Three-dimensional magnetopause and tail current model of the magnetosphere of Ganymede

The encounters of Ganymede by the Galileo spacecraft have shown the existence of an intrinsic magnetic field as well as a magnetosphere for Ganymede [Kivelson et al., 1996, 1997, 1998; Kurth et al., 1997; Volwerk et al., 1999; Williams et al., 1997a, 1997b]. The magnetic field in the region of Ganymede was first modeled by Kivelson et al. [1996] as a simple dipole, excluding external field sources (namely, a field due to magnetopause and magnetotail currents), superposed on the Jovian background magnetic field. We call this model M1 herein. This paper presents an alternative model with magnetopause and tail field current contributions, along with an internal multipole model for Ganymede, that has been derived by refitting the internal and external source model coefficients to the magnetic field data local to the Galileo spacecraft using the methods of Choe and Beard [1974a, 1974b]. For brevity we will refer to this model as the Stone-Choe-Beard (SCB) model. Both models are then compared to the original magnetic field data for the G2 and G7 encounters. The SCB model represents the magnetic field and its variations at the spacecraft, especially at the magnetopause crossings, more accurately than M1. A method that tests both models throughout the magnetosphere of Ganymede utilizing particle absorption signatures will be presented in a manuscript in progress.