Ionospheric Current Variations by Empirical Orthogonal Function Analysis: Solar Activity Dependence and Longitudinal Differences

The solar quiet (Sq) ionospheric current variations exhibit spatial and temporal patterns that can be identified by the prevailing eigenmodes based on the empirical orthogonal function (EOF) analysis. In this study, the Sq current function over the American and European/African sectors are derived using ground magnetometer data from 2006 to 2019 based on the spherical harmonic analysis technique. Subsequently, we decomposed the Sq current function into eigenmodes by applying the EOF analysis, where the first three eigenmodes capture 96% of the overall Sq current variance. Additionally, these eigenmodes are utilized to model the Sq current function and compare its properties between the two longitudinal sectors. We observed that the EOF model could reconstruct the observed Sq current function with the first three eigenmodes in both longitudinal sectors. Moreover, the EOF model unveils a clear association of the Sq current function with several driven features, such as magnetic latitude, local time, season, and solar activity. Both longitudinal sectors had comparable Sq current patterns under varying solar activity, while their amplitudes varied. Besides, the newly developed model could reproduce a refined Sq current variability over the two longitudinal sectors as long as the observed Sq variations have sufficient duration. Our EOF model shows that the variations of the Sq current function with solar activity can be explained by the first three eigenmodes, which could be used as a basis for further numerical modeling of the Sq current variations.