Rapid Injections of MeV Electrons and Extremely Fast Step‐Like Outer Radiation Belt Enhancements

Rapid injection of MeV electrons associated with strong substorm dipolarization has been suggested as a potential explanation for some radiation belt enhancement events. However, it has been difficult to quantify the contribution of MeV electron injections to radiation belt enhancements. This paper presents two isolated MeV electron injection events for which we quite precisely quantify how the entire outer‐belt immediately changed with the injections. Tracking detailed outer‐belt evolution observed by Van Allen Probes, for both events, we identify large step‐like relativistic electron enhancements (roughly 1 order of magnitude increase for ∼2 MeV electron fluxes) for L ≳ 3.8 and L ≳ 4.6, respectively, that occurred on ∼30‐min time scales nearly instantaneously with the injections. The enhancements occurred almost simultaneously for 10s keV to multi‐MeV electrons, with the lowest L of enhancement region located farther out for higher energy. The outer‐belt stayed at these new levels for ≳several hours without substantial subsequent enhancements. Plain Language Summary Since the discovery of the Earth's radiation belts, the populations of energetic particles trapped by the magnetic fields, understanding the origin of radiation belt relativistic electrons has been one of the primary goals in space physics. One proposal for their source is that relativistic electrons are rapidly injected from outside the radiation belts associated with dramatic changes of the background magnetic field. To explore the importance of this process, we conduct a case study of two isolated injection events. With a new analysis technique, we quite precisely determine how the entire radiation belt electron population immediately changed with the injections. For both events, we find large, step‐like relativistic electron enhancements in a significant part of the outer radiation belt. The enhancements occurred promptly with the injections on extremely fast time scales that are comparable to the injection process. Furthermore, they occurred almost simultaneously for a wide energy range, showing an energy‐dependent feature that can be explained by injection process. After the enhancements, the outer‐belt relativistic electron intensities stayed at the enhanced levels for at least several hours without substantial subsequent enhancements, indicating that the enhancements occurred discretely with the injections. Key Points Quantitative determination of prompt and global outer‐belt response