Dusk-brightening Event in Saturn's H[FORMULA][F][SUP]+[/SUP][INF]3[/INF][/F][/FORMULA] Aurora

We report on a unique dusk-brightening event within Saturn's aurorae. Measurements of the H-3(+) infrared aurora using the CSHELL instrument on NASA's Infrared Telescope Facility (IRTF), made in 2005 December, show an auroral intensity structure unlike anything previously detected. The aurora has a significantly brighter dusk sector over three Earth nights, a period in excess of 5 Saturnian days, suggesting a consistent source for this emission, stable in position within the magnetosphere. However, unlike previously detected dawn-brightening events, the overall auroral brightness remains low and the ion wind structure appears unaffected. Using the location of magnetopause crossings as a proxy for the solar wind pressure, the solar wind appears to be exceptionally rarefied. This leads us to conclude that the dusk-brightening event is strongly linked with the unusual solar wind conditions at the time of the observations.