Experiences with a three-current ionization chamber as primary detector standard for absolute calibration in space

Context: With ISS-SOLAR-SolACES, a new approach in terms of calibrating solar spectral irradiance (SSI) data was validated during the mission period from 2008 to 2017: an ionization chamber (IC) as primary detector standard, operated in space, allowed daily calibration measurements. Issue: Though primary photo-ionization efficiencies and higher-order ionization effects from photoelectrons are constant with time, filter transmissions and signal contributions from solar x-rays are to be determined for the SSI data evaluation, requiring a deeper investigation. Aims and Methods: The experiences made with ionization chambers in the laboratory and aboard the ISS-SOLAR-SolACES mission are presented. They include the determination of higher-order ionization effects, the measured transmission of the filters with time, and the treatment of the solar soft x-ray background. Recommended combinations of IC filling gases and filter materials as well as laboratory and in-space measurements will provide correction procedures for different levels of solar activity to achieve further improvement of SSI data accuracy in the spectral range from 1 nm to 133 nm. Results and Perspectives: The usage of the IC technology aims for establishing absolute SolACES-type calibration equipment in space, providing reference SSI data sets in solar and solar-terrestrial science, as well as in related applications such as global navigation satellite system signal evaluation.