Abstract
Preseismic enhancement of ionospheric total electron content (TEC) was found to have started ~40min before the 2011 Tohoku-oki earthquake from the dense network of Global Positioning System (GPS) receivers in Japan [Heki, 2011]. In response to Kamogawa and Kakinami [2013], who doubted the reality of the enhancement, Heki and Enomoto [2013] employed two approaches. The first one is to convert slant TEC to vertical TEC (VTEC), which is free from apparent TEC variations due to satellite movements. They showed that preseismic increases are comparable to postseismic drops; i.e., there were no net postseismic TEC decreases as claimed by Kamogawa and Kakinami [2013]. The other approach is to compare TEC changes with data from three different sensors, i.e., E region critical frequencies (foEs), electron density profile by GPS radio occultation, and geomagnetic field data. Heki and Enomoto [2013] suggested that both foEs at Kokubunji and geomagnetic declination at six stations in eastern Japan changed simultaneously with VTEC.
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