HST/STIS observation of Ganymede’s aurora: Investigating the variability of the auroral ovals

Abstract

We examine Space Telescope Imaging Spectrograph (STIS) observations of Ganymede’s auroral emissions acquired during two visits in 2010 and 2011 with the Hubble Space Telescope (HST) when Ganymede was at eastern elongation (ID 12244). The observations clearly show two auroral ovals in the polar regions as expected from previous observations (Feldman et al., 2000, McGrath et al., 2013, Saur et al., 2013). In our work we analyze the variability of the location of its ovals in detail.

As the only known satellite in the solar system with an intrinsic magnetic field, Ganymede has a so-called mini-magnetosphere, which interacts with the surrounding Jovian magnetosphere. Simulations of different authors suggest that the most likely excitation mechanism for auroral emissions at Ganymede is electron acceleration by strong field-aligned currents that coincide with the location of the so-called separatrix (i.e., the boundary area between open and closed magnetic field lines). The position of the intersection line between this separatrix and Ganymede‘s surface in turn depends on the local plasma interaction: As the mini-magnetosphere is compressed upstream (i.e., trailing hemisphere) the auroral oval is shifted to higher latitudes. Downstream (i.e., leading hemisphere) the magnetosphere is stretched out and auroral emission occurs at lower latitudes. In order to understand how the locations of the auroral ovals change and whether this effect is controlled by the plasma interaction, we analyze in a first step their latitudinal positions. We provide a mapping of auroral emission distribution with the purpose of determining spatial, i.e. latitudinal, variability of the ovals as a function of Ganymede‘s position in Jupiter’s magnetosphere.