A Survey of Electron Anisotropies in the Magnetosphere: Dependence on Solar Wind Pressure and Kp
Abstract
We present a survey of electron distributions measured by THEMIS within Earth's equatorial magnetosphere at L values ≤ 14. Particular attention is focused on the spatial distribution and occurrence of magnetic field-aligned and perpendicular anisotropies in electron distribution functions and how these depend on solar wind dynamic pressure and magnetospheric activity. Using 161,300 3-s resolution electron measurements from THEMIS A and D, we find that field-aligned anisotropies are most prominent at low energies (30--200 eV) and occur over nearly all L and MLT, with the largest values concentrated in the dayside/postnoon plasmasphere, while perpendicular anisotropies dominate at higher energies (1--30 keV) and are largest and most probable in a narrower band from L ∼ 5--10 with a pronounced dayside/prenoon bias. A localized region near dawn at 4≲ L≲ 8--9 is found to be typically devoid of field-aligned electrons over the entire 30--200~eV band. Both anisotropy types shift toward lower L shell and become more probable under elevated solar wind dynamic pressure and Kp, and their spatial and energy dependence is largely complementary: perpendicular anisotropies coincide spatially with regions of quasi-parallel chorus generation, while field-aligned anisotropies coincide with oblique chorus and time domain structure (TDS) generation regions. In addition to being a useful diagnostic for electron population types, our results provide a useful basis for comparison with global distributions of wave modes which arise from these anisotropies, and offer an anisotropy-based observational framework for interpreting their dependence on magnetospheric activity.
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BibTeX
@misc{hull2026survey,
title = {{A Survey of Electron Anisotropies in the Magnetosphere: Dependence on Solar Wind Pressure and Kp}},
author = {A. J. Hull and O. V. Agapitov and F. S. Mozer},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.28745},
eprint = {2609.28745},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.28745},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - A Survey of Electron Anisotropies in the Magnetosphere: Dependence on Solar Wind Pressure and Kp
AU - A. J. Hull
AU - O. V. Agapitov
AU - F. S. Mozer
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.28745
UR - https://arxiv.org/abs/2609.28745
AB - We present a survey of electron distributions measured by THEMIS within Earth's equatorial magnetosphere at L values ≤ 14. Particular attention is focused on the spatial distribution and occurrence of magnetic field-aligned and perpendicular anisotropies in electron distribution functions and how these depend on solar wind dynamic pressure and magnetospheric activity. Using 161,300 3-s resolution electron measurements from THEMIS A and D, we find that field-aligned anisotropies are most prominent at low energies (30--200 eV) and occur over nearly all L and MLT, with the largest values concentrated in the dayside/postnoon plasmasphere, while perpendicular anisotropies dominate at higher energies (1--30 keV) and are largest and most probable in a narrower band from L ∼ 5--10 with a pronounced dayside/prenoon bias. A localized region near dawn at 4≲ L≲ 8--9 is found to be typically devoid of field-aligned electrons over the entire 30--200~eV band. Both anisotropy types shift toward lower L shell and become more probable under elevated solar wind dynamic pressure and Kp, and their spatial and energy dependence is largely complementary: perpendicular anisotropies coincide spatially with regions of quasi-parallel chorus generation, while field-aligned anisotropies coincide with oblique chorus and time domain structure (TDS) generation regions. In addition to being a useful diagnostic for electron population types, our results provide a useful basis for comparison with global distributions of wave modes which arise from these anisotropies, and offer an anisotropy-based observational framework for interpreting their dependence on magnetospheric activity.
ER -