Ultra low frequency emissions ranging from 0.1 to 3 Hz in circumpolar areas

Abstract

We examine the characteristics of oscillations of two types in the high-frequency edge of the ULF range (0.1–3 Hz), serpentine emission (SE), and discrete frequency dispersed signals (DS). Oscillations of both the types are observed in the polar caps exclusively with induction magnetometers. Since these instruments are currently practically absent at high latitudes, the analysis has been carried out from records obtained at the stations Vostok and Thule close to the geomagnetic poles in 1968–1971. The DS occurrence rate is shown to have a sharp peak at local magnetic noon. This fact indicates that DS emergence is rigidly tied to the geomagnetic field line passing through the observation station. At the same time, the seasonal variation in the frequency of DS occurrence has a main peak in local summer and an additional peak in local winter. We have revealed before that at least a part of DS is excited in the foreshock region. Taking this into account, we can assume that the wave packets incident to the magnetopause fall on the external field lines mainly in the noon region and propagate along these lines in both directions, eventually reaching Earth’s surface in the polar regions. Unlike DS, the SE occurrence rate has neither a daily nor a seasonal variation. We have tested and confirmed indirectly the hypothesis put forward earlier about the excitation of SE by cyclotron instability of protons in the solar wind, simulating frequency variations in ion-cyclotron waves at different levels of interplanetary plasma perturbation and comparing the results with the SE frequency variations observed under similar conditions. We conclude that it is necessary to resume continuous observations of ULF emissions, using induction magnetometers installed in polar caps near the projections of cusps and near geomagnetic poles.