Topside Ionosphere Variability and Response to Solar EUV Using Empirical Orthogonal Function Analysis

Abstract:

The topside ionosphere is highly variable and is known to correlate on long time scales with the F10.7 flux, a common proxy for the solar EUV radiation. However, it is not generally known which EUV wavelength bands contribute most to the ionization, and most models use the proxy rather than incorporating real solar EUV data on a wavelength-by-wavelength basis. In addition, the ion density signal is modulated by a seasonal oscillation due to the changing solar zenith angle (SZA) which is more dramatic at high latitudes. We have used empirical orthogonal function (EOF) analysis to explore the variability in the topside ionosphere. We found that the first principal component correlates strongly with solar EUV, and the second principal component correlates with a seasonal oscillation due to the SZA, indicating that these two variables explain most of the variance. We use EOFs to remove the SZA oscillation in order to begin to investigate the response of the topside ionosphere to the solar EUV on a wavelength-by-wavelength basis.