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

The solar quiet (Sq) current in the American and European/African sectors are derived from ground magnetometer data from January 2006 to December 2019 based on the spherical harmonic analysis (SHA) technique. We decomposed the Sq current function into basic modes by applying the empirical orthogonal function (EOF) analysis, and the first three eigenmodes capture about 96% of the total variance in the Sq current. We observed that the first mode (EOF1) is modulated by the solar activity, while the second (EOF2) and third (EOF3) modes show annual and semiannual patterns characterized by positive and negative variations. However, these eigenmodes are utilized to reconstruct the Sq current function. In the American sector, total Sq current intensity (Jtotal) of low solar minimum (F10.7 = 80 sfu) is about 58% of high solar maximum (F10.7 = 200 sfu). This percentage reduces to about 52% in the European/African sector. At 12:00 LT, the summer current vortex is amplified and penetrates the south into the winter hemisphere. Such a deep crossequatorial Sq current flow is not evident in the American sector at F10.7 = 200 sfu. In both sectors, the southern current vortex is higher than the northern current vortex, with average foci near 40o magnetic latitude. The northern focus hour appears to be earlier than the southern focus hour. Our EOF model with only three eigenmodes suggests a dependence of Sq current variability on solar activity variations, which could be used as a basis for further numerical modeling of the Sq current variations.