Scientists are finally peering inside the sun's middle corona

Scientists are one step closer to solving a key mystery about the sun.

Researchers have discovered a complex, dynamic, web-like plasma structure in the sun's middle atmosphere, suggesting that the solar wind — the constant stream of charged particles that flows off the sun — forms within this web. The finding comes from rare observations of an elusive region of the sun and pave the way for a NASA mission due to launch next year.

"We had no idea if it would work or what we would see," Dan Seaton, a solar physicist at the Southwest Research Institute in Colorado and one of the study's authors, said in a statement. "The results were very exciting. For the first time, we have high-quality observations that completely unite our observations of the sun and the heliosphere as a single system."

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The outer layer of the sun's atmosphere is called the corona and is typically only visible to humans during total solar eclipses, when the moon blocks out most of the sun's light. The middle corona, which is about 650,000 miles (1 million kilometers) above the sun's surface, has been the least monitored region of the solar atmosphere, partly because there haven't been observations of the sun's corona at heights below 1.3 million miles (2 million km).

In August 2021, researchers announced first-of-their-kind observations of the middle corona in extreme ultraviolet wavelengths. Using advanced instruments with extended fields of view, the scientists found that solar wind structures originate in the middle corona. This provided the first comprehensive insight into the emergence of solar winds — a glimpse into understanding an important phenomenon that influences every planet in the solar system.

Solar wind is one example of what scientists call space weather, a collection of ways that the sun and its outbursts can affect Earth. Space weather can destroy electronic equipment, weaken radio communication, add noise to cell phone reception and knock out power grids. However, scientists don't understand the exact mechanisms that eject solar winds and other space weather, making these events nearly impossible to forecast.

Now, new observations coupled with data from additional space probes and computer models have revealed a complex plasma network in the middle corona. 

An international team of researchers analyzed data from a month-long observation campaign using the Solar Ultraviolet Imager (SUVI) on NOAA's GOES-17 weather satellite. The satellite's extended field of view captured images of not only the center of the sun but also its sides, providing an unusual glimpse at this region's structure and evolution.

"We had the rare opportunity to use an instrument in an unusual way to observe a region that has not really been explored," Seaton said in a second statement. "We didn't even know if it would work, but we knew if it did, we'd make important discoveries."

Using this data, the team tracked a pair of coronal holes — patches of the sun that appear dark and mark where the sun's magnetic field is open to space and expelling material — as they rotated across the solar disk. When the coronal holes appeared at the sun's east and west edges, the researchers discovered the middle corona to be a highly structured coronal web called the "S-web." 

This complex web is made of magnetized plasma structures that continuously interact and reconnect. Researchers saw this mechanism at play as it sparked solar wind streams above the coronal web.

The findings from this study could be useful for a NASA mission targeting launch in 2025, the Polarimeter to Unify the Corona and Heliosphere (PUNCH). This two-year mission will use 3D imaging to capture the sun's outer corona and the region of the solar system nearest to it, with a goal to better understand how the corona generates solar wind.

"Now that we can image the sun's middle corona, we can connect what PUNCH sees back to its origins and have a more complete view of how the solar wind interacts with the rest of the solar system," Seaton, who is the science operations center lead for PUNCH, said in the statement. "Prior to these observations, very few people believed you could observe the middle corona to these distances in UV. These studies have opened up a whole new approach to observing the corona on a large scale."

The research is described in a paper published Nov. 24 in the journal Nature Astronomy.

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