Abnormally low F-layer ionization over Ahmedabad during the night of 26-27 November 1984

A case study of the critical frequency of F2 layer (foF2) over Ahmedabad (Geog latitude 23°N, longitude 72.4°E) during 17-30 November 1984, a geomagnetic quiet period, has been carried out. An interesting feature observed is very low values of foF2 (2.3 MHz) around 2100 hrs LT on 26 November. The quarter hourly ionograms during the night of 26-27 November 1984 were subjected to true height analysis to study the features of F-layer. The variations of hm show rapid rise of about 100 km at 2100 hrs LT followed by rapid fall of similar magnitude at 0100 hrs LT. The quasi-periodic variations with periods of about 90 min are also seen. The variation of the semi-thickness of F-layer also shows similar wave-like features. The wave-like perturbations of about 12 km in F-layer height and about 16 km in the semi-thickness of F-layer point to gravity waves in neutral winds and temperature as their source.