Local time, annual, latitude, and seasonal variations of total electron content over Japan

Total Electron Content(TEC)data are being accumulated by using about 1000 GPS receivers under the coordination of Japan Geographical Survey Institute to monitor the movement of Japanese islands which experience frequent earthquake. General pictures of diurnal, seasonal, and annual variations of TEC at three receiving stations, Okinawa (25degrees N), Kokubunji (35degrees N), and Wakkanai (45 degrees N) have been obtained by using the data acquired in the period of January 2000 January 2004. Seasonal variation shows two peaks in March and September which start to appear clearly in Okinawa at 08 JST (Japan Standard Time) and disappears at 04 JST. The magnitude of the two peaks is nearly the same until 22 JST and the September peak starts to reduce the magnitude from 02 JST and disappears at 06 JST. Whilst at Kagoshima and Aomori, two peaks appear between 08 JST and 16 JST. After 16 JST the September peak starts decreasing and disappears at 18 JST, which is much earlier than at Okinawa. Daytime TEC in January in low latitude is nearly equal to the TEC in July. Daytime TEC increases as solar radio flux increases. This tendency is the smallest in summer. Nighttime TEC also shows a slight increase as the solar radio flux increases.