Mass-loading Effects at Ganymede

Mass-loading effects can be important in the interactions between Jupiter's plasma wind and the Galilean moons. The modifications to the inflow depend on the ionization mechanism and the pickup rate. At Ganymede water molecules are thought to be primarily sputtered from the polar regions by energetic ions from Jupiter's magnetosphere. Paranicas et al. [1999] have estimated the total production rate to be ~ 10^{26} s-1. While this is less than the ionization source rate for the sulfur and oxygen ions (~ 10^{28} s-1) at Io [Bagenal, F., 1997] it still can influence the interaction between the Jovian flow and the moon. Ionization effects are most important when Ganymede is close to Jupiter's central plasma sheet where the background plasma is relatively dense. During Galileo's G8 flyby of Ganymede, the magnetic field magnitude starts to increase gradually above the background at a distance of about seven Ganymede radii as the spacecraft moves towards Ganymede's magnetosphere from the upstream flank side. The G8 encounter occurred when Ganymede was very close to the central plasma sheet therefore we suggest that the pile-up of the magnetic field is due to the mass-loading of the exospheric neutrals in the vicinity of the moon. The pickup ions will decrease the average temperature of the flow if the flow speed is less than the plasma thermal speed and will increase it if the flow speed is greater than the plasma thermal speed. Therefore the slowing down of the background flow, the increase in plasma density and the change in the flow temperature may cause a Mach number variation as a function of the distance from Ganymede. This may possibly account for the existence of a transient shock structure we have identified in the observations. We plan to conduct MHD simulations including the mass-loading effects to study the non-linear response of the plasma to the addition of ions near Ganymede.