AbstractBy examining the compression‐induced changes in the electron phase space density and pitch angle distribution observed by two satellites of Van Allen Probes (RBSP‐A/B), we find that the relativistic electrons (>2 MeV) outside the heart of outer radiation belt (L* ≥ 5) undergo multiple losses during a storm sudden commencement. The relativistic electron loss mainly occurs in the field‐aligned direction (pitch angle α < 30° or >150°), and the flux decay of the field‐aligned electrons is independent of the spatial location variations of the two satellites. However, the relativistic electrons in the pitch angle range of 30°–150° increase (decrease) with the decreasing (increasing) geocentric distance (|ΔL| < 0.25) of the RBSP‐B (RBSP‐A) location, and the electron fluxes in the quasi‐perpendicular direction display energy‐dispersive oscillations in the Pc5 period range (2–10 min). The relativistic electron loss is confirmed by the decrease of electron phase space density at high‐L shell after the magnetospheric compressions, and their loss is associated with the intense plasmaspheric hiss, electromagnetic ion cyclotron (EMIC) waves, relativistic electron precipitation (observed by POES/NOAA satellites at 850 km), and magnetic field fluctuations in the Pc5 band. The intense EMIC waves and whistler mode hiss jointly cause the rapidly pitch angle scattering loss of the relativistic electrons within 10 h. Moreover, the Pc5 ULF waves also lead to the slowly outward radial diffusion of the relativistic electrons in the high‐L region with a negative electron phase space density gradient.
