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 distributions nearly concentric with the planet. The hydrogen torus is continuous in local time, but the oxygen torus has a gap from dawn to noon. When fitted to circles, the H torus has a mean radius of 11.0 +/- 0.5 R-S, while the O torus has a mean radius of 7.9 +/- 0.8 R-S (1 R-S = 60268 km). Both tori display peaks at just before midnight at local times of 23.6 h (H) and 21.8 h (O). These maxima seem to be regular features over the 120-day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn's magnetotail, as would be the case during substorms. However, the persistence of the ENA emissions suggests they are continuously driven by processes internal to Saturn's magnetosphere. When mapped along magnetic field lines to Saturn's ionosphere, both H and O emissions appear equatorward of the aurora and are distinct from it.