Unexpected rain in the sun links two solar mysteries – Review of Eurasia



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For five months in mid-2017, Emily Mason did the same thing every
day. Arriving at his office at NASA's Goddard Space Flight Center in
Greenbelt, Maryland, she sat at her desk, opened her computer and
I looked at images of the sun – every day, every day. "I probably looked like
by means of data of three or five years, "estimates Mason. Then, in
October 2017, she stopped. She realized she was looking at the
wrong thing all the time.

Mason, a graduate student at the Catholic University of America in Washington, DC, was looking for coronal rain: giant plasma globes, or electrified gas, that drip from the Sun's outer atmosphere back to its surface. But she hoped to find him in helmet coils, the million-mile magnetic lugs – named for their resemblance to a knight's pointed helmet – that can be seen rising out of the sun during a solar eclipse. Computer simulations predicted that coronal rain could be found there. Observations of the solar wind, the gas that escaped from the Sun and went out into space, suggested that the rain might be happening. And if she could find it, the underlying physics of the rain would have major implications for the 70-year mystery of why the outer atmosphere of the Sun, known as the crown, is much hotter than the surface. But after nearly half a year of searching, Mason simply could not find him.

"It was very beautiful," Mason said, "for something that never happened after all."

The problem, it turned out, was not what she was looking for, but where. In an article published in Astrophysical journal lettersMason and his coauthors describe the first observations of coronal rain in a smaller, previously neglected type of magnetic loop in the sun. After a long and tortuous search in the wrong direction, the discoveries forge a new connection between the anomalous warming of the crown and the source of the slow solar wind – two of the biggest mysteries solar science faces today.

How it rains in the sun

Observed through high-resolution telescopes mounted on
SDO spacecraft, the Sun – a hot plasma ball, full of magnetic
field lines traced by giant, fire loops – seems to have few
similarities with the Earth. But our planet provides some useful resources
it guides in the analysis of the chaotic turmoil of the Sun: among them, the rain.

On Earth, rain is only one part of the larger water cycle, one
endless tug of war between the heat impulse and the force of gravity. this
begins when liquid water, collected on the surface of the planet in the oceans,
lakes, or streams, is warmed by the sun. Some of it evaporates and rises
in the atmosphere, where it cools and condenses in clouds.
Eventually, these clouds become heavy enough for the force of gravity to become
irresistible and the water falls back to Earth like rain, before
process begins again.

In the sun, Mason said, coronary rain works similarly, "but instead
of 60 degrees of water, you're dealing with a million-degree plasma. "Plasma,
an electrically charged gas, does not accumulate as water, but rather
traces the magnetic loops that emerge from the surface of the Sun as a
roller coaster on the slopes. At the foot points of the loop, where it connects
the surface of the Sun, plasma is superheated from a few thousand to
more than 1.8 million degrees Fahrenheit. Then it expands the loop and
gathers at its peak, away from the source of heat. As the plasma cools,
condenses and gravity draws the legs of the loop like coronal rain.

Mason was looking for coronary rain in helmet streamers, but she
motivation to look there had more to do with this underlying warm-up
and cooling cycle than rain itself. Since at least the mid-1990s,
scientists know that helmet streamers are one of the sources of the slow
solar wind, a relatively slow and dense stream of gas escaping from the
Sun separately from its fast moving counterpart. But measurements of
the slow solar wind gas revealed that it had already been heated to a
extreme degree before cooling down and escaping from the sun. The cyclical process
of heating and cooling behind coronal rain if it were happening inside
the pennants of the helmet, would be a piece of the puzzle.

The other reason connects to the coronal heating problem – the mystery of how and why the outer atmosphere of the Sun is about 300 times warmer than its surface. Surprisingly, the simulations have shown that coronal rainfall only forms when heat is applied to the bottom of the loop.

"If a cycle has coronal rain, it means that 10 percent of the bottom, or less, is where coronal heating is happening," Mason said. Raining ties provide a measuring rod, a cut-off point to determine where the corona is heated. Starting the quest for the biggest loops they could find – huge helmet streamers – seemed like a modest goal and would maximize their chances of success.

She had the best data for the work: images taken by NASA's Solar Dynamics Observatory, or SDO, a spacecraft that photographed the Sun every twelve seconds since its launch in 2010. But nearly half a year after the survey, Mason has not yet had watched a single drop of rain on a helmet streamer. She had, however, noticed a series of small magnetic structures, those with which she was unfamiliar.

"They were really brilliant and kept getting my attention," Mason said. "When I finally looked at them, they certainly had dozens of hours of rain at a time."

At first, Mason was so focused on his search for helmet serpentine that she did not do any of the remarks.

"She came to a group meeting and said," I've never met her – I see that all the time in those other structures, but they're not helmet coils, "said Nicholeen Viall, solar scientist at Goddard and coauthor of the paper. "And I said," Wait … hang in there, where do you see that? "I do not think anyone has ever seen it before!"

A measuring bar for heating

These structures differed from helmet streamers in various ways. But the most striking thing about them was the size of them.

"These circuits were much smaller than we were looking for," he said.
Spiro Antiochos, who is also a solar physicist at Goddard and co-author
the paper. "So this tells you that the warming of the corona is very
more localized than we were thinking. "

While the findings do not say exactly how the corona is heated, "they push the floor where coronal heating can happen," Mason said. She had found rainy ties about 30,000 miles high, a mere two percent of the height of some of the ribbons of the helmet she was originally looking for. And the rain condenses the region where the main coronal heating may be happening.

"We still do not know exactly what is heating the crown, but we know that this needs to happen at this layer," Mason said.

A new source for the slow solar wind

But some of the observations did not match the previous ones
theories. According to the current understanding, only coronal rainfall
forms in closed loops, where the plasma can collect and cool without any
ways to escape. But as Mason scoured the data, she found cases
where the rain was forming on the lines of open magnetic field. Anchored to the sun
in only one end, the other end of these open field lines fed
space and plasma could escape into the solar wind. To explain the
anomaly, Mason and the team developed an alternative explanation – a
which connected the rain on these tiny magnetic structures to the origins of the
the slow solar wind.

In the new explanation, rainy plasma begins its journey in a
circuit, but switches – through a process known as magnetic
reconnection – to an open one. The phenomenon happens frequently in the
Sol, when an open circuit collides with an open field line and the system
is reconfigured. Suddenly, the superheated plasma in the closed circuit
lies in an open field line, like a train that has changed
tracks. Some of this plasma will expand rapidly, cool and fall
back in the sun as coronal rain. But other parts of it will escape.
forming, they suspect, a part of the slow solar wind.

Mason is currently working on a computerized simulation of the new explanation, but she also hopes that observational evidence will soon confirm this. Now that the Parker Solar Probe, launched in 2018, is traveling closer to the Sun than any previous spacecraft, it can fly through gusts of slow solar wind that can be traced back to the Sun – potentially one of the coronal rain events of Mason. After observing the coronal rain in an open field line, the outgoing plasma, escaping the solar wind, would normally be lost to posterity. But not anymore.

"Potentially, we can make that connection to the Parker Solar Probe and say that was it," Viall said.

Digging Through the Dice

How about finding coronal rain in helmet streamers? The search
continued. The simulations are clear: the rain must be there. "Perhaps
Is it so small that you can not see? "Said Antiochos. "We really do not
to know."

But again, if Mason had found what she was looking for, she
may not have made the discovery – or have spent all this time
learning the ins and outs of solar data.

"Sounds like a slog, but honestly it's my favorite thing," he said.
Bricklayer. "I mean, that's why we built something that takes a lot of images
of the Sun: So we can look at them and find out. "

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