High time resolution measurement of multiple electron precipitations with energy‐time dispersion in high‐latitude part of the cusp region

The SS‐520‐2 sounding rocket skimmed over the high‐latitude part of the cusp region and observed fine‐scale field‐aligned electron precipitations in the vicinity of the inverted‐V structures with the Low Energy Particle‐Electron Spectrum Analyzer (LEP‐ESA). There are at least two types of fine‐scale electron precipitations, namely “edge‐type electron bursts” and “multiple energy‐time dispersions.” Edge‐type electron bursts were observed only at the edge of the inverted‐V region, whereas multiple energy‐time dispersions were observed separately from the inverted‐V region as well as within or overlapping it. The latter was characterized by field‐aligned precipitations with falling energies from ∼200 eV down to ∼20 eV at a repetition rate of 1–2 Hz. Source altitudes were estimated using the energy‐time and pitch angle‐time dispersions. As a result, we found that the source altitudes were distributed along the geomagnetic field at altitudes of several thousand kilometers, depending on the accelerated energies of electrons. Higher‐energy electrons are generated at higher altitudes. The source temperature of the energy‐time dispersion was much higher than that of ionospheric cold electrons. We suggest that electrons injected from the magnetosheath were accelerated by inertial Alfvén waves at altitudes of several thousands of kilometers.