When Voyager 2 swept past Uranus in 1986, passing within about 81,500 kilometers of the planet’s cloud tops, it delivered a surprise.
Instruments aboard the spacecraft detected an unexpectedly intense belt of high-energy electrons, far stronger than scientists anticipated for an ice giant so distant from the Sun and tipped nearly 98 degrees on its side. For nearly four decades, the origin of that radiation has remained an open question.
A new study from researchers at the Southwest Research Institute suggests Voyager 2 arrived at exactly the wrong—or perhaps right—moment. By reanalyzing the Uranus flyby data and comparing it with modern observations of Earth’s radiation belts, the team argues that Uranus was likely experiencing an episode of extreme space weather. Specifically, a fast-moving solar wind structure known as a co-rotating interaction region, or CIR, may have slammed into the planet’s magnetosphere just as Voyager passed through.
On Earth, such disturbances are known to trigger powerful geomagnetic storms. Under those conditions, electromagnetic emissions called chorus waves ripple through the magnetosphere. Rather than simply scattering charged particles, these waves can act as accelerators, boosting electrons to extreme energies. The new analysis suggests the same process occurred at Uranus, explaining why Voyager 2 recorded some of the strongest wave activity—and most intense radiation—of its entire mission.
If correct, the findings imply that Uranus’s magnetosphere is far more dynamic than a single flyby could ever reveal. They also strengthen the case for a dedicated mission to the ice giant, one capable of observing how its magnetic environment responds over time to the Sun’s variable output. Such insights would not only reshape understanding of space weather at distant planets, but also inform studies of Uranus-like worlds orbiting other stars.
RESEARCH PAPER
Robert C. Allen et al., “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 (2025)
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