GPS-TEC Variation during Low to High Solar Activity Period (2010-2014) under the Northern Crest of Indian Equatorial Ionization Anomaly Region

The measurements of ionospheric TEC (total electron content) are conducted at a low latitude Indian station Surat (21.16°N, 72.78°E Geog.), which lies under the northern crest of the equatorial anomaly in Indian region. The data obtained are for a period of five years from low to high solar activity (2010- 2014) using GPS (Global Positioning System) receiver. In this study, we report the diurnal and seasonal variation of GPS-TEC, dependence of GPS-TEC with solar activity, geomagnetic condition and EEJ strength. From the seasonal analysis, it is found that greater values of the GPS-TEC are observed during equinox season followed by winter and summer. The appearance (in the year 2011 and 2014) and disappearance (in the year 2010 and 2012) of “winter anomaly” have been observed at the station. From the correlation of GPS-TEC with different solar indices, i.e. solar EUV flux, F10.7 cm solar radio flux and Zurich sunspot number (SSN), it is concluded that the solar index EUV flux is a better controller of GPS-TEC, compared to F10.7 cm and SSN. Further, it is observed that there is no effect of rising solar activity on correlation. Moreover, the percentage variability of GPS-TEC and the standard deviation of GPS-TEC obtained for quiet and disturbed days show that dependence of GPS-TEC on geomagnetic condition is seasonal. Also, there is a positive correlation observed between GPS-TEC and EEJ strength.

Anomalous Variation in GPS Based TEC Prior to the 5 Earthquakes in 2009 and 2010

The present paper reports the analysis of GPS based TEC measurement corresponding to 5 earthquakes (M > 6.0) that have occurred in 2009 and 2010 during low solar activity period and quiet geomagnetic conditions. The No. of satellites registering high TEC at low elevation angles is observed prior to these earthquakes. The earthquakes are 1) Loyalty Island (22o36'S, 170o54'E) M-6.6 on 19 January 2009, 2) Izu Island (32o48'N, 140o36'E) M-6.6 on 12 August 2009, 3) Ryukyu Islands (23o30'N, 123o30'E) M-6.7 on 17 August 2009, 4) Haiti (18o24'N, 72o30'W) M-7.0 on 12 Jan 2010, 5) Chile (34o12'S, 71o48'W) M-6.9 on 11 March 2010. To identify anomalous behavior of TEC, we compute interquartile range associated with median TEC so as to get upper and lower bound of TEC variation. The results show occurrence of anomalies in VTEC 1 - 13 days prior to the earthquake. The VTEC shows enhancement for all the earthquakes. Enhancement in No. of satellites registering high TEC at low elevation has been observed prior to earthquakes.

Modification in Atmospheric Refractivity and GPS Based TEC as Earthquake Precursors

The present paper reports the modification in GPS TEC (total electron content) and atmospheric refractivity prior to the Iran earthquake that had occurred on 16 April 2013 in Iran (28.10°N, 62.05°E). The analysis of GPS-based TEC from two GPS receivers, Surat (21.16°N, 72.78°E) and Lucknow (26.91°N, 80.95°E ) and results of atmospheric refractivity profile for radiosonde observation stations (Shiraz, Iran and Delhi, India) around the fault line are presented in this paper. It is seen that atmospheric refractivity gets modified from 8 to 6 days prior to the earthquake at Shiraz, Iran and Delhi, India. The GPS TEC showed variations a few days prior to the earthquake. We conclude that in search of precursory signatures for an earthquake, both GPS TEC and refractivity are important parameters.