Low-density plasma as a key catalyst for electron acceleration in the Van Allen radiation belts

The near-Earth space environment is populated by the most energetic electrons with velocities very close to the speed of light, reaching ultra-relativistic energies. These electrons present a serious hazard to the Earth-orbiting spacecraft and are referred to as the Van Allen radiation belts. The question of how these particles are accelerated to such energies is still unanswered. Examining the 20 April 2017 geostorm, we show that such acceleration is achievable only under extremely low plasma density conditions. The global model of radiation belts with a statistical model of plasma density fails to produce the acceleration to such high energies, whereas the model with observed plasma density variations accurately reproduces the observed acceleration at all radial locations and energies. This study demonstrates that electrons are accelerated to multi-MeV by taking energy from plasma waves when the conditions for such acceleration are preferential. It also reveals the intricate interplay between cold plasma and the enhancements of ultra-relativistic electrons that are millions of times more energetic than plasma particles. Similar acceleration may occur in planetary radiation belts, for lab plasmas, at exoplanets, and in other magnetized astrophysical objects.

Abstract licence: CC BY-NC-ND

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