Stimulated emission of AKR in regions of a weakly unstable electron velocity distribution

Abstract

It is generally believed that auroral kilometric radiation (AKR) originates in regions in which energetic auroral electrons possess a velocity distribution with a loss‐cone feature. According to the cyclotron maser theory of AKR, the resulting instability can be substantial in some regions of the auroral zone, leading to the generation of radiation with many features that are consistent with the observations. However, in situ measurements of velocity distributions obtained with satellites only show evidence of moderate positive gradients in the perpendicular velocity component υ⊥. Such distributions prove to be only marginally unstable, and thus imply a low level of radiation. From this point of view it is of interest to study the amplification of externally generated radiation. This radiation might originate from adjacent regions with a more unstable velocity distribution or from remote sources in space. In this work we perform numerical simulation studies using alternatively an observed electron velocity distribution, obtained from DE 1 measurements, and an idealized loss‐cone distribution. The results show the expected amplification of X mode radiation as well as effects which go beyond the behavior of a simple amplifying medium. Whereas wave growth is exponential for the idealized loss‐cone distribution, it is initially linear in time in the weakly unstable case, following the increase of the external wave power in the simulation. At later stages the collective properties of the plasma lead to interesting effects, such as the continuation of wave growth after cessation of the external wave in cases which do not exhibit growth otherwise, and a threshold power level before growth occurs. We suggest that these phenomena are relevant to the understanding of the observed enhancement and triggering of AKR due to solar radio bursts.