Abstract
We present a detailed analysis of three electromagnetic wave events observed by the Cluster Wide-Band Data instruments at frequencies of a few kilohertz, which exhibit a periodic time modulation of the wave intensity (quasiperiodic emissions). The events were measured when crossing the geomagnetic equator at radial distances of about 3-4 R-E during the close separation campaign in July-October 2013.
Coincident compressional magnetic field pulsations with the magnitude on the order of a few tenths of nanotesla were detected in two events. Their period corresponded to the modulation period of the quasiperiodic emissions in one event, while it was about double in the other.
A possible explanation for this discrepancy between the two periods is given. Although Cluster 1, Cluster 3, and Cluster 4 were close to each other, Cluster 2 was located by more than 0.5 R-E closer to the Earth.
The same quasiperiodic modulation was observed by all Cluster spacecraft, but the emissions detected by Cluster 2 had a noticeable time delay. This can be explained by the wave propagation from the generation region located at larger radial distances, close to the plasmapause.
The large range of L shells where the emissions are detected thus appears to be due to the unducted propagation from a spatially limited source. The frequency-time structure of individual elements forming the events is evaluated, and it is shown that the time dependence of the intensity modulation can be approximated by a simple model.