Variation of ionospheric total electron content in Taiwan region of the equatorial anomaly from 1994 to 2003

Abstract The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionospheric anomaly (EIA) region is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a chain of nine observational sites clustered around Taiwan (21.9–26.2°N, 118.4–112.6°E). In this study, we present results from a statistical study of seasonal and geomagnetic effects on the EIA during solar cycle 23: 1994–2003. It is found that TEC at equatorial anomaly crests yield their maximum values during the vernal and autumnal months and their minimum values during the summer (except 1998). Using monthly averaged I c (magnitude of TEC at the northern anomaly crest), semi-annual variations is seen clearly with two maxima occurring in both spring and autumn. In addition, I c is found to be greater in winter than in summer. Statistically monthly values of I c were poorly correlated with the monthly Dst index ( r  = −0.22) but were well correlated with the solar emission F 10.7 index ( r  = 0.87) for the entire database for the period during 1994–2003. In contrast, monthly values of I c were correlated better with Dst ( r  ⩾ 0.72) than with F 10.7 ( r  ⩽ 0.56) in every year during the low solar activity period (1994–1997). It suggests that the effect of solar activity on I c is a longer term (years), whereas the effect of geomagnetic activity on I c is a shorter term (months).