Longitudinal variations in the F region ionosphere and the topside ionosphere-plasmasphere: Observations and model simulations

Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) observations of the total electron content (TEC) above and below 800 km are used to study the local time and seasonal variation of longitude structures in both the F region ionosphere as well as the topside ionosphere and plasmasphere. The COSMIC observations reveal the presence of distinct longitude variations in the topside ionosphere-plasmasphere TEC, and these further exhibit a seasonal and local time dependence. The predominant feature observed at all local times in the topside ionosphere-plasmasphere TEC is a substantial maximum (minimum) during Northern Hemisphere winter (summer) around 300°-360° geographic longitude. Around equinox, at a fixed local time, a wave 4 variation in longitude prevails in the daytime F region TEC as well as the topside ionosphere-plasmasphere TEC. The wave 4 variation in longitude persists into the nighttime in the F region; however, the nighttime topside ionosphere-plasmasphere TEC exhibits two maxima in longitude. The COSMIC observations clearly reveal the presence of substantial longitude variations in the F region and topside ionosphere-plasmasphere, and to elucidate the source of the longitude variations, results are presented based on the coupling between the Global Ionosphere Plasmasphere model and the Thermosphere Ionosphere Electrodynamics General Circulation Model. The model simulations demonstrate that the orientation of the geomagnetic field plays a fundamental role in generating significant longitude variations in the topside ionosphere-plasmasphere but does not considerably influence longitude variations in the F region ionosphere. The model results further confirm that nonmigrating tides are the primary mechanism for generating longitude variations in the F region ionosphere. The coupled model additionally demonstrates that nonmigrating tides are also of considerable importance for the generation of longitude variations in the topside ionosphere-plasmasphere TEC.