Extreme ultraviolet spectral irradiance measurements since 1946

摘要

In the physics of the upper atmosphere the solar extremeultraviolet (EUV) radiation plays a dominant role controlling most of thethermospheric/ionospheric (T/I) processes. Since this part of the solarspectrum is absorbed in the thermosphere, platforms to measure the EUVfluxes became only available with the development of rockets reachingaltitude levels exceeding 80 km. With the availability of V2 rockets used inspace research, recording of EUV spectra started in 1946 using photographicfilms. The development of pointing devices to accurately orient thespectrographs toward the sun initiated intense activities insolar–terrestrial research. The application of photoelectric recordingtechnology enabled the scientists placing EUV spectrometers aboardsatellites observing qualitatively strong variability of the solar EUVirradiance on short-, medium-, and long-term scales. However, as moremeasurements were performed more radiometric EUV data diverged due to theinherent degradation of the EUV instruments with time. Also, continuousrecording of the EUV energy input to the T/I system was not achieved. It is onlyat the end of the last century that there was progress made in solving the seriousproblem of degradation enabling to monitore solar EUV fluxes with sufficientradiometric accuracy. The data sets available allow composing the dataavailable to the first set of EUV data covering a period of 11 years forthe first time. Based on the sophisticated instrumentation verified in space,future EUV measurements of the solar spectral irradiance (SSI) are promisingaccuracy levels of about 5% and less. With added low-cost equipment,real-time measurements will allow providing data needed in ionosphericmodeling, e.g., for correcting propagation delays of navigation signals fromspace to earth. Adding EUV airglow and auroral emission monitoring byairglow cameras, the impact of space weather on the terrestrial T/I systemcan be studied with a spectral terrestrial irradiance camera (STI-Cam) andalso be used investigating real-time space weather effects and deriving moredetailed correction procedures for the evaluation of Global NavigationSatellite System (GNSS) signals. Progress in physics goes with achievinghigher accuracy in measurements. This review historically guides thereader on the ways of exploring the impact of the variable solar radiation inthe extreme ultraviolet spectral region on our upper atmosphere in thealtitude regime from 80 to 1000 km.