'Traffic jams' around Uranus could solve the mystery of its weak radiation belts

Scientists may have solved a lingering mystery surrounding the ice giant Uranus and its weak radiation belts. It's possible the belts' weakness is linked to the planet's curiously tilted and lopsided magnetic field; the field could be causing "traffic jams" for particles whipping around the world.

The mystery dates back to Voyager 2's visit to Uranus in January 1986, far before the probe left the solar system in 2018. The spacecraft found that Uranus' magnetic field is asymmetric and tilted roughly 60° away from its spin axis. Additionally, Voyager 2 found that the radiation belts of Uranus, consisting of particles trapped by this magnetic field, are about 100 times weaker than predicted.

The new research, based on simulations made using Voyager 2 data, suggests these two strange aspects of the ice giant are related.

"It has a magnetic field like no other in the solar system. Most planets that have strong intrinsic magnetic fields, like Earth, Jupiter and Saturn. They have a very 'traditional' magnetic field shape, which is known as a dipole," lead author Matthew Acevski told Space.com. "This is the same magnetic field shape as you would expect from your everyday bar magnet. At Uranus, this is not the case; Uranus' field is highly asymmetric — and it becomes increasingly so closer to the planets’ surface."

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Acevski explained this research highlights how Uranus' magnetic asymmetry warps the structure of the planet's proton radiation belts, especially near the region passed by Voyager 2.

"My hypothesis was that the magnetic asymmetry was warping the proton radiation belts, forming regions around the planet where the radiation belts were more compressed," Acevski said, "and, thus, with stronger intensity; other regions where they were more spread out, leading to weaker intensity.

"If Voyager 2 flew through a region where the radiation belts were more spread out, that could explain its observations of weaker-than-expected proton radiation belts."

A solar system anomaly

The coldest planet in the solar system and the seventh planet from the sun, Uranus is odd among the other worlds of our planetary system. The ice giant rolls around like a cosmic ball, tilted in one direction at a 97-degree angle from the plane of its orbit. That means, when it rotates, it does so kind of "sideways." It's the only solar system planet that does this.

The tilt, which is believed to be the result of a collision with an Earth-size object in the distant past, causes Uranus to have the most extreme seasons in the solar system, with a winter that lasts 21 years. Completing an orbit once every 84 Earth years, Uranus is also only one of two planets in the solar system (the other is Venus) that rotates around the sun in the opposite direction to all the other planets.

About four times wider than Earth and located around 19 times as far from the sun as our planet sits, Uranus is looped by 13 faint rings and at least 28 moons. Uranus also has auroras, similar to the northern and southern lights of Earth, but because of the planet's tilted magnetic field, these don't appear over its poles as they do over our planet, Jupiter and even Saturn.

Like with all planets that have magnetic fields, there are trapped charged particles around Uranus, creating radiation belts — but why these radiation belts seem so weak has remained a puzzle for five decades.

The team's simulation abandoned the idea that Uranus' magnetic field acts as a dipole and used a more complex quadrupole magnetic field to replicate its lopsided nature.

This revealed that particles accelerate and decelerate as they pass through regions of different field strengths. The particles' changes in speed cause them to bunch up in some regions and become more dispersed in others. This effect only appears when a single, complex quadrupole magnetic field is factored into the simulation, which is why it had never been seen before.

"We found that Uranus' magnetic asymmetry could result in regions around the planet where the protons drift slower and are more compressed and other regions where they drift faster and are more spread out," Acevski said. "This is analogous to how traffic jams form on a ring road. When cars travel slower, it causes more dense traffic; if cars travel faster, the traffic is more spread out."

Acevski and colleagues theorize that when Voyager 2 visited Uranus, it passed through a weak area of the ice giant's radiation belt.

"We projected Voyager 2's trajectory onto this profile and found that the spacecraft did, in fact, fly through a region of 'fast drift,' which would imply it should have observed lower than normal proton radiation belt intensity," Acevski said. "It is important to note that our particle simulations show this result accounts for a maximum variation of approximately 20% of the proton intensity around the planet."

That means the team's model can't fully account for the 100 times lower intensity observed by Voyager 2.

"It is possible that whatever primary effect did cause these much weaker proton radiation belts could have been compounded by this effect that we found," Acevski continued. "We were extremely surprised by the results. It is amazing to see how much of an influence magnetic asymmetry can have on radiation belt structure. This is something that was not previously known."

Acevski pointed out that the results obtained by himself and the team could help inform future spacecraft missions to Uranus. Thus far, Voyager 2 is the only spacecraft to visit the ice giant. This means direct data about the world is extremely limited.

Plans are underway at NASA to launch a mission to Uranus as soon as 2030. Such a mission could help experimentally verify the conclusion of this simulation.

"What we need to verify these simulations is a flagship spacecraft mission to Uranus to get new, in-situ measurements of the planet over the course of several years rather than just a few hours as Voyager 2 did," Acevski said. "A new mission could also allow us to uncover new physics that we couldn't even predict with simulations.

"As this is a planet with a magnetic field that we have never seen before, it is entirely possible that completely new phenomena are found, which would broaden our understanding of planetary science."

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Acevski certainly is not finished with this strange solar system world yet. The ice giant is a particular fascination for the researcher.

"Uranus presents a unique challenge to science, one which I am finding great enjoyment in tackling. It is truly fascinating how much you can uncover with so little data, and we are quite literally only scratching the surface," Acevski concluded. "As of today, there are not many people researching the icy giant planets, Uranus and Neptune, despite the fact that they exhibit such strange features, particularly in their magnetic fields, and so drawing attention to the strange phenomena that can occur there is a very exciting prospect to me."

The team's research was published in June in the journal Geophysical Research Letters.