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Abstract

Ultra-relativistic (>3 MeV) electrons are considered as a novel, separate population in the Earth's radiation belts since their loss and acceleration features are distinct from relativistic (∼MeV) electrons. The dominant acceleration mechanism of ultra-relativistic electrons remains a subject of ongoing debate. Some studies suggest that the acceleration mechanism of ultra-relativistic electrons is energy-dependent: local acceleration dominants the enhancement of ∼3–5 MeV electrons while two-step acceleration process leading by radial diffusion effects for ∼7 MeV electrons. However, a recent study (https://doi.org/10.1126/sciadv.abc0380) theoretically demonstrated that local acceleration could accelerate electrons up to >7 MeV directly under the extreme plasma depletion. In this study, we report four enhancement events of ultra-relativistic electrons that occurred in September and October 2017. Analysis of phase space density (PSD) radial profiles and contours demonstrate that local acceleration plays the dominant role in the enhancements of ∼7 MeV electrons in mid-September and mid-October, supported by persistently growing peaks in electron PSD. While enhancements of ultra-relativistic electrons in other two events still show energy-dependent phenomenon, our results provide the observation evidence that one-step local acceleration process can lead to the enhancement of ∼7 MeV electrons in some events. We suggest that the acceleration process of ultra-relativistic electrons may be subject to the efficiency of local acceleration.