MODELING STUDIES OF THE LONGITUDINAL VARIATIONS IN TEC AT EQUATORIAL-ANOMALY LATITUDES

The longitudinal differences in total electron content (TEC) in summer in the northern equatorial-anomaly region are studied using observations from Wuhan (an east-Asian longitude station) and Palehua (a mid-Pacific longitude station), and values from the Sheffield University Plasmasphere-Ionosphere Model. The model results show that the differences in the observed values of TEC result from the longitudinal differences in the displacement of the magnetic and geographic equators and in the magnetic declination angle. During daytime, the TEC values are larger at Palehua mainly because the neutral air wind velocity in the magnetic meridian is equatorward at Palehua whereas it is poleward at Wuhan. Closer agreement between the modelled and observed daytime values of TEC is obtained when the phase of the daytime meridional neutral air wind velocity, as given by the HWM90 thermospheric wind model, is shifted towards the morning sector by 3 h. The observed longitudinal differences in nighttime TEC, particularly in the enhancements (which are most frequent and strongest at Wuhan), are caused principally by the longitudinal differences in the prereversal enhancement of the E x B drift velocity. The model calculations for Wuhan show that the time of peak enhancement in TEC, which occurs before midnight in the absence of neutral air winds, is shifted to after midnight by the neutral air wind.