Anomalous variation in total electron content (TEC) associated with earthquakes in India during September 2006–November 2007

Employing a GPS receiver we have monitored ionospheric total electron content (TEC) at Agra (Geographic Lat. 27.2°N, Long. 78°E), India since 24 June, 2006. In this paper we have analysed 15 months of data for the period between 01 September, 2006 and 30 November, 2007 statistically. The results show anomalous depletions and enhancements which are examined in the light of magnetic storms and earthquakes. As magnetic conditions have remained quiet throughout the period of data except some sporadic cases, majority of the TEC anomalies are interpreted as due to earthquakes. We find that out of 43 cases of moderate and high magnitude earthquakes (M ⩾ 5.0) there are 23 cases in which both depletions and enhancements occurred and in the remaining 17 cases (no data available in three cases) either of enhancement or depletion occurred. In general the anomalies occurred 0–9 days before the earthquakes as precursors. The correlation coefficient of 0.8 is found between the two sets of data which satisfy null hypothesis. A mechanism responsible for TEC anomalies under the influence of earthquakes is discussed also.