Auroral plasma turbulence and the cause of auroral kilometric radiation fine structure

Abstract

Electron holes excited in the auroral kilometric radiation (AKR) source region in presence of a very dilute cool electron background are interpreted as causing the observed fine structure in AKR radiation. Using high time and frequency resolution measurements of the FAST wave tracker, we demonstrate that a substantial part of the AKR emission consists of a large number of elementary radiation events that we interpret as traveling electron holes that may have resulted from the nonlinear evolution of electron acoustic waves and have the properties of Bernstein‐Green‐Kruskal modes. Estimates of the propagation velocity of these structures are in good agreement with theory. Power estimates show that each elementary radiation event may contribute ≃ 10 3–4 W of power to AKR implying that a moderately large number of elementary radiators is required to reproduce the total AKR emission. The elementary radiation structures are sometimes reflected from the acceleration potential or become trapped in larger structures like ion acoustic waves or ion holes. The observations indicate that the radiation efficiency is highest at the turning point where the velocity of the elementary radiators with respect to the reflector system vanishes. Monitoring the time variation of the frequency drift of the elementary radiators allows to qualitatively infer about the mesoscale motion of the AKR source region and the spatial extension of the mesoscale field‐aligned electric potential drops.