Io's Plasma Environment during the Galileo Flyby:
Global 3D MHD Modeling
with Adaptive Mesh Refinement

Michael R. Combi, Konstantin Kabin, Tamas I. Gombosi, Darren L. De Zeeuw
 Space Physics Research Laboratory, University of Michigan, Ann Arbor
 Kenneth G. Powell
 Department of Aerospace Engineering, University of Michigan, Ann Arbor

Exerpts of a paper which appears in the Journal of Geophysical Research (Space Physics), Vol. 103, No. A5, Pages 9071-9081, May 1, 1998

ABSTRACT:

The first results for applying a 3D multiscale ideal MHD model for the mass-loaded flow of Jupiter's corotating magnetospheric plasma past Io are presented. The model is able to consider simultaneously physically realistic conditions for ion mass loading, ion-neutral drag, and intrinsic magnetic field in a full global calculation without imposing artificial dissipation. Io is modeled with an extended neutral atmosphere which loads the corotating plasma torus flow with mass, momentum, and energy. The governing equations are solved using adaptive mesh refinement (AMR) on an unstructured Cartesian grid using an upwind scheme for MHD. For the work described in this paper we explored a range of models without an intrinsic magnetic field for Io. We compare our results with particle and field measurements made during the December 7, 1995, flyby of Io, as published by the Galileo Orbiter experiment teams. For two extreme cases of lower boundary conditions at Io, our model can quantitatively explain the variation of density along the spacecraft trajectory and can reproduce the general appearance of the variations of magnetic field and ion pressure and temperature. The net fresh ion mass-loading rates are in the range of ~300-650 kg s-1, and equivalent charge exchange mass-loading rates are in the range ~540-1150 kg s-1 in the vicinity of Io.

FIGURES:




Figure 1 a & b.

Magnetic Field and Streamlines for Mass-Loading Flow Past Io. Above (top) is shown the plane aligned along the upstream magnetic field direction. The draped field lines (generally vertical lines) and velocity stream lines (generally horizontal lines with arrows) are shown. Above (bottom) is shown the plane perpendicular to the upstream magnetic field direction. Models were run for three cases of lower boundary conditions: (1) no mass-loading and reflective boundary conditions, (2) mass loading and reflective boundary conditions, and (3) mass loading and fixed boundary conditions. Figures here are shown for the third case of mass loading and fixed boundary conditions. The spectral color table indicates the magnitude of magnetic field. The increased magnetic field perturbation in the wake is apparent in the mass-loaded cases. The spatial scale covers from -10 to +12 Io radii horizontally and to 18 Io radii vertically.




Figure 2.

Plasma Density for the Mass-loading Flows Past Io. The plasma density is shown for the model with mass-loading and fixed boundary conditions. Mass-loading produces a strong density peak in the wake behind Io as was measured by the Galileo spacecraft. The symmetry line through Io covers distances from -4 to +8 Io radii.




Figure 3.

MHD Current for the Mass-Loading Flow Past Io. The current density is shown for the model with mass-loading and fixed boundary conditions. Current is derived from the curl of the magnetic field vector. An estimate of the integrated current gives a value of several million amperes which is comparable to that required to account for the Jupiter-Io current system.




Figure 4.

Comparison of the Mass-Loading Models with Galileo Particle and Field Measurements. The magnetic field measurements in (a) from Kivelson et al. [1996, Science 274, 296] are shown with the model values. The plasma density (b), pressure (c), and temperature (d) from the model are shown with those from Frank et al. [1996, Science 274, 394] obtained from the plasma ion measurements. The solid lines give the results for the model with mass-loading and reflective boundary conditions. The dashed lines give the results for the model with mass-loading and fixed boundary conditions.




Figure 5.

Velocity Flow Vectors along the December 1995 Galileo Flyby Trajectory. Shown along the projected trajectory in Io's orbit plane are the velocity vectors (direction and magnitude) from our mass-loaded MHD model calculations. The results for the fixed boundary model are shown but the reflective boundary model is virtually identical. Both the variations in direction and variation in magnitude of the velocity are quite similar to those by Frank et al. [1996, Science 274, 394] obtained from the Galileo plasma ion measurements. The absolute velocities from the measurements are uncertain at this time.