Response of the Ionospheric F2-region Over Irkutsk and Hainan to Strong Geomagnetic Storms

The ionospheric responses to two strong storms on 17-19 August 2003 and 22-23 January 2004 are studied, using the data from Irkutsk (52.5°N, 104°E) and Hainan (19.5°N, 109°E) ionospheric stations. The analysis of variations in relative deviations of the critical frequency Δf0F2 revealed that at middle latitudes (Irkutsk) negative disturbances were observed in the summer ionosphere; positive and negative ones, in the winter ionosphere during the main and recovery phases respectively. At low latitudes (Hainan), the disturbances were positive in all the cases considered. Mechanisms of the disturbances were analyzed with the aid of empirical models of the neutral atmosphere NRLMSISE-00 and thermospheric wind HWM07. The main factors determining Δf0F2 variations at middle latitudes during the storms were demonstrated to be the disturbed equatorward thermospheric wind transporting the disturbed atmospheric composition, the increase in the atomic oxygen concentration, and the passage of internal gravity waves. At low latitudes, the effects associated with neutral composition variations are less significant than those of the thermospheric wind and electric fields.