| dc.description.abstract | This dissertation studies ionospheric electron density variations by means of ionosonde and global position System (GPS). The ionospheric variations during Chi-Chi earthquake are the event study observed by 2 ionosondes Chungli, Taiwan and Kokunbunji, Japan, 4 local GPS receivers in Taiwan, and one at Okinawa, and 5 virtual GIM (Global Ionospheric Maps) 30˚E, 60˚E, 90˚E, 120˚E and Kokunbunji. The spatial analyses are employed to identify whether the earthquake or the geomagnetic disturbances cause the anomalous reductions. Moreover, we analyze the peak frequency and the peak electron density in the F2 layer (foF2 and NmF2), the peak density and the total electron content (TEC) during 1997 and 1999 associated with 144 M ≥ 5.0 earthquakes in Taiwan. The statistical results show that the anomalous reductions in NmF2 and TEC appear more frequently before than those after the earthquakes. By contrast, the anomalous enhancements appear less frequently before than those after the earthquakes. In general, the pre-earthquake departure of NmF2 and TEC from their associated medians is proportional to the earthquake magnitude. In Taiwan, the anomalous reductions appear not only at 1000-1800LT but also at 1600-2000LT. We further examine GIM-TEC during M8.0 Wenchuan earthquakes, and observe both the anomalous enhancement and reduction before the earthquake. It is found that the enhance anomaly appear less frequently than the reduce one. Therefore, the anomalous reductions appear on day 13, day 6, and day 5 before the earthquake while the anomalous enhancement appears on day 3 before the earthquake. The spatial analysis reveals that the depletion appears over the epicenter and its conjugate point, while the enhancement simply occurs around the epicenter. Apply the similar statistical analysis on the GIM-TEC of 33 M ≥ 6.0 earthquakes occurring in China during 1 May 1998 and 11 May 2008. Results show that the anomalous GIM-TEC reductions occur more frequently than those of the enhancements before the earthquakes. The precursors in Taiwan mainly appear during 1000-1800LT or 1600-2000LT while that in China mainly generally occurs during 2000-0400LT. | en_US |