substantial periods of time that the Galileo spacecraft is located in the plasma sheet offer the unique opportunity to view the responses of the Jovian magnetosphere to the volcanic activity on Io. If specific Io volcanic eruptions can be identified with temporal fluctuations in densities, composition, and motions of the plasma sheet, remarkable advances in our knowledge of the transport of mass and momentum in the Jovian magnetosphere are envisioned.

Simultaneous observations of three-dimensional plasma velocity distributions and of plasma waves with the Galileo spacecraft allow the first studies of wave-particle interactions in the wide-ranging types of plasmas in the Jovian magnetosphere. A discussion of measurements of plasma waves during the Voyager encounters has been given by Gurnett and Scarf (1983). For example, the velocity distributions of ions can be examined to determine whether or not resonant acceleration by ion cyclotron waves is an important mechanism for ion heating in the torus and plasma sheet. Further the amplitudes of broadband electrostatic noise can be compared with plasma velocity distributions to determine the importance of the anomalous resistivity in plasma heating. Free energy sources, e.g., ring distributions in the electron velocity distributions, for the generation of electron cyclotron or upper hybrid waves may be identified and related to the wave amplitudes observed with the plasma wave instrument. In general the direct measurement of the plasma density and other parameters gives the growth, propagation and resonance conditions for plasma waves in wave-particle interactions. Thus the mechanisms for providing Jupiter with intense radio sources and particle precipitation into the auroral ionosphere can be further understood.

The existence of the magnetospheric wind at radial distances > 130 RJ in the dawn sector of the magnetosphere offers exciting goals for the orbit into the distant magnetotail. The origins of this wind are unknown. It is possible that the magnetospheric wind develops near the Alfven point, where the corotational speed is equal to the Alfven speed. The actual position must be determined from considerations of the tangential stress balance (cf. Vasyliunas, 1983). Thus magnetic bubbles could be slung radially outwards into the magnetotail. The low pressures in the magnetotail would produce super-Alfvenic radial outflow. On the other hand, the outflow wind might be thermally powered by the hot plasmas in the plasma sheet inside the Alfven point. A third possibility is that the magnetospheric wind is the signature of reconnection of magnetotail field lines in a convection pattern controlled by dayside magnetic merging rates. The responses of the magnetotail to fluctuating internal plasmas, e.g., Iogenic plasmas, or to a varying solar wind are unknown. Is the magnetotail characterized by spectacular, explosive activity or a mere quiescent outflow of plasmas? The exploratory orbit into the magnetotail will indeed answer many questions concerning the origins and dynamics of this immense and little understood plasma region.