Multi-Event Studies of Sudden Energetic Electron Enhancements in the Inner Magnetosphere and Its Association With Plasmapause Positions
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Sudden enhancements of energetic electrons (tens of keV to <500 keV) at low L-shells have been frequently observed by Van Allen Probes. It is, however, still unclear how deep these electrons can reach and what the responsible mechanisms are. Here we seek to determine the inner boundary of initial enhancements of these energetic electrons and its link with the plasmapause location to offer a new, insightful perspective on this topic. Previous statistical studies have revealed a remarkably consistent observation that initial enhancements of energetic electrons occur mostly at higher L-shell than the innermost plasmapause locations (L-pp), though a few outlier events were also identified. In this study, we examine the identified outlier events where electron enhancements were reported inside the plasmapause (either in situ L-pp or simulated innermost L-pp). More detailed investigation indicates that the appearance of enhancements inside the in situ L-pp is often associated with duskside observations where the actual initial enhancement likely occurs somewhere else, outside the innermost plasmapause location. We thereby urge caution when using the relationship between the observed enhancement locations and the in situ L-pp to infer the effect of electric fields on energetic electrons. In summary, our findings suggest that the innermost L-pp remains the innermost limit of the initial sudden enhancements of energetic electrons. Combining with phase space density analysis, our result implies the role of a large-scale electric field that leads to sudden electron enhancements and the erosion of the plasmasphere.