Wave-3 and wave-4 patterns in the low- and mid-latitude ionospheric TEC

The JPL global ionospheric maps (GIMs) from January 1, 1999 to December 31, 2013 are analyzed using the space-time spectral analysis method to obtain the tidal patterns in the ionospheric TEC. The results show that noticeable wave-3 and wave-4 signatures can be detected near the crests of equatorial ionospheric anomaly (EIA) regions. Wave-4 structure is comprised predominately by DE3 component, while wave-3 structure is comprised mainly by DE2 component. SPW4 (SPW3) component also accounts for the wave-4 (wave-3) signature. The absolute amplitude of all the aforementioned tidal and SPW signatures shows a saturation effect on the solar activity. The tidal components have significant seasonal variations, with the prominent latitudinal asymmetry. The relative amplitude of zonal mean TEC, however, shows persistent patterns with little solar activity dependence. DE3 is intense in March and September/October, and is weak in northern winter and southern summer. DE2 is strong in the southern hemisphere, and is weak in boreal winter. The amplitude of tide DE3 in TEC is smaller than that of DE2.