Observing drift compressional waves in the nightside ionosphere using the Ekaterinburg radar

Abstract

A review of researches related to ultra low frequency (ULF) waves studied using a midlatitude coherent decameter radar located near Ekaterinburg (EKB), Russia is presented. The radar was constructed similar to SuperDARN radars. During the experiment conducted in 2013–2015 three of its beams were surveyed successively with the integration time of 6 s providing time resolution of 18 s at each beam. In a few cases, data received in the nighttime ionosphere showed signatures of the drift compressional mode. In one of them, the wave exhibited clear dependence of frequency on azimuthal wave number m, which corresponds with the theory of drift compressional waves. In another case, merging of the drift compressional and Alfvén modes at some critical value of m was shown. A number of the observed waves’ frequencies were compared with the Alfvén ones inferred from the spacecraft data. The results showed that in a large part of cases waves had sub-Alfvénic frequencies. This finding complies with the assumption that at least a part of the waves observed with the radar should be identified with the drift compressional mode, whose frequency can be lower than frequency of field line resonance (FLR). Additionally, a case of simultaneous spacecraft and radar wave observation was presented. As the wave had poloidal structure, diamagnetic properties, and its frequency was considerably lower than the minimal frequency of the FLR, it was assumed that it was some kind of compressional mode, apparently the drift compressional wave.