Rapid outward extension of electron radiation belt driven by substorm injection and chorus waves

The electron radiation belts undergo drastic changes on a variety of timescales ranging from minutes to decades. Here we concentrate on the rapid evolution of outer electron radiation belt on a timescale of hours observed by RBSP and THEMIS satellites on 26 May 2013. The outer boundary of the electron radiation belt moved from L=5.5 to L>6.07 during about 6 hours. Near the outer boundary (L=6.0), the 30keV-5MeV electron fluxes increased by up to four orders of magnitude. We propose a two-step scenario involving substorm injection and the subsequent chorus-driven acceleration to explain this rapid extension event. The substorm injection alone can cause 100% and 20% of the total subrelativistic (<0.2MeV) and relativistic (2-5MeV) electron flux enhancements within a few minutes. Such injection process was sequentially recorded by the two radially displaced RBSP satellites. The subsequent stochastic acceleration by chorus waves can yield 60%-80% of the total energetic (0.2-5.0MeV) electron flux enhancement within about 6 hours. The strong chorus waves were simultaneously detected by the RBSP and THEMIS satellites, and such local acceleration process is supported by the data-driven STEERB simulations. The current results clearly illustrate the respective importance of substorm injection and chorus waves for the evolution of radiation belt electrons at different energies on a relatively short timescale.