Electrons with energies below 100 keV constitute a significant part of the inner magnetosphere population. Electrons within 10-50 keV energy range that surround spacecraft can cause surface charging, which is a build-up of electricity on the outer components of the satellite (e.g., Matéo-Vélez et al., 2018). A sudden discharge can result in material damage and/or an electromagnetic disturbance around the spacecraft. These electrons are the seed population for the radiation belts, being accelerated to MeV energies by various processes in the Earth’s inner magnetosphere (e.g., Jaynes et al., 2015). They precipitate into the upper atmosphere (causing auroral emissions) (Artemyev et al., 2024), and the distribution in space, time, and energy of these electrons plays a big part in the ionization and heating that occurs in the high- and mid-latitudes in the ionosphere. The electron energy determines how far the electron can penetrate, for example, 30 keV – 1 MeV electrons can reach altitudes of 50-90 km. Electron precipitation can lead to enhancements in the atmospheric neutral composition at high polar latitudes which results in production of HOx and NOx (e.g., Chapman-Smith et al., 2023). They are important species in chemical reactions playing a key role in the ozone balance of the mesosphere and stratosphere, respectively.
At present, the behavior of keV electrons is a puzzle in many ways. Their fluctuations occur on a time scale of minutes and their fluxes are local time (MLT)-dependent (e.g., Dubyagin et al., 2024). For this reason, studying them using daily/orbit averaging – as it is often done for MeV electrons – misses much of their variations (e.g., Boynton et al., 2019; Wu et al., 2023). Increasing geomagnetic activity does not necessarily lead to the enhancements of their fluxes. Widely accepted understanding that geomagnetic storms are associated with increases in energetic particle fluxes in the inner magnetosphere is not valid for keV electrons (e.g., Ganushkina et al., 2021). Changes of 2-3 orders of magnitude in fluxes have been observed during smaller disturbances (e.g., isolated substorms) but without any clear dependence on other measures of the substorm strength.
For keV electrons, there are no simple answers as to which of the solar wind parameters drive their variations. A better understanding of keV electron flux behavior is needed. While a prediction model may hint at the drivers and mechanisms (e.g., Simms et al., 2022), no matter how well it may forecast (Simms et al., 2023; Swiger et al., 2023), it is not a valid tool for effectively testing hypotheses about physical drivers. The models struggle to reproduce the fast-time-scale variations of keV electrons (Perlongo et al., 2017; Denton et al., 2019; Castillo et al., 2019; Swiger et al., 2022; Simms et al., 2022, Yu et al., 2022). If there is no clear picture about what controls keV electrons in the inner magnetosphere, it is very hard to find definite dependencies on drivers for the precipitating component. For example, what are the conditions when the ionization can increase due to the increased precipitation of keV electrons and are there any dependencies on solar wind parameters for HOx and NOx? We don’t know the answer to such questions.
References
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