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

Abstract

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. 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. 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 1nm to 133nm. 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.