Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation

Abstract

[1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 RE do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects. Copyright 2005 by the American Geophysical Union.