Looking into solar coronal prominence cavities

by Karen C. Fox

Image of a coronal mass ejection as taken by SDO. › View larger
Scientists want to understand what causes giant explosions in the sun’s atmosphere, the corona, such as this one. The eruptions are called coronal mass ejections or CMEs and they can travel toward Earth to disrupt human technologies in space. To better understand the forces at work, a team of researchers used NASA data to study a precursor of CMEs called coronal cavities. Credit: NASA/Solar Dynamics Observatory (SDO)

The sun’s atmosphere dances. Giant columns of solar material – made of gas so hot that many of the electrons have been scorched off the atoms, turning it into a form of magnetized matter we call plasma – leap off the sun’s surface, jumping and twisting. Sometimes these prominences of solar material, shoot off, escaping completely into space, other times they fall back down under their own weight.

The prominences are sometimes also the inner structure of a larger formation, appearing from the side almost as the filament inside a large light bulb. The bright structure around and above that light bulb is called a streamer, and the inside “empty” area is called a coronal prominence cavity.


Such structures are but one of many that the roiling magnetic fields and million-degree plasma create in the sun’s atmosphere, the corona, but they are an important one as they can be the starting point of what’s called a coronal mass ejection, or CME. CMEs are billion-ton clouds of material from the sun’s atmosphere that erupt out into the solar system and can interfere with satellites and radio communications near Earth when they head our way.

“We don’t really know what gets these CMEs going,” says Terry Kucera, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “So we want to understand their structure before they even erupt, because then we might have a better clue about why it’s erupting and perhaps even get some advance warning on when they will erupt.”

Kucera and her colleagues have published a paper in the Sept. 20, 2012, issue of The Astrophysical Journal on the temperatures of the coronal cavities. This is the third in a series of papers — the first discussed cavity geometry and the second its density — collating and analyzing as much data as possible from a cavity that appeared over the upper left horizon of the sun on Aug. 9, 2007 (below). By understanding these three aspects of the cavities, that is the shape, density and temperature, scientists can better understand the space weather that can disrupt technologies near Earth.

The faint oval hovering above the upper left limb of the sun in this picture is known as a coronal cavity. › View larger
The faint oval hovering above the upper left limb of the sun in this picture is known as a coronal cavity. NASA’s Solar and Terrestrial Relations Observatory (STEREO) captured this image on Aug. 9, 2007. A team of scientists extensively studied this particular cavity in order to understand more about the structure and magnetic fields in the sun’s atmosphere. Credit: NASA/STEREO

The Aug. 9 cavity lay at a fortuitous angle that maximized observations of the cavity itself, as opposed to the prominence at its base or the surrounding plasma. Together the papers describe a cavity in the shape of a croissant, with a giant inner tube of looping magnetic fields — think something like a slinky — helping to define its shape. The cavity appears to be 30% less dense than the streamer surrounding it, and the temperatures vary greatly throughout the cavity, but on average range from 1.4 million to 1.7 million Celsius (2.5 to 3 million Fahrenheit), increasing with height.

Trying to describe a cavity, a space that appears empty from our viewpoint, from 93 million miles away is naturally a tricky business. “Our first objective was to completely pin down the morphology,” says Sarah Gibson, a solar scientist at the High Altitude Observatory at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. who was an author on all three cavity papers. “When you see such a crisp clean shape like this, it’s not an accident. That shape is telling you something about the physics of the magnetic fields creating it, and understanding those magnetic fields can also help us understand what’s at the heart of CMEs.”

To do this, the team collected as much data from as many instruments from as many perspectives as they could, including observations from NASA’s Solar Terrestrial Relations Observatory (STEREO), ESA and NASA’s Solar and Heliospheric Observatory (SOHO), the JAXA/NASA mission Hinode, and NCAR’s Mauna Loa Solar Observatory.

They collected this information for the cavity’s entire trip across the face of the sun along with the sun’s rotation. Figuring out, for example, why the cavity was visible on the left side of the sun but couldn’t be seen as well on the right held important clues about the structure’s orientation, suggesting a tunnel shape that could be viewed head on from one perspective, but was misaligned for proper viewing from the other. The cavity itself looked like a tunnel in a crescent shape, not unlike a hollow croissant. Magnetic fields loop through the croissant in giant circles to support the shape, the way a slinky might look if it were narrower on the ends and tall in the middle – the entire thing draped in a sheath of thick plasma. The paper describing this three-dimensional morphology appeared in The Astrophysical Journal on Dec. 1, 2010.

Next up, for the second paper, was the cavity’s density. Figuring out density and temperature was a trickier prospect since one’s point of view of the sun is inherently limited. Because the sun’s corona is partially transparent, it is difficult to tease out differences of density and temperature along one’s line of sight; all the radiation from a given line hits an instrument at the same time in a jumble, information from one area superimposed upon every other.

Using a variety of techniques to tease density out from temperature, the team was able to determine that the cavity was 30% less than that of the surrounding streamer. This means that there is, in fact, quite a bit of material in the cavity. It simply appears dim to our eyes when compared with the denser, brighter areas nearby. The paper on the cavity’s density appeared in The Astrophysical Journal on May 20, 2011.

“With the morphology and the density determined, we had found two of the main characteristics of the cavity, so next we focused on temperature,” says Kucera. “And it turned out to be a much more complicated problem. We wanted to know if it was hotter or cooler than the surrounding material – the answer is that it is both.”

Ultimately, what Kucera and her colleagues found was that the temperature of the cavity was not – on average – hotter or cooler than the surrounding plasma.

However, it was much more varied, with hotter and cooler areas that Kucera thinks link the much colder 10,000 degrees Celsius (17,000 F) prominence at the bottom to the million to two million degrees Celsius (1.8 million to 3.6 million degrees Fahrenheit) corona at the top. Other observations of cavities show that cavity features are constantly in motion creating a complicated flow pattern that the team would like to study further.

While these three science papers focused on just the one cavity from 2007, the scientists have already begun comparing this test case to other cavities and find that the characteristics are fairly consistent. More recent cavities can also be studied using the high-resolution images from NASA’s Solar Dynamics Observatory (SDO), which launched in 2010.

“Our point with all of these research projects into what might seem like side streets, is ultimately to figure out the physics of magnetic fields in the corona,” says Gibson. “Sometimes these cavities can be stable for days and weeks, but then suddenly erupt into a CME. We want to understand how that happens. We’re accessing so much data, so it’s an exciting time – with all these observations, our understanding is coming together to form a consistent story.”

Related Links – NASA’s Solar Fleet

› STEREO website

› SDO website

› SOHO website

› Hinode website

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26 thoughts on “Looking into solar coronal prominence cavities

  1. Lot to read and more to understand. Understanding solar activity’s effects on the Earth through the geomagnetic fluctuations may finally resolve dilemma of the climate change.

  2. OT I know, but I nearly spilt my coffee just now – the advertisment at the top of the WUWT page just turned to an offer of GB shares in a company which makes renewable energy things, like wind turbines.

  3. It’s a step in the right direction, but “Our point with all of these research projects into what might seem like side streets” Side streets from what? The Man Made Global Warming silliness coming from NASA?
    When they understand the sun, maybe then they can rattle on about Earth’s warming and cooling periods. It seems backwards to me.

  4. Very interesting. It wasn’t so long ago that we were happy just to see the sun come up every morning. :)

  5. It is very clear that what we see ( visual image from earth) of the sun is not a surface so the sun can not be a black body. The sun clearly has some sort of atmosphere. No one at this time knows if there is a solid or liquid core (which would provide a surface) and what is the temperature at the surface and what is the surface area. The Stefan-Boltzman equation application to the sun is a mathematical construct which is likely to be incorrect. A black body by definition has a surface. The S-B equation correctly only applies to a black body in a vacuum.

  6. Karen Fox is one of NASA’s best writers as this article shows. I read this one last night. Gives me a warm fuzzy knowing that their are still real scientists doing real science and real science writers writing about real science, not that Karen Fox hasn’t added the mandated narrative supporting propaganda statement a time or two.

  7. Another few puzzle pieces that we may or may not ever quite understand and less then likely to model with much true accuracy. Short term predictions probably. I think that takes the fun out it. I mean, today we need to be on our toes to not get caught with our electronics and grid pants down. It would seem more practical to design and build stuff that can withstand these kinds of fluxes especially in our electrical grid systems. Were it not for satellites and continental wide electrical grid systems no one would care. I like lots of other is fascinated with the sun and with plasmas, neat stuff. I wounder, if in reality this is a few large and lucrative industries that are looking for tax payer bailouts for a problem they themselves created.

  8. It appears to this unqualified observer that the slinky like plasma structures of the sun are behaving like giant electromagnets that experience inductive voltage spikes, detonations and cme’s.

  9. Fascinating stuff. Thank you Karen Fox for writing it, and Anthony for putting it here and reminding us that majestic Old Sol can still inspire awe and wonder.

  10. tallbloke says:
    September 21, 2012 at 4:23 am

    the team was able to determine that the cavity was 30% less than that of the surrounding streamer.

    30% less what? Density?

    Because the earlier part of the sentence discussed deriving the density from temperatures, I’d surmise that “cavity” should be in possessive form, i.e. “cavity’s”. Thus, yes, 30% less density.

    cheers,

    gary

  11. @tallbloke:
    “Using a variety of techniques to tease density out from temperature, the team was able to determine that the cavity was 30% less than that of the surrounding streamer.”

    Yes, it seems they are talking about density.

  12. The Second Law of Thermodynamics (maximization of entropy) strictly applies only to energetically closed systems. When a heat source (such as nuclear fusion) is present, thermal gradients exist, and a consequence of thermal gradients is segregation of materials according to their physical properties. E.g. Archimedes’ Law states that in a gravitational field materials with lower density will be buoyant in comparison to materials of greater density. Similarly the electric properties of different materials will sort them quickly in an intense magnetic field – an effect magnetic sector mass spectrometers exploit routinely.
    This segregation of matter produces local inhomogeneities which can trigger further phenomena – such as coronal cavities, solar storms, sunspots. A high resolution examination of the spectra of the flares, the cavities, etc. shows they have different composition. In the solar atmosphere, the electrical and magnetic properties will be highly important.

  13. A slinky-like shape sounds as if an inductor effect is created.
    I guess it would have to return through the middle and a change in source-voltage or of spacing between the coils could trigger a CME. Or as MontyPython says–something completely different.

  14. @ John Day, please read what I wrote. It is a mathematical construct. The temperature of about 5800K assumes that a) the S-B equation applies b) the sun is a black body. & c) the area is calculated from the visible projection. Some people make similar calculation about the earth assuming that the radiation emitting temperature of about 220K is somewhere at the top of the atmosphere. Where there is an atmosphere calculations and measurements of radiation involving (or assuming) the S-B equation are likely to be incorrect and certainly not to the accuracy claimed by some climate scientists who have no understanding of heat and mass transfer (which is an engineering subject)

  15. John Day says:
    September 21, 2012 at 6:16 am

    “@cementafriend
    > … so the sun can not be a black body
    Then how do you explain the fact that measured solar radiation (“irradiance”) closely approximates a black body with a temperature of 5800 Kelvins?”

    I suspect you could have answered your own question. The sun is so hot that there are many energy transitions available for electrons to jump to that the spectral lines all meld together to give the blackbody spectrum. In fact, most of the electrons are free and the solar surface is actually a dissociated plasma, but the quantum jumps are still available.

  16. The energy in solar flares and CMEs comes from stored energy in the strong EM fields around the solar surface. This EM energy is stored in electric double layers around the solar surface but occasionally dumps when the double layer catastrophically collapses. It’s called a Langmuir burst. This same effect is observed around high energy high voltage environments on Earth, e.g. high energy DC power lines, which require special design to stop the strong EM fields around them suddenly collapsing like on the solar surface.

  17. cementafriend says:
    September 21, 2012 at 6:12 pm

    @ John Day, please read what I wrote. It is a mathematical construct. The temperature of about 5800K assumes that a) the S-B equation applies b) the sun is a black body. & c) the area is calculated from the visible projection. Some people make similar calculation about the earth assuming that the radiation emitting temperature of about 220K is somewhere at the top of the atmosphere. Where there is an atmosphere calculations and measurements of radiation involving (or assuming) the S-B equation are likely to be incorrect and certainly not to the accuracy claimed by some climate scientists who have no understanding of heat and mass transfer (which is an engineering subject)

    I did read what you wrote, but it’s somewhat incoherent. First you said the Sun cannot be a black body because it does not have surface. Now you’re saying that the Sun does behave like a black body, radiating at 5800K, if we assume it is a black body. Then you switch gears again on to say that because the Sun has an atmosphere then those observations of radiation are not correct.

    For your information the definition of black body does not depend on the body having a solid flat surface. It depends only that the body absorb all impinging electromagnetic radiation, and that it also be a perfect radiator, such that no other kind of body can radiate more energy, at a given temperature.

    It is an idealized concept (like a Carnot Engine or ideal gases) to illustrate the principles of thermal radiation. Perfect black bodies don’t exist in the real world.

    The best realization of a black body is not a radiating surface, but a hole or cavity in a large chamber with opaque walls such that energy entering the chamber through the hole cannot escape (if the hole is small enough).

  18. John Day, I am sorry to say that it may be your lack of understanding that you find what I wrote is incoherent. Stefan used the work of Fourier to develop his equation. If you look at the original work it was done with surfaces in a vacuum. The definition of black and gray bodies does include the word “surface” . You will find in Perry’s Chemical Engineering Handbook the following “The characteristic properties of a blackbody are that it absorbs all the radiation incident on its surface and the quality and intensity of the radiation it emits are completely determined by its temperature.” This may help you further http://physicsworld.com/cws/article/news/2012/sep/14/plancks-law-violated-at-the-nanoscale . Note the statement “Planck assumed that all radiation striking a black body will be absorbed at the surface of that body, which implies that the surface is also a perfect emitter.” . With respect to a cavity please note that the inside of the cavity has a surface.
    In space there are things called “blackholes”.(a poor name) which have very dense cores (having a surface). “Blackholes” appear to be at very high temperature because they do emit x-rays and gamma rays. It is unclear if the “Blackholes” have an atmosphere.which affects emission.
    From various literature the sun has a core (ie a surface) but no one knows its size, density, composition (Xenon has been mentioned), or temperature. The photo above and much other information clearly shows that the sun has an atmosphere. The supposed surface temperature of the sun (about 5800K) is a mathematical construct. to give an average emission temperature of the core & atmosphere using the S-B equation. It is likely to be in error.
    The real problem is that so-called climate scientists are not real scientists collecting empirical data and then let the data point to physical processes that maybe occurring. They make up relationships and then select some data or manipulate data to suit their biased hypotheses. The twisting of the S-B relationship is one indication of climate science practice.
    Engineers on the other hand always base their calculations and design on actual measured data. Registered professional engineers need to understand what they are doing, they can be criminally liable for their decisions.Heat & mass transfer is an engineering subject which should be left to qualified registered engineers.

  19. “cementafriend says:
    September 22, 2012 at 5:27 pm
    John Day, I am sorry to say that it may be your lack of understanding that you find what I wrote is incoherent. Stefan used the work of Fourier to develop his equation. If you look at the original work it was done with surfaces in a vacuum. The definition of black and gray bodies does include the word “surface” . You will find in Perry’s Chemical Engineering Handbook the following “The characteristic properties of a blackbody are that it absorbs all the radiation incident on its surface and the quality and intensity of the radiation it emits are completely determined by its temperature.” This may help you further http://physicsworld.com/cws/article/news/2012/sep/14/plancks-law-violated-at-the-nanoscale . Note the statement “Planck assumed that all radiation striking a black body will be absorbed at the surface of that body, which implies that the surface is also a perfect emitter.” . With respect to a cavity please note that the inside of the cavity has a surface.
    In space there are things called “blackholes”.(a poor name) which have very dense cores (having a surface). “Blackholes” appear to be at very high temperature because they do emit x-rays and gamma rays. It is unclear if the “Blackholes” have an atmosphere.which affects emission.
    From various literature the sun has a core (ie a surface) but no one knows its size, density, composition (Xenon has been mentioned), or temperature. The photo above and much other information clearly shows that the sun has an atmosphere. The supposed surface temperature of the sun (about 5800K) is a mathematical construct. to give an average emission temperature of the core & atmosphere using the S-B equation. It is likely to be in error.
    The real problem is that so-called climate scientists are not real scientists collecting empirical data and then let the data point to physical processes that maybe occurring. They make up relationships and then select some data or manipulate data to suit their biased hypotheses. The twisting of the S-B relationship is one indication of climate science practice.
    Engineers on the other hand always base their calculations and design on actual measured data. Registered professional engineers need to understand what they are doing, they can be criminally liable for their decisions.Heat & mass transfer is an engineering subject which should be left to qualified registered engineers.”

    I can’t tell whether you’re just a troll trying to poke the WUWT anthill, or if you are just misinformed. Being misinformed is OK, provided you try to recognize and attempt to correct your lack of understanding.

    The ‘surfaces’ you reference above are idealized surfaces, i.e. infininitely thin, infinitely apsorbtive, infinitely emissive. It’s OK to use these ideals for illustration, provided you don’t make them a requirement for realization of a ‘real’ black body, because such idealized surfaces obviously can’t be perfectly realized, only approximated. For example, the laws of thermodynamics don’t require us to build a perfect Carnot Engine. Nor do we expect atmospheric gases to always behave as perfect ‘ideal’ gases, even though many of the physics laws we use are based on these idealized concepts.

    So you cannot and should not require the Sun to have a “surface”, merely to make it realize a goal of perfect black body behavior. It already talks and walks like a 5800K black body, so for all practical purposes it is a black body. Nobody expects it to be perfect.

    In fact, in the real world, objects with solid surfaces tend not to behave like black bodies, because they reflect or otherwise leak out the absorbed rays, which violate the definition. For example, carbon exists in many forms, but the form which behaves most like a black body is a lamp-black (“soot”). So the lamp black “surface” (at visible-to-IR wavelengths) looks more a like a random structures of diffuse amorphous cavities than a solid surface. That’s how impingin thermal energy (and light) get trapped simulating a black body.

    In other words, soot looks more like the “surface” of the Sun (than graphite, diamond or other forms of carbon). Indeed, the nooks,crannies, cavities etc forming the Sun’s surface help make the Sun more like a black body.

    :-|

  20. a question:
    in the orange based photograph at the head of the post around the eruption there are at least three series of paralel lines of “light” .

    are these artifacts from the imaging systems and programs used or are they evidence of “shockwaves” or what?

    C

  21. @pk
    >…are these artifacts …?

    Yes, the radial spikes are artifacts of telescope’s optics and the parallel ridges are artifacts of digital filtering used to enhance the imagery.

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