A ‘traffic jam’ around Uranus could solve the mystery of its faint radiation belts

Scientists may have solved an ongoing mystery surrounding the ice giant Uranus and its faint radiation belts. It is possible that the weakness of the belts is linked to the magnetic field of the planet which is tilted badly; the field could cause a “traffic jam” for particles whipping around the world.

The mystery dates back to Voyager 2’s visit to Uranus in January 1986, long before the probe left the solar system in 2018. The spacecraft discovered that Uranus’ magnetic field is asymmetric and tilted about 60° from its spin axis. In addition, Voyager 2 discovered that Uranus’ radiation belts, made up of particles trapped by this magnetic field, are about 100 times weaker than predicted.

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

“It has a magnetic field like no other in the solar system. Most of the planets with strong intrinsic magnetic fields, such as Earth, Jupiter and Saturn. They have a ‘traditional’ magnetic field shape, called a dipole ,” lead. author Matthew Acevski told Space.com. “This is the same magnetic field shape as you would expect from your everyday supermagnet. At Uranus, this is not the case; Uranus’ field is highly asymmetric – and it gets closer to the planet’s surface.”

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Acevski explained that this research shows how the magnetic asymmetry of Uranus thickens the structure of the planet’s proton radiation belts, especially near the region Voyager 2 orbits.

“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,” said Acevski, “and, therefore, with stronger intensity; regions others where they were more dispersed, resulting in a weaker intensity.

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

An anomaly in the solar system

The coldest planet in the solar system and the seventh planet from the sun, Uranus is an oddity among the other worlds of our planetary system. The ice giant rolls around like a cosmic ball, tilted in one direction at an angle of 97 degrees from the plane of its orbit. That means, when it rotates, it does the kind of “side.” It is the only solar system planet to do this.

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

About four times wider than Earth and located about 19 times as far from the sun as our planet, Uranus is surrounded by 13 faint rings and at least 28 moons. Uranus also has auroras, similar to the northern and southern lights of Earth, but due to the planet’s tilted magnetic field, these are not seen over its poles as they are over our planet, Jupiter and even Saturn.

Like all planets with magnetic fields, charged particles are trapped around Uranus, creating radiation belts – but the fact that these radiation belts are so weak remains a puzzle for five decades.

A light blue orb next to a striped, yellow and red orb with a black circle in the middle.  The background is black

A light blue orb next to a striped, yellow and red orb with a black circle in the middle. The background is black

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

This showed that particles accelerate and decelerate as they pass through regions of different field strengths. Changes in the speed of the particles cause them to pile up in some regions and become more dispersed in others. This effect is only seen when a single complex quadrupole magnetic field is included in the simulation, which is why it has never been seen before.

“​​​​​​​We discovered that Uranus’ magnetic asymmetry may lead to regions around the planet where the protons flow more slowly and are more compressed and other regions where they flow faster and are more dispersed ,” Acevski said. “This is similar to how congestion occurs on a ring road. When cars are traveling more slowly, it causes more dense traffic; if cars are traveling faster, the traffic is more spread out.”

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

“We projected the trajectory of Voyager 2 on this profile and found that the spacecraft was, in fact, flying through a region of ‘fast current’, which would suggest that it should have been under significantly lower than the normal proton radiation belt intensity,” said Acevski. “It is important to note that our particle simulations show that this result is responsible for a maximum variation of about 20% of the proton intensity on all over the planet.”

That means the team’s model cannot fully account for the 100 times lower intensity observed by Voyager 2.

“It is possible that these much weaker proton radiation belts could be exacerbated by this effect we found,” continued Acevski. “We were very surprised by the results. It is amazing to see how much magnetic asymmetry can influence the structure of radiation belts. This is something that was not known before.”

Gray machinery with a white circular dish on the front and a black and brown backgroundGray machinery with a white circular dish on the front and a black and brown background

Gray machinery with a white circular dish on the front and a black and brown background

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

NASA has plans to send a mission to Uranus as early as 2030. Such a mission could help to experimentally verify the conclusion of this simulation.

“What we need to verify these simulations is a primary spacecraft mission to Uranus to obtain new in-situ measurements of the planet over several years rather than just a few hours as Voyager 2 did,” Acevski said. . “A new mission could also allow us to discover new physics that we could not even predict with simulations.

“Since this is a planet with a magnetic field that we have never seen before, it is possible to discover completely new phenomena, which would expand our understanding of planetary science.”

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Acevski is definitely not done with this strange solar system life yet. The ice giant is of particular interest to the researcher.

“Uranus presents a unique challenge to science, one that I’m really enjoying tackling. It’s really interesting what you can uncover with so little data, and we’re just scratching the surface,” Acevski said. “As of today, not many people are researching the giant icy planets, Uranus and Neptune, despite the fact that they show such strange features, especially in their magnetic fields, thus drawing attention to the phenomena as strange as it can be. A very exciting prospect for me.”

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

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