Climatological study of GPS total electron content variations caused by medium-scale traveling ionospheric disturbances

Using global positioning system (GPS) data taken from the International GNSS Service (IGS), we investigated total electron content (TEC) perturbations associated with medium-scale traveling ionospheric disturbances (MSTIDs). We analyzed TEC data taken from four or five GPS receivers in each of six regions ( Japan, Europe, eastern United States, western United States, Australia, and South America) in 3 years ( 1998, 2000, and 2001). To derive perturbation components of TEC (I'), we subtracted the 1-hour running average form the time sequence of TEC for each satellite-receiver pair. Standard deviation of I' within 1 hour, delta I, was calculated every hour, and MSTIDs activity were defined as delta I/(I) over bar, where (I) over bar is the 1-hour average of absolute vertical TEC. We found that MSTIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependences. Daytime MSTIDs activity are high in winter in all six regions. On the other hand, seasonal variation of nighttime MSTIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MSTIDs are most active near the June solstice, whereas it is most active near the December solstice in the European longitudinal sector. Nighttime MSTIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MSTIDs activity. These results suggest that mechanisms causing MSTIDs could be different between daytime and nighttime.