Modeling Decadal and Centennial Solar UV Irradiance Changes

TL;DR

Addressing decadal-to-centennial solar UV irradiance variability and its climate impact, the authors develop an empirical framework that separates cycle-driven active-region effects from centennial trends tied to the open solar flux $F_{0}$, with a cycle-averaged parameter $P_k$ linking cycle shape to $F_{0}$. They reconstruct four UV bands (FUV 115–180 nm, MUV 180–310 nm, UV1 100–243 nm, UV2 243–308 nm) and the FUV–MUV color index by calibrating a plage–network model to SSI3 data and introducing a long-term modulation $F_{LT}$ derived from an IMF-based decomposition of $F_{0}$. Millennium reconstructions (971–2020 CE) combine plage coverage from chatzistergos20, cycle parameters, and $F_{LT}$ to produce smoothed 22-year UV histories that generally agree with SSI3, with notable wavelength-dependent deviations from SATIRE in MUV/UV2. The work advances our understanding of solar UV variability and provides datasets and methods for improving atmospheric chemistry, climate forcing, and exoplanet habitability studies over centennial timescales.

Abstract

Reconstructions of solar spectral irradiance - especially in the ultraviolet (UV) range - are crucial for understanding Earth's climate system. Although total solar irradiance (TSI) has been thoroughly investigated, the spectral composition of solar radiation offers a deeper insight into its interactions with the atmosphere, biosphere, and climate. UV radiation, in particular, plays a key role in stratospheric chemistry and the dynamics of stratospheric ozone. Reconstructing solar irradiance over the past centuries requires accounting for both the cyclic modulation of active-region coverage associated with the 11-year solar cycle and the longer-term secular trends, including their centennial variability. This study utilizes an empirical framework, based on a 1000-year record of Open Solar Flux, to characterize the various temporal components of solar irradiance variability. We then combine these components to reconstruct Solar UV irradiance variations in spectral bands crucial for Earth's atmospheric studies.

Figures (7)

• Figure 1: An example of a SSI3 composite spectrum, in logarithmic scale, from 100 nm to 310 nm with different bands highlighted by different colors.
• Figure 2: The correlation between FUV-MUV color and the Bremen Mg II index is shown for monthly values (red crosses) and for yearly averaged data (blue points) for the period 1980-2016. The Pearson's coefficient is r=-0.985 for monthly values and r=-0.989 for yearly values.
• Figure 3: Comparison of the monthly averages of the reconstruction (blue line) and the SSI3 composite data (red line) for the four UV bands: FUV (top-left), MUV (top-right), $\mathrm{UV_{1}}$ (bottom-left), and $\mathrm{UV_{2}}$ (bottom-right).
• Figure 4: Top panel: Reconstruction of plage area coverage from 971 CE to the present. Bottom Panel: The reconstructed time series (solid curve) and its uncertainty range (grey region) are shown from 1900 to the present. The actual measured monthly plage area coverage by chatzistergos20 is shown as dotted line.
• Figure 5: Smoothed 22-year reconstructions from 971 CE to the present, obtained in this work (blue line), along with their uncertainty ranges (shaded blue regions). This uncertainty arises from the propagation of errors in the plage coverage and in the coefficients in Tab.\ref{['tab2']}. The red lines represent the corresponding reconstructions from jungclaus2016. The five panels show: FUV (top-left), MUV (top-right), $\mathrm{UV_{1}}$ (middle-left), $\mathrm{UV_{2}}$ (middle-right) and the color index FUV-MUV (bottom).