Predominance of Ech Wave Contribution to the Diffuse Auroral Precipitation in Earth’s Outer Magnetosphere
Thursday, 18 December 2014
Due to its importance for global energy dissipation in the ionosphere, the diffuse aurora has been intensively studied in the past 40 years. Its origin (precipitation of 0.5-10 keV electrons from the plasma sheet without potential acceleration) has been generally attributed to whistler-mode chorus wave scattering in the inner magnetosphere (R<~8 RE), while the scattering mechanism beyond that distance remains unresolved. By modeling the quasi-linear diffusion of electrons with realistic parameters for the ambient magnetic field, loss-cone size and electrostatic electron cyclotron harmonic (ECH) wave intensity (obtained from THEMIS observations as a function of magnetospheric location), we estimate the loss-cone filling ratio and ECH wave-induced electron precipitation systematically throughout the entire data set, from 6 RE out to 31 RE (the THEMIS apogee). By comparing the wave-induced precipitation directly with the equatorially mapped energy flux distribution of diffuse aurora from ionospheric observations (OVATION Prime model) at low altitudes, we quantify the contribution of auroral energy flux from ECH wave scattering. Although the wave amplitudes drop, as expected, with distance from the Earth, due to the small loss cone size and stretched magnetic field topology, ECH waves are still capable of causing sufficient scattering of plasma sheet electrons to account for the observed diffuse auroral dissipation. Our results demonstrate that ECH waves are the dominant driver of diffuse aurora in the outer magnetosphere (beyond ~8 RE).