Extreme ultraviolet spectral irradiance measurements since 1946

Abstract

Abstract. In the physics of the upper atmosphere the solar extreme ultraviolet (EUV) radiation plays a dominant role controlling most of the thermospheric/ionospheric (T/I) processes. Since this part of the solar spectrum is absorbed in the thermosphere, platforms to measure the EUV fluxes became only available with the development of rockets reaching altitude levels exceeding 80 km. With the availability of V2 rockets used in space research, recording of EUV spectra started in 1946 using photographic films. The development of pointing devices to accurately orient the spectrographs toward the sun initiated intense activities in solar-terrestrial research. The application of photoelectric recording technology enabled the scientists placing EUV spectrometers aboard satellites observing qualitatively strong variability of the solar EUV irradiance on short-, medium-, and longterm scales. However, as more measurements were performed more radiometric EUV data diverged due to the inherent degradation of the EUV instruments with time. Also, continuous recording of the EUV energy input to the T/I system was not achieved. It is only at the end of the last century that there was progress made in solving the serious problem of degradation enabling to monitore solar EUV fluxes with sufficient radiometric accuracy. The data sets available allow composing the data available to the first set of EUV data covering a period of 11 years for the first time. Based on the sophisticated instrumentation verified in space, future EUV measurements of the solar spectral irradiance (SSI) are promising accuracy levels of about 5 % and less. With added low-cost equipment, real-time measurements will allow providing data needed in ionospheric modeling, e.g., for correcting propagation delays of navigation signals from space to earth. Adding EUV airglow and auroral emission monitoring by airglow cameras, the impact of space weather on the terrestrial T/I system can be studied with a spectral terrestrial irradiance camera (STI-Cam) and also be used investigating real-time space weather effects and deriving more detailed correction procedures for the evaluation of Global Navigation Satellite System (GNSS) signals. Progress in physics goes with achieving higher accuracy in measurements. This review historically guides the reader on the ways of exploring the impact of the variable solar radiation in the extreme ultraviolet spectral region on our upper atmosphere in the altitude regime from 80 to 1000 km.

Making of a solar spectral irradiance dataset I: observations, uncertainties, and methods

“…As Schmidtke (2014) describes in his historical overview of SSI measurements, the first attempts at obtaining SSI were not suitable for long-term recording of the variable solar spectrum. While E. Regener and V. H. Regener, in 1932, used balloons flying up to an altitude of 33 km to obtain a spectrum of the visible to the far ultraviolet (UVC, 200 nm-280 nm) region, Baum et al (1946, cited by Schmidtke) used V2 rockets to obtain the first SSI measurements in the extreme ultraviolet (EUV) band.…”

Where does the Thermospheric Ionospheric GEospheric Research (TIGER) Program go?

“…However, in this spectral range the optics and detectors steadily suffer from loss of the initial calibration due to degradation. Degradation of each optical component is caused by diversity and unique diverse physical processes so that it is not possible to model the resulting efficiency changes -it must be measured (Schmidtke, 2015). The more components involved, the more difficult it is to determine the efficiency of the instrument.…”

“…If crosscalibration capability becomes available, a compilation of instruments out of the following missions will fulfill the requirements explained in the preceding sections: SOHO, TIMED, GOES-13/-15, SDO, P-2, and CORONAS-PHOTON with EUV/UV spectrometers, broadband detectors (silicon detectors with thin film filters; Spherical EUV and Plasma Spectrometer (Schmidtke, 2015)), and/or poly-chromators.…”

“…Space missions recording solar EUV emissions from 1946 until 2014 have been reviewed by Schmidtke (2015) showing that there is no coherent data set covering the full EUV spectral region from 5-105 nm as relevant for the T/I system over a full period of a solar cycle, yet. The NASA TIMED Solar EUV Experiment (SEE) is closest to meeting this need with its daily spectral measurement between 27 nm and 190 nm from 2002 to 2015.…”

Spectroradiometry with space telescopes

“…Monitoring the solar EUV irradiance for use in modelling the thermosphere and the ionosphere, however, is an operational issue that cannot wait until a clean platform becomes available: real-time measurements of the solar VUV irradiance provide data needed for ionospheric modelling that in turn permits correcting propagation delays of navigation signals from space to Earth. 8 In an aperçu on the history of VUV solarirradiance measurements since 1946, Schmidtke (2015) points out that it was only at the end of the twentieth century that enough progress had been made to solve the serious problem of degradation. The use of a primary standard in orbit now permits measuring the solar VUV irradiance with sufficient radiometric accuracy.…”