Low‐energy electron measurements at Ganymede with the Galileo spacecraft: Probes of the magnetic topology

Abstract

During the close flyby of Ganymede by the Galileo spacecraft on 6 September 1996 a plasma analyzer was used to obtain comprehensive measurements of the thermal electron plasmas in the vicinity of this moon. Our initial analyses are directed toward the character of energy influxes into Ganymede's polar caps and the pitch angle distributions for warm electrons in the energy range of 70 eV to 4.5 keV. The source of these electrons is Jupiter's plasma sheet within which Ganymede was embedded during the flyby. These electrons were present along the entire trajectory and provide the opportunity to use their pitch angle distributions with respect to the magnetic field in order to investigate the magnetic field topology at Ganymede. Observations of the loss cones for these pitch angle distributions are a simple means of determining whether or not a given magnetic field line intersects the moon's surface. It is found that the observed pitch angle distributions are inconsistent with the current magnetostatic model for the magnetic fields in the vicinity of Ganymede. Thus a major revision of this model is required that accounts for the contributions of the plasma dynamical interaction in order to achieve a more accurate assessment of an intrinsic magnetic moment in the interior of this moon. In addition, the electron energy fluxes into the polar cap at the time of flyby were measured to be about 1 erg/cm²‐s, or a total of approximately 5 × 1016 ergs/s over each polar cap available for excitation and ionization of any atmosphere near the moon's surface.