Ganymede, the largest satellite in the solar system, is unique among the planetary satellites not only because of its great size but also because it is the only satellite in the solar system known to possess an intrinsic magnetic field. Ganymede's intrinsic field is sufficiently strong to stand off Jupiter's magnetospheric plasma above its surface and form a mini-magnetosphere embedded in Jupiter's magnetosphere. In this dissertation, we focus on understanding the interactions between Ganymede's magnetosphere and Jupiter's magnetospheric plasma from the perspective of both observations and numerical simulations.

First, we have used all available observations to establish an overall picture of Ganymede's magnetosphere and to investigate the characteristics of in-situ observations from the Galileo spacecraft obtained on individual flybys. We find that the time of magnetopause crossings inferred from the signatures in different instruments agree well with each other for five of the six Galileo close encounters but not during the G8 encounter. This disagreement leads us to speculate that the background flow during the time when the G8 encounter occurred may have been supermagnetosonic unlike that on the other passes and hence a transient shock formed in front of the magnetosphere.

Second, we have conducted a series of three-dimensional MHD simulations to further our understanding of Ganymede's magnetosphere. Our simulations show that, in addition to the familiar structures such as the magnetopause and equatorial current sheet, Ganymede's magnetosphere extends into an Alfv én wing that mediates the interaction of Ganymede with the plasma and ionosphere of Jupiter. The field-aligned currents in the Alfv én wing close not only through the moon and its ionosphere but also through the magnetopause and tail current sheets. Our model reproduces quite closely the magnetic field and plasma observations from multiple Galileo passes. Moreover, our MHD simulations enable us to interpret the observed boundary fluctuations as direct evidence of magnetic reconnection at the magnetopause. By obtaining evidence in comparing our model results with the observations, we conclude that even under steady upstream conditions, upstream reconnection is intermittent. Using our simulation results to provide a realistic description of the external magnetospheric fields, we have been able to refine earlier determinations of Ganymede's internal field and set better constraints on properties of a likely subsurface ocean.