Sun has cooling gas showers in tiny magnetic loops at 17,500 ° F

They say do not look at the sun – but for five months, physicist Emily Mason has spent every day doing nothing else, examining images of the surface of our star.

Ms. Mason and her colleagues discovered parts of the sun where the overheated gases were cooling down to fall on the star's surface as rain.

The results create a link between two of the sun's greatest mysteries: the nature of heating that makes the sun's outer atmosphere about 300 times warmer than the underlying surface and the source of the slowest and most dense solar wind.

The sun is a huge ball of plasma – extremely hot, electrically charged gases – from which magnetic field lines form gigantic magnetic fire loops in space.

The notion of rain on the sun may seem absurd – but the familiar meteorological phenomenon is an excellent analog for some of the processes that take place on the surface of our neighborhood star.

On Earth, rain is a key part of the water cycle. The liquid water on the surface of the Earth – that it is accumulated in rivers, lakes or oceans – evaporates when it is heated by the sun's energy . After getting high in the atmosphere, the water cools and concentrates in clouds.

When the droplets of water in the clouds become too heavy to remain suspended in the air, they fall back on the Earth in the form of rain. And so, the cycle repeats itself.

On the sun, the coronal rain works in the same way, explains Ms. Mason. "But instead of 60-degree water, you use a million-degree plasma," she said.

However, rather than clustering on the ground, the plasma travels magnetic loops that extend into arches from the sun's surface.

At the end of each loop, where lines emerge from the star's surface, the plasma is overheated from a few thousand to over a million degrees Celsius (1.8 m F).

The heated plasma dilates its loop until it meets at the top – farthest from the heat source – where it can cool, condense and fall back into the loop as coronal rain.

Coronal rain had already been observed as a result of solar flares, when the heat associated with a solar flare suddenly stops, allowing the erupting plasma to cool and fall back onto the surface of the sun.

Based on previous computer simulations and observations of the solar wind, Ms. Mason also expected to find coronal rain in what are called "helmet streamers" – magnetic loops of several million kilometers high that exceed the sun during solar eclipses.

Helmet streamers are so named because of their appearance to the knight's helmet.

Scientists know since the mid-90s that helmet streamers are one of the sources of the slow solar wind – the relatively calm and dense gas stream that escapes from the sun regardless of its counterpart to motion plus quick.

The measurements of the gas in the slow solar wind had shown that it had already been extremely heated before cooling off and escaping from the sun.

Since coronal rain was expected to undergo similar warming and cooling, it seemed likely that this rain would play a role in the activity along helmet streamers and the resulting slow wind formation.

At the same time, the simulations predict that coronal rain can only be formed when heat is applied to the bottom of the host's magnetic loop – the lower 10% or less – the identification of the size of a loop containing rain would provide a form of measuring rod to determine the limited area above the surface of the sun in which the crown is heated.

The search for coronal rain in the helmet streamers was launched.

Ms. Mason studied images taken by NASA's Solar Dynamics Observatory – a spacecraft that has been photographing the sun every 12 seconds since its launch in 2010.

However, she spent almost a year looking for coronal rain in helmet streamers without success.

"I have probably looked at data of a duration of three or five years," Ms. Mason said. "There was a lot of research for something that never happened."

Instead, Ms. Mason continued to find coronal rain in other magnetic structures – small loops at a height of 48,000 km (30,000 miles), a percentage of the size of helmet streamers on which she intended to concentrate.

"They were really bright and they continued to catch my eye," said Ms. Mason. "When I finally looked at them, they sure had dozens of hours of rain at a time.

However, being focused on her goal of locating rain in the helmet streamers, she initially rejected these results – until her colleagues realized that her observations were relatively new.

"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 scientist and author of the paper.

"And I said," Wait … hold on well. Where do you see it? I do not think anyone has ever seen this before! & # 39;

The discovery of rain in the smallest magnetic loops has made it possible to define the region in which the coronal heating must take place.

"We still do not know exactly what is heating the crown, but we know that this must happen in this layer," Ms. Mason said.

Some of the discoveries have also forced researchers to reconsider previous theories.

It had been thought that coronal rain could only form on closed magnetic loops – where the superheated plasma can congregate at the end of a loop and cool without any means of escape.

However, while Ms. Mason was studying the data on small loops, she also discovered cases where rain appeared to form on open magnetic field lines.

Such open lines are anchored to the sun only at one end – the other end extending into space, feeding the solar wind.

The researchers now suggest that the plasma always starts with a closed magnetic loop – but can switch to an open loop according to a process called magnetic reconnection.

Reconnection occurs frequently on the Sun when closed loops hit open field lines, changing the way the field is arranged.

Like train switching paths, overheated plasma can be in an open loop – which means that during cooling, it inevitably falls on the sun in the form of coronal rain, but the rest is free of s & # 39; escape.

Scientists suggest that it is so that part of the slow solar wind is released.

Ms. Mason is working on creating a computer simulation to support this new explanation.

The Parker Solar Probe, launched in 2018, can also provide additional evidence. She will be able to fly over the slow solar wind and trace her original point on the sun, as perhaps one of Mrs. Mason's coronal rain events.

"We can potentially establish this connection with Parker Solar Probe and say: that's it," said Dr. Viall.

Despite these fortuitous discoveries, the search for coronal rain in the helmet streamers continues – simulations always predicting that both phenomena must be found together.

"Maybe it's so small that you can not see it?" Explains Spiro Antiochos, solar physicist and paper author, NASA's Goddard Space Flight Center. He admits, "We really do not know."

The complete results of the study were published in the journal Astrophysical Journal Letters.