Evolution of electron pitch angle distribution due to interactions with whistler mode chorus following substorm injections

Abstract

During substorms, pitch angle distribution (PAD) of freshly injected plasma sheet electrons can develop into the typical pancake distribution from the nearly isotropic distribution, which has previously been suggested as the result of resonant scattering driven by electrostatic cyclotron harmonic (ECH) and whistler mode waves in the equatorial regions. In this paper, using the two-dimensional bounce-averaged Fokker-Planck equation, we present a quantitative analysis of the electrons PAD evolution at L = 6 due to interactions with upper (ω/Ωe/ > 0.5) and lower (ω/Ωe/ < 0.5) band chorus based on the observed wave characteristics during substorms. It is found that the upper band chorus can efficiently scatter the electrons with energies 0.1-2 keV into loss cone and drive the electrons with energies above 2 keV toward loss cone. The lower band chorus can only cause precipitation loss of the electrons with energies above 1 keV by resonant scattering. The PADs of electrons with energies above 0.1 keV can develop into the pancake-shaped distributions due to the combined resonant scattering by upper and lower band chorus, and the pancake index PI, defined as the flux ratio between 90° and 70°, can reach 6 after 5 hours since substorm injections. The PI variation timescale and amplitude are consistent with previous statistical results. The numerical results suggest that the resonant scattering of electrons by the whistler mode chorus waves can be a substantial mechanism responsible for the formation of a pancake distribution outside L = 6. Besides, the rapid precipitation of electrons (∼keV) driven by resonant scattering of chorus waves may also be responsible for the diffuse aurora. Copyright 2009 by the American Geophysical Union.