Multi-point Cluster Observations of VLF Risers, Fallers, and Hooks at and near the Plasmapause.
Résumé
The four Cluster Wideband (WBD) plasma wave receivers occasionally observe electromagnetic triggered wave emissions at and near the plasmapause. We present the remarkable cases of such observations. These triggered emissions consist of very fine structured VLF risers, fallers and hooks in the frequency range of 1.5 to 3.5 kHz with frequency drifts for the risers on the order of 1 kHz/s. They appear to be triggered out of the background whistler mode waves (hiss) that are usually observed in this region, as well as from narrowband, constant frequency emissions. Occasionally, identical, but weaker, emissions are seen to follow the initial triggered emissions. When all the Cluster spacecraft are relatively close (<800 km, with interspacecraft separations of around 100–200 km), the triggered emissions are correlated across all the spacecraft. The triggered emissions reported here are observed near the perigee of the Cluster spacecraft (around 4–5 RE) within about 20 degrees, north or south, of the magnetic equator at varying magnetic local times and generally at times of low to moderate Kp. In at least one case they have been observed to be propagating toward the magnetic equator at group velocities on the order of 5−9×107 m/s. The triggered emissions are observed in the region of steep density gradient either leading up to or away from the plasmasphere where small-scale density cavities are often encountered. Through analysis of images from the EUV instrument on board the IMAGE spacecraft, we provide evidence that Cluster may sometimes be immersed in a low density channel or other complex structure at the plasmapause when it observes the triggered emissions. Examples of the various types of triggered emissions are provided which show the correlations across spacecraft. Supporting density data are included in order to determine the location of the plasmapause. A nonlinear gyroresonance wave-particle interaction mechanism is discussed as one possible generation mechanism.