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Re: [Phys-l] a magnetic filament snapped.

Please forgive the following rambling piece. It's hot (relatively: 29 C at 12:30, going to 37 C later today) here in Burnaby, and something may have snapped in my head. I should be out painting my deck, but I have to wait for the sunlight to be off it. I digress frequently below, but it's fun to reminisce!

On 22-Jul-06 Chuck Britton wrote:

from http://spaceweather.com/ (a nasa site) I read:

Yesterday on the sun, a magnetic filament snapped.

I would like to think that some of my sharp HS students might wonder
about the 'basic physics' involved here.

I like to visualize 'lines of magnetic flux' as Faraday did.
Basic physics says that these flux lines have no ends - so how can they 'snap'?

Your intuition is, as usual, right on. Magnetic field lines can't snap for just the reason you give.

I haven't read Faraday on his field model, but I probably should have done so when I was teaching. While this description served Faraday well, it is an endless* source of misconception for students. They and the author of NASA's colorful language have unintentionally reified Faraday's model beyond its elastic limit, and like a Slinky toy, having been treated in that manner it can no longer be expected to work. Getting my Sunbeam into this metaphor, magnetic field lines should never be expected to behave like rubber bands**, though there is an understandable temptation to interpret them in that way. The NASA blurb tempts one further to imagine that the CME represents the release of energy stored like the elastic potential energy in a rubber band, compounding the cognitive error by reifying energy as well.

I'm pretending to have an idea about how this could be, but would
like some other input.

1st assumption: nasa's statement has some merit, at least from a
descriptive point of view.

Nope! Clearly it didn't fool you, but it has the potential* to fool your unsophisticated students.

2nd assumption: we're talking about a very complex turbulent system
of high speed charged particles associated with this flux filament.

While the system which gives rise to the CME is indeed turbulent, it is not reasonable to dismiss its explanation in this manner. I do not know a good model for CME production, and there may not be one in the literature, but implying that the mechanism can only be understood by invoking turbulence is probably not justifiable. Your appeal to Feynman reminds me that the system that propels a canoe, where the paddle interacts with the lake, is also turbulent, but one need not invoke solutions of great complexity to understand the phenomenon of propulsion****.I expect it's like that with CME's, too.

You are dangerously close to reification when you refer to a "flux filament". There ain't no such thing, at least in free space, and it is not useful to describe a magnetic field in such terms. In free space the protons and electrons which would be associated with each such filament might be expected to be flowing in the surfaces of tubes roughly concentric with the filament. In reality there is no reason to believe that such spatial order exists in the Sun's environment. The protons and other charged particles are distributed with no long range order, and they execute individual approximately helical orbits in the magnetic field wherever they are. No filament exists, even conceptually, having a length greater than the length of one of those helical orbits, and I think almost none of those approximately helical orbits could be endless***.

I alluded to the possible exception to my dismissal of flux filaments. These do have some utility in some Type 2 superconductors, but read on...

I can visualize a simple system that might have a bearing on this 'snap'.

Two disk magnets put face-to-face will have a 'simple' B-field around them.

If these two disks are forced apart from each other, the 'simple,
single' field will separate into two 'simple' fields. This transition
from one field to two separate fields is very complicated and I will
invoke some complex turbulence to make an otherwise smooth transition
into the 'snap'.

Totally bogus oversimplification on my part???

Don't hide behind turbulence! Turbulence certainly has no role whatever to play in the physical situation you describe here.

Pulling the two disks apart will immediately suggest a non-Hookeish property of these rubber bands. The attractive, restorative force diminishes with extension. Clearly the Faraday picture is not helpful here.

In 1964 or so I used the two disk magnet picture in a seminar at Cal to demonstrate that a physical explanation published in PRL by the Bell Labs speaker for a phenomenon observed in Type 2 superconductors could not be correct. He had implicitly assumed that flux pinning, which arguably does occur in the solid state, persisted in the free space surrounding the superconductor. Using his suggestion I pointed out that, were that true, a very high magnetic field could be produced by simply winding up the flux filaments between two Type 2 superconducting disk magnets with trapped fluxons. This could be accomplished by spinning one of the disks about their common axis. The PRL referees had not picked up on that, and my thesis supervisor was quite pleased with my performance in that seminar.

I like to pretend that large scale physical systems obey the basic
laws of physics and that some sort of intuitive connection is
possible.

You know that you needn't pretend. Just because I don't know a particular explanation, and perhaps no one else does, either, it is still a central tenet of my belief system that physical systems (on whatever scale) obey the basic laws of physics. I would add to this the caveat that we also may not yet know all of those laws, and perception by intuition illudes everyone in the present generation of physicists. Take, for example, the explanation of observations of the apparent luminosity Type Ia supernovae as a function of their radial velocities - the expansion of the universe!

(WWFS - What Would Feynmann Say?)

We all wish that he was still around to ask. I wish I'd asked him about canoe propulsion in He II. I missed out on a teachable moment.

(It's now 13:45, and it's 31.2 C out there.)

Respectfully submitted, without proofreading!

Leigh

* Sorry, I couldn't resist.

** "elastic bands" as Faraday and other Brits and some Canucks would say

*** I recall an "electron smoke ring" mode that was once proposed as a proton accelerator. The idea was to bind a proton to a torus of electrons (and their associated magnetic flux) and then accelerate the overwhelmingly negatively charged entity. The velocities achieved would approximate those obtainable for electrons, but the proton would share that velocity, and would attain a kinetic energy 2,000 times as great as an electron's at that velocity.

**** Of course I know that a canoe cannot be paddled in an inviscid fluid, but I still believe I understand the phenomenon of canoe propulsion, as Feynman did. I sure could use a supefluid helium lake out there today!