A MODEL STUDY OF THE SEASONAL AND SOLAR ACTIVITY VARIATIONS OF THE ENHANCED ELECTRON DENSITY REGIONS IN THE NIGHT-TIME IONOSPHERIC F2-LAYER AND PLASMASPHERE

Enhanced electron density regions (EEDR) in the night-time ionospheric F2-layer and plasmasphere have been modeled using the global Upper Atmosphere Model (UAM) for eight selected days representing four seasons and two levels of the solar activity. The modeling calculations have been performed in two versions: 1) by using the empirical thermospheric NRLMSISE-00 model for the neutral component calculations and 2) by using the theoretically calculated thermosphere parameters. The results were compared with the empirical model IRI-2001 predictions. All model calculations show the noticeable MLT, UT, seasonal and solar activity effects. The latitudinal and MLT locations of the EEDR are established. These regions are better expressed in equinoxes under low solar activity. The strong UT dependence exists in all model runs with maximum of the effect in the 12-24 UT sector. The EEDR extend to the plasmasphere along the geomagnetic field lines forming the tubes with enhanced electron density. The electromagnetic drift influences on the latitudinal locations of the high-latitude sides of these tubes, moving them to the lower latitudes. The main cause of the occurrence of the night increases of the electron density is the equatorward thermospheric wind driving the F2-layer plasma to the higher altitudes thus decreasing the ion loss rate. Seasonal and solar activity effects of the night-time EEDR are formed by the corresponding variations of the thermospheric wind velocity.