Interaction evidence between Enceladus' atmosphere and Saturn's magnetosphere

Abstract

Saturn's ultraviolet (UV) auroras are highly variable, with much of the activity believed to be controlled by conditions in the solar wind. Like Jupiter, Saturn is also expected to have an electrodynamic interaction between the planet's ionosphere and its satellites' atmospheres, a process that is responsible for the moon's auroral footprint components at Jupiter. Recent observations from several instruments on the Cassini spacecraft, e.g., the UltraViolet Imaging Spectrograph (UVIS) and the Cassini Plasma Spectrometer (CAPS), show strong evidence that Enceladus makes significant plasma contributions to Saturn's magnetosphere. Modeling by Pontius and Hill (2006) suggests that Enceladus' mass loading region may be comparable in extent to Io's, whose auroral footprint emissions are clearly detected. However, the upper limits of Enceladus' magnetic footprint emissions are predicted to be only a few tenths of kilo‐Rayleighs (kR). We present results of a search for Enceladus' auroral footprint using series of UV images of Saturn's aurora taken by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in January 2004 and the Advanced Camera for Surveys (ACS) between February 2005 and January 2007. We detected no auroral evidence of the electromagnetic connection between Enceladus' atmosphere and Saturn's ionosphere, in agreement with modeling predictions. As our study could detect no signature of currents linking Saturn's ionosphere and Enceladus' interaction region, the implication is that the field‐aligned currents associated with mass loading at Enceladus are too weak to cause auroral emission.