Abstract. Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moon's surface or atmosphere. The plasma effects vary with Europa's height above the center of Jupiter's extended plasma disk. Europa is comet-like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiter's ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europa's radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time-varying part of Jupiter's magnetospheric field at Europa's orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiter's magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of-10 km or more in thickness would explain the observations.

Abstract. Ganymede presents a unique example of an internally magnetized moon whose intrinsic magnetic field excludes the plasma present at its orbit, thereby forming a magnetospheric cavity. We describe some of the properties of this mini-magnetosphere, embedded in a sub-Alfv6nic flow and formed within a planetary magnetosphere. A vacuum superposition model (obtained by adding the internal field of Ganymede to the field imposed by Jupiter) organizes the data acquired by the Galileo magnetometer on four close passes in a useful, intuitive fashion. The last field line that links to Ganymede at both ends extends to -2 Ganymede radii, and the transverse scale of the magnetosphere is -5.5 Ganymede radii. Departures from this simple model arise from currents flowing in the Alfv6n wings and elsewhere on the magnetopause. The four passes give different cuts through the magnetosphere from which we develop a geometric model for the magnetopause surface as a function of the System III location of Ganymede. On one of the passes, Ganymede was located near the center of Jupiter's plasma disk. For this pass we identify probable Kelvin-Helmholtz surface waves on the magnetopause. After entering the relatively low-latitude upstream magnetosphere, Galileo apparently penetrated the region of closed field lines (ones that link to Ganymede at both ends), where we identify predominantly transverse fluctuations at frequencies reasonable for field line resonances. We argue that magnetic field measurements, when combined with flow measurements, show that reconnection is extremely efficient. Downstream reconnection, consequently, may account for heated plasma observed in a distant crossing of Ganymede's wake. We note some of the ways in which Ganymede's unusual magnetosphere corresponds to familiar planetary magnetospheres (viz., the magnetospheric topology and an electron ring current). We also comment on some of the ways in which it differs from familiar planetary magnetospheres (viz., relative stability and predictability of upstream plasma and field conditions, absence of a magnetotail plasma sheet and of a plasmasphere, and probable instability of the ring current).

Abstract. Following Galileo's arrival at Jupiter in fall 1995, a total of six spacecraft have now sampled the Jovian magnetosphere. Using these data sets to investigate the average location and shape of the Jovian boundaries, we fit ellipse profiles to the observations, allowing for the disk-like shape of the magnetosphere and taking account of variable solar wind pressure. We find that the subsolar magnetopause location varies with solar wind dynamic pressure to power between -1/5 and -1/4, in contrast to the terrestrial -1/6 power; this is a well-known difference attributed to the presence of hot plasma and centrifugal stretching in the Jovian magnetodisk that lessens the pressure gradients in the outer magnetosphere, resulting in its unusual responsiveness to compression. The magnetopause is less flared than the bow shock as expected, and the magnetopause shape is especially streamlined (least flared and more bulletlike) during the higher solar wind dynamic pressure conditions encountered. The average subsolar shock-to-magnetopause standoff ratio is approximately 6/5, while at low incident solar wind dynamic pressure the ratio rises to around 4/3 suggesting a blunter Earth-type magnetopause shape under these conditions. In particular, our analysis confirms that the magnetopause boundary shape is influenced by the radially inflated magnetodisk, as has been previously inferred from the stretched magnetic field lines seen within the magnetosphere. Our fits to the observations reveal that the average magnetopause boundary is indeed contracted on the northsouth axis about the magnetic equator. The bow shock is not found to be so asymmetric in shape, suggesting that there is little effect of external magnetic field direction, and supporting our conclusion that the internal magnetodisk shape is the cause of the magnetopause polar flattening.

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