[1] A global ionospheric total electron content (TEC) model based on the empirical orthogonal function (EOF) analysis method is constructed using the global ionosphere maps provided by Jet Propulsion Laboratory during the years 1999-2009. The importance of different types of variation to the overall TEC variability as well as the influence of solar radiation and geomagnetic activity toward TEC can be well represented by the characteristics of EOF base functions E k and associated coefficients P k . The quick convergence of EOF decomposition makes it possible to use the first four orders of the EOF series to represent 99.04% of the overall variance of the original data set. E 1 represents the essential feature of global spatial and diurnal variation of the TEC. E 2 contains a hemispherically asymmetric pattern manifesting the summer-to-winter annual variation. E 3 and E 4 can well reflect the equatorial anomaly phenomenon. P 1 contains an obvious solar cycle variation pattern as well as annual and semiannual variation components. P 2 mainly includes an annual fluctuation component. P 3 has a strong annual variation and a weak seasonal variation pattern. P 4 has both evident annual and semiannual oscillation components. The Fourier series as a combination of trigonometric and linear functions are used to represent the solar cycle, annual, and semiannual variation of the coefficients. Therefore the global TEC model is established through incorporating the modeled EOF series. The accuracy and quality of the model have been validated through the model-data comparison, which indicates that the model can reflect the majority of the variations and the feature of temporal-spatial distribution of the global ionospheric TEC.

The thermospheric densities derived by CHAMP/STAR accelerometer within the time period from 01 May 2001 to 31 December 2007 are utilized to investigate the hemispheric asymmetry in response to strong storm driving conditions. The geomagnetic storms of 03–07 April 2004 are first studied since the storms occurred close to the vernal equinox, allowing the seasonal asymmetry to be eliminated to the greatest extent. The averaged density enhancements in the southern polar region were much larger than that in the northern polar region. The comparisons of density versus Dst and Apindex indicate a strong linear dependence with the slopes of the fitted lines in the southern hemisphere being 50% greater than that in the northern hemisphere. This effect can possibly be attributed to the non‐symmetric geomagnetic field. 102 storm events are used to conduct a statistical analysis. For each storm, a linear fit is made between the averaged mass density and theDst and Ap indices independently in each hemisphere. The seasonal variation of the intercepts and the slopes of the fitted lines are further explored. The baseline is strongly dependent on season, with the hemisphere receiving the larger amount of sunlight having larger density. The slopes showed considerable hemispheric differences around the vernal equinox yet no statistical differences around other seasons. It is speculated that competing mechanisms cancel each other during the solstices, while during the equinoxes, the lower magnetic field in the southern hemisphere may allow stronger ion flows, thereby causing more Joule heating. It is uncertain why the vernal equinox would be favored in this explanation though.

A sea temperature data assimilation system for the China Seas and adjacent areas Chinese Science Bulletin 48, 70 (2003); Evaluation of regional ionospheric grid model over China from dense GPS observations Geodesy and Geodynamics 7, 361 (2016); Regional TEC modelling over Africa using deep structured supervised neural network Geodesy and Geodynamics 11, 367 (2020);. RESEARCH PAPER. SCIENCE CHINA Information Sciences