GANYMEDES MAGNETOSPHERE - MAGNETOMETER OVERVIEW

Ganymede presents a unique example of an internally magnetized moon wh ose intrinsic magnetic field excludes the plasma present at its orbit, thereby forming a magnetospheric cavity. We describe some of the prop erties of this mini-magnetosphere, embedded in a sub-Alfvenic flow and formed within a planetary magnetosphere. A vacuum superposition model (obtained by adding the internal field of Ganymede to the field impos ed by Jupiter) organizes the data acquired by the Galileo magnetometer on four close passes in a useful, intuitive fashion. The last field l ine that links to Ganymede at both ends extends to similar to 2 Ganyme de radii, and the transverse scale of the magnetosphere is similar to 5.5 Ganymede radii. Departures from this simple model arise from curre nts flowing in the Alfven 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 functio n of the System Ln location of Ganymede. On one of the passes, Ganymed e was located near the center of Jupiter's plasma disk. For this pass we identify probable Kelvin-Helmholtz surface waves on the magnetopaus e, After entering the relatively low-latitude upstream magnetosphere, Galileo apparently penetrated the region of closed field lines (ones t hat link to Ganymede at both ends), where we identify predominantly tr ansverse fluctuations at frequencies reasonable for field line resonan ces. We argue that magnetic field measurements, when combined with flo w measurements, show that reconnection is extremely efficient. Downstr eam 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 planet ary 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 m agnetotail plasma sheet and of a plasmasphere, and probable instabilit y of the ring current).