The "ice giants" of the Solar System – Uranus and Neptune – remain the least explored of any planets orbiting our Sun.
Thanks to the sheer distance between them and Earth, the
first probe to ever study them was the Voyager 2 probe, which remains the only
mission ever to conduct a flyby.
What this probe revealed led to numerous mysteries about
both worlds, their systems of moons, and other characteristics. For instance,
when Voyager flew past Uranus, it recorded an electron belt of a much higher
energy level than expected.
Since then, scientists have studied thousands of gas giants beyond the Solar System and made comparisons that have bolstered the mystery of how the Uranian system could support so much trapped electron radiation.
In a recent study, scientists at the Southwest Research
Institute (SwRI) hypothesized that the Voyager 2 observations may have been the
result of a solar wind structure.
Similar to how Earth experiences processes driven by solar
wind storms, they believe a "co-rotating interaction region" was
passing through the system when Voyager 2 made its historic flyby.
The research was led by Dr. Robert C. Allen, a space
physicist and the Lead Scientist of the SwRI's Space Sciences Division. He was
joined by SwRI Lead Scientist Sarah Vines, and Senior Program Manager George C.
Ho.
The paper describing their research, "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,"
recently appeared in Geophysical Research Letters.
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Diagram of the space weather impacts of a fast solar wind
structure (first panel) driving an intense solar storm at Earth in 2019 (second
panel) with conditions observed at Uranus by Voyager 2 in 1986 (third panel).
(Allen et al., Geophys. Res. Lett., 2025) |
To date, the Voyager 2 probe has provided the only direct
measurements of the radiation environment at Uranus. This led to the
predominantly accepted characterization of the system as having a weaker ion
radiation belt and a very intense electron radiation belt.
However, when the team reanalyzed the probe's data, they
discovered hints that the probe's observations did not occur during normal
solar wind conditions. Instead, they suggest that the probe's flyby coincided
with a transient solar wind event passing through the system.
This event, they argue, produced the most powerful
high-frequency waves observed during the Voyager 2 mission. At the time,
scientists thought that these waves would scatter electrons that would be lost
to Uranus's atmosphere.
However, scientists have since learned that, under certain
circumstances, these waves can also accelerate electrons and add additional
energy to planetary systems. To this end, the team compared the Voyager 2
observations to similar events observed at Earth and noted similarities.
"Science has come a long way since the Voyager 2 flyby.
We decided to take a comparative approach, looking at the Voyager 2 data and
comparing it to Earth observations we've made in the decades since," said
Dr. Allen in an SwRI press release.
"In 2019, Earth experienced one of these events, which
caused an immense amount of radiation belt electron acceleration," added
Dr. Vines.
"If a similar mechanism interacted with the Uranian system, it would explain why Voyager 2 saw all this unexpected additional energy."
Their comparative approach suggests that interactions
between solar wind and Uranus' magnetosphere could have driven high-frequency
waves capable of accelerating electrons to energies close to the speed of
light.
They also raise many additional questions about the
fundamental physics behind these intense waves and the sequence of events that
led to them.
"This is just one more reason to send a mission
targeting Uranus. The findings have some important implications for similar
systems, such as Neptune's," said Dr. Allen.