Scientists Identify Solar Storm That Supercharged Uranus

Lead

When NASA’s Voyager 2 swept past Uranus in January 1986 it recorded an electron radiation belt far stronger than models predicted, while the ion belt was modestly weaker. A new reanalysis of the Voyager 2 dataset, published in 2025, finds that a transient space‑weather disturbance — likely a co‑rotating interaction region (CIR) in the solar wind — can account for the unusually intense electron measurements recorded during the flyby. By comparing Voyager observations to detailed Earth‑orbit measurements from a 2019 event, researchers conclude the extra electron acceleration was short‑lived but powerful enough to push Uranus’s electron belt near its theoretical limit. The result explains a four‑decade puzzle and underlines how variable planetary radiation environments can be when hit by solar transients.

Key Takeaways

Voyager 2 flew by Uranus on January 24, 1986, and measured an electron radiation belt nearly at the maximum intensity Uranus can sustain, while ion fluxes were slightly below expectations.
A 2025 reanalysis (Allen et al., Geophysical Research Letters) compared the Voyager 2 Uranus data with Earth’s radiation‑belt response to a 2019 co‑rotating interaction region and found strong parallels in electron acceleration patterns.
The team attributes the Voyager anomaly to a CIR — a region where fast solar wind overtakes slower streams — which can compress the magnetosphere and energize electrons on timescales of hours to days.
Evidence supporting this interpretation includes timing and spectral signatures in the Voyager particle and field records that match CIR‑driven events seen at Earth in 2019.
If CIRs commonly affect ice‑giant magnetospheres, radiation levels at Uranus and Neptune may fluctuate more than previously thought, with implications for spacecraft design and mission planning.
Authors recommend an orbital mission to Uranus to map how its magnetosphere responds across seasons and to test the CIR hypothesis directly.

Background

Uranus presents a challenging magnetospheric environment: its rotation axis is tilted about 98 degrees relative to its orbital plane, and its magnetic dipole is significantly offset and misaligned with the spin axis. These geometrical peculiarities produce extreme seasonal and diurnal variations in how the planet’s magnetic field interacts with the solar wind. When Voyager 2 arrived in 1986, models and limited precedent for ice giants left scientists unprepared for the magnitude of the electron belt that was observed.

Voyager’s particle detectors and magnetometer recorded a contrast: ion populations were less intense than predicted, but relativistic electrons reached intensities near theoretical upper bounds for stable trapping by Uranus’s magnetic field. Over the ensuing decades, hypotheses ranged from instrument calibration errors to internal magnetospheric processes; none fully reconciled all features in the data. Advances in space‑weather monitoring at Earth and improved kinetic models of particle acceleration motivated a fresh comparative study of the old Voyager records.

Main Event

The new study reprocessed Voyager 2 particle and field measurements using modern analysis tools and then cross‑compared those signatures with data from Earth’s orbit collected during a pronounced CIR in 2019. On Earth, that 2019 event produced rapid electron acceleration in the outer radiation belt, documented by satellite instruments that reveal characteristic temporal and spectral changes when a CIR passes.

At Uranus, the reanalysis identified analogous features in Voyager’s electron data: a sudden enhancement in flux, energy‑dependent acceleration consistent with wave‑particle interactions, and a timing pattern that fits a transient driver rather than a long‑term steady state. The authors argue these indicators are most consistent with a CIR sweeping through the Uranian system just prior to or during Voyager’s closest approach.

Co‑rotating interaction regions form when persistent fast solar wind catches up with slower wind ahead of it, creating compressions and recurrent shocks in interplanetary space. Such compressions can increase particle fluxes and amplify whistler‑mode and other waves that efficiently energize electrons, rapidly inflating a planet’s electron radiation belt. The research team models this chain of effects and finds it can plausibly raise Uranus’s electron population to the levels Voyager recorded.

Analysis & Implications

If CIRs caused the Voyager 2 anomaly, then the observed radiation belt strength represents a transient maximum rather than an equilibrium state of Uranus’s magnetosphere. That distinction matters for how scientists interpret past observations and plan future missions: a probe that sampled at a different time might measure far lower electron intensities, or encounter other transient enhancements tied to solar wind structure.

The finding prompts a reassessment of how comparable processes operate at other giant planets. Neptune, with its own irregular magnetic geometry, could respond similarly to solar wind transients; understanding these responses is essential for estimating spacecraft radiation exposure and for planning long‑lived orbital operations. The result also highlights the value of comparative planetology — using well‑observed terrestrial cases to decode sparser remote encounters.

For space‑weather science, the study reinforces that CIRs, not only major coronal mass ejections (CMEs), can drive substantial particle acceleration at distant planets. Because CIRs are recurrent and span large heliospheric longitudes, they may be a dominant driver of variability at outer planets during particular solar cycles. That has implications for models that forecast radiation conditions beyond Earth orbit.

Comparison & Data

Parameter	Typical Pre‑Flyby Expectation	Voyager 2 Observation (1986)	Earth CIR Event (2019)
Electron belt intensity	Moderate, below theoretical max	Near theoretical maximum for Uranus	Large, rapid enhancement recorded in outer belt
Ion belt intensity	Modelled moderate	Slightly weaker than expected	Variable; not the primary accelerated population
Driver	Steady solar wind / internal processes	Transient driver suspected (CIR)	Co‑rotating interaction region confirmed

The table summarizes qualitative comparisons because absolute numeric intensities for Uranus are limited by Voyager’s single pass and instrument calibration constraints. Nevertheless, the correspondence in relative behavior and timing between Voyager’s electron signatures and Earth’s documented CIR response supports the CIR interpretation. The authors complement the comparison with spectral and timing diagnostics rather than relying on single numeric values.

Reactions & Quotes

“Science has advanced since Voyager; a comparative approach with Earth data helps fill gaps in interpreting the 1986 observations,” noted Robert Allen, a space physicist at the Southwest Research Institute and co‑author of the study.

Robert Allen, Southwest Research Institute (co‑author)

“The 2019 CIR at Earth produced substantial electron acceleration. A similar interaction at Uranus would explain the extra energy Voyager measured,” said Sarah Vines, co‑author and space physicist at SwRI.

Sarah Vines, Southwest Research Institute (co‑author)

Mission planners and magnetospheric scientists welcomed the analysis as a plausible resolution to a long‑standing anomaly while stressing that only in‑situ, multi‑point observations at Uranus can fully confirm the mechanism.

Community response (scientific commentary)

Explainer: Co‑rotating Interaction Regions and Radiation Belts

Co‑rotating interaction regions (CIRs) form where fast solar wind streams overtake slower ones, producing compressed plasma and enhanced magnetic fields that co‑rotate with the Sun. These compressed regions can persist for several solar rotations and drive waves and shocks that accelerate charged particles. Planetary radiation belts are populations of trapped charged particles encircling a planet’s magnetic field; electron belts are particularly sensitive to transient wave activity. At tilted, offset magnetospheres like Uranus, the geometry of field lines alters how and where particles are trapped and energized, so a CIR’s impact can differ markedly from one planet to another.

Unconfirmed

Definitive proof that a specific CIR passed Uranus exactly during Voyager 2’s flyby is not available because of sparse in‑situ solar wind monitors in the outer heliosphere at that time.
Other internal magnetospheric processes or coincident solar events could have contributed to the electron enhancement; disentangling simultaneous drivers requires additional data.

Bottom Line

The 2025 reanalysis offers a credible and testable explanation for Voyager 2’s unexpectedly intense electron measurements at Uranus: a transient CIR in the solar wind likely provided the additional energy that briefly inflated the planet’s electron radiation belt. This interpretation resolves a long‑standing discrepancy between models and Voyager observations by showing how external space‑weather drivers can dominate short‑term radiation conditions at ice giants.

Confirming the CIR hypothesis and mapping the full variability of Uranus’s magnetosphere will require a dedicated orbital mission with modern particle, field and plasma imagers sampling the system over extended periods and different seasons. Until then, the study underscores a simple lesson for planetary exploration: single flybys capture valuable snapshots, but transient space‑weather events can make those snapshots atypical.

Scientists Identify Solar Storm That Supercharged Uranus’s Radiation in 1986