Three-dimensional MHD simulations of Ganymede's magnetosphere

Ganymede is unique among planetary moons because it has its own magnetic field strong enough to form a magnetosphere within Jupiter's magnetospheric environment. Here we report on our three-dimensional global magnetohydrodynamic (MHD) simulations that model the interaction between Ganymede's magnetosphere and the corotating Jovian plasma. We use the measured field and particle properties to define our boundary conditions. Our simulations show that, in addition to the familiar structures such as the magnetopause and equatorial current sheet, Ganymede's magnetosphere extends into an Alfven wing that mediates the interaction of Ganymede with the plasma and ionosphere of Jupiter. The field-aligned currents in the Alfven wing close on themselves not only through the moon and its ionosphere. They also close through the magnetopause and tail current sheets. The pattern of the field-aligned currents varies according to the orientation of the external magnetic field and asymmetries in the intensities of the parallel currents are organized by the clock angle of the ambient field in the plane perpendicular to the incident flow. The simulations reproduce quite closely the magnetic field structure measured by the Galileo magnetometer for all six close encounters. The magnetopause currents are well resolved in our high resolution simulations, producing sharp rotations in the field orientation consistent with the observations. However, the discrepancies between our model results and the data, such as the weaker field strength near closest approach in multiple simulated flybys, suggest the possibility that Ganymede's intrinsic magnetic field may be stronger than the accepted value. The magnetosphere produced in our simulations can provide us with realistic estimates of the moon's magnetic environment thereby enabling us to refine our determination of Ganymede's internal magnetic field and to better understand the energetic particle behavior.