Ganymede as a Laboratory for Multiscale Physics

There are multiple scales on which the local physics of collisionless magnetic reconnection operate. These include the ion scale and electron scales defined by the associated inertial lengths. In Earth's magnetosphere, even the ion scale is inaccessible to even very large-scale numerical simulations, as it is just a tiny fraction of the total system size. Therefore we primarily rely on resistive MHD simulations when studying Earth's magnetosphere. In contrast, Ganymede's magnetosphere is much smaller, and the ion inertial length is much larger compared to the system's size. The relatively large scales make it possible to model Ganymede's magnetosphere with more sophisticated approaches while adequately resolving the relevant physical scales. Ganymede is therefore an ideal laboratory for studying both the multiple scales involved in collisionless reconnection as well as the consequences for the global magnetosphere; it is a real magnetosphere with available in situ data that is also accessible to multiple modeling approaches. We present recent Hall MHD simulations with the BATSRUS code, and comparisons to Galileo observations, of this small magnetosphere and demonstrate the global importance of ion scale effects. We further extract magnetic separators and compare the results with resistive MHD, Hybrid and PIC results. As with the GEM reconnection challenge, we find that including ion scale physics is the minimum extension of MHD to capture most basic features of the magnetosphere.