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Abstract In the inner magnetosphere, fast magnetosonic waves (MS waves) are known to resonantly interact with ring current protons, causing these protons to gain energy preferentially in the direction perpendicular to the background magnetic field. An anisotropic distribution of enhanced ring current protons is a necessary condition to excite electromagnetic ion cyclotron (EMIC) waves which are known to facilitate a rapid depletion of ultra‐relativistic electrons in the outer radiation belt. So, when a simultaneous observation of high‐frequency EMIC (HFEMIC) waves, anisotropic low‐energy protons, and MS waves was first reported, a chain of energy flow from MS waves to HFEMIC waves through proton heating was naturally proposed. In this study, we carry out a statistical analysis using Van Allen Probes data to provide deeper insights into this energy pathway. Our results show that the occurrence of HFEMIC waves exhibits good correlation with the enhanced flux and anisotropy of low‐energy protons, but the correlation between the low‐energy protons and the concurrent MS waves is rather poor. The latter result is given support by quasilinear diffusion analysis, indicating negligible momentum diffusion rates at sub‐keV energies, unless MS wave frequency gets very close to the proton cyclotron frequency (which constitutes only a small number of the cases). The fact that the first chain of the coupling is statistically inconclusive calls for an alternative explanation for the major source of the low‐energy anisotropic proton population in the inner magnetosphere.
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This content will become publicly available on February 28, 2026
Observations of Fast Local Loss of High‐Energy Ring Current Protons Associated With Deepening Local Minimum in Phase Space Density
Abstract The development of a deepening local minimum in phase space density (PSD)‐ profile indicates fast local loss potentially caused by wave‐induced scattering. The identification and characterization of proton PSD deepening minima are important for investigating the ring current loss and overall dynamics. Using multiyear Van Allen Probes observations, we analyze ∼10–100s keV proton PSD and report >100 keV proton deepening PSD minima for the first time. The overall occurrence rates of proton deepening local minimum peaks at ∼3%, mainly located at = 4.5–5.0 near the plasmapause. The occurrence rate increases with the decrease of AL index and increase of solar wind dynamic pressure. The theoretical resonance energy of protons with typical He‐band electromagnetic ion cyclotron (EMIC) waves agrees with the energy of protons with deepening PSD minima. Thus, EMIC waves are the likely cause of the deepening PSD minimum and contribute to the fast local loss of ring current protons.
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- Award ID(s):
- 2131012
- PAR ID:
- 10634390
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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