Drifting ELF/VLF Emissions Observed at Low Latitude Ground Station During Geomagnetic Storm

We have reported for the first time total seven strong events of drifting ELF/VLF discrete emissions observed on 28th–29th April, 1990 in the pre-midnight sector at Varanasi (Geomag. lat. 14°55′N, long. 154°E, L = 1.07). The events exhibit a regular increasing as well as decreasing frequency drifts and are mainly discrete periodic emissions of riser, faller and hook types observed during a geomagnetic storm period, with minimum Dst-index −98 nT and K p -index ≥ 5. The frequency drift in ELF/VLF emissions at low latitudes seems to be a rare phenomenon. The repetition period and the frequency drift rate have been evaluated for all the recorded events. The frequency drifts have been interpreted in terms of a combined effect of L-shell drift of interacting energetic electrons and the change in convection electric fields during the storm developments. The computed maximum spectral power density ⟨B_f^2⟩_max of the wave varies between 1.8 × 10 −21 and 4.08 × 10 −22 Gauss 2 /Hz, whereas frequency drift rates are in agreement with the observed values.