Statistical Study of the Energetic Electron Microinjections at the High‐Latitude Magnetosphere

Understanding the formation of the seed population for the energetic electrons trapped within the Earth's Van Allen radiation belts has been under debate for decades. The magnetic reconnection in the Earth's magnetotail during the substorms is the main process of accelerating the electrons to the tens to hundreds of keV. These electrons are further injected toward the radiation belts, where they get further accelerated to relativistic energies. Recently, it has been suggested that another source could come from the dayside diamagnetic cavities where electrons and ions can be locally energized to hundreds of keV energies. It has been shown that the physical mechanism within the cavities can create a strong acceleration perpendicular to magnetic field, which can lead to temperature anisotropy and drift mirror instability. The electron fluxes localized within the troughs of the mirror mode waves exhibit the counter‐streaming “microinjection” signature. To investigate the origin of microinjections and their dependence on solar wind conditions, here we have performed an event search and a statistical study of their properties encompassing a total of ∼165 hr (47 microinjection events) of Magnetospheric Multiscale observations at the pre‐dusk sector high‐latitude boundary layer. The ultralow frequency range magnetic field fluctuations coincided with the counter‐streaming energetic electron fluxes. For most events, the interplanetary magnetic field was duskward and anti‐sunward; over 60% of these microinjections satisfy the criteria of the drift mirror instability, which indicates the temperature anisotropy could play an important role for the microinjection. Plain Language Summary Studying the source and acceleration mechanisms of the high‐energy particles is crucial for understanding the radiation environment in the near‐Earth space. The Van Allen radiation belts, with the shape of donuts around the Earth from an altitude of 0.2–2 RE (inner belts) and 3–8 RE (outer belt), are formed by trapped energetic particles. Several mechanisms are proposed as seed population for the radiation belt electrons which can be then accelerated to relativistic energies. Recently dayside structures at the high‐latitude magnetosphere called diamagnetic cavities are suggested to be one of the possible sources both for the high‐energy particles (ten to hundreds of keV) and the temperature anisotropy to the magnetosphere. A recent Magnetospheric Multiscale observation has shown that th