2024 THEMIS SCIENCE NUGGETS
Rapid Transport of Energetic Electrons to Low $L$-shells: the Key Role of Electric Fields
Anton Artemyev
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles
Introduction
Energetic electrons in the Earth’s inner magnetosphere largely originate from plasma sheet electrons, which are injected around midnight beyond the geostationary orbit, at L>7. These injected electrons then drift azimuthally around the Earth and are diffusively transported to lower L-shells due to scattering by ultra-low-frequency waves. The rate of this diffusive transport rapidly decreases with decreasing L-shell. This effect, combined with the effective scattering and losses of energetic electrons by plasmaspheric whistler-mode hiss waves, forms the inner edge of the outer radiation belt around the plasmapause, at L~5, with much smaller energetic electron fluxes in the slot region and the smaller, more stable inner belt. During strong substorms, however, energetic electrons can be transported deep into the inner magnetosphere, reaching the slot region or even smaller L-shells. This transport is usually too fast to be explained by radial diffusion.
Near-equatorial spacecraft observations of the radial electron transport to low L-shells are limited by the time resolution in tracing electron flux radial dynamics: the orbital period of these spacecraft is usually several hours, allowing the monitoring of the entire radial profile of electron fluxes only with a few hours' resolution. This limitation may be mediated by low-altitude spacecraft, which can cross the entire inner magnetosphere within a few minutes and collect an almost instantaneous radial profile of energetic fluxes. Moreover, the large fleet of low-altitude Operational Environmental Satellites (POES) can improve the time resolution between successive collections of radial electron flux distributions. The Defense Meteorological Satellite Program (DMSP) satellite fleet offers similar advantages, providing better temporal and spatial (latitude) resolutions of ionosphere electric field distributions, which project to the equatorial plane and drive electron radial transport. The primary limitation of POES measurements of energetic electron fluxes, namely low energy resolution, can be addressed by using data from the low-altitude Electron Losses and Fields Investigation (ELFIN) CubeSat, which provides high energy resolution measurements of energetic electrons. This study aims to investigate the radial transport to low L-shells of energetic electrons and the associated spatial and temporal distribution of electric fields using low-altitude ELFIN and DMSP measurements in combination with equatorial THEMIS measurements.
Results
We investigate the fast and radial transport of energetic electrons into the slot region, extending even deeper into the inner radiation belt. The basic scenario explaining such transport is shown in schematic Figure and can be summarized as (1) During storm time, the substorms plasma sheet injections form strong electric field regions at sub-auroral latitudes; this field is detected as enhanced ionospheric drift measured by DMSP spacecraft and confirmed by equatorial THEMIS observations. (2) These sub-auroral flows are azimuthal but connect to the equatorward flows near midnight; DMSP and SuperDARN show penetration of weaker flows to latitude as low as 55o. (3) Electric fields associated with observed ionospheric drifts are projected along magnetic field lines to the equatorial plane and drive radial transport of energetic (<300keV) electrons. (4) This transport is sufficiently fast to overcome electron losses by hiss waves, traditionally forming electron flux minimum within the slot region. (5) When the electric field penetrates to sufficiently low latitudes, the associated ExB drift can transport energetic electrons into the inner radiation belt
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| Figure 1. Schematic view of the scenario of the radial transport to low -shells: (1) plasma sheet injection forms strong electric field in the sub-auroral latitudes, near the plasmasphere, (2) this electric field penetrates to low latitudes and project along magnetic field lines to the equator, (3) ExB drift provides fast transport of energetic electrons into the plasmasphere and inner radiation belt. |
Conclusion
The dynamics of the outer radiation belt are traditionally associated with wave-particle resonant interactions, which provide local electron acceleration and losses through very low-frequency waves, and electron radial transport by ultra-low frequency waves. However, these processes cannot explain observations of rapid radial transport of energetic electrons (on a time-scale of a couple of hours), deep into the inner magnetosphere (down to L~2, mapping to 45o magnetic latitude in the ionosphere). This transport is likely associated with strong convection electric fields forming around the plasmapause. To investigate these rapid, low-latitude electron transport, we combine low-altitude observations of energetic electron fluxes by the ELFIN CubeSats and DMSP satellites, SuperDARN measurements of ionospheric plasma flows (electric fields), and equatorial measurements of energetic electrons and electric fields by THEMIS. Our findings demonstrate that the rapid filling of the slot region by <300 keV electrons is directly associated with electric field penetration to low latitudes (down to 40-50o). The proposed electron transport scenario, the direct penetration of energetic electrons by strong, localized electric fields, underscores the importance of ionosphere-magnetosphere coupling in radiation belt dynamics.
References
Artemyev, A., Nishimura, Y., Angelopoulos, V., Zhang, X.-J., and Bortnik, J. (2024). Rapid transport of energetic electrons to low L-shells: The key role of electric fields. Journal of Geophysical Research: Space Physics, 129, e2024JA033136. doi:10.1029/2024JA033136Biographical 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