A modelling study of the longitudinal variations in the north-south asymmetries of the ionospheric equatorial anomaly

The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to study the effects of neutral winds of the north-south asymmetries in the ionospheric equatorial anomaly at longitudes 120°E, 200°E, 283°E and 330°E. The model results obtained for magnetically quiet equinoctial conditions at solar maximum produce north-south asymmetries in the equatorial anomaly in agreement with the observations. The study shows that the neutral wind causes the north-south asymmetries and that the asymmetries have longitudinal variation in accord with the longitudinal variations in the displacement of the geographic and geomagnetic equators and in the magnetic declination angle. At longitudes 120°E and 283°E, where the magnetic declination angle is small and the magnetic equators are located in opposite geographic hemispheres, the asymmetries in the anomaly are caused mainly by the asymmetries in the meridional wind. On the other hand, at longitudes 200°E and 330°E, where the geographic and geomagnetic equators are almost coincident and the magnetic declination angles are eastward and westward, respectively, the asymmetries in the anomaly arise mainly from the zonal wind. During daytime, in the hemisphere of stronger poleward wind, the crest values in TEC are weaker than in the conjugate hemisphere at all longitudes considered. The values of NmF2 can be stronger or weaker, depending on the competition between the effects of increased chemical loss rate and the downward flow of plasma from the plasmasphere caused by the stronger poleward wind. At night, after the magnetic meridional wind has changed direction and has been in that direction for some time, the stronger crests in both TEC and NmF2 occur in the hemispheres of stronger equatorward wind.