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We have applied the empirical orthogonal function (EOF)analysis to examine the climatology of the total ion density Ni at 840 kmduring the period 1996-2004, obtained from the Defense MeteorologicalSatellite Program (DMSP) spacecraft. The data set for each of the localtime (09:30 LT and 21:30 LT) is decomposed into a time mean plus the sum ofEOF bases Ei of space, multiplied by time-varying EOF coefficientsAi. Physical explanations are made on the first three EOFs, whichtogether can capture more than 95% of the total variance of the original dataset. Results show that the dominant mode that controls the Ni variability isthe solar EUV flux, which is consistent with the results ofRich et al. (2003). The second EOF, associated with the solar declination, presents anannual (summer to winter) asymmetry that is caused by the transequatorialwinds. The semiannual variation that appears in the third EOF for theevening sector is interpreted as both the effects of the equatorialelectric fields and the wind patterns. Both the annual and semiannual variationsare modulated by the solar flux, which has a close relationship with theO+ composition. The quick convergence of the EOF expansion makes itvery convenient to construct an empirical model for the original data set.The modeled results show that the accuracy of the prediction depends mainlyon the first principal component which has a close relationship with thesolar EUV flux

annales geophysicae

Statistical characteristics of the total ion density in the topside ionosphere during the period 1996-2004 using empirical orthogonal function (EOF) analysis