What is the cause of coronal loops?

[Michael Mozina]

I'm trying to ascertain if there is any "consensus" on the cause of coronal loops in current theory?

 

[Nereid]

IIRC, tusenfem (aka Martin)*, gave a detailed answer to a similar question, in another forum (I can't find it just now).

You might like to check out his posts in that forum.

*he's a astrophysicist, specialising in the plasma physics of the IPM, and has worked on/is working on Double Star, Galileo, Venus Express, and Cluster.

 

[Michael Mozina]

Do you happen to recall which forum it was in?

 

[Garth]

I'm trying to ascertain if there is any "consensus" on the cause of coronal loops in current theory?

Magnetic fields.

Garth

 

[Michael Mozina]

Magnetic fields.

Garth

According to Dr. Hannes Alfven, the magnetic fields in plasma are held in place by the flow of current through the plasma. Wouldn't that suggest that electrical currents plays a vital role in this process as the University of Maryland suggests in the following paper?

http://www.astro.umd.edu/~white/papers/03_norh_020723.pdf

 

[Garth]

The corona is a plasma of fast following ions constrained by magnetic fields, of course there are electric currents - its astrophysical MHD.

Garth

 

[Michael Mozina]

The corona is a plasma of fast following ions constrained by magnetic fields, of course there are electric currents - its astrophysical MHD.

Garth

I agree, although IMO, the role of electricity never seems to get much attention or mention. If the coronal loops are simply electrified columns of rising hot plasma, why does the "heat source" of the corona remain such an enigma?

 

[Michael Mozina]

The corona is a plasma of fast following ions constrained by magnetic fields, of course there are electric currents - its astrophysical MHD.

Garth

Would it not then be fair to assume that coronal loops are a type of plasma filament, much as we might find in a toy plasma ball? All the components and tell tale signs are there. We have a plasma atmosphere, and electrical currents flowing through plasma. Neutron release is often a byproduct of z-pinch forces in laboratory plasma discharges. We see evidence of neutron capture signatures in the loops. The loops emit x-rays just as a plasma filaments will emit x-rays. All the signs would suggest that coronal loops are caused by electrical discharges through plasma. These discharges create plasma filaments in the solar atmosphere that we call coronal loops.

The heat released in these filamaments as they rise into the corona is the heat source of the corona.

 

[Michael Mozina]

This brings me back to one issue that seems to be in considerable dispute.

If these are MHD loops, where do they originate? Do they begin below the photosophere or do they originate higher in the solar atmosphere?

 

[Garth]

This brings me back to one issue that seems to be in considerable dispute.

If these are MHD loops, where do they originate? Do they begin below the photosophere or do they originate higher in the solar atmosphere?

You might find this paper useful http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHPAEN000001000005001684000001&idtype=cvips&prog=normal

Fresh magnetic fluxes are continually injected into the low corona from below by the solar dynamo. Flaring activities are obviously the hydromagnetic and plasma responses of the corona to this flux injection, although the nature of these complex processes are not understood well.’ Observation suggests that a significant part of the emergent magnetic flux is in the form of lines of force with the two ends anchored in the dense photosphere. Hence, the magnetic field in the corona continues to be energized by the photosphere through the turbulent transport ( ~1/2 km/s) of the magnetic footpoints. This is the basis for the popular notion that magnetic energy can build up on a time scale long compared to the typical coronal Alfven transit time of the order of minutes, to be released in spurts of 10^31-32 ergs as sporadic flares.

Or if you want a picture from APOD Heating Coronal Loops.

Yes, there is discussion and new evidence encourages new ideas to be properly tried and tested.


Garth

 

[Michael Mozina]

Thanks. I'll read the paper fully and comment once I've finished. It looks very interesting.

I have a few "concerns" about a couple of the assumptions made on the first few pages, but I'll wait till I've had time to study the paper to comment.

I appreciate the link very much. If there are any other papers on MHD structures in the solar atmosphere that you think might be helpful, I would be most grateful to any and all suggestions.

 

[Astronuc]

Thanks for that paper, Garth!

The MHD effect is primarily IxB and plasma waves. One may wish to look into plasma instabilities.

Also - NASA is redoing websites, including Goddard's. Links from other sites outside NASA might become invalid.

http://www.nasa.gov/centers/goddard/2004solar.html

http://www.gsfc.nasa.gov/topstory/20020515tracewind.htm

TRACE Website - http://trace.lmsal.com/

Images from TRACE - http://trace.lmsal.com/POD/

IIRC, there are some animations of flares and prominances. The APOD picture linked by Garth shows a loop that is about 30 Earth diameters across!

 

[Michael Mozina]

Thanks for that paper, Garth!

The MHD effect is primarily IxB and plasma waves. One may wish to look into plasma instabilities.

Also - NASA is redoing websites, including Goddard's. Links from other sites outside NASA might become invalid.

http://www.nasa.gov/centers/goddard/2004solar.html

I have found many images on NASA's website showing the coronal loops originating underneath the photosphere and coming through the photosphere such as this image from the page you listed:

http://www.gsfc.nasa.gov/gsfc/spacesci/pictures/insidesun/SS1m.jpg

Here's a movie of that process:

http://www.nasa.gov/mpg/124357main_flare_320.mpg

My question is ultimately oriented around the placement of the "base" of the coronal loops as imaged by Trace and SOHO spacecraft . According to Lockheed Martin, the base of these MHD loops seen in 171A and 195A are located in the corona, but much of NASA's material suggests the base of these MHD loops must begin much lower in the solar atmosphere. I would think that the STEREO system launching in June/July should be able to tell us if the base of these loops originates and becomes visible underneath the photosphere as the movie suggests, or if the base only becomes visible in the lower corona as Lockheed suggests.

 

[Nereid]

How do these images "show the coronal loops originating underneath the photosphere and coming through the photosphere ..." (my bold)?

I mean, by definition, isn't the photosphere the zone on the Sun beneath which images cannot be obtained (from Earth), because the mean free path of (optical, NIR, UV) photons is small?

 

[Michael Mozina]

How do these images "show the coronal loops originating underneath the photosphere and coming through the photosphere ..." (my bold)?

From the link that Garth was so kind to provide:

Observation suggests that a significant part of the emergent magnetic flux is in the form of lines of force with the two ends anchored in the dense photosphere. Hence, the magnetic field in the corona continues to be energized by the photosphere through the turbulent transport ( -$ km s-i) of the magnetic footpoints.

The notion that these MDH loops are anchored into the dense photosphere makes a lot of sense. Thicker plasma from below can be driven by the electromagnetic currents and rise up through the photosphere.

The animation movie I cited in my last post shows a couple of these MHD loops piercing the photosphere. It suggests that the flow of these MHD loops causes sunspots to form on the surface of the photosphere, that "evolve" into two sunspots as the loop goes higher into the solar
atmosphere.

NASA's point of view makes a lot of sense, since thicker plasma driven by electromagnetic current could in fact come from below and right into the corona. That seems rather logical if the driving force for these loops are magnetic fields deep in the core. The currents would flow around there MDH loops long before it reached the surface of the photosphere.

I mean, by definition, isn't the photosphere the zone on the Sun beneath which images cannot be obtained (from Earth), because the mean free path of (optical, NIR, UV) photons is small?

That is a "theory" that is based on a number of assumptions. The animation of course is not limited to this perpective, nor is the driving force of current limited to an optical view. These things could exist whether or not we "see" them. The question is "can we see them before they reach the surface of the photosphere? Based on everything I've seen, I believe we can.

 

[Michael Mozina]

You might find this paper useful http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHPAEN000001000005001684000001&idtype=cvips&prog=normal

Or if you want a picture from APOD Heating Coronal Loops.

Yes, there is discussion and new evidence encourages new ideas to be properly tried and tested.


Garth

From that reference you cited earlier...

The steepening to a tangential discontinuity leads to its resistive dissipation, despite the high conductivity. Thus a small part of the built-up energy goes to heat the corona. This is a remarkable process in which high conductivity itself promotes resistive dissipation. Coronal
heating is a central problem that is far from resolved, observationally and theoretically

Try as I might, I can't make heads or tails of these few sentences. I wonder if you might enlighten me here a bit on this point, because frankly these statements seem more than just "remarkable". They sound downright illogical from my perspective. My background in electricity would suggest that ideal conductors generate minimal amounts of heat. If the plasma in the corona is an ideal conductor, why would it promote heat build up in the atmosphere above a plasma surface of the photosphere that is only 5800K, and a chromosphere that is only 10's of thousands of degrees?

Because I see high energy photons coming from the coronal loops, I can appreciate that the coronal loops themselves must reach very extreme temperatures. On the other hand, I see little if any evidence to suggest that the entire corona reaches the same temperatures as the material inside the coronal loops.

 

[Garth]

The problem of heating the corona, and coronal loops within it, has not been solved although there are two main suggested processes that might be responsible: wave energy or magnetic reconnection.

The waves might be sound waves, magneto-acoustic waves or Alfven waves. Magnetic reconnection depends on the fact that the Sun's surface has strong magnetic fields and is in violent motion.

A combination of all these sources may provide the answer, this paper was just discussing the MHD contribution.

The energy in the coronal loops is enormous, but as the corona is very tenuous it needs only a small amount of energy to heat it to high temperature, about 1 Kw/metre² of the Sun's surface area, or about 0.000016% of the total solar energy emitted.

The MHD heating is thought to arise when the magnetic field lines reconnect after being disrupted by plasma motion. As you quoted from that paper: "Coronal heating is a central problem that is far from resolved, observationally and theoretically."

Garth

 

[Michael Mozina]

The problem of heating the corona, and coronal loops within it, has not been solved although there are two main suggested processes that might be responsible: wave energy or magnetic reconnection.

The waves might be sound waves, magneto-acoustic waves or Alfven waves.

Well, I'm inclined to go with Alfven waves personally, since I can "see" that the photon output is concentrated inside the coronal loops. Furthermore the University of Maryland suggests that there are strong electrical currents involved in some of these output wavelengths.

http://www.astro.umd.edu/~white/papers/03_norh_020723.pdf

The energy in the coronal loops is enormous, but as the corona is very tenuous it needs only a small amount of energy to heat it to high temperature, about 1 Kw/metre² of the Sun's surface area, or about 0.000016% of the total solar energy emitted.

I agree that density issues apply, but I am inclined to believe that the small amount of energy seen in the darker areas of the corona in Yohkoh images comes from the heat concentrated inside the dense plasma filaments that carry the electrical current from one base of the loop to the other. While the coronal (current) loops are "bright", the surrounding material is typically rather "dark". Since we are looking at a plasma atmosphere, the likelihood of scattering of light from the coronal loops precludes me from "assuming" that the entire corona is the same temperature range as the loops themselves. The loops themselves appear to be the heat source of dark regions of the corona.

 

[Michael Mozina]

I could use some help with understanding MDH waves and understanding the skism that seems to have developed between modern MHD concepts that are applied to astronomy and the work of Hannes Alfven. While Alfven first proposed the math to explain how MHD waves could propogate though plasma. He suggested that current flowing through the plasma would tend to align the charged plasma into coherent moving waves. However many astronomers talk today about magnetic fields being "frozen" in light plasma, something that Hannes Alfven specifically rejected in his later work. In fact he seemed to regret having used the term "frozen" as it relates to magnetic fields in plasma because of the apparent misuse and misunderstading of this concept by many astronomers.

I'm just getting my feet wet with MHD theory, and I obviously don't have a firm grasp on the field of science just yet. There are some things that do seem to be obvious to me however. One of the observations I've seen repreatedly in solar images is the fact that plasmas, particularly solar plasmas tend to be very "active" and mobile, and contain kinetic energy at an atomic level. We can for instance observe a "boiling effect" that occurs at the surface of the photosophere. It seems to me that this light, relatively energetic plasma contains far too much kinetic energy for a magnetic field to be "frozen" into the plasma, since the plasma itself is full of kinetic energy. At best case, current might tend to flow along a certain path related to an inner magnetic field, but that would tend to be a moving "flux rope", not unlike a plasma filament in a plasma ball. There would necessarily be a lot of movement involved.

From observing the filaments of a plasma ball, I can understand how current flow can create current flow "patterns" that seem to become "frozen" into the plasma. The notion however that magnetic fields in plasma are "frozen" in moving particles however is rather misleading IMO.

The "frozen" idea that Alfven tried to convey in his work seems to be mostly related to, and limited to, very *dense* plasmas with *high* levels of current flow. Alfven did not seem to believe that the "frozen" magnetic field concept worked very well for lighter, less dense plasma with dispersed and kinetically active ions. It seems as though astronomy today only partially accepted Alfven's full set of teachings as it relates to plasma movement and current flow in plasma.

This issue comes directly into play in the coronal loops. We've observed magnetic fields from these loops suggesting they are moving filaments within the plasma atmosphere of the sun, but that would require *massive* current flow sustained over a very long period of time.

 

[MadScientist 1000]

Magnetic fields.

Garth

Magnetic Gauge Field Lines actually. Its a much more refined answer. I think there was something about prominces or coronal loops in a National Geographic in 2004, so you might want to check that out.