Comparison of GPS Integrated Electron Content Measurements with Electron Density Values acquired by the DEMETER Satellite in Japan

The DEMETER satellite, designed with the objective of investigating ionospheric phenomena before earthquakes, measures the in-situ electron density at ~700 km height using a Langemuir probe. On the other hand, dual-frequency GPS data can be used to retrieve the Integrated Electron Content (IEC) along the ray path from ground stations to GPS satellites. We compare electron density data acquired by DEMETER with IEC data derived from GEONET, an ultra dense GPS network in Japan, on days preceding two M>7 earthquakes and along the the DEMETER ground tracks. Previous studies have reported irregularities of the same order of magnitude in the DEMETER electron density over the epicentral areas 2 and 8 days prior to the earthquakes. In both cases, we find anomalies in the GPS-derived IEC that are coincident with those found in the DEMETER-derived electron density, but of significantly different amplitude. We also find a time delay of 38 seconds in the onset of the anomaly between the IEC and DEMETER data. This probably results from the different measurement geometry and sampling altitude between the two techniques. During one of the DEMETER passes (and in the absence of upcoming earthquake), the spatial distribution of the IEC data shows a stationary, wavelike structure similar to the one reported by (Saito et al., 2001)