Detecting undocumented trends in solar irradiance observations

Abstract

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.

SPAM: Solar Spectrum Prediction for Applications and Modeling

“…The point is that the Earth's atmosphere completely absorbs solar irradiance in the range of 0-190 nm, so measurements can only be carried out on board the spacecraft, which makes it difficult to continuously control the quality of received data. The sensitivity of the photosensors may suffer during a prolonged operation of instruments in space, leading to systematic errors and false trends in observations [55]. To build an accurate model, it is necessary to exclude data distorted due to sensor degradation.…”

Rome Precision Solar Photometric Telescope: precision solar full-disk photometry during solar cycles 23–25

“…Fröhlich, 2013;Kopp, 2016Kopp, , 2021Montillet et al, 2022); SSI changes in the UV in phase with the solar cycle are also well established and assessed to reach several tens of percent for the radiation below 200 nm (e.g. Deland and Cebula, 2008;Woods et al, 2018;Marchenko et al, 2019;Woods et al, 2022), while there is still an uncertainty in both the phase and variability of the SSI changes in the visible and infrared ranges (Ermolli et al, 2013;Coddington et al, 2019;Dudok de Wit, 2022). Still debated is also the TSI trend on the timescales longer than the solar cycle (e.g.…”

“…We recall that the TSI is the solar radiative energy flux per unit area integrated over the entire spectrum, measured at normal incidence at the top of the Earth's atmosphere and at a mean Sun-Earth distance of one astronomical unit, given in units of W m −2 , while the SSI is the analogous quantity but spectrally resolved, given in units of W m −2 nm −1 . All existing measurements show a clear TSI change by ≈ 0.1% in phase with the 11-year solar activity cycle and TSI fluctuations by up to 0.2-0.3% on timescales shorter than a few days (e.g., Fröhlich, 2013;Kopp, 2016Kopp, , 2021Montillet et al, 2022); SSI changes in the UV in phase with the solar cycle are also well established and assessed to reach several tens of percent for the radiation below 200 nm (e.g., Deland and Cebula, 2008;Woods et al, 2018;Marchenko et al, 2019;Woods et al, 2022), while there is still an uncertainty in both the phase and variability of the SSI changes in the visible and infrared ranges (Ermolli et al, 2013;Coddington et al, 2019;Dudok de Wit, 2022). Still debated is also the TSI trend on the timescales longer than the solar cycle (e.g., Kopp, 2021;Schmutz, 2021).…”