Change of Ionospheric Crest in China Sector Based on GPS-TEC

The variations of ionospheric TEC (total electron content) of equatorial anomaly are studied using the data from seven GPS sites of China region in 2004, including the occurrence latitude and time of the crest and the ionospheric gradient. Results show that the latitude of the crest varies from 17.5°N to 22.5°N in different seasons. It is higher in equinoxes and lower in winter, and the mean of occurrence latitude is about 20°N. The occurrence time of the crest varies also between 13 h and 16 h in local time (LT). It is earlier in winter and later in summer, and the mean time is about 14 h LT for the former and between 15 and 16h LT for the latter. It is found that there is the gradient of ionospheric TEC along the longitudinal chain, which vary with the local time and seasons. The minimum of gradient occur in nightside about 4.5 h LT and the maximum appears in dayside between 13 h and 16 h LT. Moreover, the variations of gradient is smaller in different seasons for the nightside and turn notable for the dayside, and the gradient is greater in equinoxes than summer and winter in dayside.