Detecting undocumented trends in solar irradiance observations

Quantifying the long-term stability of solar irradiance observations is crucial for determining how the Sun varies in time and for detecting decadal climate change signals. The stability of irradiance observations is challenged by the degradation of instrumental sensitivity in space and by the  corrections that are needed to mitigate this degradation. We propose a new framework for detecting instrumental trends, based on the existing idea of comparing the solar irradiance at pairs of dates for which a proxy quantity reaches the same level. Using a parametric model we then reconstruct the trend and its confidence interval at all times. While this method cannot formally prove the instrumental origin of the trends, the \modif{observation} of similar trends with different proxies provides strong evidence for a non-solar origin. We illustrate the method with spectral irradiance observations from the Solar Radiation and Climate Experiment (SORCE) mission, using various solar proxies such as sunspot number, MgII index, F10.7 index. The results support the existence of non-solar trends that exceed the level of solar cycle variability. After correcting the spectral irradiance for these trends, we find the difference between the levels observed at solar maximum and at solar minimum to be in better agreement with irradiance models.