A Study Of Ionospheric Response To The Flares

During solar flares, sudden increases of extreme ultraviolet (EUV) and X-ray flux produce abrupt increases of total electron content in the sunlit part of the ionosphere. Using the total electron content (TEC) increment, ATECf and its variation rate increment, A(dTECf/dt) obtained from a global GPS network and a time-dependent theoretical model, we discuss the features of electron content variations during solar flares.At first, according to the continuity equation for electron, we calculate the evolvement of the TEC during solar flares. It is found that both ATECf and A(dTECf/dt) are proportional to the solar flare's maximal flux and inversely proportional to the Chapman function ch(x). Theâ–³(dTECf/dt) is also correlated with the durations of the solar flares.Second, analyzing the TEC data of some GPS stations, we find that bothâ–³TECfandâ–³(dTECf/dt) are closely related with the maximum values of solar flare's X-ray flux and its location on the solar disc. After eliminating the effect of the maximal X-ray fluxes and the locations on solar disc of different flares, results show that theâ–³TECf is smaller in summer than in spring and autumn, and larger in winter than in spring and autumn.â–³(dTECf/dt) is observed to be correlated with the duration of the solar flare, which is in agreement with our theoretical calculation.Using a time-dependent theoretical model with an assumed time-dependent solar flux, we find that the increases of electron content caused by solar flares occur mainly at 115 km (E region) and 250 km (F region). During solar flares, the increase of electron density in the F region is thought to be responsible for a large fraction of the SITEC. The electron content increments caused by solar flares at both regions have characteristics in seasonal variation. They are larger in winter and smaller in summer. At E region, the altitude at which the maximum appeared is highest in winter and lowest in summer. At F region, the altitude at which the maximum appeared is lowest in winter, highest in spring or autumn. The seasonal variation of the electron content increment at 250 km is more prominent than that at 115 km. Model study also shows that the electron content increments have relationship with the durations of the flares which is supported by our observation results.In addition, this paper studies the relationship between the electron content increment caused by the solar flare and the local time. It is obtained that the increase of electron content at 115 km is more dependent on the local time than that at 250 km. Another result is that the increases of electron content caused by solar flares are different between the dawn and dusk regions. Above 400 km, it is larger in the dusk region. Below 400 km, it is larger in the dawn region.In conclusion, this paper studies the evolvement of the electron content during solar flares by both the statistic analysis and the simulation analysis. Our investigation can help us understand the electron content during the solar flare in more detail.