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.
This paper points out certain discrepancies when the incoherent scatter radar observations at Jic~marca and the electron density models, (the Bent and IRI) were compared. In all cases the bottomside semi-thickness of the observed profile was significantly greater than that of both Bent and IRI models. The decay constants associated with the topside ionosphere for the observed profiles were generally greater than that of the two models, the difference being more pronounced in the I~! model. The results call for a revision of the models so that they accurately represent the electron density distribution at the equator.
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