Strong pitch-angle scattering of energetic electrons in the presence of electrostatic waves above the ionospheric trough region

Abstract

Simultaneous measurements of energetic electrons and electrostatic waves in the midlatitude trough region indicate that events characterized by strong pitch-angle diffusion in the altitude range 2000-4500 km occur in the presence of ELF electrostatic waves attributed to a current-driven ion cyclotron instability. Scattering by the electrostatic waves is sufficient to account for the strong pitch-angle scattering commonly observed by low-altitude satellites. The resulting precipitation creates a sharply defined, high-latitude boundary for energetic electrons as observed by low-altitude satellites. This boundary is similar to the feature generally referred to as the high-latitude trapping boundary. However, the precipitation boundary occasionally occurs on closed field lines at a significantly lower latitude than the trapping boundary. When present, the precipitation boundary is most sharply defined in the nighttime hemisphere.