Solar wind probably regulates Uranus’s thermosphere

Nov. 15 () –

Unpredictable long-term changes in the solar wind (the stream of particles and energy that comes from the sun) explain the cooling observed in the upper atmosphere of Uranus.

Research led by scientists at Imperial College London further predicts that Uranus’s 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.

Lead researcher Dr Adam Masters, from Imperial’s Department of Physics, said in a statement: “This apparently very strong control of Uranus’s upper atmosphere by the solar wind is unlike what we have seen on any other planet in our solar system.

“It means that planets outside the solar system could be in the same situation. This knowledge could help researchers studying exoplanets by shedding light on the types of signals that could be detected coming from similar planets around distant stars.”

The research was published in the journal Geophysical Research Letters.

The last and only spacecraft to pass by Uranus was Voyager 2 in 1986, on its way out of the solar system. It was able to take the temperature of the upper part of Uranus’ atmosphere, called the thermosphere.

Since then, ground-based telescopes have been able to measure the temperature of Uranus’s thermosphere regularly, and in that time, its overall temperature has been reduced by approximately half.

Earth also has a thermosphere, but it hasn’t experienced the same dramatic global temperature change, and neither have other planets in the solar system with monitored thermospheres.

Scientists wondered if it could be due to the “solar cycle” of 11 years of sunspot activity, but after 30 years of collecting data, no pattern detected except steady decline. A simple seasonal effect was also ruled out, since the Uranus equinox came and went in 2007.

The mystery was finally solved when the paper’s authors, then working in slightly different fields, met at a conference. They realized that the explanation It could have to do with gradual changes in the properties of the solar wind over the same period of time.

In Earth’s thermosphere, temperature is predominantly controlled by sunlight, with photons (light particles) providing energy and causing certain reactions. The intensity of these photons coming from the sun waxes and wanes with the 11-year solar cycle.

However, the solar wind flowing from the sun into space has also been changing in a different way, over a longer time scale. The annual mean external solar wind pressure has been decreasing slowly but significantly since about 1990, although it shows little correlation with the 11-year cycle. However, This decrease closely reflects the decrease in the temperature of Uranus’s thermosphere.

This suggested to the team that the temperature of Uranus’s thermosphere is not controlled by photons, like that of Earth. Instead, it appears that decreasing solar wind pressure has been causing the typical size of Uranus’ protective magnetic “bubble” to grow larger.

Since this bubble, known as the magnetosphere, is an obstacle for the solar wind to reach the planet’s surface, a bigger bubble means a bigger obstacle. This drives the flow of energy through space around Uranus, which eventually reaches the planet’s thermosphere and appears to strongly control its overall temperature.

The result suggests that for planets closest to their parent star, as Earth is to the Sun, their thermosphere is controlled by starlight. But for the more distant planets, which may have much larger magnetospheres, the incident energy of the stellar wind may be a much stronger factor.

Dr. Masters is part of an international team defining scientific goals for a future NASA mission to Uranus, scheduled to launch in the 2030s. The cooling of Uranus’s thermosphere was an unsolved mystery, but with little idea of ​​the possible cause, It had been difficult to come up with a theory that the mission could test.

That has now changed, with this discovery predicting how Uranus’ thermosphere should continue to evolve and revising the scientific goal of this future mission to focus on how solar wind power actually reaches Uranus’ unusual magnetosphere. The team is also interested in knowing if a similar situation exists on Neptune, which also hasn’t been visited since Voyager in the 1980s.

Meanwhile, the discovery could help characterize exoplanets. Wherever the situation is like that on Uranus, emissions from the exoplanet’s upper atmosphere, including auroras, should be very sensitive to how the incident stellar wind is evolving. The team suggests that observers should focus more on exoplanets farther from their parent star and/or on systems with strong stellar winds, where emissions may have been underestimated until now.

Dr Masters explained: “This strong star-planet interaction in Uranus could have implications for establishing whether different exoplanets generate strong magnetic fields in their interiors, an important factor in the search for habitable worlds outside our solar system.”