EUV data processing methods of the Solar Auto-Calibrating EUV Spectrometers (SolACES) aboard the International Space Station

Since 2008, the Solar Auto-Calibrating EUV Spectrometer (SolACES) experiment monitors the solar EUV radiation. SolACES is part of the SOLAR mission, a device mounted on the COLUMBUS module on the International Space Station (ISS). The SolACES device is capable of measuring the EUV radiation of the full solar disk, with three grazing incidence spectrometers, covering the spectral range from 17 to 135 nm. Degradation and ageing effects of hardware components are a major issue in the field of EUV measurements. In addition, propellant pollution from ISS logistic vehicles strongly complicate the instrumental operation as well as the data evaluation. To correct these effects, two double ionization chambers have been mounted onto the device frame. These are considered as a primary radiometric detector standard. By refilling the ionization chambers, the absolute EUV flux can be measured. Efficiency changes, being a severe issue in the field of measurements of EUV radiation, can corrected via an in-situ auto-calibration. The use of this method requires a thorough understanding of electron multiplier degradation and processes inside an ionization chamber. Aims The main focus of this paper lies on the correction of hardware ageing effects in the spectrometer readout, as well as on the usage of ionization chamber measurements for absolute calibration. Additionally, we intend to demonstrate the long term stability of the in-orbit calibration principle of EUV spectrometers with double ionization chambers. Methods In this paper, we first briefly present the instrument and its components. Second, we list, explain, and correct various degradation effects within the spectrometer units. The core part are the adjustments between spectrometer and ionization chamber data, for calibration purposes. Results and perspectives The proof of concept of the in-orbit calibration, via ionization chambers, is given, by intercomparison of EUV data from SolACES with other experiments, over almost one decade. The presented data can be used for further investigations in climate modelling, as well as contribute to a SSI composite.