Scientists may have solved the lingering mystery surrounding the ice giant Uranus and its faint radiation belts. It is possible that the weakness of the belts is related to the planet’s strangely inclined and tilted magnetic field; field could cause “traffic jams” for particles whipping around the world.
The mystery dates back to Voyager 2’s visit to Uranus in January 1986, well before the probe left the solar system in 2018. The probe found that Uranus’ magnetic field is asymmetric, tilted roughly 60° from its rotation axis. In addition, Voyager 2 found that Uranus’ radiation belts, consisting of particles trapped by this magnetic field, are about 100 times weaker than previously thought.
New research based on simulations using data from Voyager 2 suggests that these two strange aspects of the ice giant are related.
“It has a magnetic field unlike any other in the solar system. Most planets that have strong internal magnetic fields, such as Earth, Jupiter and Saturn. They have a very ‘traditional’ magnetic field shape, which is known as a dipole,” the lead told Space.com by Matthew Acevski. “This is the same magnetic field shape you would expect from your everyday bar magnet. This is not the case with Uranus; Uranus’ field is highly asymmetric – and it is getting closer to the surface of the planets.”
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Acevski explained that this research highlights how Uranus’ magnetic asymmetry distorts the structure of the planet’s proton radiation belts, particularly near the region around Voyager 2.
“My hypothesis was that the magnetic asymmetry distorts the proton radiation belts, creating regions around the planet where the radiation belts were more compressed,” Acevski said, “and therefore with stronger intensity; other regions where they were more spread out, leading to weaker intensity.
“If Voyager 2 flew through an area where the radiation belts were more spread out, this could explain its observation of weaker proton radiation belts than expected.”
Anomaly of the solar system
The coldest planet in the solar system and the seventh planet from the sun, Uranus is special among the other worlds of our planetary system. The ice giant rolls like a cosmic ball, tilted in one direction at an angle of 97 degrees from the plane of its orbit. This means that when it turns, it does so “sideways”. It is the only planet in the solar system that can 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 a winter that lasts 21 years. Uranus, which completes an orbit once every 84 Earth years, is also one of only two planets in the Solar System (the other being Venus) that orbits the Sun in the opposite direction to all the other planets.
Uranus is about four times wider than Earth and is about 19 times farther from the Sun than our planet, surrounded by 13 faint rings and at least 28 moons. Uranus also has auroras, similar to Earth’s northern and southern lights, but because of the planet’s tilted magnetic field, they don’t appear over its poles like our planet, Jupiter, and even Saturn.
As with all planets that have magnetic fields, charged particles are trapped around Uranus to create radiation belts – but why these radiation belts appear so weak has remained a mystery for five decades.
The team’s simulation abandoned the idea that Uranus’ magnetic field acts like a dipole and used a more complex quadrupole magnetic field to replicate its unbiased nature.
This revealed that particles speed up and slow down as they pass through regions of varying field strength. Changes in particle velocity cause them to clump together in some areas and become more dispersed in others. This effect only appears when a single, complex quadrupole magnetic field is included in the simulation, which is why it has never been observed before.
“We found that the magnetic asymmetry of Uranus could result in regions around the planet where protons move more slowly and are more compressed, and other regions where they move faster and are more spread out,” Acevski said. “It’s an analogy to how traffic jams form on a ring road. When cars go slower, it causes more traffic, if cars go faster, the traffic is spread out.”
Acevski and colleagues believe that when Voyager 2 visited Uranus, it passed through a weak region of the ice giant’s radiation belt.
“We projected Voyager 2’s trajectory onto this profile and found that the spacecraft actually flew through the ‘fast drift’ region, which would mean it should have observed a lower-than-normal intensity of the proton radiation belt,” Acevski said. “It is important to note that our particle simulations show that this result represents a maximum variation of about 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 caused these much weaker proton radiation belts could have been multiplied by this effect we found,” Acevski continued. “We were extremely surprised by the results. It is amazing to see how much influence magnetic asymmetry can have on the structure of the radiation belt. This is something that was not known before.”
Acevski pointed out that the results he and the team obtained could help inform future spacecraft missions to Uranus. Voyager 2 is the only spacecraft to visit the ice giant so far. This means that direct data about the world is extremely limited.
Plans are underway at NASA to launch a mission to Uranus as early as 2030. Such a mission could help experimentally verify the conclusion of this simulation.
“What we need to validate these simulations is a flagship spacecraft to Uranus to get new, in-situ measurements of the planet over several years, rather than just a few hours, as Voyager 2 did,” Acevski said. “The new mission could also allow us to uncover new physics that we couldn’t predict even with simulations.
“Since this is a planet with a magnetic field that we have never seen before, it is entirely possible that entirely new phenomena have been found that would expand our understanding of planetary science.”
Acevski is certainly not done with this strange world of the solar system. The ice giant is a special fascination for researchers.
“Uranium presents a unique challenge for science that I’m excited to tackle. It’s really fascinating how much you can uncover with so little data, and we’re literally just scratching the surface,” Acevski concluded. “To date, not many people have studied the ice giant planets Uranus and Neptune, even though they exhibit such strange features, especially in their magnetic fields, and therefore raise awareness of the strange phenomena that may occur there. a very exciting prospect for me.”
The team’s research was published in June in the journal Geophysical Research Letters.