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- Title
Preferential Energization of Lower-Charge-State Heavier Ions in the Near-Earth Magnetotail.
- Authors
Keika, K.; Kasahara, S.; Yokota, S.; Hoshino, M.; Seki, K.; Amano, T.; Kistler, L. M.; Nosé, M.; Miyoshi, Y.; Hori, T.; Shinohara, I.
- Abstract
O+ ions make a significant contribution to plasma pressure in the inner magnetosphere during magnetic storms. The storm-time O+ enhancements are primarily caused by enhanced supply from the ionosphere and preferential energization in the magnetotail. In order to characterize the magnetotail process that dominates the energization, we examine differences in energy spectra of energetic 10-180 keV/q ions between different ion species, namely H+, He++, He+. O++, and O+. We use observations made by the MEP-i instrument on the Arase (ERG) spacecraft on the nightside in the radial distance range of ~5-~7 Re during the main and early recovery phases of the May and July 2017 storms. The comparisons of energy spectra show that, for the same charge states, heavier ions are more energized than lighter ions. For the same mass, lowercharge-state ions are more energized than higher-charge-state ions. The spectra exhibit a sharp decrease at high energies for all ion species, while the spectra for more energized ions were shifted toward higher energies, compared to those for less energized ions. The results suggest that the preferential energization is due to temperature increases rather than the generation of energetic ions in the high-energy tail. Considering temporal and spatial scales of heavy ion kinetic motions, we conclude that the preferential energization of lowercharge-state heavier ions occurs during the course of dipolarization, likely due to non-adiabatic heating in the near-Earth plasma sheet, effective trapping during transport by localized flow channels, and/or non-adiabatic acceleration within the near-Earth flow-braking region. Plain Language Summary Plasma in the Earth's magnetosphere consists primarily of electrons, protons, and singly charged oxygen ions of ionospheric origin, O+, but other ions such as He++ of solar wind origin and He+ and O++ of ionospheric origin exist and are observable. The shape and evolution of an energy spectrum, which presents the number of particles as a function of particle kinetic energy, help us identify physical processes of plasma transport, heating, and acceleration in the magnetosphere. This study compares energy spectra between different mass or charge state ions observed by the Arase spacecraft during two magnetic storms that occurred in 2017. The differences seen in the spectral shape indicate that lower-chargestate heavier ions gain more energy in the near-Earth magnetotail. The preferential energization likely occurs during the course of magnetic field reconfiguration (from stretched to dipole-like), suggesting the importance of local kinetic motions of heavy ions in global magnetotail-inner magnetospheric dynamics.
- Subjects
PLASMA pressure; MAGNETOTAILS; MAGNETOSPHERE; IONOSPHERE; MAGNETIC storms
- Publication
Journal of Geophysical Research. Space Physics, 2022, Vol 127, Issue 1, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029786