Topside ionospheric response to solar EUV variability

Abstract

We present an analysis of 23 years of thermal plasma measurements in the topside ionosphere from the Defense Meteorological Satellite Program (DMSP) spacecraft. The H+/O+ ratio and density vary dramatically with the solar cycle; cross‐correlation coefficients between E10.7 and the daily averaged densities are greater than 0.85. The ionospheric parameters also vary dramatically with season, particularly at latitudes away from the equator where the solar zenith angle varies greatly with season. There are also 27 day solar rotation periodicities in the density, associated with periodicities in the directly measured solar EUV flux. Empirical orthogonal function analysis captures over 95% of the variation in the density in the first two principal components. The first principal component (PC1) is clearly associated with the solar EUV while the second principal component (PC2) is clearly associated with the solar zenith angle variation. The magnitude of the variation of the response of the topside ionosphere to solar EUV variability is shown to be closely related to the ionospheric 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 amplified by a factor of e at an altitude a scale height above the F2 peak. When the topside ionosphere is H+ dominated during solar minimum, DMSP may be much less than a scale height above the F2 peak while during solar maximum, when it is O+ dominated, DMSP may be several scale heights above the F2 peak.

WN4 variability in DMSP ion densities across season, solar cycle, and local time

“…Journal of Geophysical Research: Space Physics 10.1002/2017JA024065 depending on thermospheric heating and winds, as well as electric fields. It can be above or below the DMSP orbit altitude depending on these conditions [Anderson and Hawkins, 2016].…”

Long-term trends in the ionospheric response to solar extreme-ultraviolet variations

“…The most widely used proxies for ionospheric applications are the F10.7 index (solar radio flux at 10.7 cm, measured in solar flux units, sfu; see Tapping, 1987, Maruyama, 2010, the Mg II index (the core-to-wing ratio of the Magnesium K line; Maruyama, 2010), and indices based on direct EUV measurements (e.g., Schmidtke, 1976;Unglaub et al, 2011;Jacobi et al, 2016) such as the Solar EUV Experiment (SEE, Woods et al, 2000) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Using the latter poses the potential problem of satellite degradation (BenMoussa et al, 2013;Schmidtke et al, 2015), which may be overcome by repeated calibration or in-flight calibration as was applied during the SolACES experiment on board the ISS (Schmidtke et al, 2014(Schmidtke et al, , 2015. The understanding and realistic estimation of solar irradiance have been an open issue for long times, but in recent times the EUV datasets (either direct measurements, composite datasets, or models) have considerably improved (e.g., Haberreiter et al, 2017).…”

ELF Whistler Dependence on a Sunlit Ionosphere

“…The drastic swings in the dominant ion species shown in Figure illustrate the variability in the composition as a result of EUV radiation (Anderson & Hawkins, ) and other plasma processes in the ionosphere (Moen et al, ). The common times between the occurrence of ELF whistlers and the presence of solar radiation along with changes in ionospheric composition is worthy of further study.…”