2024 THEMIS SCIENCE NUGGETS
Energetic Particle Precipitation in Sub-Auroral Polarization Streams
Anton Artemyev
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles
Introduction
Sub-auroral polarization streams (SAPS), and their more intense counterpart, sub-auroral ion drifts (SAID), are often associated with the earthward transport of hot ions, their decoupling from electron motion due to diamagnetic drifts, and the formation of strong polarization electric fields and ionospheric flows. SAPS/SAID are formed between the inner edge of electron and ion plasma sheet, close to the plasmapause. Although the magnetosphere-ionosphere coupling within SAPS/SAID is mostly attributed to the precipitation of <30 keV particles (plasma sheet electrons and low-energy ring current ions), the physics of SAPS/SAID at the equatorial magnetosphere likely involves the dynamics of energetic (>50keV) particles. To date, studies of energetic (>50 keV) particle precipitation into the low-altitude (ionosphere) within SAPS/SAID are very limited. This stems, in part, from the lack of energetic particle measurements on low-altitude satellites capable of measuring plasma flows, such as the Defense Meteorological Satellite Program (DMSP) and Swarm, the main observational datasets for SAPS/SAID investigations. Energetic particle precipitation may significantly alter the physics of magnetosphere-ionosphere coupling within SAPS. Relativistic electrons will extend the altitude range of ionization enhancement to <100 km, while energetic ion precipitation will extend its latitudinal range. We study energetic particle precipitation during SAPS using observations from the low-altitude ELFIN CubeSats in conjugation with near-equatorial THEMIS measurements. We combine those with low energy measurements from DMSP also at low altitude. We thus can investigate the full energy range of ion and electron precipitation along the entire flux-tube connected to SAPS/SAID.
Results
We examined the energetic (up to relativistic energies) electron and ion precipitation within the SAPS region. The comparison of DMSP and ELFIN measurements shows that SAPS includes (is embedded with) strong precipitation of [100,1000] keV ions within the region of ion scattering by magnetic field line curvature and strong precipitation of >100 keV electrons at the electron isotropy boundary (the transition region between outer radiation belt and the plasma sheet, with strong curvature scattering). Within the region of ion scattering by magnetic field line curvature, ELFIN also detected relativistic electron precipitation bursts, likely resulting from electron scattering by electromagnetic ion cyclotron (EMIC) waves. Within the uncertainty level of ELFIN and DMSP latitudinal profiles, [10,1000] keV ion (observed by DMSP and ELFIN) and [100,3000] keV electron precipitation (observed by ELFIN) are likely inside the main SAPS region.
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| Figure 1. Comparison of the density profile (from DMSP measurements) and integrated energy flux of precipitating electrons and ions (from ELFIN measurements) within SAPS region identified from DMSP and THEMIS measurements. Electron isotropy boundary IBe (region of energetic electron scattering by magnetic field line curvature effect) and EMIC-driven precipitation regions are indicated. |
Conclusion
Magnetosphere-ionosphere coupling and dynamics are modified by precipitating ion and electron energy fluxes that alter the ionosphere's characteristics. Sub-auroral polarization streams (SAPS) are a classical example of this coupling, encompassing precipitation of plasma sheet (<30 keV) particles, intensification of field-aligned currents, and ionospheric feedback in the form of enhanced electric fields. This study demonstrates, for the first time, that SAPS are associated with precipitation of energetic ions (50-1000 keV) and relativistic (up to a few MeV) electrons. The 50-1000 keV ions may dominate the precipitating energy flux at low latitudes, where scattering of plasmasheet ions is significantly reduced. Relativistic electron precipitation significantly extends ionization enhancement effects to altitudes below 100 km, where these electrons deposit their energy.
References
Artemyev, A. V., Zou, Y., Zhang, X.-J., Meng, X., and Angelopoulos, V. (2024). Energetic particle precipitation in sub-auroral polarization streams. Geophysical Research Letters, 51, e2023GL107731, doi:10.1029/2023GL107731Biographical note
Anton Artemyev is a researcher in the Department of Earth, Planetary, and Space Sciences at UCLA. His research focuses on various aspects of magnetotail dynamics and magnetosphere-ionosphere coupling, including effects of energetic particle precipitations into the atmosphere due to various scattering mechanisms operating within the equatorial magnetosphere.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu