A 3-DIMENSIONAL MHD SIMULATION OF PLASMA-FLOW PAST IO

We describe a time-dependent three-dimensional magnetohydrodynamic (MHD) simulation of plasma flow past Jupiter's satellite Io. The simulation starts with a uniform, magnetized flow interacting with a finite conducting sphere (representing Io), and the integration is carried forward in time until a quasi-steady state develops. Previously we have reported on the presence of standing slow mode perturbations in the flow, attached to Io. In this paper we present details of the simulation model and describe the overall features of the interaction. Field-aligned currents are found in regions extending downstream from the obstacle at an angle to the background field; these currents from the Alfven wings, an expected result. The plasma flow diverts both around Io and the Alfven wings. An "Alfven ellipse," the intersection of a cylindrical Alfven wing with an arbitrary plane above Io, describes the obstacle that an Alfven wing presents to the flow. In addition to the Alfven mode, which carries the field-aligned currents, perturbations carried by the other MHD modes are also present. Standing slow mode perturbations are the most significant of these, but a fast mode signature also appears. Near Io the interaction is clearly nonlinear; all three MHD modes interact, and the local Alfven and sound speeds are altered. However, estimates of the spatial location of the standing MHD mode disturbances based on the background characteristic velocities are qualitatively accurate.