Solar wind power likely controls Uranus’ thermosphere temperature

Uranus’ upper atmosphere has been cooling for decades, and now scientists have discovered why. Observations from Earth show that Uranus’ upper atmosphere has been cooling for decades, but there is no clear explanation.

Now, a team led by scientists from Imperial College London has determined that unpredictable long-term changes in the solar wind – the flow of particles and energy coming from the sun – are behind this decline.

The researchers predict that depending on how the solar wind changes over the next few years, Uranus’ upper atmosphere should continue to cool, or the trend should reverse and become warmer again.

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

“This means that planets outside our solar system may be in the same situation. Therefore, these insights are an important addition to the signal that could be detected from similar planets around distant stars. Determining the type may help researchers investigating exoplanets.”

The study was published in the journal Geophysical Research Letters on November 14th.

cool mystery

The last and only spacecraft to fly past Uranus was Voyager 2, which attempted to leave the solar system in 1986. We were able to measure the temperature in the upper part of Uranus’ atmosphere, called the thermosphere.

Since then, Earth-based telescopes have been able to regularly measure the temperature of Uranus’ thermosphere, during which time the planet’s overall temperature has decreased by about half.

Although Earth has a thermosphere, it has not experienced such dramatic global temperature changes, and no other planet in the solar system has a monitored thermosphere.

Scientists thought it might be due to an 11-year “solar cycle” of sunspot activity, but despite 30 years of data collection, no pattern other than a steady decline could be detected. Uranus’s equinox came and went in 2007, so simple seasonal effects were also ruled out.

The mystery was finally solved when the paper’s authors, who were working in slightly different fields at the time, met for a conference. They realized that the explanation could be related to gradual changes in the properties of the solar wind within the same time frame.

changing influence

In Earth’s thermosphere, temperature is primarily controlled by sunlight, where photons (particles of light) provide energy and cause specific reactions. The intensity of these photons coming from the sun increases and decreases depending on the 11-year solar cycle.

However, the solar wind that flows out into space from the Sun also changes in other ways over longer timescales. The annual average outward pressure of the solar wind has been decreasing slowly but significantly since about 1990, but with little correlation to the 11-year cycle. However, this decrease closely reflects the decrease in Uranus’ thermosphere temperature.

This suggested to the researchers that the temperature of Uranus’ thermosphere is not controlled by photons as it is on Earth. Instead, lower solar wind pressure appears to be increasing the typical size of Uranus’ protective magnetic “bubbles.”

This bubble, known as the magnetosphere, creates an obstacle for the solar wind to reach the planet’s surface, so the larger the bubble, the greater the obstruction. This facilitates the flow of energy through the space around Uranus, eventually reaching the planet’s thermosphere and appearing to exert powerful control over its overall temperature.

This result suggests that the thermosphere of a planet close to its parent star is controlled by starlight, much like the Earth is close to the sun. However, more distant planets may have much larger magnetospheres, and the incident energy from the stellar wind may be a stronger factor.

To Uranus and beyond

Dr. Masters is part of an international team defining the science goals for future NASA missions to Uranus, planned for launch in the 2030s. The cooling of Uranus’s thermosphere has been a mystery to be solved, but with little idea of ​​possible causes, it has been difficult to come up with a theory that can be tested by a mission.

That has now changed, with this discovery leading to a new project that aims to predict how Uranus’ thermosphere will continue to evolve, as well as focus on how solar wind energy actually gets absorbed into Uranus’ unusual magnetosphere. This resulted in a thorough review of future mission science goals. The researchers are also interested in whether a similar situation exists on Neptune, which has not been visited since Voyager in the 1980s.

In the meantime, the discovery could help characterize exoplanets. In a Uranus-like situation, emissions from an exoplanet’s upper atmosphere, including its auroras, should be very sensitive to how the incoming stellar wind evolves. The researchers suggest that observers should focus more on exoplanets that are far from their parent stars, or on systems with strong stellar winds whose emissions may have been underestimated.

Dr Masters said: “This strong star-planet interaction at Uranus could influence the establishment of whether different exoplanets generate strong internal magnetic fields. “This is an important element in the quest for a habitable world.”

But the mystery of why the solar wind itself changes as its pressure decreases over decades is a question even other scientists have.