Anomalous enhancement of ionospheric F2 layer critical frequency and total electron content over low latitudes before three recent major earthquakes in China

This paper reports unusual variations in the ionospheric total electron content (TEC) and the critical frequency of the F2 layer ( foF2) a few days before the main shock of three major earthquakes (M > 6). Epicenters of these earthquakes are distributed in China. Ionospheric data, recorded by an Indian network of ionosonde and GPS receivers at Delhi (28.6°N, 77.2°E), Bhopal (23.29°N, 77.46°E), and Trivandrum (8.4°N, 76.6°E) are analyzed to find seismoionospheric signatures of three recent major earthquakes in China. The results clearly indicate large enhancements in foF2 and TEC on geomagnetic quiet days, observed mainly at a low‐latitude station in Delhi, which is nearest to the earthquake epicenters. The anomalous behavior of foF2 and TEC appeared 1–4 days before the main shock and especially during 1100–1700 LT. However, the ionospheric foF2 and TEC variability at the equatorial station Trivandrum and equatorial anomaly station Bhopal did not show any significant changes, thereby indicating the localized nature of such unusual ionospheric variations. The observed results suggest that the unusual enhancements of ionospheric foF2 and TEC over Delhi a few days before the main shock of each earthquake are most likely due to a seismoionospheric link