Abstract
Quantifying the long-term stability of solar irradiance observations is crucial for determining how the Sun varies in time and detecting decadal climate change signals. The stability of irradiance observations is challenged by the degradation of instrumental sensitivity in space and by the post-launch corrections 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 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 good agreement with irradiance models.
Highlights
One of the major challenges in solar irradiance studies is the production of long records that are accurate and stable in time
We provide a mathematical framework for extracting undocumented trends from solar irradiance data by comparing the observed variability with several solar proxies such as the F10.7 index, the MgII index and the sunspot number
Compared to previous approaches by Morrill et al (2014) and Woods et al (2018), we provide a parametric expression of the correction, which 1) allows the correction to be estimated at any time and 2) provides confidence intervals, which are vital for testing the significance of the results
Summary
One of the major challenges in solar irradiance studies is the production of long records that are accurate and stable in time. For example, rely on the solar UV input into the atmosphere to quantify ozone production in the tropical stratosphere, which is one of the levers by which solar variability affects climate (Gray et al, 2010). Solar physicists need to understand long-term solar variability to properly constrain past solar variations (Coddington et al, 2016), while satellite operators need an accurate UV flux to estimate satellite drag (Vourlidas & Bruinsma, 2018). Of particular interest for all these users is the spectrally-resolved solar irradiance, or solar spectral irradiance (SSI), because different wavelengths impact the Earth’s environment in distinct ways.
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