Some recent developments in understanding auroral electron acceleration processes

Abstract

Discrete auroral arcs and the associated inverted-V-type electron precipitation are accompanied by potential structures, upgoing ion beams and plasma density depletions in the acceleration region altitude and above. However, exactly how these phenomena depend on altitude in various Kp and solar illumination conditions as well as at various magnetic local time (MLT) and invariant latitude (ILAT) is not so well known. We review the main results of our recent large statistical studies of the altitude dependence of these parameters using Polar data. We also include similar statistical results on some additional parameters that appear to have major importance in inverted-V auroral acceleration. These parameters include middle-energy (/spl sim/50-500 eV) electron anisotropies, certain types of electrostatic wave bursts as well as ion shell distributions. One of the most interesting results of the statistical studies is that many of the parameters mentioned change their behavior rather abruptly at around 4 R/sub E/ radial distance. We draw the conclusion that the region for auroral energization processes seems to take place within a closed potential structure below /spl sim/3-4 R/sub E/. These conclusions lead to a new scenario for auroral plasma physics and the energy flow in the auroral acceleration process. The observational results are in agreement with simulations that we also summarize in this paper.