Pitch angle diffusion in morningside aurorae: 1. The role of the loss cone in the formation of impulsive bursts of precipitation

Abstract

Pitch angle diffusion of energetic electrons is important in morningside diffuse aurorae, equatorward of the auroral oval, where the plasma is shielded from the influence of large‐scale electric fields. Morningside aurorae exhibit complex spatial and temporal structure. The temporal structure is addressed in a treatment which includes the principal elements of the feedback between pitch angle diffusion and VLF wave growth. A time‐dependent bounce‐averaged pitch angle diffusion equation is derived for the electron flux in and near the loss cone. The diffusion equation includes backscatter from the atmosphere. The atmospheric backscatter is represented by an empirical model in which the pitch angle distribution in the loss cone is approximated by a Taylor series in the sine of the pitch angle. The backscatter model is adjusted to match detailed computations performed with a Fokker‐Planck code for the scattering and slowing of energetic electrons in the atmosphere. Solutions of the pitch angle diffusion equation—both steady state and time‐dependent—are used to compute anisotropies and estimate growth rates of VLF whistler mode waves. The electrons' anisotropy is most sensitive to the diffusion rate near the transition from weak to strong diffusion, where the anisotropy may become so low that VLF wave growth is halted. The principal effect of loss cone filling during an episode of strong diffusion is a sharp increase in the minimum resonant energy for wave growth and a rapid decrease of the growth rate. The characteristic time scales for wave growth and quenching due to changes in the anisotropy are of the order of 0.1–1 s and 2–10 s, respectively. The interaction of growth and decay of waves leads to precipitation bursts on a time scale of 1–50 s. The theory explains many observations of impulsive electron precipitation throughout the outer trapping regions at energies above 20–25 keV. Whether the theory can explain auroral precipitation pulsations at energies below 25 keV without invoking other wave modes remains unresolved.