Study On Modeling Of Mid/Low-latitude Ionosphere And Its Relative Anomalous Phenomenon

The investigation on ionospheric model is of great significance for design of communication ciruit and understanding of ionospheric mechanisms. The aim of this dissertation is to establish empirical and physical models to describe the variation of mid-latitude ionosphere, as well as low-latitudes with empirical models. Based on the physical model, the storm-time variation is simulated and pre-earthquake anomalous variation is investigated preliminarily. As basis, a brief review is given on mainly sources affecting on ionosphere and corresponding variation of ionosphere, firstly. The main topics and results are as follows:First, extracting of periodic components and long-term prediction of the indices of solar activity and geomagnetic activity are investigated by use of EMD (empirical mode decomposition) and AR (auto-regressive model). With EMD method, sunspot number R and geomagnetic indices, i.e. Ap, aa and Dst, are decomposed and physical mechanisms of their periodic components are discussed. Furthermore, EMD and AR are applied to the long-term prediction of sunspot number and the lead time reaches up to 11 years, which gives a possibility to realize the long-term prediction of ionosphere with ionospheric models.Second, hourly values of the critical frequency of the ionospheric F region, foF2, observed at the nine ionosonde stations in China about two solar cycles have been used to investigate the dependence of the monthly median foF2 on solar activity and geomagnetic activity, and to construct single-station model (SSM) using Fourier expansion. The results of the present analysis show that there is a significant nonlinear relationship between monthly median foF2 and sunspot number R especially at daytime in each month and nighttime in summer and the nonlinear relationship are more evident as the latitude decreases. Furthermore, introducing geomagnetic index Ap further improves the description of variation of monthly median foF2, and the standard deviations decrease also dependently on time and month. Based on the nonlinear relationships, we develop a new SSM for the each station and the SSMs are compared with IRI, which show a much better agreement with observation.Third, with the time weighted accumulation index ap(Ï„), an empirical storm-time model of the relative deviation of foF2 from its monthly median is established. Because of low correlation between variation of foF2 and geomagnetic indices, the model has low accuracy. The method used widely in weather, hydrography, et al., Kalman filter, is used to update the coefficients of the empirical model and expected to make better improvement. The proposed method is tested by the data of hourly values from 1986 to 1995 over Changchun station. Simultaneously the method is compared with IRI2001 including the STORM model for several disturbed events, which shows this method has a powerful ability of updating the model coefficients and prospect of application.Fourth, one-dimensional time-dependent theoretical ionospheric model for mid-latitude is established with which variation of storm-time ionosphere are discussed. Thoroughly taking the mainly kinetic and photochemical process into account, the model can give six ionic species, O+, O2+, NO+, N2+, N+, He+ and electron through solution of continuity and momentum equations. The foF2 over Beijing station is simulated in undisturbed condition. Moreover, the variation of ionospheric density profile during geomagnetic storms, considering the disturbances of the thermospheric temperature, composition and neutral wind are discussed in detail.Fifth, ionospheric anomalous disturbances of earthquakes are investigated. The characteristics of ionospheric perturbations on temporal and spatial distribution are represented briefly. Then the hourly ionospheric data foF2 are analyzed before Wenchuan great earthquake over four ionospheric observatories Kunming, Chongqing, Lhasa and Lanzhou. The possible mechanisms that could be responsible for the observed pre-earthquake ionospheric anomalies are discussed with the physical model of mid-latitude ionosphere and the hypothesis of abnormal vertical atmospheric electric fields.