Longitudinal variations of topside ionospheric and plasmaspheric TEC

The upward looking ionospheric total electron content (TEC) from the MetOp-A and TSX satellites during 2008-2015 has been used to systematically study the longitudinal variations of the topside ionosphere and plasmasphere. The results of this study are summarized as follows: (1) There are significant longitudinal variations in the topside ionosphere and plasmasphere at low latitudes. The TEC maximum during the June solstice over the Western and Central Pacific Ocean corresponds to a TEC minimum at the same location during the December solstice, but the opposite behavior occurs over South America and the Atlantic Ocean. (2) During the solstices, the relative longitudinal variations in the geomagnetic equatorial region do not have a strong dependence on local time and solar activity. (3) The TEC in the winter hemisphere decreases with increasing solar activity, especially at higher altitudes and at night. The topside TEC depletion with solar activity depends on longitude. (4) The solstice-like longitudinal pattern lasts much longer than the equinox-like patterns, with the June solstice pattern lasting the longest. Furthermore, the equinox-like longitudinal patterns occur in March when expected, whereas they extend from the autumnal equinox until the end of October. (5) The longitudinal variations of upward looking TEC are different from the corresponding longitudinal variations of electron densities around the F-2 peak and orbital altitudes. This indicates that the topside ionosphere structure is strongly influenced by the physical processes in the topside region, rather than being a pure reflection of the ionospheric F-2 peak structure.