Reconstructing solar irradiance from historical Ca II K observations

Abstract

Context. Knowledge of solar irradiance variability is critical to Earth’s climate models and understanding the solar influence on Earth’s climate. Direct solar irradiance measurements have only been available since 1978. Reconstructions of past variability typically rely on sunspot data. However, sunspot records provide only indirect information on the facular and network regions, which are decisive contributors to irradiance variability on timescales of the solar cycle and longer. Aims. Our ultimate goal is to reconstruct past solar irradiance variations using historical full-disc Ca II K observations to describe the facular contribution independently of sunspot observations. Here, we develop the method and test it extensively by using modern CCD-based (charge-coupled device) Ca II K observations. We also carry out initial tests on two photographic archives. Methods. We employ carefully reduced and calibrated Ca II K images from 13 datasets, including some of the most prominent series, such as those from the Meudon, Mt Wilson, and Rome observatories. We convert them to unsigned magnetic field maps and then use them as input to the adapted Spectral and Total Irradiance Reconstruction (SATIRE) model to reconstruct total solar irradiance (TSI) variations over the period 1978–2019, for which direct irradiance measurements are available. Results. The reconstructed irradiance from the analysed Ca II K archives agrees well with direct irradiance measurements and existing reconstructions. The model also returns good results on data taken with different bandpasses and images with low spatial resolution. Historical Ca II K archives suffer from numerous inconsistencies, but we show that these archives can still be used to reconstruct TSI with reasonable accuracy provided the observations are accurately processed and the effects of changes in instrumentation and instrumental parameters are identified and accounted for. The reconstructions are relatively insensitive to the TSI reference record used to fix the single free parameter of the model. Furthermore, even employment of a series, itself reconstructed from Ca II K data, as a reference for further reconstructions returns nearly equally accurate results. This will enable the Ca II K archives without an overlap with direct irradiance measurements to be used to reconstruct past irradiance. Conclusions. By using the unsigned magnetic maps of the Sun reconstructed from modern high-quality Ca II K observations as input into the SATIRE model, we can reconstruct solar irradiance variations nearly as accurately as from directly recorded magnetograms. Historical Ca II K observations can also be used for past irradiance reconstructions but need additional care, for example identifying and accounting for discontinuities and changes in the quality of the data with time.

Full-disc Ca ii K observations—A window to past solar magnetism

“…For most applications of Ca II K data one observation per day is typically sufficient, however having multiple observations per day from various sources is extremely important. On the one hand, there are applications for which the exact time of observation might be needed, for instance for comparing Ca II K images to magnetograms to select data pairs as close in time as possible (Chatzistergos et al, 2019d), or to recover TSI variations within a day (Chatzistergos et al, 2021a). On the other hand, complementary observations from different sites over the same days allow assessment of the quality of the available datasets and identification of potential inconsistencies within the archives.…”

“…This approach produces unsigned magnetograms and does not recover the magnetic field strength in sunspot regions. Chatzistergos et al (2021a) used this approach to reconstruct unsigned magnetograms from 13 Ca II K archives, including the historical photographic datasets from Mt Wilson and Meudon. We note that the spatial resolution is an important factor when recovering magnetograms, since low spatial resolution results in high probability of "missing" part of the magnetic flux due to sub-pixel cancellation of opposite-polarity elements (Krivova and Solanki, 2004).…”

“…Use of historical photographic data for irradiance reconstructions has so far been rather limited, with only five TSI reconstructions from historical Ca II K observations known to us, namely, by Ambelu et al (2011), Foukal (2012, Penza et al (2022), Xu et al (2021), and Chatzistergos et al (2021a). Ambelu et al (2011), Foukal (2012), and Xu et al (2021 used the photometrically uncalibrated Mt Wilson, Mt Wilson, and Kodaikanal Ca II K data, respectively to reconstruct TSI variations with a linear regression model.…”

“…All of these factors lets by Chatzistergos et al (2019b) to reconstruct TSI over the last five centuries with an empirical model. Chatzistergos et al (2021a) and Chatzistergos et al (2021b) used 13 Ca II K archives, including historical photographic archives from Meudon and Mt Wilson, to reconstruct past irradiance variations with the Spectral and Total Iiradiance REconstruction (SATIRE Krivova et al, 2003) model. Figure 13 shows the reconstructed TSI series from four Ca II K archives by Chatzistergos et al (2021a), in particular those from Rome/PSPT, San Fernando CFDT2, Meudon K3, and Meudon K1v (see Figure 4 for the correspondence of the K3 and K1v parts of the Ca II K line).…”

“…Chatzistergos et al (2021a) and Chatzistergos et al (2021b) used 13 Ca II K archives, including historical photographic archives from Meudon and Mt Wilson, to reconstruct past irradiance variations with the Spectral and Total Iiradiance REconstruction (SATIRE Krivova et al, 2003) model. Figure 13 shows the reconstructed TSI series from four Ca II K archives by Chatzistergos et al (2021a), in particular those from Rome/PSPT, San Fernando CFDT2, Meudon K3, and Meudon K1v (see Figure 4 for the correspondence of the K3 and K1v parts of the Ca II K line). The reconstructions done by Chatzistergos et al (2021a) were limited to the periods with overlap to the direct TSI measurements, since their aim was to assess the quality of the reconstruction.…”