When and how it rains in the sun | Space

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Coronal rain in prominence, or solar flare, seen by the Solar Dynamics Observatory (SDO) in 2012. Gif via Solar Dynamics Observatory / Scientific Visualization Studio / Tom Bridgman (Lead Animator).

The rain comes in various forms throughout the solar system: water on Earth, methane / ethane on Titan and sulfuric acid on Venus. But did you know too it is raining Sun? Good type of. Large drops of plasma – electrified gases – can flow protruding (magnetic magnetic loops of gas) in the outside atmosphere of the sun, the crown, on the hot surface. This unusual phenomenon, called coronal rain, was the subject of a study conducted by a NASA undergraduate student who wanted to document it better.

In a new peer-reviewed document Letters from the Astrophysical Journal – Published April 5, 2019 – Emily Mason at NASA's Goddard Space Flight Center explains how she found evidence of coronal rain, but in an unexpected place.

When Mason began looking for evidence of coronal rain, she first studied helmet banners – heights of one million kilometers named for their resemblance to a sharp knight's helmet – visible from the sun during a total solar eclipse. Computer simulations had suggested that this would be the place most likely to receive coronal rain. Mason searched for nearly six months … but found no sign of plasma rain, even though she had images taken by NASA's Solar Dynamics Observatory (SDO), which photographed the sun every 12 seconds since its launch in 2010. As Mason said:

I've probably gone through three or five years of data. There was a lot of research, something that never finally happened.

Giant plasma gas loops on the sun, called helmet streamers. Coronal rain was thought to be there, but the search turned out to be empty. Image via Pice Observatory / Vojtech Rušin (1994) / Miloslav Druckmüller (2007).

So where was it? he should to be there, Mason knew. In the end, Mason and his colleagues had searched the wrong place. Instead of helmet streamers, rain was found in a kind of smaller magnetic loop on the sun. It was there, but not at the place where the researchers were waiting to find it. This discovery should help scientists understand the unusual warming of the solar corona and the source of the slow solar wind, two mysteries on the sun that make scientists even more puzzled. Rain has been observed in the form of a multitude of small magnetic structures, Mason said:

They were really bright and they continued to catch my eye. When I finally looked at them, he was certain that they had had dozens of rainy hours at a time.

As Spiro Antiochos, solar physicist at Goddard and co-author of the article explains:

These loops were much smaller than we were looking for. This tells you that the heating of the crown is much more localized than we thought. Maybe that [the coronal rain] is so small that you can not see it? We really do not know.

As Nicholeen Viall, a solar energy scientist from Goddard, noted, it took a while to recognize the structures of what they were:

She came to the group meeting and said, "I never found it. I see it all the time in these other structures, but it's not a helmet banner. "And I said," Wait … wait. Where do you see him? I do not think anyone has ever seen this before!

Coronal rain has been found in smaller magnetic loops like these, the new Raining Null-Point (RNTP) topologies. Image via Solar Dynamics / Emily Mason observatory.

So, how is this "rain" going? In general terms, the process is similar to what happens on Earth, Mason said:

But instead of 60-degree water, you use a million-degree plasma.

Unlike water, plasma is an electrically charged gas. It follows the protuberances emerging from the sun's surface and is extremely hot – from a few thousand to over 1.8 million degrees Fahrenheit (1 million degrees Celsius). At the peak of importance, it condenses and then falls back to the surface of the sun in the form of coronal rain.

In the hydrological cycle of the Earth, water evaporates on the surface and rises into the atmosphere. It then cools and condenses into clouds, and when there is enough moisture in the clouds, it falls to the surface as rain. Coronal rain is a somewhat similar process, but with a composition completely different from the rain itself.

To help solve some mysteries of the sun, coronal rain provides clues.

The measurement of gas in the slow solar wind – separated from the other rapidly moving solar wind – indicated that it had been heated to extreme levels before cooling off and escaping the sun. But how? If the process occurred in the streamers of the helmet, the cyclical process of heating and cooling behind the coronal rain could explain that. But as rain seems to occur more in small loops, this idea may require a little more work.

The many parts of the sun, including the crown and protuberances. Image via Astronomy / Roen Kelly.

In previous theories, it was thought that coronal rain occurred only in closed loops, where the plasma heated and cooled, but could not escape in space. Mason's work suggests, however, that rain begins in a closed loop, then goes through a process called magnetic reconnection to an open process, such as a train switching circuit. Part of the plasma will escape then, but another part will fall to the surface in the form of rain. The plasma that escapes is part of the slow solar wind.

Then comes the other puzzle that explains why the outside atmosphere of the sun is 300 times hotter than its surface. At first glance, this observed fact does not seem to make any logical sense. As Mason discovered, rain in the loops can provide a cutoff point to determine how much the crown is heated:

If a loop has coronal rain, it means that the coronal warming occurs in the lower 10 percent, or less.

These scientists are now hoping that NASA's Parker Solar Probe, which performs the closest sun passes of all time – even through the crown – will help confirm or not Mason's results.

In summary: The rain on the sun may seem absurd, but it is real – albeit in a form unlike anything on Earth – and can help solve some persistent puzzles about how our sun works.

Source: Observations of solar coronal rain in zero point topologies

Via NASA