SAN ANTONIO — December 3, 2025 — Southwest Research Institute (SwRI) scientists believe they may have resolved a 39-year-old mystery about the radiation belts around Uranus.
In 1986, when Voyager 2 made the first and only flyby of Uranus, it measured a surprisingly strong electron radiation belt at significantly higher levels than anticipated. Based on extrapolations from other planetary systems, Uranus' electron radiation belt was off the charts. Since then, scientists have wondered how the Uranian system could support such an intense trapped electron radiation belt, at a planet unlike anything else in the solar system.
Based on new analyses, SwRI scientists theorize that Voyager 2 observations may have more in common with processes at Earth driven by large solar wind storms. Scientists now think a solar wind structure — known as a co-rotating interaction region — was likely passing through the Uranian system. This could explain the extreme energy levels Voyager 2 observed.
"Science has come a long way since the Voyager 2 flyby," said SwRI's Dr. Robert Allen, lead author of a paper outlining this research. "We decided to take a comparative approach looking at the Voyager 2 data and compare it to Earth observations we've made in the decades since."
This new study indicates that the Uranian system may have experienced a space weather event during the Voyager 2 visit that led to powerful high-frequency waves, the most intense observed over the entirety of the Voyager 2 mission. In 1986, scientists thought that these waves would scatter electrons to be lost to Uranus's atmosphere. But since then, Allen said, scientists have learned that those same waves under certain conditions can also accelerate electrons and feed additional energy into planetary systems.
"In 2019, Earth experienced one of these events, which caused an immense amount of radiation belt electron acceleration," said SwRI's Dr. Sarah Vines, a co-author of the paper. "If a similar mechanism interacted with the Uranian system, it would explain why Voyager 2 saw all this unexpected additional energy."
But these findings also raise a lot of additional questions about the fundamental physics and sequence of events that would enable these intense wave emissions.
"This is just one more reason to send a mission targeting Uranus," Allen said. "The findings have some important implications for similar systems, such as Neptune's."
The paper "Solving the mystery of the electron radiation belt at Uranus: Leveraging knowledge of Earth's radiation belts in a re-examination of Voyager 2 observations" is published in Geophysical Research Letters and is accessible at DOI: 10.1029/2025GL119311.
