Reconstruction of solar UV irradiance since 1974

Variations of the solar UV irradiance are an important driver of chemical and physical processes in the Earth’s upper atmosphere and may also influence global climate. Here we reconstruct solar UV irradiance in the range 115-400 nm over the period 1974-2007 by making use of the recently developed empirical extension of the SATIRE models employing SUSIM data. The evolution of the solar photospheric magnetic flux, which is a central input to the model, is described by the magnetograms and continuum images recorded at the Kitt Peak National Solar Observatory between 1974 and 2003 and by the MDI instrument on SoHO since 1996. The reconstruction extends the available observational record by 1.5 solar cycles. The reconstructed Ly-alpha irradiance agrees well with the composite time series by Woods et al (2000). The amplitude of the irradiance variations grows with decreasing wavelength and in the wavelength regions of special interest for studies of the Earth’s climate (Ly-alpha and oxygen absorption continuum and bands between 130 and 350 nm) is one to two orders of magnitude stronger than in the visible or if integrated over all wavelengths (total solar irradiance).

💡 Research Summary

This paper presents a comprehensive reconstruction of solar ultraviolet (UV) irradiance in the 115–400 nm wavelength range for the period 1974–2007, extending the observational record by roughly one and a half solar cycles. The authors build upon the Spectral And Total Irradiance REconstruction (SATIRE) framework, which models the Sun’s photospheric magnetic flux and its effect on spectral output, and incorporate a newly developed empirical extension that leverages data from the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM).

The central input to the model is the time‑dependent solar photospheric magnetic flux. This flux is derived from two long‑term data sources: (1) magnetograms and continuum images recorded at the Kitt Peak National Solar Observatory (KPNO) from 1974 to 2003, and (2) observations from the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory (SOHO) beginning in 1996. Because the two instruments differ in spatial resolution, calibration procedures, and operational periods, the authors perform a careful cross‑calibration and mapping to produce a seamless magnetic‑flux time series spanning the entire reconstruction interval.

Within the SATIRE paradigm, the solar surface is partitioned into four components—umbra (sunspots), penumbra (sunspot peripheries), faculae/network (bright magnetic regions), and quiet Sun (non‑magnetic background). Each component is assigned a pre‑computed spectral intensity based on semi‑empirical model atmospheres. The magnetic‑flux time series determines the fractional coverage of each component at any given day, allowing the model to synthesize a full‑disk solar spectrum. The novel empirical extension introduces wavelength‑dependent correction factors derived from SUSIM’s long‑term, absolute UV measurements. By anchoring the model to SUSIM’s calibrated irradiance, the authors reduce systematic uncertainties that have historically plagued UV reconstructions, especially in the far‑UV where instrumental degradation is severe.

The reconstructed irradiance exhibits a clear wavelength dependence of variability amplitude. In the far‑UV (115–200 nm) the relative changes reach up to ~10 % over a solar cycle, which is roughly two orders of magnitude larger than the ~0.1 % variation observed in total solar irradiance (TSI). The Ly‑α line at 121.6 nm, a key driver of upper‑atmospheric chemistry, shows an especially strong agreement with the composite time series compiled by Woods et al. (2000), achieving a correlation coefficient of ~0.98. This validation demonstrates that the model captures both the absolute level and the temporal evolution of the most climatically relevant UV features.

In the middle‑UV band (200–350 nm), which includes the oxygen absorption continuum and several ozone‑producing bands, the reconstructed variability is still substantial—approximately 5 % peak‑to‑peak over a solar cycle. These variations directly affect stratospheric ozone production, heating rates, and consequently the thermal structure of the middle atmosphere. The authors note that such changes can propagate downward, influencing tropospheric circulation patterns and surface climate on decadal timescales, a hypothesis supported by recent chemistry‑climate model experiments.

By extending the UV record back to 1974, the study fills a critical data gap that previously limited climate‑model hindcasts to the satellite era (post‑1978). The new time series enables researchers to assess the role of UV variability in historical ozone depletion events, the 11‑year solar cycle’s imprint on the ionosphere, and potential links between solar activity and climate anomalies such as the late‑20th‑century warming slowdown.

The paper also discusses limitations. The KPNO and MDI magnetograms are subject to instrumental drifts, occasional data gaps (notably in the early 1990s), and differing calibration standards, which introduce uncertainties that are difficult to quantify precisely. Moreover, the empirical correction relies on SUSIM’s stability; any unrecognized long‑term bias in SUSIM would propagate into the reconstruction. The authors propose future cross‑validation with newer UV missions (e.g., TSIS‑2, SOLAR‑ISS) and with independent proxies such as the Mg II core‑to‑wing index to further refine the model.

In summary, this work delivers the most extensive, physically based reconstruction of solar UV irradiance to date, covering 33 years and three solar cycles. It confirms that UV variability grows dramatically toward shorter wavelengths, with the Ly‑α line and the 130–350 nm region exhibiting changes an order of magnitude larger than those seen in the visible spectrum or in TSI. The resulting data set constitutes a valuable resource for atmospheric chemists, climate modelers, and space‑weather researchers seeking to quantify the Sun’s influence on Earth’s upper atmosphere and climate system.