ABSTRACT
The accurate measurement of solar spectral irradiance (SSI) from space is challenging because it requires a system that can reliably exclude the impact of degradation on the instruments for the full duration of a long-term mission. The new Autocalibrating XUV-IR Spectrometer System (SOLACER) presented here meets this requirement. It is a compact and moderate-cost instrument that allows repeated onboard calibration of the spectrometers (SPs) providing SSI data of significantly increased accuracy. To this end, absolute radiometers and ionization chambers with proven long-term stability serve as primary irradiance detectors to determine the absolute SSI fluxes passing through a series of narrow- and medium-band filters with updated transmission. Consecutively cross-calibrated bolometers (BOSs) and highly sensitive photomultiplier tubes are used as secondary irradiance detectors. The new SOLACER instrument is designed to cover the spectral range from about 2-2800 nm with eight planar grating SPs. Adding total solar irradiance and BOS sensors adapted to observe the Earth's infrared emission and the reflected solar radiation, the global energy budget at the top of the atmosphere can be measured to provide data of higher absolute accuracy for climate modeling, too.
ABSTRACT
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.
