Preliminary results on equatorial spread F irregularities by VHF and HF radars

Abstract

Preliminary results obtained from a coordinated campaign involving simultaneous operation of VHF (53 MHz) and HF (18 MHz) radars, located at Gadanki and Trivandrum, on the structures and dynamics of equatorial spread-F irregularities are reported. Such kind of measurement involving 18 MHz radar is attempted, probably, for the first time. The structures as revealed by both VHF and HF radars are more or less similar. However, the Doppler velocities transverse to Earth's magnetic field of the 3 m irregularities as observed by VHF radar reveal a maximum velocity of 200/ms in the central regions of plume structures, while the corresponding velocities of 8.3 m irregularities observed by HF radar reveal a maximum of around 50/ms in contrast to the earlier expectations. This suggests significant dispersive nature indicating that different physical processes are operational in this wavelength regime. Further the observations by VHF radar revealed the plume structures which are generally found to be vertically erect in the altitude region of 300-450 km without having much tilt indicating that the shears in the zonal plasma drift are not significant in this altitude region during equinoctial period over Indian longitude.