Annual TEC variation in the equatorial anomaly region during the solar minimum: September 1996-August 1997

The ionospheric total electron content (TEC) in the equatorial anomaly region is studied by analyzing dual-frequency signals from the global position system (GPS) acquired from a meridional chain of 9 observational sites clustered around Taiwan (21.9°-26.2°N, 118.4°-121.6°). This relatively dense GPS chain provides a powerful tool for studying ionospheric TEC in the northern hemispheric equatorial anomaly region with an unprecedented spatial resolution. Specifically, we studied seasonal and geomagnetic effects on the equatorial ionospheric anomaly during the solar minimum period between September 1996 and August 1997. We found that the surveyed data indicated semiannual variations in the magnitude of TEC, I_c, at the anomaly, with maxima in the equinoxes, similar to the semiannual variation of geomagnetic activity. The values of I_c were found to maximize in April, 1997 and minimize in July, 1997. Statistical studies indicate that the monthly values of I_c do not correlate with the planetary K_p magnetic index (r = 0.41) but correlate well with the D_st geomagnetic activity index (r = 0.72). This suggests that variations of TEC are mainly driven by a low-latitude forcing for the surveyed period. The most likely time for I_c to occur was 1400 local time (∼ 30%) at 20°N geographic latitude (∼ 37%).