Vertical TEC over seismically active region during low solar activity

Abstract

Using data of Japanese dense array of GPS receivers (GEONET) we analyzed variations of total electron content (TEC) over Japan during a period of lowest solar and geomagnetic activity in AprilSeptember 2008. During that time, five large earthquakes with magnitude M>6.8 occurred around the Japanese Islands. We calculated vertical TEC with time resolution 1 hour and we compared the TEC over Japan with TEC over southwestern USA. In order to detect abnormal signals, we processed daily TEC based on median and quartile of preceding 15 days. Knowing that the decisive role in the ionosphere state is performed by space weather effects, we compared the estimated TEC values with time series of the B z component of the interplanetary magnetic field (IMF), UV solar radiation, index of geomagnetic activity Dst and planetary index Kp. We have found that though the analyzed time period falls on the minimum of solar activity, the observed positive anomalies in the regional TEC correspond to fluctuations of IMF B z and to the following increase in Kp and Dst indices. Other case of ~25% TEC increase of duration of four days (DOY: 140-143) is, most likely, caused by increase in the solar UV radiation and coincides with growth of the global TEC. Other positive or negative abnormal TEC variations coincide with fluctuations of the global mean TEC or are of large spatial scale, i.e. much larger than the earthquake preparation zone. Therefore, it is rather difficult to find correlation between the observed anomalies and the five large earthquakes. Our results do not disprove the possibility of precursory phenomena but show the difficulties in identifying earthquake precursors in the ionosphere TEC on the background of TEC changes produced by space weather effects, even during solar minimum.

Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: Mw dependence of the disturbance amplitudes

“…9 insufficient ground station coverage, GIM sometimes gives unrealistic values. For example, the vertical TEC at the time of the 2009 July 15 New Zealand earthquake (9:22 UT) is ∼1.4 TECU according to GIM, but we revised it to ∼4.1 TECU by analysing GPS data of the 'west' station following Astafyeva & Heki (2011).…”

Analysis of pre-earthquake ionospheric anomalies before the global <i>M</i> = 7.0+ earthquakes in 2010

“…Recently, pre-earthquake ionospheric anomalies have drawn intensive attention (Astafyeva and Heki, 2011;Calais and Minster, 1995;Chavez et al, 2011;Dautermann et al, 2007;Hasbi et al, 2011;Kim and Hegai, 1999;Oyama et al, 2008). Earthquake ionospheric effects are becoming a hotspot of earthquake precursor information exploration.…”

Disturbances in the F2 region critical frequency before the earthquake of September 11, 2008 off the coast of Hokkaido, Japan, and during a moderate magnetic storm based on data from ground-based vertical ionosphere sounding stations

“…It is remarkable that analysis of total electron con tent (TEC) before the considered earthquake in the vicinity of its epicenter (Astafyeva and Heki, 2011) has not shown anomalous TEC variations. This is seem ingly related to the method limitations on revealing possible short term ionospheric anomalies preceding earthquakes from TEC variations, and this had been indicated by the authors themselves (Astafyeva and Heki, 2011).…”