“…The advantages of the EOF analysis method have made it one of the preferred methods in the meteorological field to analyze the dominant components of temporal and spatial variations [Lorenz, 1956]. The EOF analysis method has also been used for empirical modeling of ionospheric and thermospheric variability [e.g., Dvinskikh, 1988;Singer and Dvinskikh, 1991;Daniell et al, 1995;Matsuo et al, 2002Matsuo et al, , 2005Marsh et al, 2004;Zhao et al, 2005;Zapfe et al, 2006;Liu et al, 2008;Mao et al, 2008;Zhang et al, 2009Zhang et al, , 2010Matsuo and Forbes, 2010;A et al, 2011;Wan et al, 2012]. From the mathematical perspective, the orthogonal base functions, also called EOFs, are discrete functions of discrete variables.…”