Analysis Of Spatiotemporal Variation Of Ionospheric TEC In The South Atlantic Anomaly Region Based On Empirical Orthogonal Functio

There are a large number of free electrons in the ionosphere,which can change the radio propagation,so it has an important influence on navigation and positioning,radio communication,and aerospace.Total electron content（TEC）is one of the important parameters of the ionosphere,and its spatial and temporal characteristics analysis is a hot topic in the field of ionosphere.At the same time,the South Atlantic Anomaly（SAA）shows many different characteristics because the magnetic field is 30%to 50%weaker than that in the same latitude region,and it is a hot area of interest in aerospace missions and earth science research.In order to increase the understanding of the ionosphere and avoid the adverse effects of the ionosphere,it is very important to study the temporal and spatial variation of ionospheric TEC in the SAA region under different solar activity conditions.In this paper,Thermosphere-Ionosphere-Electrodynamics General Circulation Model（TIEGCM）of the National Center for Atmospheric Research was used to conduct statistical research on ionospheric TEC simulation data of the solar maximum（2002）and solar minimum（2008）.The main work and conclusions are as follows:Firstly,based on the global ionospheric TEC data obtained from TIEGCM simulations,preliminary statistical analysis of its daily variation,seasonal variation,annual variation in temporal variation and latitude and longitude variation in spatial variation is carried out using descriptive statistical methods,and the influencing factors of ionospheric TEC are analyzed and discussed.By comparing the global distribution map of ionospheric TEC under different solar and geomagnetic conditions,it is found that the value of ionospheric TEC in the solar maximum is significantly higher than that in the solar minimum,and the value of ionospheric TEC increases with the increase of geomagnetic activity intensity,and the low latitude area is more affected.In addition to being affected by solar and geomagnetic activity,ionospheric TEC is also affected by many factors such as earthquakes,typhoons and eclipses.Then,the temporal and spatial characteristics of ionospheric TEC in the SAA region in the solar maximum and the solar minimum were compared by using an empirical orthogonal function（EOF）analysis.The results show that the first three EOFs accounted for 94.8%and93.86%of the variability in the data in 2002 and 2008,respectively.The ionospheric TEC variations of the first three EOFs were generally consistent in 2002 and 2008.The first mode showed the equatorial anomaly caused by plasma drift and the east-west asymmetry possibly caused by the change in geomagnetic declination and zonal wind;EOF₂ exhibited the zonal variation influenced by the solar EUV radiation and the semiannual variation possibly controlled by the[O/N₂],solar zenith angle,and atmospheric circulation.EOF₃ suggested an equatorial anomaly and winter anomaly influenced by the[O/N₂]variation.However,the variation of ionospheric TEC values and amplitudes were significantly greater in the solar maximum than that in the solar minimum,and the spring-autumn asymmetry of the TEC is more obvious in the solar minimum.Finally,we used the EOF method again to extract the diurnal and annual variation characteristics of the time coefficient,and used the correlation analysis between the annual variation coefficient and the solar activity index F10.7,the geomagnetic activity index Dst and Ap.The results show that the ionospheric TEC in SAA region in 2002 was mainly affected by solar EUV radiation,and had a strong correlation with F10.7（r=0.7348）.However,ionospheric TEC in the SAA region in 2008 was mainly affected by geomagnetic activities,and had a strong correlation with Dst（r=-0.7898）.All the above correlations passed the significance test of 99%.Through statistical analysis of the model data,this paper not only successfully applied the EOF method in meteorological statistics,but also further understood the temporal and spatial variation of ionospheric TEC in the SAA region,influencing factors and other aspects of information.In addition,it also had a further understanding of the characteristics of the model,which laid a foundation for the subsequent comparison with the observation data of the AUKO-1 satellite.