Occurrence of type I plasma waves in the equatorial electrojet during morning and evening hours

Abstract

Type I waves are generally observed under strong electrojet (EEJ) conditions because of the threshold condition on the electron drift velocity (with respect to ions) for their generation. In this paper we present observation of type I waves during morning and evening hours under weak (as revealed from the low values of mean drift velocity of type II waves and also from low ground magnetic field values) EEJ conditions. In these conditions, type I waves are observed when the plasma turbulence in the EEJ as revealed by the width of the Doppler spectra is high even though the electron drift velocity as given by the mean Doppler frequency is much smaller than the ion acoustic velocity. This suggests that there are regions within the volume probed by the radar wherein the electron drift velocity exceeds the ion acoustic velocity Cs (even though the mean electron drift velocity is very much less than Cs) due to the irregular electric fields of the primary irregularities.