Jovian plasma interaction with Ganymede's magnetosphere

Abstract:

We use a three-dimensional hybrid plasma model to study the global aspects of Jovian plasma interaction with Ganymede. Ganymede, the largest moon of Jupiter, is a unique body for several reasons: (1) it is the only known moon that has its own intrinsic magnetic field, (2) its dipole magnetic moment is large enough to form an embedded magnetosphere within the magnetosphere of Jupiter, and (3) it has a bound neutral atmosphere and an ionosphere, mainly composed of molecular and atomic oxygen, that interact with the co-rotating plasma of Jupiter. Since Jupiter's magnetic dipole moment tilts nearly 10^o from its rotation axis, Ganymede passes two distinct plasma environments on its orbit around Jupiter (which is slightly inclined to the Jovian equator): (1) the plasma sheet, where the plasma density is nearly 5 cm^-3 and plasma beta is slightly larger than one, and (2) outside the plasma sheet where the plasma density is lower than that in the sheet and plasma beta is smaller than one. The sonic and Alfvénic Mach numbers, however, are both smaller than one in both of these regions. Thus, the formation of a bow shock upstream of Ganymede is not expected. The plasma interaction with Ganymede has been studied before using MHD simulations and the formation of a magnetopause, magnetotail, and Alfvén wings were examined. We use our three-dimensional hybrid model to compare the global effects of the two plasma regimes on the interaction with Ganymede. We compare our simulation results with Galileo flyby observations, and explain differences between our mode with MHD simulation results. We also provide a global map of plasma precipitation into the surface of Ganymede which has direct impact on Ganymede's atmosphere/exosphere formation.