Saturn's secondary auroral ring

In January 2004, Saturn’s spin axis was strongly tilted away from Earth and revealed most of the southern polar cap region. We took advantage of this viewing geometry to observe very faint UV auroral emissions with the Hubble Space Telescope. Recent Cassini UVIS observations of Saturn’s night side aurora appear to confirm the presence of this emission. In HST images, these auroral features appear near the planetary limb where they are amplified by limb brightening effect (Fig. 1). They form a distinct secondary ring of emission outside the main auroral ring, near the 67° S parallel. This outer auroral emission maps to a region of the equatorial plane between 4 and 11 RS [1]. We suggest that a population of suprathermal electrons observed by Cassini can provide more than the required energy flux without the need for field aligned acceleration. This auroral UV emission may also be associated with ENAs originating from the energetic protons and O+ of magnetosphere, and/or with a secondary infrared auroral oval.