Smithsonian Scientists Help Uncover How the Solar Wind Gets its Energy

Since the 1960s, astronomers have wondered how the Sun's supersonic "solar wind," a stream of energetic particles that flows out into the Solar System, continues to receive energy once it leaves the Sun. Now, thanks to a lucky line up of two spacecraft currently in space studying the Sun, they may have discovered the answer.

A paper in the journal Science, led by researchers at the Smithsonian Astrophysical Observatory (SAO) that is part of Center for Astrophysics | Harvard & Smithsonian (CfA), provides conclusive evidence that the fastest solar winds are powered by magnetic "switchbacks," or large kinks in the magnetic field, near 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," said Yeimy Rivera of the CfA who co-led the study. "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."

Previously, NASA's Parker Solar Probe found that these switchbacks were common throughout the solar wind. When Parker became the first craft to enter the Sun's magnetic atmosphere in 2021, scientists observed that switchbacks become more distinct and more powerful as Parker approached the atmosphere's outer edge. Up to now, however, scientists lacked experimental evidence that this interesting phenomenon actually deposits enough energy to be important in the solar wind.

"About three years ago, I was giving a talk about how fascinating these waves are," said co-author Mike Stevens, also at the CfA . 'At the end, an astronomy professor stood up and said 'that's neat, but do they actually matter?'"

To answer this, the team of scientists had to use two different spacecraft: Parker is built to fly through the Sun's atmosphere, or "corona." Scientists and engineers at the Smithsonian Astrophysical Observatory (SAO), which is part of the CfA, partially built one of the key instruments aboard Parker. Meanwhile, ESA's Solar Orbiter mission is also on an orbit that takes it relatively nearby to the Sun with complementary instruments onboard that measure the solar wind at larger distances.

This discovery was made possible because of a coincidental alignment in February 2022 that let both the Parker Solar Probe and Solar Orbiter measure the same solar wind stream within two days of each other. Solar Orbiter was almost halfway to the Sun while Parker was skirting the edge of the Sun's magnetic atmosphere.

"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," said Samuel Badman of the CfA, the other co-lead of the study. "When we connected the two, that was a real eureka moment."

Scientists have long known that energy is moved throughout the Sun‘s corona and the solar wind, at least in part, through what are known as "Alfven waves.” These waves that transport energy through a plasma, the superheated state of matter that makes up much of the Sun.

However, how much the Alfven waves evolve and interact with the solar wind between the Sun and Earth couldn't be measured until these two missions were sent closer to the Sun than ever before together. Now, scientists can directly determine how much energy is stored in the magnetic and velocity fluctuations of these waves near the corona and how much less energy is carried by the waves further from the Sun.

The new research shows that the Alfven waves in the form of switchbacks provide enough energy to account for the heating and acceleration documented in the faster stream of the solar wind as it flows away from the Sun. The knowledge is a crucial piece of the puzzle that will help scientists better forecast solar activity between the Sun and Earth as well as understanding how Sun-like stars and stellar winds operate everywhere.

"It took over half a century to confirm that Alfvenic wave acceleration and heating are important processes, and they happen in approximately the way we think they do," said John Belcher, emeritus professor from the Massachusetts Institute of Technology who was not involved in this study. “This will be a classic paper and it helps fulfill one of the main goals of the Parker Solar Probe."

The paper describing these results, co-led by Dr. Rivera and Dr. Badman, appears in the August 30, 2024 issue of the journal Science and is available at http://www.science.org/doi/10.1126/science.adk6953

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