Scientists at the Southwest Research Institute (SwRI) believe they’ve cracked a decades-old mystery surrounding Uranus’ unexpectedly powerful electron radiation belt. New analysis of data collected by Voyager 2 during its 1986 flyby suggests the planet was likely experiencing a significant space weather event – a co-rotating interaction region – at the time of the observation, temporarily boosting the intensity of the radiation belt far beyond typical levels.
The discovery, published in November 2025 in Geophysical Research Letters, offers a potential explanation for a reading that baffled researchers for nearly 40 years. Voyager 2 measured an electron radiation belt intensity close to the maximum Uranus could sustain, a level far exceeding predictions based on extrapolations from other planetary systems.
Radiation belts, also known as Van Allen belts, are regions of trapped, energetic charged particles surrounding a planet. They form through the interaction of a planet’s magnetic field with the solar wind – a continuous stream of particles emitted by the Sun. For planets with a global magnetic field, like Earth and Uranus, these particles become trapped within the magnetosphere. The intensity of these belts can fluctuate significantly based on solar activity.
The challenge with Uranus has always been explaining the sustained high energy levels observed by Voyager 2. “Science has come a long way since the Voyager 2 flyby,” said Dr. Robert Allen, a space physicist at SwRI and lead author of the research, in a statement. “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.”
Earthly Parallels and Co-Rotating Interaction Regions
The key to unlocking the mystery, the researchers found, lay in similarities between the Uranian observation and a powerful space weather event that impacted Earth in 2019. Both events appear to be linked to a phenomenon called a co-rotating interaction region (CIR). A CIR occurs when a faster-moving stream of solar wind overtakes a slower one, creating a compression and disturbance in the interplanetary medium. This interaction can generate intense waves and accelerate charged particles.
“If a similar mechanism interacted with the Uranian system, it would explain why Voyager 2 saw all this unexpected additional energy,” explained Dr. Sarah Vines, a space physicist at SwRI and co-author of the study.
Previously, scientists believed that the high-frequency waves observed by Voyager 2 would primarily scatter electrons, causing them to be lost to Uranus’ atmosphere. However, research since the Voyager 2 flyby has revealed that these same waves, under certain conditions, can also accelerate electrons, injecting additional energy into the planetary system. The 2019 Earth event provided a compelling example of this acceleration process.
The team’s analysis suggests that Voyager 2 arrived at Uranus during a period when a CIR was passing through the planet’s magnetosphere, triggering a surge in electron acceleration and dramatically increasing the intensity of the radiation belt. This explains why the observed levels were so much higher than anticipated.
The findings have significant implications for our understanding of Uranus and other ice giants like Neptune. The researchers emphasize that a dedicated mission to Uranus is crucial to further investigate these phenomena. Such a mission could provide continuous monitoring of the planet’s magnetosphere, allowing scientists to study the long-term effects of solar wind interactions and the stability of the radiation belts, particularly given Uranus’ unique axial tilt and resulting extreme seasonal variations.
“This is just one more reason to send a mission targeting Uranus,” Allen concluded. “The findings have some important implications for similar systems, such as Neptune’s.”
Allen, R. C., Vines, S. K., & Ho, G. C. (2025). 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. Geophysical Research Letters, 52(22). https://doi.org/10.1029/2025gl119311
