On Europa's magnetospheric interaction: A MHD simulation of the E4 flyby

The global three-dimensional interaction of Europa with the Jovian magnetos phere is modeled by using a complete set of ideal magnetohydrodynamic (MHD) equations. The model accounts for exospheric mass loading, ion-neutral cha rge exchange, recombination, and a possible intrinsic dipole magnetic field of Europa. The single-fluid MHD equations are solved by using a modern, fi nite volume, higher-order, Godunov-type method on an adaptively refined uns tructured grid, which allows detailed modeling of the region near Europa wh ile still resolving both the upstream region and the satellite's wake. The magnetic field and plasma density measured during Galileo's E4 flyby of Dec ember 19, 1996, are reproduced reasonably well in the simulation. We find t he agreement between the data and our model particularly convincing if we a ssume that the plasma velocity during the E4 flyby deviated from the nomina l corotation direction by approximately 20 degrees. Evidence from the Galil eo energetic particle detector also supports this assumption. In this case, we can fit the data using a dipole with orientation close to that of an in duced dipole arising from the interaction of a hypothetical conducting subs urface layer on Europa with the: periodically changing magnetic field of Ju piter. However, the magnitude of the dipole in our model is somewhat smalle r (70%) than that suggested by Khurana et al. [1998]. The total mass loadin g and ion-neutral charge exchange rates are consistent with the estimates o f Europa's atmosphere and ionosphere.