Radar observations of 2.8 m equatorial spread-F irregularities

Abstract

VHF radar observations made of two intense equatorial spread-F events at Gadanki (geographic 13.5 °N, 79.2 °E; geomagnetic 6.3 °N) are presented in the form of Doppler spectra and height-time variations of signal intensity, Doppler velocity and spectral width. The Doppler spectra are found to be discrete and narrow during the early phase of instability evolution and fairly broad at a later stage of evolution, involving rising plasma bubbles and turbulent irregularities. The most intense backscattered signals are associated with the plume structures with peak power values at 42 dB above the noise level. The Doppler velocities, presented only for the bottomside in view of the restricted Doppler window, are found to be in accord with the slopes observed of the scattering structures in the height-time-intensity plots. The drifts are predominantly downward with a peak value of about 70 m s^-1 which is significantly above that of the background plasma. The downdrafting structures in both the events are found to descend all the way to the E region, an unusual feature not seen in the RTI plots observed over the magnetic equator. The spectral widths are of the order of 10-20 m s^-1 during the initial phase and exceed 100 m s^-1 at a later stage of the spread-F evolution.