Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India

The variation in nighttime ionospheric E- and F-region critical frequencies (foEs and foF2) is examined for the months of August 1988, September 1993 and May 1997 in which major earthquakes (6<M<6.6) occurred in Indian seismic zones of Bihar-Nepal border, Latur in West India, and Jabalpur in Central India. The ionospheric data are obtained from Ahmedabad (latitude 23.0 1 degrees N, longitude 72.36 degrees E). The percent deviations of foF2 and foEs from their monthly median are determined for pre-midnight (18:00-00:00 h LT) and post-midnight (00:0006:00 h LT) periods and studied over a span of 20 days before the occurrence of earthquakes. The results show that foF2 are reduced in both the time sectors prior to the occurrence of main shocks. In pre-midnight sector the reduction is between 24 and 30%, 0 to 4 days before the main shocks and in the post-midnight sector it is between 18 and 26%, 1 to 15 days before the main shocks. The foEs show enhancements by 100 to 155% during the same periods. The effects of magnetic storms (KP and Dst variations) on the data are identified clearly but they do not vitiate the effects of earthquakes. The earthquake related anomalies are interpreted in terms of electromagnetic coupling between the lithosphere and ionosphere during earthquakes preparation processes.