What powers the sun's supersonic solar wind?

It has been a longstanding mystery since the 1960s. How does the Sun’s supersonic “solar wind,” an energetic stream of particles flowing into the solar system, continue to receive its energy after leaving the Sun?

Thanks to the alignment of two spacecraft missions currently studying the Sun – one led by NASA and the other led by the European Space Agency (ESA) – a significant breakthrough may be on the horizon.

The findings could provide a crucial missing piece of the puzzle to help scientists better forecast solar activity between the Sun and Earth.

Solar wind and magnetic switchbacks

A recent paper published in the journal Science provides compelling evidence that the fastest solar winds draw their power from magnetic “switchbacks,” or substantial twists in the magnetic fields close to the Sun.

“Our study addresses a huge open question about how the solar wind is energized and helps us understand how the Sun affects its environment and, ultimately, the Earth,” explained Yeimy Rivera, co-leader of the study and a postdoctoral fellow at the Smithsonian Astrophysical Observatory.

“If this process happens in our local star, it’s highly likely that this powers winds from other stars across the Milky Way galaxy and beyond and could have implications for the habitability of exoplanets.”

The unseen energies of solar wind

Previously, NASA’s Parker Solar Probe had found that switchbacks were prevalent throughout the solar wind.

Parker, which became the first spacecraft to enter the Sun’s magnetic atmosphere in 2021, observed that switchbacks became more distinct and potent closer to the Sun.

However, experimental evidence showing that these magnetic waves contributed enough energy to be significant in the solar wind had been lacking – until now.

Observations from two distinct spacecraft

“About three years ago, I was giving a talk about how fascinating these waves are,” said co-author Mike Stevens, an expert at the Center for Astrophysics. “At the end, an astronomy professor stood up and said, ‘that’s neat, but do they actually matter?’”

Answering this complex question required combining observations from two distinct spacecraft. Parker was designed to navigate the Sun’s atmosphere (corona), while the Solar Orbiter mission measures solar winds at larger distances. In a stroke of luck, both spacecraft measured the same solar wind stream within two days of each other in February 2022.

This coincidental alignment allowed the Solar Orbiter, located almost halfway to the Sun, and Parker, near the edge of the Sun’s magnetic atmosphere, to provide complementary data.

“We didn’t initially realize that Parker and Solar Orbiter were measuring the same thing at all. Parker saw this slower plasma near the Sun that was full of switchback waves, and then Solar Orbiter recorded a fast stream which had received heat and with very little wave activity. When we connected the two, that was a real eureka moment,” noted co-author Samuel Badman, an astrophysicist at the Center for Astrophysics.

Alfvén waves: The energy transmitters

Alfvén waves – which transport energy through plasma (the superheated matter that makes up the solar wind) – play a major role in moving energy around the Sun’s corona and the solar wind.

The new research shows that Alfvén waves, in the form of switchbacks, provide enough energy to account for the heating and acceleration observed in the faster solar wind streams flowing away from the Sun.

John Belcher is an emeritus professor at the Massachusetts Institute of Technology who co-discovered Alfvén waves in solar wind, but was not involved in the current study.

“It took over half a century to confirm that Alfvénic wave acceleration and heating are important processes, and they happen in approximately the way we think they do,” said Belcher.

Beyond forecasting solar activity

The implications of the research extend beyond forecasting solar activity and space weather. The insights will shed light on other mysteries of the universe, including the operation of sun-like stars and stellar winds.

Adam Szabo, the Parker Solar Probe mission science lead at NASA, concluded: “This discovery is one of the key puzzle pieces to answer the 50-year-old question of how the solar wind is accelerated and heated in the innermost portions of the heliosphere, bringing us closer to closure to one of the main science objectives of the Parker Solar Probe mission.”

The study is published in the journal Science.

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