Solar wind likely regulates Uranus’ thermosphere
Uranus captured by the Webb Telescope’s near-infrared camera. – NASA, ESA, CSA, STSCI
MADRID, November 15 (EUROPE PRESS) –
Unpredictable long-term changes in the solar wind (the flow of particles and energy coming from the Sun) explain the cooling observed in the upper atmosphere of Uranus.
The research, led by scientists at Imperial College London, also predicts that Uranus’ upper atmosphere should continue to cool, or reverse the trend and become hotter again, depending on how the solar wind changes in the coming years.
This was reported by lead researcher Dr Adam Masters from Imperial’s Department of Physics. in the statement: “This apparently very strong control of the upper atmosphere of Uranus by the solar wind is unlike anything we have seen on any other planet in our solar system.
“This means that planets outside the solar system could face the same situation. This knowledge could help researchers studying exoplanets by shedding light on the types of signals that could be detected emanating from similar planets around distant stars.”
The study was published in the journal Letters on Geophysical Research.
The last and only spacecraft to fly past Uranus was Voyager 2 in 1986, heading out of the solar system. He was able to measure the temperature of the upper part of Uranus’ atmosphere, called the thermosphere.
Since then, ground-based telescopes have been able to regularly measure the temperature of Uranus’ thermosphere, and during this time his overall temperature dropped by about half.
Earth also has a thermosphere, but it has not experienced such a dramatic global temperature change as other planets in the solar system with a controlled thermosphere.
Scientists wondered if this could be related to the “solar cycle” of 11-year sunspot activity, but after 30 years of data collection no pattern was found other than a steady decline. A simple seasonal effect was also ruled out because the Uranian equinox came and went in 2007.
The mystery was finally solved when the authors of the paper, who were working in several different fields at the time, met at a conference. They realized that the explanation This could be due to a gradual change in the properties of the solar wind over the same period of time.
In Earth’s thermosphere, temperature is primarily controlled by sunlight, with photons (particles of light) providing energy and causing certain reactions. The intensity of these photons emanating from the Sun waxes and wanes with the 11-year solar cycle.
However, the solar wind moving from the Sun into space also changed differently, on a longer time scale. The mean annual outer solar wind pressure has been decreasing slowly but significantly since about 1990, although it has little correlation with the 11-year cycle. However, This decrease closely reflects the decrease in the temperature of Uranus’ thermosphere.
This led the team to believe that the temperature of Uranus’ thermosphere is not controlled by photons, as it is on Earth. Instead, it appears that the decrease in solar wind pressure has caused the typical size of Uranus’s protective magnetic “bubble” to increase.
Because this bubble, known as the magnetosphere, is an obstacle to solar wind reaching the planet’s surface, a larger bubble means a larger obstacle. This results in a flow of energy through the space around Uranus, which eventually reaches the planet’s thermosphere and appears to strongly control its overall temperature.
The result suggests that planets closest to their parent star, like Earth to the Sun, have their thermosphere controlled by starlight. But for more distant planets, which may have a much larger magnetosphere, falling stellar wind energy may be a much stronger factor.
Dr. Masters is part of an international team defining the science goals for NASA’s future mission to Uranus, scheduled to launch in the 2030s. The cooling of Uranus’ thermosphere was an unsolved mystery, but he had few ideas about a possible cause. It was difficult to come up with a theory that the mission could test.
Now the situation has changed: this discovery predicts how Uranus’ thermosphere should continue to evolve, and revises the science goal of this future mission to focus on how solar wind energy actually reaches Uranus’ unusual magnetosphere. The team is also interested in finding out if a similar situation exists on Neptune. which also has not been visited since Voyager in the 1980s.
Meanwhile, this discovery could help characterize exoplanets. Wherever the situation is on Uranus, emissions from the exoplanet’s upper atmosphere, including auroras, should be very sensitive to how the falling stellar wind develops. The team suggests that observers should focus more on exoplanets farther from their parent star and/or systems with strong stellar winds. where emissions may have so far been underestimated.
Dr Masters explained: “This strong interaction between the star and the planet on Uranus may have implications for establishing whether different exoplanets generate strong magnetic fields in their cores, which is an important factor in the search for habitable worlds beyond our solar system.”