Ultraviolet (UV) radiation from the Sun has a major influence on Earth’s environment and, in particular, on our planet’s atmosphere. It drives the chemistry that forms and sustains the stratospheric ozone layer, affects the thermal balance of the upper atmosphere, and impacts the drag experienced by satellites in low Earth orbit. To understand how our atmosphere has evolved not only in recent decades but over centuries, we need to know how the Sun’s UV radiation has changed in the past.
By analyzing one thousand years of solar magnetic activity—reconstructed through the study of radioisotopes in tree rings—and combining it with a global archive of solar images, a team of scientists from the University of Rome Tor Vergata, with contributions from researchers at the U.S. National Science Foundation’s National Solar Observatory (NSO) and the Institute of Computational Life Sciences in Zurich (ZHAW), has provided the clearest picture to date of how solar variability has shaped Earth’s atmospheric chemistry. The study, led by Raffaele Reda, a researcher in the Department of Physics, stems from a multi-decade collaboration coordinated by Francesco Berrilli of the University of Rome Tor Vergata dedicated to investigating solar variability and its impact on Earth. Over the years, the collaboration has also involved climatologists from Sapienza University of Rome, CNR/ISAC, the University of Trento, and HAO/NCAR.
Indeed, the Sun is constantly changing. Its magnetic field strengthens and weakens in an 11-year cycle, producing the familiar rise and fall of sunspot numbers. But the Sun also undergoes long-term variations, such as extended periods of quiet, including the Spörer Minimum (1400–1500) and the Maunder Minimum (1600–1700), during which solar activity drops dramatically. Until now, scientists had only rough estimates of how UV radiation behaved during these Grand Minima. The new research tackles this challenge directly. Using an empirical model that links UV radiation to magnetic structures on the solar surface, the team reconstructed four UV bands that are crucial for ozone chemistry. The results show that during the Grand Minima, UV radiation decreases significantly, altering the conditions under which the ozone layer forms.
One of the most unexpected findings concerns the middle-ultraviolet (MUV) band between 180 and 300 nm. Previous models predicted only modest long-term variations in this band. The new reconstruction instead shows that MUV variability is substantially larger when the long-term behavior of the Sun’s quiet magnetic network is correctly taken into account. This suggests that small-scale magnetism—far less dramatic than sunspots—plays a crucial role in determining the Sun’s UV output. Since MUV radiation influences both ozone production and stratospheric heating, this discovery has important implications for climate modeling.
For further details, see the National Solar Observatory press release: https://nso.edu/blog/suns-ultraviolet-mood-swings/
Reda Raffaele, Penza Valentina, Criscuoli Serena, Bertello Luca, Cantoresi Matteo, Lucaferri Lorenza, Ulzega Simone, Berrilli Francesco, Modeling Decadal and Centennial Solar UV Irradiance Changes, Solar Physics, 300, id.173 (2025) DOI: 10.1007/s11207-025-02572-3
