Improved solar spectral irradiance from SCIAMACHY satellite observations using optimised degradation corrections

The SCIAMACHY instrument on board ESA’s environmental satellite Envisat was operated from 2002 to 2012 and provided spectrally resolved measurements of the Earth’s atmosphere. In addition, it measured daily solar spectral irradiances (SSI), forming a valuable database for solar studies.
The first result of this dissertation is a new SCIAMACHY reference solar spectrum covering 235 to 2384 nm at a spectral resolution of 0.2 to 1.5 nm. The spectrum was recorded before substantial optical degradation set in. Instrumental changes between on-ground calibration and in-orbit operation were corrected using measurements of SCIAMACHY’s internal white light source (WLS). The resulting solar spectrum agrees well with other SSI references for most parts of the spectral range and adequately accounts for substantial throughput losses due to detector icing in the near infrared (NIR). The results significantly contribute to solving controversies in the NIR and support recent studies indicating that the ATLAS-3 composite is biased in the NIR.
The second part investigates the full SCIAMACHY SSI data record and its suitability to study SSI variability and trends. One main limitation for long-term space-based measurements is the optical degradation of the instrument. This work focuses on improving the degradation correction by combining WLS measurements with direct solar measurements. This allows to better separate instrument variations and natural solar variability. Before, the WLS emission changes with time were successfully accounted for. The re-calibrated SCIAMACHY SSI shows an overall improvement with some remaining instrumental issues.
Finally, a solar proxy model using the MgII index and the photometric sunspot index was applied to the re-calibrated SCIAMACHY SSI. The approach can eliminate remaining instrumental drifts or offsets. The modelled time series (1979 to 2014) agrees well with other SSI reconstructions on shorter, i.e. solar rotational, timescales but indicate limitations for solar cycle timescales, especially at higher wavelengths. Relative changes between solar cycle maximum and minimum are consistent with reference data sets.
Overall, this work highlights the importance of careful calibration and the value of an internal WLS for degradation monitoring.