Io's nonlinear MHD-wave field in the heterogeneous Jovian magnetosphere

Abstract

Io's relative motion in the plasma torus strongly perturbs the incident magnetoplasma. The waves generated by Io then propagate through the dense plasma torus, the low-density magnetospheric plasma and finally reach the Jovian ionosphere producing the well-known Io footprint. Direct spacecraft observations by the Voyager and Galileo spacecraft demonstrated that Io's interaction is nearly fully saturated, i.e. the plasma flow close to Io is nearly brought to a halt in conjunction with a strong magnetic field perturbation. Here we use a nonlinear, three-dimensional, time-dependent MHD model to examine how the Io-generated waves propagate, are partly reflected at plasma density gradients, and nonlinearly interact. In this work, we concentrate on the basic properties of the wave propagation based on a simplified magnetic field geometry. Despite the idealization, structural features such as the shape and morphology of the Io footprint and its wake can be qualitatively compared to measured data. We show that a strong and saturated interaction fundamentally modifies Io's wave field from the linear wave morphology picture traditionally studied. In particular, we find that due to the strong and thus nonlinear interaction the standard law of reflection completely breaks down. Io's Alfvén waves are reflected in Jupiter's ionosphere nearly anti-parallel to the incident wave. We also notice overlapping and blending together of the multiply reflected Alfvén wings with increasing strength of Io's interaction. This could be a possible explanation for the disappearance of multiple footprints when Io moves to the torus center. Copyright 2007 by the American Geophysical Union.