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Mason searched for coronal rain in helmet streamers, like the one that appears to the left of this image, taken during the 1994 eclipse seen from South America. A smaller pseudostreamer appears on the western member (right side of the image). Named for their resemblance to the Knight's sharp helmet, the helmet streamers extend far into the shallow crown of the Sun and are more easily seen when the light from the shining surface of the Sun is obscured. Credits: © 1994 Observing Service and Vojtech Rušin, © 2007 Miloslav Druckmüller
Emily Mason did the same thing every day for five months, in mid-2017. Arriving at her office at NASA's Goddard Space Flight Center in Greenbelt, Maryland, she sat down at her desk, opened her computer and stared at the sun – all day, every day. "I've probably gone through three or five years of data," Mason said. Then, in October 2017, she stopped. She realized that she had always looked at the wrong thing.
Mason, a graduate student at the Catholic University of America in Washington, was looking for coronal rain: giant plasma globes, or electrified gas, that come back from the Sun's external atmosphere on its surface. But she expected to find it in the helmet streamers, the magnetic loops of several kilometers, named for their resemblance to the pointed helmet of a knight, visible from the Sun during a solar eclipse. Computer simulations have predicted that coronal rain could be found there. Observations of solar wind, gas escaping from the Sun and coming out into space, suggest that it might rain. And if she could just find it, the underlying rain physics would have major implications for the 70-year-old mystery about why the Sun's outer atmosphere, called the crown, is so much hotter than its surface. But after almost six months of searching, Mason just could not find it. "There was a lot of research," said Mason, "for something that never happened."
It turned out that the problem was not what she was looking for, but where. In an article published today in Astrophysical Journal Letters, Mason and his coauthors describe the first observations of coronal rain in a smaller, previously neglected magnetic loop type on the Sun. After a long and winding search in the wrong direction, the results establish a new link between the anomalous warming of the corona and the source of the slow solar wind – two of the greatest mysteries of solar science to date.
Coronal rain, like the one shown in this NASA SDO movie in 2012, is sometimes observed after solar flares, when the intense heat associated with a solar flare stops abruptly after the eruption and the remaining plasma cools down. and falls back to the solar surface. Mason was looking for coronal rain not associated with eruptions, but caused by a cyclical process of heating and cooling similar to the cycle of water on Earth. Credits: NASA Solar Dynamics Observatory / Scientific Visualization Studio / Tom Bridgman, Lead Facilitator
How it's raining on the sun
Observed through high-resolution telescopes mounted on NASA's SDO satellite, the Sun – a hot plasma ball filled with magnetic field lines traced by giant and inflamed loops – seems to have little physical similarity to the Earth. But our home planet provides some useful guides to analyze the chaotic tumult of the Sun: among them, rain.
On Earth, rain is only part of the larger water cycle, an endless tussle between heat and gravity. It begins when the liquid water accumulated on the surface of the planet in oceans, lakes or streams is heated by the sun. Part evaporates and rises into the atmosphere, where it cools and condenses into clouds. Finally, these clouds become heavy enough that the gravitational attraction becomes irresistible and the water falls back onto the Earth in the form of rain, before the process begins again.
On the Sun, says Mason, coronal rain works the same way, "but instead of water at 60 degrees, it's a million-degree plasma." Plasma, an electrically charged gas, does not accumulate like water, but traces the magnetic loops that emerge from the surface of the sun like a big eight on tracks. At the foot of the loop, where it attaches to the surface of the Sun, the plasma is overheated from a few thousand to over 1.8 million degrees Fahrenheit. He then extends the loop and gathers himself at his peak, far from the source of heat. As the plasma cools, it condenses and gravity attracts it along the legs as coronal rain.
Mason was looking for coronal rain in the helmet streamers, but his motivation to look at it had more to do with the underlying heating and cooling cycle than the rain itself. Since at least the mid-90s, scientists know that helmet streamers are one of the sources of the slow solar wind, a relatively slow and dense gas stream that escapes from the Sun separately from its equivalent . But measurements of the slow solar wind revealed that it had already been extremely heated before cooling off and escaping the sun. The cyclic process of heating and cooling under the coronal rain, if it occurred inside the helmet streamers, would be a piece of the puzzle.
The other reason is related to the problem of coronal heating – the mystery of how and why the outside atmosphere of the Sun is about 300 times hotter than its surface. Strikingly, simulations have shown that coronal rain is only formed when heat is applied to the bottom of the loop. "If a loop has coronal rain on it, it means the bottom 10%, or less, is the one where coronal warming occurs," said Mason. Rainbows provide a measuring rod, a cutoff point to determine where the crown is heated. Starting their search in the biggest possible loops – giant helmet banners – seemed to be a modest goal, which would maximize their chances of success.
She had the best data for her work: images taken by NASA's Solar Dynamics Observatory, or SDO, a spacecraft that photographed the Sun every 12 seconds since its launch in 2010. But nearly six months later, Mason still had not observed a single drop of rain in a helmet streamer. She had, however, noticed a multitude of tiny magnetic structures, which she did not know. "They were really bright and they continued to catch my eye," said Mason. "When I finally looked at them, they sure had dozens of hours of rain at a time."
At first, Mason was so focused on her quest for a helmet streamer that she did nothing about the sightings. "She came to the group meeting and said:" I never found it. I see it all the time in these other structures, but these are not helmet banners, "said Nicholeen Viall, a solar scientist at Goddard, and co-author of the paper. "And I said, 'Wait … wait. Where do you see him? I do not think anyone has ever seen this before!
In his article, Mason analyzed three observations of the topology Raining Null Point Topologies, or RNTP, previously ignored magnetic structure, presented here in two wavelengths of extreme ultraviolet light. The coronal rain observed in these relatively small magnetic loops suggests that the crown can be heated in a much smaller area than expected. Credits: NASA Solar Dynamics Observatory / Emily Mason
A measuring rod for heating
These structures differ from helmet streamers in several ways. But the most striking thing about them was their size.
"These loops were much smaller than we were looking for," said Spiro Antiochos, a solar physicist at Goddard and co-author of the newspaper. "So it tells you that the heating of the crown is much more localized than we thought."
Although the results do not say exactly how the crown is heated, "they are pushing the ground where crown heating could occur," said Mason. She had discovered loops of rain of about thirty kilometers, barely 2% of the height of some of the helmet streamers that she was looking for originally. And the rain condenses the area where key coronal warming can take place. "We still do not know exactly what is heating up the crown, but we know it has to happen in this layer," said Mason.
A new source for slow solar wind
But some of the observations do not correspond to the previous theories. According to current understanding, coronal rain is formed only on closed loops, where the plasma can collect and cool without any means of escape. But as Mason scanned the data, she discovered cases of rain forming on open magnetic field lines. Anchored to the Sun at one end only, the other end of these cleared field lines elongating in space, the plasma being able to escape in the solar wind. To explain the anomaly, Mason and the team developed another explanation: an explanation that connected the rain to these tiny magnetic structures at the origins of the slow solar wind.
In the new explanation, the rainy plasma begins its course in closed loop, but passes – via a process called magnetic reconnection – to an open process. The phenomenon occurs frequently on the Sun, when a closed loop strikes an open field line and the system reconnects. Suddenly, the overheated plasma on the closed loop is found on an open field line, like a train that has changed lanes. Part of this plasma will expand rapidly, cool down and fall back to the sun as coronal rain. But other parts of it will escape, forming, according to them, part of the slow solar wind.
Mason is currently working on a computer simulation of the new explanation, but she also hopes that future evidence will confirm this. Now that Parker Solar Probe, launched in 2018, is closer to the Sun than any other spacecraft, it can now fly through gusts of slow solar wind that can be traced back to the Sun – potentially, until 'at one of Coron's coron rain events. After observing the coronal rain on an open field line, the outgoing plasma, escaping the solar wind, would normally be lost to posterity. But no longer. "We can potentially make that connection with Parker Solar Probe and say that's it," Viall said.
Digging in the data
As for finding coronal rain in helmet streamers? The search continues. The simulations are clear: the rain should be there. "Maybe it's so small that you can not see it?" Said Antiochos. "We really do not know."
But again, if Mason had found what she was looking for, she might not have made the discovery – or spent all that time learning the ins and outs of solar data.
"It sounds like a manual job, but honestly, it's my favorite thing," Mason said. "I mean, that's why we built something that takes so many images of the Sun, so we can look at them and understand them."
Publication: E.I.Mason, et al., "Observations of solar corona rain in zero point topologies", ApJL, 2019; doi: 10.3847 / 2041-8213 / ab0c5d
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