Variability of Jupiter’s main auroral emission and satellite footprints: A survey of Galileo-era HST observations

HST images of Jupiter’s aurora show that the main emission occasionally contracts or expands, shifting toward or away from the magnetic pole, by several degrees (e.g. Grodent et al., 2008). These latitudinal shifts have been linked to both changes in the solar wind dynamic pressure (Grodent et al., 2003) and Io’s volcanic activity (Bonfond et al., 2012). They are sometimes accompanied by motion of a satellite footprint, indicating a change in the magnetic field configuration that shifts the ionospheric mapping of a given radial distance in the equatorial magnetosphere. As a field line becomes increasing radially stretched its ionospheric footprint shifts equatorward compared to its initial mapping. However, in some cases, the main emission has been observed to shift independently of the satellite footprints, indicating that the variability stems from some other change in the corotation enforcement current (CEC) system that is responsible for the main auroral emissions at Jupiter. Here we analyze HST auroral images from the Galileo era (1996-2003) and compare how latitudinal shifts of the Ganymede footprint compare to concurrent shifts in the main auroral emission. We focus on images with overlapping Galileo measurements because concurrent information is available about the current sheet strength, which indicates the amount of field line stretching that can affect the position of both the main emission and satellite footprints (Vogt et al., 2017). We find that the Ganymede footprint and main auroral emission typically, but do not always, display similar poleward or equatorward motion. Additionally, we find that the auroral shifts are weakly linked to changes in the current sheet strength measured by Galileo. We discuss implications of the observed auroral shifts in terms of the expected changes to the CEC system and magnetospheric mapping.