Source of the Bursty Bulk Flow Diffuse Aurora: Electrostatic Cyclotron Harmonic and Whistler Waves in the Coupling of Bursty Bulk Flows to Auroral Precipitation

Abstract

Electron cyclotron harmonic (ECH) and whistler chorus waves are recognized as the two mechanisms responsible for the resonant wave-particle interactions necessary to precipitate plasma sheet electrons into the ionosphere, producing the diffuse Aurora. Previous work has demonstrated ECH waves dominate electron scattering at L shells >8, while whistler chorus dominates scattering at L shells L < 8. However, we find from Time History of Events and Macroscale (THEMIS) Interactions during Substorms observations of fast flows at L = 12 that oblique whistler chorus emissions play the dominant role in scattering electrons. Previous works have identified whistler-mode waves within fast flows that are produced by an electron temperature anisotropy Te,⊥/Te,> 1, consistent with electron betatron acceleration. Here, however, we find whistler chorus emissions throughout an interval of fast flows where Te,⊥/Te,< 1. Parallel electron beams account for the enhanced parallel electron temperature and serve as the instability mechanism for the whistler chorus. The parallel electron beams and associated cigar-shaped distributions are consistent with Fermi acceleration at dipolarizations in fast flows. We demonstrate that the scattering efficiency of the whistler chorus exceeds that of ECH waves, which THEMIS also detects during the fast flows. The obliquity of the whistler waves permits efficient scattering of lower-energy electrons into the diffuse aurora. We conclude that Fermi acceleration of electrons provides one important free-energy source for the wave-particle interactions responsible for coupling plasma sheet electrons into the diffuse aurora during substorm conditions. ©2019. American Geophysical Union. All Rights Reserved.