Studying the seed population of Saturn's <1 MeV ion radiation belts using Cassini/MIMI measurements
Abstract
Characterizing the seed population of Saturn's radiation belts is essential, since the seed particles control the structure and dynamics of the radiation belt particles. At Saturn, the ion radiation belts not produced by Cosmic Ray Albedo Neutron Decay (CRAND), are dominated by hydrogen and oxygen and do not extend above 1 MeV or inward of ~L=4. These constraints are presumably caused by the interaction of the seed population with Saturn's neutral gas cloud. However, past observations with a limited Cassini dataset hint that there may be deviations from the behavior that this interaction dictates. In this presentation, we extend the analysis of the seed ion population to most of the Cassini energetic ion dataset and describe their moments using κ-distribution functions. The κ-distribution function provides a detailed description of energetic charged particle distributions and is an invaluable aid in the determination of physical parameters that are fundamentally important for studying the source, loss, transport and acceleration of energetic plasma inside the magnetosphere. We use κ-distribution fits to combined CHEMS (3 to 236 keV/e), LEMMS (0.024 < E < 18 MeV), and INCA (5.2 to >220 keV) H+ and O+ energetic ion spectra to calculate the >20 keV energetic ion moments inside Saturn's magnetosphere. We find that (a) the 9<L<20 region corresponds to a local equatorial acceleration region, where sub-adiabatic transport of H+ (Γ~1.25) and quasi-isothermal behavior of O+ (Γ~0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi-isothermal (Γ <1); (c) the energetic ion injections in the outer parts of Saturn's magnetosphere (especially beyond 17-18 Rs) produce durable signatures in the energetic ion moments; and (d) the plasma sheet does not seem to have a "ground thermodynamic state", but instead the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSM33C3213R
- Keywords:
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- 2756 Planetary magnetospheres;
- MAGNETOSPHERIC PHYSICS;
- 2774 Radiation belts;
- MAGNETOSPHERIC PHYSICS;
- 2780 Solar wind interactions with unmagnetized bodies;
- MAGNETOSPHERIC PHYSICS;
- 2784 Solar wind/magnetosphere interactions;
- MAGNETOSPHERIC PHYSICS