The Sun’s UV Ups and Downs Over the Last 1,000 Years

A team of scientists led by the Solar and Space Physics group at University of Rome Tor Vergata has recently reconstructed the Sun’s ultraviolet (UV) output over the past millennium, spanning the period from 971 to the present. This work is the result of an international collaboration involving researchers from the National Solar Observatory, funded by the U.S. National Science Foundation (NSF), as well as from the Zurich University of Applied Sciences. The results of this study have been published in Solar Physics as Reda, R., Penza, V., Criscuoli, S. et al. (2025), “Modeling Decadal and Centennial Solar UV Irradiance Changes” https://doi.org/10.1007/s11207-025-02572-3.

Direct monitoring of solar UV irradiance began only in the 1980s; therefore, the only way to investigate its earlier variability is through reconstruction techniques. Reconstructing solar irradiance over long timescales requires accounting for both the cyclic 11-year modulation associated with the emergence of active regions and related features, as well as longer-term secular trends that describe variations beyond the 11-year cycle.

By combining these two components, the team reconstructed solar UV irradiance in four spectral bands: FUV (115–180 nm), MUV (180–310 nm), UV₁ (100–243 nm), and UV₂ (243–308 nm). Variations in these UV bands are critically important for studies of Earth’s atmosphere, as UV radiation controls the production and destruction of ozone and influences the properties of the upper atmosphere (i.e., the thermosphere), with direct consequences for satellite orbits and communication systems.

While Total Solar Irradiance (TSI) typically varies by only about 0.1% between solar minimum and maximum, it has varied by about 0.2% from the Spörer Minimum to the present day. In contrast, reconstructed ultraviolet (UV) irradiance exhibits substantially stronger variability. The study finds a decrease of approximately 15% in the far-ultraviolet (FUV) band during the Spörer and Maunder minima relative to present-day levels, while variations in the middle-ultraviolet (MUV) band exceed 1.6%.