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 related to the planet’s curiously tilted and tilted magnetic field; field can 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 spacecraft discovered that Uranus’ magnetic field is asymmetric and tilted roughly 60° off its spin axis. Additionally, Voyager 2 found 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 aspects of the ice giant are connected.
“It has a magnetic field like no other in the solar system. Most planets that have strong internal magnetic fields, such as Earth, Jupiter and Saturn. They have a very ‘traditional’ form of magnetic field, which is known as a dipole.” , he says. author Matthew Acevski told Space.com. “This is the same shape of the magnetic field as you would expect from your everyday bar magnet. On Uranus, this is not the case; Uranus’ field is highly asymmetric – and it gets closer and closer to the planets surface.”
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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 passed by Voyager 2.
“My hypothesis was that the magnetic asymmetry was distorting the proton radiation belts, forming regions around the planet where the radiation belts were more compressed,” Acevski said, “and, thus, of stronger intensity; other regions where they were more widespread, leading to weaker intensity.
“If Voyager 2 flew through a region where the radiation belts were more widespread, this could explain its observations of weaker-than-expected proton radiation belts.”
An anomaly of 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 rotates like a cosmic ball, tilted in one direction at an angle of 97 degrees from the plane of its orbit. That is, when it rotates, it does so somewhat “sideways”. It is the only planet in the solar system to do so.
The tilt, which is 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 lasting 21 years. Completing an orbit once every 84 Earth years, Uranus is also only one of two planets in the solar system (the other being Venus) that orbits the sun in the opposite direction to all the other planets.
About four times wider than Earth and located about 19 times farther from the sun than our planet, Uranus is 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, these do not appear over its poles as they do over our planet, Jupiter and even Saturn.
As with all planets that have magnetic fields, there are charged particles trapped around Uranus, creating radiation belts – but why these radiation belts appear so faint 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 unidirectional nature.
This revealed that particles speed up and slow down as they pass through regions of different field strengths. Changes in the speed of particles cause them to accumulate in some regions 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 seen before.
“We found that Uranus’ magnetic asymmetry can 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. “This is analogous to how traffic jams form on a ring road. When cars travel slower, it causes denser 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 region 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 ‘fast slip’ region, which would mean that it should have observed lower-than-normal intensity of the radiation belt. proton,” said Acevski. “It is important to note that our particle simulations show that this result accounts for a maximum change of about 20% in the proton intensity around the planet.”
This means the team’s model cannot fully account for the 100 times lower intensity observed by Voyager 2.
“It is possible that whatever primary effect caused these much fainter belts of proton radiation may have been compounded by this effect we found,” Acevski continued. “We were extremely surprised by the results. It’s 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 noted 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 live 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 completion of this simulation.
“What we need to verify these simulations is a spacecraft mission to Uranus to take 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 may 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 entirely possible that entirely new phenomena will be found, which would expand our understanding of planetary science.”
Acevski is certainly not done with this strange world of the solar system yet. The ice giant is a special fascination for the researcher.
“Uranus presents a unique challenge for science, which I’m excited to tackle. It’s really fascinating how much you can discover with so little data, and we’re literally just scratching the surface,” Acevski concluded. “Until today, not many people research the icy giant planets Uranus and Neptune, despite the fact that they exhibit such strange features, especially in their magnetic fields, thus drawing attention to strange phenomena that can happening there is a very exciting prospect for me.”
The team’s research was published in June in the journal Geophysical Research Letters.