Energization of radiation belt electrons by ring current ion driven ULF waves

Abstract

In this paper we examine the hypothesis that MeV energy radiation belt electrons can be energized by ULF waves, the ULF waves themselves being excited by drift‐bounce resonance with energetic ring current ions. We show that this energization is most likely to occur during depleted plasmatrough conditions and when the plasmapause lies on a low L‐shell. For example, when the plasmasphere has been eroded so that the plasmapause lies at L = 3, then efficient energization for electrons with energies >1 MeV at L ≃ 3–4 can be caused by ULF waves generated by the N = 2 drift‐bounce resonance with either ≃ 10–15 keV H+, or ≃ 100–300 keV O+, ring current ions respectively. We illustrate this transfer of energy from the ring current to the radiation belt by considering a radiation belt electron drift resonance with fundamental field‐aligned mode m ≃ +20 guided poloidal Alfvén waves, the waves being excited by westward drifting ring current ions. Our results are significant in that they offer a new paradigm for the ULF wave energization of MeV radiation belt electrons, suggesting that ring current ion driven ULF waves may play an important role in radiation belt energization by ULF waves. Given the prevalence of ∼100 keV O+ ions in the ring current at L ∼ 3–4 during storm‐times, and the storm‐time erosion of the plasmapause, the conditions required for efficient ULF wave energy transfer from the ring current to >1 MeV electrons in the radiation belt will be satisfied during storm‐times.