Topside Ionospheric Response to Solar EUV Variability

Abstract:

We present an analysis of 23 years of thermal plasma measurements in the topside ionosphere from several DMSP spacecraft at ~800 km. The solar cycle variations of the daily averaged densities, temperatures, and H⁺/O⁺ ratios show a strong relationship to the solar EUV as described by the E10.7 solar EUV proxy with cross-correlation coefficients (CCCs) with the density greater than 0.85. The H⁺/O⁺ varies dramatically from solar maximum when it is O⁺ dominated to solar minimum when it is H⁺ dominated. These ionospheric parameters also vary strongly with season, particularly at latitudes well away from the equator where the solar zenith angle (SZA) varies greatly with season. There are strong 27-day solar rotation periodicities in the density, associated with the periodicities in the solar EUV as measured by the TIMED SEE and SDO EVE instruments, with CCCs at times greater than 0.9 at selected wavelengths. Empirical Orthogonal Function (EOF) analysis captures over 95% of the variation in the density over the 23 years in the first two principle components. The first principle component (PC1) is clearly associated with the solar EUV showing a 0.91 CCC with the E10.7 proxy while the PC1 EOFs remain relatively constant with latitude indicating that the solar EUV effects are relatively independent of latitude. The second principle component (PC2) is clearly associated with the SZA variation, showing strong correlations with the SZA and the concomitant density variations at latitudes away from the equator and with the PC2 EOFs having magnitudes near zero at the equator and maximum at high latitude. The magnitude of the variation of the response of the topside ionosphere to solar EUV variability is shown to be closely related to the composition. This is interpreted as the result of the effect of composition on the scale height in the topside ionosphere and the "pivot effect" in which the variation in density near the F2 peak is expected to be amplified by a factor of e at an altitude a scale height above the F2 peak. When the topside ionosphere is H⁺ dominated, DMSP may be much less than a scale height above the F2 peak while when it is O⁺ dominated, DMSP may be several scale heights above the F2 peak.