Abstract
Abstract. In this study, the empirical orthogonal function (EOF) decomposition technique was utilized to analyze the similarities and differences of the spatiotemporal characteristics between the total electron content (TEC) of the International GNSS Service global ionospheric map (GIM) and that derived from the International Reference Ionosphere 2016 (IRI-2016) model in 2013. Results showed that the main spatial patterns and time-varying features of the data set have good consistency. The following four main spatiotemporal variation features can be extracted from both data sets through EOF decomposition: the variation with the geomagnetic latitude reflecting the daily averaged solar forcing, the diurnal and semidiurnal periodic changes with longitude due to local time, and the interhemispheric asymmetry caused by the annual variation of the inclination angle of the Earth's orbit. The differences between the spatial patterns represented by the EOF base functions of IRI-2016 and GIM TECs were analyzed by extracting the same time-varying coefficients. The deviations of the interhemispheric asymmetry component between the two data sets showed roughly equal values throughout the Southern or Northern Hemisphere, whereas those of the other spatial modes were mainly concentrated on the equatorial region. The differences of the time-varying characteristics between the IRI-2016 and GIM TECs were also compared by extracting the same EOF base functions. Although the EOF coefficients of the two data sets presented consistent seasonal variations, the magnitude of IRI-2016 TEC changes over time was less than that of GIM TEC. The diurnal variation of the daily averaged solar forcing component and the annual variation of the interhemispheric asymmetry component exhibited relatively large deviations between the two data sets. Considering the variance contribution of the different EOF components and their average relative deviations, both analyses showed that the daily averaged solar forcing and interhemispheric asymmetry components were the main factors for the deviation between the IRI-2016 and GIM TECs.
Highlights
The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth and stretches from a height of approximately 60 km to more than 1000 km
The global ionospheric map (GIM) Total electron content (TEC) used in this study is the official International GNSS Service (IGS) combined final product provided by the Crustal Dynamic Data Information System
We described the coefficients of the base function according to the changes in Universal Time (UT) and day of year (DOY) in Fig. 4 to reflect the seasonal changes effectively
Summary
The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth and stretches from a height of approximately 60 km to more than 1000 km. The IRI is one of the most accepted standard global empirical ionosphere models This model can be used to estimate the values of electron density and temperature, ion temperature and composition, and TEC at altitudes ranging from approximately 50 to 2000 km at a particular location, at a particular time, and on a particular day. Several assessments of the IRI models have been conducted, few studies on the comprehensive evaluation of the temporal and spatial distribution prediction performance of the IRI model are available. The predictive performance of the IRI model for ionospheric temporal and spatial changes should be evaluated using efficient analytical methods. GIM TEC data in 2013 were selected as reference values, and the EOF method was introduced to analyze the global TEC prediction performance of IRI-2016. Results provide a reference for the further understanding of the differences between the IRI2016 and the GIM TECs at a global scale
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