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

Electron holes excited in the auroral kilometric radiation (AKR) source reg ion in presence of a very dilute cool electron background are interpreted a s 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 t hat may have resulted from the nonlinear evolution of electron acoustic wav es and have the properties of Bernstein-Green-Kruskal modes. Estimates of t he propagation velocity of these structures are in good agreement with theo ry. Power estimates show that each elementary radiation event may contribut e similar or equal to 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 elemen tary radiators with respect to the reflector system vanishes. Monitoring th e 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 held-aligned electric potential drops.