While rain on Earth is associated with water, precipitation on the Sun comes as giant clumps of plasma, or supercharged gas, which drizzle down from the star's atmosphere on to its surface. Though coronal rain has been observed on numerous occasions, its source, which researchers believed would help them better understand how the Sun's outer atmosphere, or corona, gets so hot, had never been discovered. Now, thanks to Emily Mason, a graduate student at The Catholic University of America in Washington, D.C., the mystery has finally been solved.
Mason's quest to discover the origin of coronal rain began in mid-2017, when she started sifting through the images taken by NASA’s Solar Dynamics Observatory, or SDO, a spacecraft that has photographed the Sun every twelve seconds since its launch in 2010. She was particularly focused on the helmet streamers — closed magnetic loops which connect regions of opposite magnetic polarity. Often visible protruding from the Sun during a solar eclipse, the massive million-mile tall loops were identified by computer simulations as the most likely source of the precipitation.
Though the researcher found no evidence of the plasma rain in the helmet streamers, Mason did observe a series of tiny magnetic structures beside them that she was unfamiliar with. “They were really bright, and they kept drawing my eye,” she said. “When I finally took a look at them, sure enough, they had tens of hours of rain at a time.”
After six months of researching, Mason announced in a group meeting, "I never found it [rain in the helmet streamers] — I see it all the time in these other structures, but they’re not helmet streamers." Nicholeen Viall, a solar scientist at NASA's Goddard Space Flight Center, and a coauthor of the paper published in the Astrophysical Journal Letters on April 5, 2019, who recalls the moment clearly, said, “And I said, ‘Wait…hold on. Where do you see it? I don’t think anybody’s ever seen that before!"
The structures Mason was referring to are called null-point topologies. These magnetic loops are very different in structure from the helmet streamers and measure just tens of thousands of miles high. “These loops were much smaller than what we were looking for,” said Dr. Spiro Antiochos, who is also a solar physicist at Goddard and a co-author of the paper. “So that tells you that the heating of the corona is much more localized than we were thinking.”
As for how the plasma rain forms? Mason believes the process is similar to that on Earth. On our planet, the water cycle begins when liquid water from the oceans, lakes and streams evaporates due to the sun's heat and rises into the atmosphere. The cooler air above causes the water vapor to condense into clouds, which eventually get heavy enough to be dragged down by gravity and fall as rain.
In the Sun's case, the electrically-charged plasma follows the magnetic loops emerging from the hot star's surface, similar to a rollercoaster on tracks. As it gets close to the endpoints, where the loop meets the Sun's surface, the thousand-degree gas gets superheated to over 1.8 million degrees Fahrenheit. This causes the plasma to expand and gather at the top of the loop. As it moves away from the sun's intense heat, the gas cools, condenses, and helped by the Sun's gravity, falls along the loop's sides as coronal rain!
Though Mason's discovery did not solve the mystery of how the Sun's atmosphere is so much hotter than the star itself, it did provide some clues. “We still don’t know exactly what’s heating the corona, but we know it has to happen in this layer,” she said.
While the main source of the rain has been found, NASA scientists are convinced there is more to be found in the helmet streamers as well. “Maybe it’s so small you can’t see it?” asked Antiochos. “We really don’t know.” While this means Mason will have to pore over thousands of additional photos, the researcher does not seem to mind. “It sounds like a slog, but honestly it’s my favorite thing,” she said. “I mean that’s why we built something [ like the SDO] that takes that many images of the Sun: So we can look at them and figure it out.”
Resources: earthsky.org, NASA.gov