SM021-07
The Statistical Morphology of Saturn’s Equatorial ENA Projections

Thursday, 10 December 2020: 07:46
Virtual
Joe Kinrade1, Alexander Bader1, Sarah Victoria Badman1, Chris Paranicas2, David Constable1, Christopher Stephen Arridge1, Stanley Cowley3, Gabrielle Provan3 and Donald G Mitchell2, (1)Lancaster University, Physics Department, Lancaster, United Kingdom, (2)Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (3)University of Leicester, School of Physics and Astronomy, Leicester, United Kingdom
Recorded Presentation
Abstract:
Saturn's magnetosphere is an efficient emitter of Energetic Neutral Atoms (ENAs), given the presence of an extended neutral cloud that originates from Enceladus. The ENA emission is symptomatic of the global circulation of plasma in Saturn's magnetosphere. Energetic ions are injected from the outer magnetosphere following magnetotail dynamics and reconnection events. These ions charge exchange with the neutral cloud (confined mostly to the spin plane), producing ENAs. The global ENA emission is dynamic, displaying sudden brightening on the nightside, and discrete rotating enhancements which circle the planet for many hours as energetic ions drift with the bulk plasma flow.

Our characterization of the ENA emission at Saturn is made possible using imagery from the Ion-Neutral Camera (INCA) that flew onboard Cassini. Observations were made over the entire mission lifetime. We present an analysis of the complete INCA image set, using equatorial projections of the flux distribution to reveal the time-averaged morphology of Saturn's ENAs. A comprehensive algorithm was used to calibrate, clean and project all high-inclination orbit days. In the average pictures, many of the projected pixels consist of tens to hundreds of days continuous exposure, captured with a line-of-sight >50° elevation and within 30 RS distance from the spacecraft.

We find clear toroidal ENA distributions in O and H, and all INCA energy bands. The emission drops off sharply inside 5 RS radial distance in all cases. Average peak intensities lie at radial distances of ~7 RS (O, 170-230 keV) to ~10 RS (H, 24-55 keV). All toroids are offset towards the dayside by several RS, most clearly in the 24-55 keV H, with a maximum intensity at ~13-14 RS on the dayside, compared to only ~10 RS on the nightside. The H ENA distribution is also enhanced around midnight local times, a net effect associated with reconnection return flows and transient ENA enhancements in this sector. We also explore possible organisation of the global ENA intensity by Saturn’s rotating current systems associated with planetary period oscillations (PPOs). We find that the ENA intensity is statistically modulated by periodic changes in expected plasma sheet thickness as controlled by field-aligned current interactions, evident in both north and south rotating system frames.

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