HF doppler observations on the occurrence of equatorial spread-F

Abstract

HF doppler observations of the vertical drift velocity and group height of the 5"5 MHz plasma frequency level of the post-sunset bottomside F-region obtained on a few ESF (equatorial spread-F) and nOn-ESF days at Trivandrum are presented. The results show that on the non-ESF days, the maximum group height attained is about 400 km and the maximum velocity is less than 30 m/see. On the ESF days, however, the corresponding values are found to be in the range of 400-650 km and 30--50 m/see. The ESF onset is found to be significantly delayed relative to the velocity peak indicating that it is more closely linked to the layer height than to the drift velocity.