Energization of relativistic electrons in the presence of ULF power and MeV microbursts: Evidence for dual ULF and VLF acceleration

Abstract

We examine signatures of two types of waves that may be involved in the acceleration of energetic electrons in Earth's outer radiation belts. We have compiled a database of ULF wave power from SAMNET and IMAGE ground magnetometer stations for 1987–2001. Long‐duration, comprehensive, in situ VLF/ELF chorus wave observations are not available, so we infer chorus wave activity from low‐altitude SAMPEX observations of MeV electron microbursts for 1996–2001 since microbursts are thought to be caused by interactions between chorus and trapped electrons. We compare the ULF and microburst observations to in situ trapped electrons observed by high‐altitude satellites from 1989–2001. We find that electron acceleration at low L shells is closely associated with both ULF activity and MeV microbursts and thereby probably also with chorus activity. Electron flux enhancements across the outer radiation belt are, in general, related to both ULF and VLF/ELF activity. However, we suggest that electron flux peaks observed at L ∼ 4.5 are likely caused by VLF/ELF wave acceleration, while ULF activity probably produces the dominant electron acceleration at geosynchronous orbit and beyond.