Unexpected rain on sun links two solar mysteries

NASA/Goddard Space Flight Center

Mason's article analyzed three observations of Raining Null-Point Topologies, or RNTPs, a previously overlooked magnetic structure shown here in two wavelengths of extreme ultraviolet light. The coronal rain observed in these comparatively small magnetic loops suggests that the corona may be heated within a far more restricted region than previously expected. Credit Credits: NASA's Solar Dynamics Observatory/Emily Mason

Mason’s article analyzed three observations of Raining Null-Point Topologies, or RNTPs, a previously overlooked magnetic structure shown here in two wavelengths of extreme ultraviolet light. The coronal rain observed in these comparatively small magnetic loops suggests that the corona may be heated within a far more restricted region than previously expected. Credit Credits: NASA’s Solar Dynamics Observatory/Emily Mason

For five months in mid 2017, Emily Mason did the same thing every day. Arriving to her office at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, she sat at her desk, opened up her computer, and stared at images of the Sun — all day, every day. “I probably looked through three or five years’ worth of data,” Mason estimated. Then, in October 2017, she stopped. She realized she had been looking at the wrong thing all along.

Mason, a graduate student at The Catholic University of America in Washington, D.C., was searching for coronal rain: giant globs of plasma, or electrified gas, that drip from the Sun’s outer atmosphere back to its surface. But she expected to find it in helmet streamers, the million-mile tall magnetic loops — named for their resemblance to a knight’s pointy helmet — that can be seen protruding from the Sun during a solar eclipse. Computer simulations predicted the coronal rain could be found there. Observations of the solar wind, the gas escaping from the Sun and out into space, hinted that the rain might be happening. And if she could just find it, the underlying rain-making physics would have major implications for the 70-year-old mystery of why the Sun’s outer atmosphere, known as the corona, is so much hotter than its surface. But after nearly half a year of searching, Mason just couldn’t find it. “It was a lot of looking,” Mason said, “for something that never ultimately happened.”

The problem, it turned out, wasn’t what she was looking for, but where. In a paper published today in the Astrophysical Journal Letters, Mason and her coauthors describe the first observations of coronal rain in a smaller, previously overlooked kind of magnetic loop on the Sun. After a long, winding search in the wrong direction, the findings forge a new link between the anomalous heating of the corona and the source of the slow solar wind — two of the biggest mysteries facing solar science today.

How It Rains on the Sun

Observed through the high-resolution telescopes mounted on NASA’s SDO spacecraft, the Sun – a hot ball of plasma, teeming with magnetic field lines traced by giant, fiery loops — seems to have few physical similarities with Earth. But our home planet provides a few useful guides in parsing the Sun’s chaotic tumult: among them, rain.

On Earth, rain is just one part of the larger water cycle, an endless tug-of-war between the push of heat and pull of gravity. It begins when liquid water, pooled on the planet’s surface in oceans, lakes, or streams, is heated by the Sun. Some of it evaporates and rises into the atmosphere, where it cools and condenses into clouds. Eventually, those clouds become heavy enough that gravity’s pull becomes irresistible and the water falls back to Earth as rain, before the process starts anew.

On the Sun, Mason said, coronal rain works similarly, “but instead of 60-degree water you’re dealing with a million-degree plasma.” Plasma, an electrically-charged gas, doesn’t pool like water, but instead traces the magnetic loops that emerge from the Sun’s surface like a rollercoaster on tracks. At the loop’s foot points, where it attaches to the Sun’s surface, the plasma is superheated from a few thousand to over 1.8 million degrees Fahrenheit. It then expands up the loop and gathers at its peak, far from the heat source. As the plasma cools, it condenses and gravity lures it down the loop’s legs as coronal rain.

Mason was looking for coronal rain in helmet streamers, but her motivation for looking there had more to do with this underlying heating and cooling cycle than the rain itself. Since at least the mid-1990s, scientists have known that helmet streamers are one source of the slow solar wind, a comparatively slow, dense stream of gas that escapes the Sun separately from its fast-moving counterpart. But measurements of the slow solar wind gas revealed that it had once been heated to an extreme degree before cooling and escaping the Sun. The cyclical process of heating and cooling behind coronal rain, if it was happening inside the helmet streamers, would be one piece of the puzzle.

The other reason connects to the coronal heating problem — the mystery of how and why the Sun’s outer atmosphere is some 300 times hotter than its surface. Strikingly, simulations have shown that coronal rain only forms when heat is applied to the very bottom of the loop. “If a loop has coronal rain on it, that means that the bottom 10% of it, or less, is where coronal heating is happening,” said Mason. Raining loops provide a measuring rod, a cutoff point to determine where the corona gets heated. Starting their search in the largest loops they could find — giant helmet streamers — seemed like a modest goal, and one that would maximize their chances of success.

She had the best data for the job: 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. But nearly half a year into the search, Mason still hadn’t observed a single drop of rain in a helmet streamer. She had, however, noticed a slew of tiny magnetic structures, ones she wasn’t familiar with. “They were really bright and they kept drawing my eye,” said Mason. “When I finally took a look at them, sure enough they had tens of hours of rain at a time.”

At first, Mason was so focused on her helmet streamer quest that she made nothing of the observations. “She came to group meeting and said, ‘I never found it — I see it all the time in these other structures, but they’re not helmet streamers,'” said Nicholeen Viall, a solar scientist at Goddard, and a coauthor of the paper. “And I said, ‘Wait…hold on. Where do you see it? I don’t think anybody’s ever seen that before!'”

A Measuring Rod for Heating

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

“These loops were much smaller than what we were looking for,” said Spiro Antiochos, who is also a solar physicist at Goddard and a coauthor of the paper. “So that tells you that the heating of the corona is much more localized than we were thinking.”

While the findings don’t say exactly how the corona is heated, “they do push down the floor of where coronal heating could happen,” said Mason. She had found raining loops that were some 30,000 miles high, a mere two percent the height of some of the helmet streamers she was originally looking for. And the rain condenses the region where the key coronal heating can be happening. “We still don’t know exactly what’s heating the corona, but we know it has to happen in this layer,” said Mason.

A New Source for the Slow Solar Wind

But one part of the observations didn’t jibe with previous theories. According to the current understanding, coronal rain only forms on closed loops, where the plasma can gather and cool without any means of escape. But as Mason sifted through the data, she found cases where rain was forming on open magnetic field lines. Anchored to the Sun at only one end, the other end of these open field lines fed out into space, and plasma there could escape into the solar wind. To explain the anomaly, Mason and the team developed an alternative explanation — one that connected rain on these tiny magnetic structures to the origins of the slow solar wind.

In the new explanation, the raining plasma begins its journey on a closed loop, but switches — through a process known as magnetic reconnection — to an open one. The phenomenon happens frequently on the Sun, when a closed loop bumps into an open field line and the system rewires itself. Suddenly, the superheated plasma on the closed loop finds itself on an open field line, like a train that has switched tracks. Some of that plasma will rapidly expand, cool down, and fall back to the Sun as coronal rain. But other parts of it will escape – forming, they suspect, one part of the slow solar wind.

Mason is currently working on a computer simulation of the new explanation, but she also hopes that soon-to-come observational evidence may confirm it. Now that Parker Solar Probe, launched in 2018, is traveling closer to the Sun than any spacecraft before it, it can fly through bursts of slow solar wind that can be traced back to the Sun — potentially, to one of Mason’s coronal rain events. After observing coronal rain on an open field line, the outgoing plasma, escaping to the solar wind, would normally be lost to posterity. But no longer. “Potentially we can make that connection with Parker Solar Probe and say, that was it,” said Viall.

Digging Through 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 you can’t see it?” said Antiochos. “We really don’t know.”

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

“It sounds like a slog, but honestly it’s my favorite thing,” said Mason. “I mean that’s why we built something that takes that many images of the Sun: So we can look at them and figure it out.”

###

From EurekAlert!

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47 thoughts on “Unexpected rain on sun links two solar mysteries

  1. Four phases of matter: solid, liquid, gas and plasma.
    Condensation is a phase change.
    Are we talking about something that is so little understood that we confuse our terms? It is either plasma or condensed matter,no? I don’t think we can have it both ways.

    • Eventually, those clouds become heavy enough that gravity’s pull becomes irresistible and the water falls back to Earth as rain, before the process starts anew.

      A pretty inept and inaccurate description of precipitation.

      • Tell us how to “resist” gravity, it could be immensely useful.

        Really who writes this garbage? I’m not surprised they are having difficultly understanding the sun.

      • I was going to comment on that, thank you for doing so so high up in the comment.

        Once cloud droplets get big enough, I assume through condensation and accretion, then they can overcome the updrafts that triggered the damn cloud to form in the first place.

      • “how to “resist” gravity” is a problem birds and planes have to cope with.

        Meanwhile “passengers” accomodate with gravity traveling on birds and planes.

    • I offer this video from a man who is often disparaged as a kook, yet here he is giving an explanation for what NASA is describing as coronal rain. Condensation reactions in the corona. Dr. Robitaille is a chemist and spectroscopist.
      IS THE CORONA ONLY GASEOUS PLASMA? EVIDENCE FOR CONDENSED MATTER!
      https://youtu.be/ItT6mx6Po3g

    • “plasma or condensed matter,no? I don’t think we can have it both ways.”

      Solid matter and plasma enjoy sharing the same space:

      Atoms in a solid copper cable “share” an wandering stream of electrons – and that’s plasma.

      The same with sun plasma: heated metal bows share a plasma of electrons.

      This electrons can leave the however solid, heated metal bows were they open to space: as energy stream of photons.

      And until that the electrons are macnetically “bound” to the however “solid” Metall.

      correct me where I’m wrong, regards.

  2. Plasma, an electrically-charged gas
    A commonly mistaken claim. The plasma is electrically neutral, not charged.
    The correct expression would “an ionized gas” where some of the electrons have been stripped of the atoms, resulting in a melange of equal number of negative charge [electrons] and positive charge [of the ions], maintaining the neutrality of the matter. Since electrons are so much lighter than the ions, they are easily moved and the plasma is then a good conductor of electricity.
    “Plasma is an electrically neutral medium of unbound positive and negative particles” [ https://en.wikipedia.org/wiki/Plasma_(physics) ]
    BTW the solar photosphere is a very weakly ionized plasma. Only one in 10,000 hydrogen atoms are ionized [the number density of charges is about the same as in ordinary sea water].

    It makes sense that the heating takes plasma close to the surface where the density and the magnetic fields are strongest.

    • Cosmic rays are 99% protons and helium nuclei, only 1% electrons. Sun and Earth have been bombarded by cosmic rays for 4.5 billion years. I wonder if the sun and Earth’s interior have slight positive charge. Is that why electricity (electrons) is attracted to the ground? Ground wiring, lightning rod, electric fence, etc.

    • Plasma is neutral in the same sense that high voltage A/C cables are neutral.

      Joule heating produced by the A/C current carried by plasma is the mystery of the hot corona.

  3. As for finding coronal rain in helmet streamers? The search continues. The simulations are clear: the rain should be there.

    The observational evidence doesn’t support the computer simulations, so the observational evidence must be wrong.

    sigh

    • That’s the problem with a computer model, it doesn’t actually physically crash and burn, and not a soul is killed or injured when it does. Events like the Titanic, the Tay and Tacoma Narrows Bridges proved that ship and bridge design needed more work, saying our model says they’re still standing wasn’t an option. The Shuttles Challenger and Columbia, the current 737 Max, London’s Millennium Bridge all proved that computer models are fallible in a highly visible way. Unfortunately humans are extremely good at disconnecting failure in one field from potential failure in their own. In this case computer models used solely in research with no direct physical risks to anyone.

  4. I would expect any such coronal rain to heat up adiabatically as it descends back towards the lower part of the loop so that would account for observations of superheating at the interface with the surface of the sun.
    Much like convective overturning within the gaseous atmospheres of planets which , I have long contended, is what really causes the observed surface temperature enhancement for planets with atmospheres.

    • would account for observations of superheating at the interface with the surface of the sun

      I think that is the expectation from their broken models, not observation.

      • Strikingly, simulations have shown that coronal rain only forms when heat is applied to the very bottom of the loop.

        The simulations from model which have just been proven not to work, but never mind, we’ll keep referring to them as is they do.

  5. “Mason is currently working on a computer simulation of the new explanation, but she also hopes that soon-to-come observational evidence may confirm it.”
    Isn’t it wonderful to see a scientist prepared to change their hypothesis when the observations change!

  6. OK. So the initial assumptions were wrong, as it is so common in science. Beware of assumptions. They are not the subject of the scientific method, so it is like introducing religion in science. Climate science is full of assumptions. A common one is to think that the effect of solar activity on climate is only through changes in TSI.

    I thought it was known that the superheating of the corona is due to Alfvén waves. Curious that the article doesn’t even mention them.

    • Alfvén ? Careful next you will be suggesting that the reason the corona is mysteriously so much hotter than the photosphere is that the source of the energy causing that heating is outside the sun 😉

      Maybe those “open field lines” are linked to electrical discharges.

      • https://en.wikipedia.org/wiki/Alfv%C3%A9n_wave#The_coronal_heating_problem

        In 1942, Hannes Alfvén proposed in Nature the existence of an electromagnetic-hydrodynamic wave which would carry energy from the photosphere to heat up the corona and the solar wind. He claimed that the sun had all the necessary criteria to support these waves and they may in turn be responsible for sun spots.

        The guy got the Nobel Prize for it. Are you sure you know what you talk about?

        • Yes, Hannes Alfvén was a top physicist. He was also a proponent of the idea that there is significant electrical activity in space. Something the “consensus” is having a lot of difficulty accepting. Though they are making baby steps in that direction , finally, it is still largely regarded witchcraft and pseudo science.

      • Greg
        April 6, 2019 at 1:43 am

        Alfvén ? Careful next you will be suggesting that the reason the corona is mysteriously so much hotter than the photosphere is that the source of the energy causing that heating is outside the sun 😉
        ————————–
        “the reason the corona is mysteriously so much hotter than the photosphere is that the matter of that heating is also outside the sun ” 😉

        Another funny way to put it. Maybe… 🙂
        E = mc2,

  7. As the sun enters (or more likely not) the new Maunder type grand minimum it will be ‘snowing’ endlessly over there.

  8. I wonder where exactly the MHD simulations are going wrong. If we understood hot plasma well enough, we would have commodity fusion power. We do have very good thermonuclear devices, which is very odd.

    Reasoning by analogy, hard to criticize for so an alien environment, is o.k. Carrying that a little further – rain here needs nucleation.
    It would be great if Svensmark had a look at GCR interaction with Solar corona plasma structures like rain.

    It could be that fusion power might need some form of nucleation (muon catalysis is one such idea).

    It should be no surprise that studying the largest nearby fusion reactor will yield vitally important insights.

    • One handy thing about that ‘nearby fusion reactor’. The containment is inside the plasma (gravity) rather than outside (magnetic fields). Now if only we could create a weak enough point singularity here, problem solved (or planet destroyed, hard to predict)

      • Somewhere there are assumptions and the reason the usual MHD code goes wrong. Which is why we keep checking. We get good results (depending on your POV) with thermonuclear weapons without a singularity.

  9. This article is a demonstration on the basic fact that you must thoroughly understand all aspects of something that you are going to make a simulation, a simulator, or model of. Rough approximations, inadequate or incomplete knowledge can not be ignored in making a simulation – a computer model. The EXACT same problem Climate Change Models have. Climate Change Models will never work until they include all parameters and variables in their model. Not if they expect to use these for 100 year projections. and that approaches the impossible because of the numbers of parameters and variables, many of which they do not even know that they do not know them – like the above article.
    Fifty years ago in my first computer science class [which was in the maths department back then] I very quickly learned GIGO. However we had the answer to our problem and knew the algorithm was WRONG. Forty years ago, working on a computer program for a Nuclear Power Plant Simulator it to our group over four years to get the simulator within the required design specs; +/_ 0.1%. Our group understood all aspects of all parameters involved and we had the data for every aspect of the nuclear power plant. Further, we had the ability to track the model through the exact same events as the NPP that we were making a simulator for.

    • We are forty years later, computer successfully prevents planes from stalling – ever, and Boeing headquarters are in Chicago.

    • +1000

      This article is a demonstration on the basic fact that you must thoroughly understand all aspects of something that you are going to make a simulation, a simulator, or model of.

      This bears repeating, so . . . This article is a demonstration on the basic fact that you must thoroughly understand all aspects of something that you are going to make a simulation, a simulator, or model of.

      That sentence alone is worthy of a whole series of articles here on WUWT.

  10. I’m a newbie to commenting here, but have followed WUWT for years. Trying tofollow the discussion I have a few questions which may be minor but would perhaps understand what is happening:

    The article says:”And the rain condenses the region where the key coronal heating can be happening”.

    Should it say “And the rain condenses IN the region where the key coronal heating can be happening.”?

    Svalgaard states: “It makes sense that the heating takes plasma close to the surface where the density and the magnetic fields are strongest.” Should it say …takes PLACE…?

    THANKS FOR ANY CLARIFICATION

  11. You too can be a solar physics explorer of this phenomenon with the easy to use SDO tools available on the website.
    A good example of this coronal rain from last Monday (20190401) can be made into a short movie for viewing on your web browser or download to your computer to keep forever.

    Step by step instruction to make your own SDO coronal rain movie:
    1. Go to this web URL: https://sdo.gsfc.nasa.gov/data/aiahmi/
    2. Set date-time as follows, start: 2019-04-01 02:00 , end: 2019-04-02 03:00
    3. Set “telescope” as AIA 304 (red).
    4. Set either “browser display” or “movie download”.
    5. Leave “one image per nth” blank
    6. hit submit. Enjoy.

    notes:
    The coronal rain prominence is on the upper left (North-eastern) limb of the solar disc. The small active region AR12737 is visible on the disc.
    At 2019-04-01 21:13 UTC image shows the rainfall really well.
    Note: the image at AIA 304 (angstroms) image is of EUV emissions at 30.4 nm. This is the Helium II emission line, which is singly ionized He plasma heated to about 80,000 Kelvin.
    This “rainfall” doesn’t appear in the even shorter wavelengths (like 193 angstrom, way too hot) as those emissions lines are too hot (like >= 1,000,000 K).

  12. “Computer simulations predicted the coronal rain could be found there.”
    =============================
    A ‘computer simulation’ that didn’t come up with the truth, the whole truth, and nothing but the truth? Why am I not surprised…..

  13. Another classic discovery, illustrating ‘the science is never settled. Consensus thinking held coronal rain would be associated with the helmet streamers. Then a gal named Emily did the unthinkable, from a consensus view. She actually looked at physical images of the corona and identified verifiable evidence that destroyed the consensus group-think… and science advanced once more.

    Well done, Emily Mason! Well Done!

  14. What’s the difference between ‘helmet streamers’ and magnetic loops? Aren’t they one and the same?

  15. Wow, one thing I like about WUWT is that it often scoops print publications and other sources. However, we got scooped on this one 10 months ago, see https://www.sciencenews.org/article/plasma-rain-sun-atmosphere-falls-surprising-places

    “My job was to find it,” Mason says. So she searched for bright blobs of plasma falling within the tall streamers in videos recorded in extreme ultraviolet light by NASA’s Solar Dynamics Observatory — but spotted none.

    She did find rain showers, however, in much shorter loops called null-point topologies, which stretch only to about 0.1 solar radii above the surface. “These things rain like crazy,” she says. Coronal rain fell in one of these smaller loops for 30 hours.

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