SM045-06
Electromagnetic Resonance Sounding Observations of Storm-induced Heavy Ion Enhancements in the Inner Magnetosphere

Tuesday, 15 December 2020: 08:50
Virtual
Peter J Chi, University of California Los Angeles, Department of Earth Planetary and Space Sciences, Los Angeles, CA, United States, Kazue Takahashi, Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, United States and Richard Eugene Denton, Dartmouth College, Department of Physics and Astronomy, Hanover, NH, United States
Recorded Presentation
Abstract:
The plasma mass in the inner magnetosphere can vary significantly due to changes in flows and forces in the coupled magnetosphere-plasmasphere-ionosphere system. During the storm recovery phase, a narrow region of O+ in the vicinity of the plasmapause (also known as the oxygen torus) usually forms, with an O+ density increased by up to two orders of magnitude. The presence of heavy ions can significantly enhance the plasma mass density and affect other physical processes in the magnetosphere, such as the reconnection rate, instability and wave growths, and the magnetospheric response time to interplanetary shocks. There are still outstanding questions regarding the morphology and the generation mechanisms of the oxygen torus.

This study applies resonance sounding techniques on satellite electromagnetic measurements to investigates the enhancements of heavy ions in the bulk plasma of the inner magnetosphere during magnetic storms. In Magnetospheric Multiscale (MMS) and Van Allen Probes (RBSP) observations, we identify the presence of heavy ions by comparing the mass density inferred from field line resonance frequencies to the charge density derived from the upper hybrid resonance frequencies. We find that, while MMS and RBSP could both observe clear enhancements of heavy ions during a magnetic storm, the degree and the width of this heavy-ion enhancement can vary with location: Some magnetic storms exhibit noticeably higher heavy ions in the morning, but this morning-afternoon asymmetry does not appear in every storm. The timing of oxygen torus occurrence in the recovery phase may also differ from storm to storm. In RBSP measurements, we also compare the bulk densities with the partial densities of low-energy ions detected by the HOPE instrument. While the average ion mass can be greater for 30 eV – 1 keV ions than that for the bulk plasma in the oxygen torus, it is evident that the majority of the heavy ions in the oxygen torus are below 30 eV, confirming the need to examine the bulk mass and charge densities through electromagnetic sounding methods.

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