Electron acceleration in the Van Allen radiation belts by fast magnetosonic waves

TLDR

Local acceleration is required to explain electron flux increases in the outer Van Allen radiation belt during magnetic storms. The study proposes that magnetosonic waves generated by unstable proton ring distributions transfer energy from the ring current to the Van Allen radiation belts. Fast magnetosonic waves accelerate electrons via Landau resonance, not Doppler‑shifted cyclotron resonance, because the waves propagate almost perpendicular to the magnetic field. Fast magnetosonic waves detected by Cluster 3 can accelerate electrons from ~10 keV to a few MeV, with pitch‑angle and energy diffusion rates comparable to whistler‑mode chorus, but they do not cause loss to the atmosphere because pitch‑angle diffusion does not reach the loss cone.

Abstract

Local acceleration is required to explain electron flux increases in the outer Van Allen radiation belt during magnetic storms. Here we show that fast magnetosonic waves, detected by Cluster 3, can accelerate electrons between ∼10 keV and a few MeV inside the outer radiation belt. Acceleration occurs via electron Landau resonance, and not Doppler shifted cyclotron resonance, due to wave propagation almost perpendicular to the ambient magnetic field. Using quasi‐linear theory, pitch angle and energy diffusion rates are comparable to those for whistler mode chorus, suggesting that these waves are very important for local electron acceleration. Since pitch angle diffusion does not extend into the loss cone, these waves, on their own, are not important for loss to the atmosphere. We suggest that magnetosonic waves, which are generated by unstable proton ring distributions, are an important energy transfer process from the ring current to the Van Allen radiation belts.