Reconsidering the inverted-V particle signature: Relative frequency of large-scale electron acceleration events

Abstract

The shape of auroral zone large-scale electron acceleration events is widely described as an inverted V, with the spectral peak gradually rising to and falling from the spatially central peak. We investigated the relative frequency of all sharp (≥3) electron energy flux enhancement events which are large-scale enough to be resolvable in the DMSP F7 particle data set. The first three days of each month of 1984 were studied, amounting to 355 passes through the nightside oval (restricted to the range 2000-2300 magnetic local time (MLT)) with a total of 1049 such events. Only 42 of these 1049 events actually fit the inverted-V morphology, with another 76 cases fitting an extremely loose definition of an inverted V. Thus, between 4.0 and 11% of all sharp energy flux enhancements are inverted Vs, depending on the strictness of the criteria. In that subset of events for which electrostatic acceleration is likely, namely those which have monoenergetic peaks and which extend over 20 km or more, inverted Vs are still just 10-29% of the total. More common are events which have relatively constant "monoenergetic peaks" (accelerating potentials). A crucial feature common to all non-V forms with monoenergetic peaks is that the maximum accelerating potential (spectral peak) is observed near one edge or both. Thus very large gradients in the potential exist near the edges of large-scale auroral forms, implying much larger electric fields than does an inverted V. The present study thus supports earlier evidence that the important physics of auroral arcs is associated with the edge of the arcs, rather than the central portion of the acceleration region. The most common type of energy flux enhancement event is a broad spectral enhancement (suprathermal burst). Contrary to popular perception, suprathermal bursts are not always narrowly confined in latitude. All categories of sharp electron energy flux enhancement events studied here exhibited a sharp high-energy drop off near the spectral peak, suggesting acceleration of one type or another (either wave-particle or electrostatic). Copyright 2000 by the American Geophysical Union.