Properties of Ganymede's magnetosphere as revealed by energetic particle observations

Abstract

Energetic particle observations made during the Galileo satellite's close encounters with Jupiter's moon Ganymede provide a measure of many characteristics of Ganymede's magnetosphere, a magnetosphere larger than that of Mercury. Changes in energetic ion anisotropy signatures, caused by Jupiter's corotating magnetic field, show that Ganymede's magnetosphere significantly slows down the ambient nearly corotating Jovian plasma. The data further indicate that this convection slowdown apparently extends at least several Jupiter radii away from Ganymede along Jovian field lines connected to the moon. The locations of these anisotropy signature changes occur at the locations of magnetometer and plasma wave magnetopause identifications that match surprisingly well the predictions of a simple model comprised of an intrinsic dipole field superimposed on Jupiter's ambient field. Loss cones measured throughout Ganymede's magnetosphere provide a quantitative estimate of the moon's surface magnetic field along the subsatellite track that agrees well with model field predictions and provides verification of Ganymede's intrinsic magnetic field. On Jovian field lines connected to Ganymede, electrons are trapped between the moon and a near‐Jupiter mirror point for several bounces as they convect across Ganymede's magnetosphere. This unique geometry allows a measurement of the amount of electron pitch angle scattering occurring in a single bounce between the Ganymede and near‐Jupiter mirror points. The energy‐dependent pitch angle diffusion coefficient and scattering lifetimes have been extracted from these observations. The consistency of these results from encounter to encounter demonstrates the stability of these processes, at least over the several month interval between encounters. Evidence for loss cone signatures established one Jovian rotation (∼10 hours) earlier indicates that the entire Jovian L shell region traversed by Ganymede provides an electron scattering environment similar to that measured on Jupiter‐Ganymede field lines. Measurements from Galileo's last close encounter with Ganymede show electron pitch angle distributions characteristic of particles trapped on closed magnetic field lines in a distorted magnetospheric configuration. Thus with its distorted magnetic field and apparent radiation belt, Ganymede takes its place as a unique member of the solar system family of magnetospheres.