Long-term trends in solar spectral irradiances from satellite observations during solar cycles 23 and 24

A controversy about the UV solar irradiance changes during the recent eleven year solar cycle and its implication on earth's climate and atmosphere has arisen. During the declining phase of solar cycle 23 (2004-2008) UV irradiance changes derived from recent SIM satellite data are unexpectedly larger than those known in previous solar cycles and expected from models. We propose to provide an independent assessment of trends during solar cycles 23 and 24 based upon re-calibrated solar spectral irradiance (SSI) data from SCIAMACHY satellite observations (2002-2012). As part of this project a new calibration scheme will be developed to improve SCIAMACHY solar spectral irradiance (SSI) data, correcting adequately the optical degradation of the instrument with time. The experience gained from SCIAMACHY will be used to extend the SCIAMACHY time series with re-calibrated SSI data from GOME and GOME-2 (1995-2020).

A large part of the work will focus on the accurate re-calibration scheme required to improve the long-term SCIAMACHY solar spectral irradiance data record (work package 1). The re-calibrated SCIAMACHY data will be extensively validated by comparisons with other satellite data and available reference data sets (work package 2). The solar trends as observed by SCIAMACHY during the descending phase f SC 23 and ascending phase of SC 24 will be investigated and compared to the trends observed by SIM (work package 3). Different statistical analyses will be carried out to compare SC behaviour of the SCIAMACHY data with respect to popular solar proxies like Mg II, Lyman alpha, and F10.7 (work package 4). Based upon the experience from SCIAMACHY, our re-calibration scheme will be adapted to GOME and GOME-2 (1995-2020) in order to provide a homogenized SSI timeseries extended in time going back to year 1995 (work package 5).