Abstract. In this paper we examine pre-earthquake ionospheric anomalies by the total electron content (TEC) derived from a ground-based receiver of the Global Positioning System (GPS). A 15-day running median of the TEC and the associated inter-quartile range (IQR) are utilized as a reference for identifying abnormal signals during all of the 20 M > = 6.0 earthquakes in the Taiwan area from September 1999 to December 2002. Results show that the pre-earthquake ionospheric anomalies appear during 18:00-22:00 LT (LT=UT+8 h) within 5 days prior to 16 of the 20 M > = 6.0 earthquakes. This success rate of 80% (=16/20%) suggests that the GPS TEC is useful to register pre-earthquake ionospheric anomalies appearing before large earthquakes.

The Global Positioning System (GPS) provides an alternative way to investigate ionospheric irregularities and their effects on the radio wave propagation. The method is based on fluctuations of the total electron content (TEC) resulted from the ionospheric plasma irregularities. Previous studies have showed the correlation between the radiowave intensity (including GPS signals) and ionospheric irregularities during magnetic storm periods. In this study, phase fluctuations derived from GPS signals are used to address aspects of the ionospheric storm events during the low irregularity activity months. We analyze data from seven GPS stations located in Central-and SouthAmerica during eight magnetic storms occurred from 1997 to 2000. It is found that, in general no significant feature in the phase fluctuation is observed during the low irregularity activity months, except during the 26 August 1998 and the 15 July 2000 storms. A detailed study shows that the GPS phase fluctuations develop when the Dst index begins to decrease significantly. This phenomenon cannot be compared directly to previous observations and model results due to the fundamental difference in the background levels of irregularity activity. To better understand the generation of ionospheric irregularities during the storm period of the low irregularity activity months, the temporal relationship between the magnetic Dst index, equatorial anomaly TEC, and the GPS phase fluctuations are examined and discussed.