History of extreme ultraviolet solar measurements in XX-XXI centuries: From balloons to the International Space Station as instrumental platforms
The Sun was one of the central points for observations as long as human cultures are known. After researchers observed the extension of the solar rainbow spectrum towards shorter wavelengths with increasing altitude they started measurements of the solar ultraviolet spectrum from balloons in 1934. The acquisition of spectra in the extreme ultraviolet and soft X-ray spectral regions was beginning after 1946 using photographic recordings aboard rockets. However, it took six more years to achieve good spectral resolution below about 220 nm because servomechanisms had to be developed to fairly accurately point the spectrographs to the Sun during the rocket flight. Another big step forward was achieved by the application of photoelectric recording technology and by using satellites as a platfor m to observe qualitatively the strong variability of the solar ultraviolet irradiance on short-, medium-and long-term scales. Until the end of the past century these measurements have been strongly impeded by the inherent degradation of the EUV instruments with time. Since there is progress solving this serious problem, measurements with sufficient radiometric accuracy allow composing the data available to the first set of ultraviolet data covering a period of eleven years. Based on the sophisticated instrumentation verified in space, future EUV measurements of the solar spectral irradiance on the International Space Station are promising accuracy levels of about 5%. Added by low-cost equipment on-line measurements will allow providing data needed in ionospheric modeling for correcting propagation delays of navigation signals from space to Earth. Adding ultraviolet airglow and auroral emission measurements the impact of space weather on the ionosphere can be studied and also be used deriving more detailed correction procedures for the evaluation of Global Navigation Satellite System (GNSS) signals.