25th anniversary study: the anomalous electromagnetic signals appear before the 21 September 1999 M7.7 Chi-Chi earthquake

Electromagnetic anomalous variations of the geomagnetic field, lightning activity, ionospheric F2-peak plasma frequency, GPS total electron content (TEC), etc. have been observed around the epicenter few days before the 21 September (local time) 1999 M7.7 Chi-Chi earthquake. The TEC over the epicenter anomalously and significantly decreases in the afternoon period on day 1, 3, and 4 before the Chi-Chi earthquake, which generally agrees with TEC decrease anomalies day 1-5 and day 10-15 appearing prior to M≥5.0 earthquakes in Taiwan during the 6-year period of 1994/1/1-1999/9/20.  Temporal and spatial analyses of the global ionospheric map (GIM) shows that TEC anomalously and significantly decrease specifically over the epicenter day 3-4 and day 10-15 before the Chi-Chi earthquake.  To find possible physical mechanisms causing the TEC decrease anomalies before the Chi-Chi earthquake, the equatorial ionization anomaly of TEC along the Taiwan longitude during September 1999 and plasma quantities of the ion density, ion temperature, and ion velocity measured by DMSP (Defense Meteorological Satellite Program) satellites are examined. It is found that westward seismo-electric fields around the epicenter area day 3-4 and day 10-15 before the Chi-Chi earthquake are essential.