1. Ionospheric irregularities observed with a GPS network in Japan TOHRU ARAMAKI[1],Yuichi Otsuka[1],Tadahiko Ogawa[1],Akinori Saito[2] and Takuya Tsugawa[2] [1]Solar-Terrestrial Environment Laboratory, Nagoya University, [2]Graduate School of Science, Kyoto University <Introduction> The Global Positioning System (GPS) provides a way to investigate ionospheric irregularities through measurements of total electron content (TEC) along a ray path from GPS satellite to receiver. To reveal the statistical characteristics of ionospheric irregularities, we have analyzed TEC data obtained from more than 1,000 GPS receivers in Japan with a time resolution of 30 s. <Conclusions> ● The summer nighttime MSTID is accompanied by irregularities with wavelengths of a few km. ● Lack of GPS phase data around Okinawa are caused by the irregularities associated with the equatorial plasma bubbles. <Acknowledgment> GPS data were provided by the Geographical Survey Institute. Fig.3 Example of medium-scale traveling ionospheric disturbance ( MSTID ) over Japan with wavelength of a few hundred km. <Analysis> ● At position1(44°N,142°E), there are three peaks in the occurrence of ionospheric irregularities, summer and equinox nighttime. The irregularities in equinox would be related to magnetic activity. ● At position2-5(38° ～ 29°N), the occurrence rate is highest in the summer nighttime. This feature is seem to be coincident with that of MSTID. Fig.4 Diurnal and seasonal variations of MSTID activity. Fig.7 TEC perturbation and lack of GPS phase data observed by the GPS receiver near Sata on the night of Oct, 18, 2002. Lack of GPS phase data was accompanied by depletion of TEC. Fig.2 Diurnal and seasonal variations of the events with ROTI ≧ 0.1 in 2000 ( bottom 6 panels ). The ROTI is defined as standard deviation of ROT ( the rate of change of TEC ) for 5 min interval. The ROTI is a measure of the occurrence of the irregularities with scale sizes from a few km to a few tens of km. We used TEC data in 2000 and revealed the statistical feature of ROTI. We also compared the results with the MSTID activity. Fig.1 Map showing locations where occurrence of the ionospheric irregularities is shown in Fig.2. ROTI is averaged within the area enclosed by solid lines. Fig.5 Day to day variation of the MSTID activity and irregularity occurrence in summer. Fig.6 Diurnal and seasonal variation of the occurrence rate of GPS phase data at Okinawa in 2000. This feature is seem to be coincident with the occurrence of plasma bubble. Lack of GPS phase data may be caused by the irregularities with scale size is 240 to 310 m. Fig.8 All-sky image of 630nm airglow at Sata, at 2144LT on Oct, 18, 2002. Depletions due to equatorial plasma bubbles can be seen in the image. ● The summer nighttime MSTID activity is consistent with the occurrence rate of irregularities. The summer nighttime MSTID is accompanied by irregularities with wavelengths of a few km. ● At position6(25°N,128°E), the occurrence rate is highest in the equinox nighttime. This feature is seem to be coincident with an activity of the equatorial anomaly. ● Lack of phase data is caused by irregularities associated with the equatorial anomaly and plasma bubble embedded in the anomaly.