Statistical Morphology of ENA Emissions at Saturn

The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft is providing the first energetic neutral particle (ENA) measurements in the magnetosphere of Saturn. Advantageous spacecraft orbits during the first 120 days of 2007 allowed ENA observations to be mapped to the equatorial plane of the planet and surveyed as a statistical ensemble in a Sun-synchronous coordinate system. When projected onto the equatorial plane, emissions from both energetic hydrogen atoms (20-50 keV) and energetic oxygen atoms (64-144 keV) form toroidal rings nearly concentric with the planet. The H ring has a mean radius of 10.2 +/- 1.3 RS, while the O ring has a mean radius of 7.3 +/- 0.8 RS (1 RS = 60268 km). The H emissions display a distinct azimuthal peak at a local time of ~0.4 hours (just past midnight), while the O emissions essentially no peak in local time. The H peak seems to be a regular feature of these emissions over the 120-day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn's magnetotail. Using a multipole model of the internal field and a ring current external field, magnetic field mapping of the ENA emissions to Saturn's ionosphere reveals that both H and O emissions appear equatorward of the aurora and are NOT connected to it.