Survey of Saturn auroral storms observed by the Hubble Space Telescope: Implications for storm time scales

We examine the ultraviolet images of Saturn obtained by the Hubble Space Telescope (HST) between 1997 and 2013 for the presence of auroral storm signatures, consisting of 2060 individual images over 74.4 h of exposure time. We find 12 storm intervals in these data, identified by bright high-latitude auroras spanning the dawn sector, which previous studies have shown are excited by strong magnetospheric compressions by the solar wind. While most of these events have previously been discussed individually, here we consider what may be deduced about the lifetime of storms, yet unobserved directly for a given event, by examining the ensemble. Specifically, we examine the presence or absence of storm signatures in successive HST observing "visits" separated by varying intervals of time. We show that the observations are consistent with a typical lifetime of similar to 1.5 Saturn rotations (similar to 16 h), within a likely range between similar to 1 and similar to 2 rotations. We suggest that this time scale and the storm evolution morphology relate to the time for hot plasma subcorotation around the planet, following injection postmidnight after a major burst of tail reconnection excited by the solar wind compression. From the overall observed similar to 12% occurrence frequency of storm signatures, we further infer an averaged storm recurrence time of similar to 5.5 days (similar to 4-7 days), although this averaged value could be shortened by the occurrence of successive storm activations within few-day disturbed solar wind intervals as observed directly in two cases.