An attempt is made to model the three-dimensional global structures and temporal variations of the ionospheric electron density (Ne) using the radio occultation (RO) Ne profile data obtained by the COSMIC/FORMOSAT-3, CHAMP and GRACE missions during the period from July 2006 to June 2017. The modeling technique we adopted is based on the empirical orthogonal function (EOF) analysis of the RO Ne dataset which is binned with grids of 2.5° in geomagnetic latitudes and 1/3 hr in geomagnetic local time (equivalent to 5° in geomagnetic longitudes) and 10 km in height. The EOF analysis decomposed the binned (gridded) Ne dataset into a series of eigen modes or basis functions (E_i) representing the variations with geomagnetic latitude, geomagnetic local time as well as height and the associated EOF amplitude coefficients (A_i) representing the variations with seasons as well as solar cycle activity. Our results showed that the EOF components (E_i, A_i) obtained by the EOF decomposition have different three-dimensional spatial structures with distinct features attributable to different factors or processes controlling the variations of the ionosphere: the first EOF component represents mainly the global mean structure of Ne and its temporal variation with seasons and solar cycle activity; the second EOF component represents mainly the seasonal control of the solar zenith angle on the ionosphere; the third EOF component is representative of the uplifting or lowering of the peak height where Ne reaches its maximum value; the fourth and fifth EOF components represent respectively the variations of the thickness of the ionosphere in the southern and northern hemispheres. It is found that the obtained eigen series converges quickly, with the first five EOF components contributing as high as 98% of the variances of the Ne dataset. We then modeled the EOF coefficients A_i obtained by the EOF decomposition using the harmonic functions representing the annual and semi-annual variations, with the solar cycle dependences being taken into account by including the changes of the harmonic amplitudes with the solar irradiance flux index F10.7. The Ne model is constructed using the A_i thus modeled and E_i obtained by the EOF decomposition. Comparison between the Ne model output results and the COSMIC-2 observational data showed that the modeled results capture well the global structures of the observational data and the model output results have very high linear correlation coefficients with the observational ones (R > 0.9), justifying the modeling technique used in our present study.

尝试使用 COSMIC/FORMOSAT-3、CHAMP 和 GRACE 任务在 2010 年期间获得的无线电掩星 (RO) Ne 剖面数据对电离层电子密度 (Ne) 的三维全球结构和时间变化进行建模。 2006 年 7 月至 2017 年 6 月。 我们采用的建模技术基于 RO Ne 数据集的经验正交函数 (EOF) 分析，该数据集以地磁纬度 2.5° 和地磁当地时间 1/3 小时（相当于地磁经度为 5°），高度为 10 公里。EOF 分析将分箱（网格化）Ne 数据集分解为一系列本征模式或基函数（E _i) 表示随地磁纬度、地磁当地时间和高度的变化以及相关的 EOF 振幅系数 (A _i )，表示随季节和太阳活动周期的变化。我们的结果表明，EOF 分量（E _i，A _i)由EOF分解得到的具有不同的三维空间结构，由于控制电离层变化的不同因素或过程具有不同的特征：第一个EOF分量主要代表Ne的全球平均结构及其随季节和太阳周期的时间变化活动; 第二个EOF分量主要代表太阳天顶角对电离层的季节性控制；第三个EOF分量代表Ne达到最大值时峰高的升高或降低；第四和第五个EOF分量分别代表南半球和北半球电离层厚度的变化。发现得到的特征级数收敛很快，前五个 EOF 分量对 Ne 数据集的方差贡献高达 98%。然后我们对 EOF 系数 A 进行建模_我使用代表年和半年变化的谐波函数通过 EOF 分解获得，并通过将谐波幅度的变化与太阳辐照通量指数 F10.7 包括在内来考虑太阳周期的依赖性。Ne 模型是使用由此建模的 A _i和通过 EOF 分解获得的E _i构建的。Ne模型输出结果与COSMIC-2观测数据对比表明，建模结果较好地捕捉了观测数据的全局结构，模型输出结果与观测数据具有很高的线性相关系数（R > 0.9），证明了我们目前研究中使用的建模技术。