# Thread: Plasma physics, MHD, Alfvén, etc

1. ## Magnetohydrodynamics

Originally Posted by sol88
Because everything I've read "mainstream" and indeed most of the links you supplied above, imply some sort of magnetic mechanisms is causing the Coronal heating difference. So how it possible to have a magnetic field without an electrical field ... So why make it more complicated than needs be? Electricity is used to accelerate particles to solar wind speeds. We can see all the electrical signatures and then choose to ignore them??
The problem here is that you do not know enough to understand what you read, and so you seriously misinterpret it. The standard, mainstream physics certainly does use electric fields to accelerate the particles. But the electric fields that do the accelerating are derived from the time variable magnetic fields, essentially via Faraday's Law. And the time variable magnetic field is derived from the electric fields of the charged particles of the solar plasma via the equations of dynamo theory.

So the basic process goes like this.
(1) Plasma in motion creates time varying magnetic field.
(2) Time varying magnetic field creates electric field.
(3) Electric field accelerates particles.

All of the scientists studying this problem know this process very well, so there is no reason for them to repeat it to each other, in papers they write to communicate with each other. Their papers consistently cast the problem in terms of the magnetic field, because in this case it is the magnetic field which is fundamental, and not the electric field. The electric field that does the particle accelerating is derived from the magnetic field, not the other way around. And since the magnetic field is relatively easy to observe, we simply start with the observed magnetic field, and work from there.

That's cool if the problem you are working on is the acceleration of solar wind particles, or perhaps the heating of the stellar corona. However, if you are working on the problem of generating the magnetic field in the first place, then assuming its existence is obviously not cool. So, the scientists working on that problem have to start with the detailed structure of the plasma (temperature, pressure, velocity, composition & etc.), and then model the generated magnetic field (including the back-reaction of the plasma to the magnetic field that it generated on itself). This is a much harder problem to approach observationally, since so much of the solar interior has been invisible, prior to the advent of helioseismology.

Originally Posted by sol88
Why??? I'm sure you are intelligent enough, unlike me, to work out the implications! I'm sure you are.
Perhaps you were thinking that mainstream science claims that magnetic fields occur in the absence of electric fields? That is possible, but not under any circumstances that we are dealing with here.

Originally Posted by sol88
As you state above these are plausible theories, what’s so implausible about the electric Sun theory?
It is contradicted by observsation, since key elements required by it, which should be observable, are not in fact observed, despite the presence of instruments fully capable of making the observations (i.e., where is the current that powers the sun?). It is also contrary to common sense physics. There has to be an ultimate source of energy for all physical processes, and none is forthcoming in the electric sun theory.

Originally Posted by sol88
How far could we have advanced if we "assumed" space was not electricaly neutral
The electrical neutrality of space is not an assumption, it is a direct observation. Why should we "assume" anything that we already know is contrary to observation?

[Moderator note: this thread was created by splitting posts from the ATM thread, The Electric Sun (so you will see some ATM ideas and references).

The posts here are not, primarily, on the ATM idea, the electric Sun, but refer mostly to basic plasma physics, MHD (magneto-hydro dynamics), and the history of its development (especially the role of the contributions of Alfvén).

As there seems to be quite a bit of confusion, among some BAUT members, regarding plasma physics, I think this thread may help to clarify it, as well as provide an opportunity for BAUT members to ask questions about this area of modern physics.]

2. ## neutrality

Originally Posted by P.Asmah
Regardless, there seems to be a contradiction between the statement above and a later statement from the same post?
There would be a contradiction only if you assume that electrical neutrality requires the complete absence of charged particles. But that is not the case. An electrically neutral plasma is one which has an equal number of positive & negative charge carriers. An electrically neutral plasma in motion will generate a magnetic field (this is an observed fact). The electrical neutrality of space is merely the observation that the number of positive & negative charge carriers is equal, not that they are absent.

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So the basic process goes like this.
(1) Plasma in motion creates time varying magnetic field.
(2) Time varying magnetic field creates electric field.
(3) Electric field accelerates particles.
The basic law is "A moving charge creates a magnetic field."
A single charge is/has an electric field.

A time varying magnetic field must cross a wire etc. to create a current flow.
Usually magnetic fields are talked about as being frozen into the plasma depending on the time scale. That means it moves with the plasma.

Plasma(charges, proton, electron, ion) in motion will usually have a driving potential. That would be a current, which is moving charges, which develop a magnetic field. So the magnetic field is a by product of electric field acceleration.

4. The basic law is "A moving charge creates a magnetic field."

But not all magnetic fields are created by moving charges.

Magnetic fields are created by the spin of the charge and when you align the spins on the electrons in an atom in a material you will produce a magnetic field. Electrons or protons moving in a uniform motion will align the spins, but you can align the electrons without a current, iron is very good at doing it, though other materials will do it as well.

The moral of this is while:

Current = Magnetic Field
Magnetic Field ≠ Current

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Originally Posted by Tim Thompson
I doubt that any significant number of scientists, in the "modern" sense, ever believed that space was truly "empty".
Well, I thought it was expected to be a perfect vacuum not so long back, but we move on.

Originally Posted by Tim Thompson
No doubt the idea of space being filled with charged particles has been batting about for the last 100 years or so. Surely over the last 50-60 years, space filled with charged particles has been the mainstream norm.
Sure, but it is the role and importance of these plasmas which is now under debate. EUrs, of course, go much further than the mainstream in this respect.

Originally Posted by Tim Thompson
I am referring specifically to the solar wind; since we are on the electric sun topic, that's the relevant space & plasma.
OK

Originally Posted by Tim Thompson
However, as a general rule, all space plasma that we can observe, is seen to behave globally as if it were charge neutral, though I should think there are bound to be charged currents flowing in small scale environments. One has to study these things on a case by case basis.
Careful, you almost uttered the words 'frozen in' there! Alfven debunked this idea. He was also the chap who came up with Magnetohydrodynamics BTW, despite you claim in another post that he was very down on the idea!

6. Originally Posted by P.Asmah
Careful, you almost uttered the words 'frozen in' there! Alfven debunked this idea. He was also the chap who came up with Magnetohydrodynamics BTW, despite you claim in another post that he was very down on the idea!
Sorry, old chap, but frozen in magnetic fields is a direct consequence of Alven's MHD assuming (almost) perfect conductivity. Or did you mean another "frozen in" as I see nowhere that Tim even remotely gets into the neighbourhood of being frozen.

* edited to correct an inaccuracy *
Last edited by tusenfem; 2006-May-31 at 12:38 PM.

7. Originally Posted by upriver
The basic law is "A moving charge creates a magnetic field."
A single charge is/has an electric field.

A time varying magnetic field must cross a wire etc. to create a current flow.
Usually magnetic fields are talked about as being frozen into the plasma depending on the time scale. That means it moves with the plasma.

Plasma(charges, proton, electron, ion) in motion will usually have a driving potential. That would be a current, which is moving charges, which develop a magnetic field. So the magnetic field is a by product of electric field acceleration.
A moving charge can be considered a current and thus produces a magnetic field. The charge is may move at constant velocity.
A single charge has an electric field around it, but as we are dealing with plasmas here, the single charge electric field does not extend over distances greater than the DeBije length. So, in all, single charge electric fields do not play a large role in the total plasma cloud, apart from creating the plasma frequency which is a collective behaviour of the plasma.

Time varying magnetic fields drive currents, also in a plasma. The fields do not have to cross a wire as the electric charges in the plasma are mobile enough and can set up currents.
Frozen in magnetic fields move with the plasma, indeed. This means that E+vxB=0, but if you look at Ohm's law you will see that there are many more terms than just this one. And uh, ... why are we discussing frozen in fields?

A driving potential is not a current, I can have a potential over empty space and not have a current. I need moving charges to have a current. Plasma in motion is just plasma in motion, e.g. in the sun's convective layer there is a temperature different, which makes the plasma well up in one channel and then down again in another channel, just like air circulation in the Earth's atmosphere.

So, now we get to the crux, it all depends on what scale you look at, just like Tim said above. If you look at the large scale there are the magnetic fields that are moving and changing in time. This time variation drives currents and electric fields via: curl(B) = mu0 J - epsilon0 dE/dt and dB/dt = - curl(E), Maxwell's equations that I hope are out of scrutiny (apart from me making mistakes in wher the mu0 and epsilon0 belong). So even if we have frozen in field, when we compress the plasma, the magnetic changes too via B2 = B1 * surf1 / surf2, where B1 and B2 are the magnetic fluxes through surfaces surf1 and surf2.

If you look at the microscopic physics of how magnetic fields are generated, you need to take another viewpoint, because then you get to the point that somewhere due to random motion of the plasma particles a mini-current is set up and creates a mini-magnetic field. This is a so-called seed field in the dynamo theory for the sun. If things are quiet, the field will contract and dissipate and nothing of it. If things are more turbulent, then we can have that the tiny magnetic loop gets sheared, and folded on itself, basically then doubling the field strength. If you do that often enough you can build up a considerable magnetic field. Naturally (I can see your comment coming, what about the current, you forget about the current) there will be currents that are creating this magnetic field, they will also increase.

So, in summary, I guess you have to be a little schizofrenic if you want to be a plasma(astro)physicist.

8. Originally Posted by Tim Thompson
There would be a contradiction only if you assume that electrical neutrality requires the complete absence of charged particles. But that is not the case. An electrically neutral plasma is one which has an equal number of positive & negative charge carriers. An electrically neutral plasma in motion will generate a magnetic field (this is an observed fact). The electrical neutrality of space is merely the observation that the number of positive & negative charge carriers is equal, not that they are absent.
Can we all agree that space is (a) quasi-neutral (b) May contain charge separation regions (wherever there is a double layer)

Regards,
Ian Tresman

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Originally Posted by tusenfem
Sorry, old chap, but frozen in magnetic fields is a direct consequence of Alven's MHD assuming (almost) perfect conductivity. Or did you mean another "frozen in" as I see nowhere that Tim even remotely gets into the neighbourhood of being frozen.

* edited to correct an inaccuracy *
He got pretty damned close to saying it, and that was my concern. And in another thread he made the bizarre claim that Alfven was somehow down on MHD, seemingly unaware that 'he wrote the book on it'! In fact he won his nobel prize for work in this area.

Anyway, this is what Alfven had to say about frozen in magnetic fields:

"The electrical conductivity of any material, including plasma, is determined by two factors: the density of the population of available charge carriers (the ions) in the material, and the mobility of these carriers. In any plasma, the mobility of the ions is extremely high. Electrons and ions can move around very freely in space. But the concentration of ions available to carry charge may not be at all high if the plasma is very low pressure or diffuse. In short, although plasmas are excellent conductors, they are not perfect. It therefore follows that weak electric fields can exist inside them, and magnetic fields are NOT frozen inside them."

10. Originally Posted by Tim Thompson
However, as a general rule, all space plasma that we can observe, is seen to behave globally as if it were charge neutral, though I should think there are bound to be charged currents flowing in small scale environments. One has to study these things on a case by case basis.
Originally Posted by P.Asmah
He got pretty damned close to saying it, and that was my concern. And in another thread he made the bizarre claim that Alfven was somehow down on MHD, seemingly unaware that 'he wrote the book on it'! In fact he won his nobel prize for work in this area.
So Tim says that space plasmas can be considered charge neutral.
But he also says there may be currents in some regions.
No mention here of magnetic fields, not not even close to "frozen in's" nephew!

11. ## Alfven

Originally Posted by P.Asmah
He got pretty damned close to saying it, and that was my concern. And in another thread he made the bizarre claim that Alfven was somehow down on MHD, seemingly unaware that 'he wrote the book on it'! In fact he won his nobel prize for work in this area.
I never said any such thing. What I did say, in this post, in this thread was this:
Originally Posted by Tim Thompson
I think you will find that a rift deveolped between Alfven & the astrophysical community. It is not terribly unlike the rift that developed between Einstein and the physics community. Just as Einstein rejected much of quantum mechanics, so did Alfven reject much of magnetohydrodynamics. And just as the world of physics regretably left Einstein behind, so did the world of plasma astrophysics leave Alfven behind. And this does not detract in any way from the evident fact that both Einstein & Alfven contributed enormously to their fields of endeavor, and both received well earned Nobel Prizes (Einstein in 1921 and Alfven in 1970). Alfven continued to believe that non-neutral electric currents were responsible for much of the magnetic phenomena in the universe. He became essentially alone in this. In the passage you quote, Alfven is selling his own point, which most astrophysicists were in the process of rejecting.
I never suggested that Alfven did not invent magnetohydrodynamics. I have a few of Alfven's books, I have studied MHD, and I have even met Alfven myself. I am well aware of his role in the creation of MHD (although I will also point out that it is unfair to single out Alfven, and not even mention the contributions of people like Parker & Cowling, and others). But what I said was true nevertheless. Alfven did not remain loyal to MHD, but went off on his own. And that's a fact.

Alfven never in his life suggested that magnetic fields were not frozen in, that is a grave misconception held by those who chose to deify Alfven, instead of actually bothering to study the physics that Alfven presented. Whether or not a field is "forzen in" depends entirely on the conductivity of the plasma. The magnetic field is literally frozen in if the plasma is perfectly conducting, which Alfven himself conclusively proved. He also showed that the higher the resistivity of the plasma, the less "forzen in" the field would be.

You miss Alfven's point entirely. He showed that the resistivity of the plasma sets a characteristic time scale for magnetic field drift through the plasma. So, let us say that the drift time scale is 109 years in some plasma. And now let us say that I want to know what's happening in that plasma, in a process that takes 104 years. The time scale I am interested in is very short compared to the drift time scale, so I can safely assume that the magnetic field is forzen into the plasma, over the time scale of my study, with no ill consequences. But, if the drift time scale of that plasma were, say 103 years, then I would be badly mistaken in assuming that the magnetic field were forzen in, because the drift time scale is short compared to my time scale of interest. Alfven knew all of this, and says so expilicitly in his books & papers. And, as a matter of fact, mainstream plasma astrophysicists today adhere to this rule, as Alfven laid it out. So there is no conflict between the way mainstream scientists treat frozen in magnetic fields, and the way Alfven said they should be treated. There is only a failure of most of Alfven's fans to understand what Alfven actually said.

Alfven's main problem was the tendency in theoretical MHD to assume perfect conductivity, simply because it is a lot easier to compute things. This is still done in studies of "ideal MHD", for the same reason. We learn most efficiently by coming to grips with the "easy" problems first, as they are approximations of the more difficult problems that do not include the approximations of ideal MHD. But most EUers seem to suffer from the misconception that ideal MHD is assumed by mainstreamers to be "real" MHD, so they rail against a ghost of their own invention.

12. Originally Posted by Tim Thompson
Alfven never in his life suggested that magnetic fields were not frozen in, that is a grave misconception held by those who chose to deify Alfven, instead of actually bothering to study the physics that Alfven presented. Whether or not a field is "forzen in" depends entirely on the conductivity of the plasma. The magnetic field is literally frozen in if the plasma is perfectly conducting, which Alfven himself conclusively proved. He also showed that the higher the resistivity of the plasma, the less "forzen in" the field would be.
Have I misunderstood the following:
In 1967 Alfvén [109] noted evidence for the presence of electric fields parallel to magnetic fields, implying that the electric conductivity is not infinite and the magnetic field lines are not “frozen in.” (Later Alfvén [110] recalled that the “frozen field” concept had given him headaches for several years.) In Stephen G. Brush, "Alfvén’s Programme in Solar System Physics" (1992)
Alfvén also published a paper whose abstract states:
The concepts of 'frozen-in magnetic field lines' and 'field-line reconnection', which are frequently used in discussions of the theory of the magnetosphere, have been criticized by Alfven and Falthammar (1971), by Heikkila (1973), and by Alfven (1975). In the present paper, it is demonstrated that both concepts are unnecessary and often misleading. The frozen-in concept is shown to belong to the pseudo-plasma formalism which is useful only in special cases. (From: Alfven, H., "On frozen-in field lines and field-line reconnection" 1967)
Alfvén also wrote:
I thought that the frozen-in concept was very good from a pedagogical point of view, and indeed it became very popular. In reality, however, it was not a good pedagogical concept but a dangerous "pseudopedagogical concept." By "pseudopedagogical" I mean a concept which makes you believe that you understand a phenomenon whereas in reality you have drastically misunderstood it.
[..]
At that time (1950) we already knew enough to understand that a frozen-in treatment of the magnetosphere was absurd. But I did not understand why the frozen-in concept was not applicable.
[..]
In 1963, Fälthammar and I published the second edition
of Cosmical Electrodynamics [12] together. [..] We analyzed the consequences of this in some detail, and demonstrated with a number of examples that in the presence of an E|| the frozen-in model broke down. On [12, p. 191] we wrote:
"In low density plasmas the concept of frozen-in lines of force is questionable. The concept of frozen-in lines of force may be useful in solar physics where we have to do with high- and medium-density plasmas, but may be grossly misleading if applied to the magnetosphere of the earth. To plasma in interstellar space it should be applied with some care."
[..]
Since then I have stressed in a large number of papers the danger of using the frozen-in concept [..] The most important criticism of the "merging" mechanism of energy transfer is due to Heikkila [40] who, with increasing strength, has demonstrated that it is wrong. In spite of all this, we have witnessed at the same time an enormously voluminous formalism building up based on this obviously erroneous concept. Indeed, we have been burdened with a gigantic pseudoscience which penetrates large parts of cosmic plasma physics. (Alfven, Hannes, "Double layers and circuits in astrophysics" 1986)

Regards,
Ian Tresman
Last edited by iantresman; 2006-May-31 at 04:58 PM. Reason: Added reference to Heikkila[40]

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Originally Posted by iantresman
Have I misunderstood the following:
In 1967 Alfvén [109] noted evidence for the presence of electric fields parallel to magnetic fields, implying that the electric conductivity is not infinite and the magnetic field lines are not “frozen in.” (Later Alfvén [110] recalled that the “frozen field” concept had given him headaches for several years.) In Stephen G. Brush, "Alfvén’s Programme in Solar System Physics" (1992)
Alfvén also published a paper whose abstract states:
The concepts of 'frozen-in magnetic field lines' and 'field-line reconnection', which are frequently used in discussions of the theory of the magnetosphere, have been criticized by Alfven and Falthammar (1971), by Heikkila (1973), and by Alfven (1975). In the present paper, it is demonstrated that both concepts are unnecessary and often misleading. The frozen-in concept is shown to belong to the pseudo-plasma formalism which is useful only in special cases. (From: Alfven, H., "On frozen-in field lines and field-line reconnection" 1967)
Alfvén also wrote:
I thought that the frozen-in concept was very good from a pedagogical point of view, and indeed it became very popular. In reality, however, it was not a good pedagogical concept but a dangerous "pseudopedagogical concept." By "pseudopedagogical" I mean a concept which makes you believe that you understand a phenomenon whereas in reality you have drastically misunderstood it.
[..]
At that time (1950) we already knew enough to understand that a frozen-in treatment of the magnetosphere was absurd. But I did not understand why the frozen-in concept was not applicable.
[..]
In 1963, Fälthammar and I published the second edition
of Cosmical Electrodynamics [12] together. [..] We analyzed the consequences of this in some detail, and demonstrated with a number of examples that in the presence of an E|| the frozen-in model broke down. On [12, p. 191] we wrote:
"In low density plasmas the concept of frozen-in lines of force is questionable. The concept of frozen-in lines of force may be useful in solar physics where we have to do with high- and medium-density plasmas, but may be grossly misleading if applied to the magnetosphere of the earth. To plasma in interstellar space it should be applied with some care."
[..]
Since then I have stressed in a large number of papers the danger of using the frozen-in concept [..] The most important criticism of the "merging" mechanism of energy transfer is due to Heikkila [40] who, with increasing strength, has demonstrated that it is wrong. In spite of all this, we have witnessed at the same time an enormously voluminous formalism building up based on this obviously erroneous concept. Indeed, we have been burdened with a gigantic pseudoscience which penetrates large parts of cosmic plasma physics. (Alfven, Hannes, "Double layers and circuits in astrophysics" 1986)

Regards,
Ian Tresman
Yes, you have misunderstood. What Tim said, and what your quotes say is that frozen in is an approximation based on the resistivity and lifetime of the plasma. In the strictest sense, fields are not frozen in since there is always a resistance. However, if the field movement is small during the lifetime of the plasma, you can do an approximation and say it is frozen.

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Originally Posted by korjik
Yes, you have misunderstood. What Tim said, and what your quotes say is that frozen in is an approximation based on the resistivity and lifetime of the plasma. In the strictest sense, fields are not frozen in since there is always a resistance. However, if the field movement is small during the lifetime of the plasma, you can do an approximation and say it is frozen.
But isn't the crux of Alfven's later work to show that MHD is only applicable in a small minority of cases (the solar interior) and nowhere else?

So, how does mainstream science today incorporate the lessons from Alfven? Any examples where the consequences can be seen?

Cheers.

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Originally Posted by VanderL
But isn't the crux of Alfven's later work to show that MHD is only applicable in a small minority of cases (the solar interior) and nowhere else?

So, how does mainstream science today incorporate the lessons from Alfven? Any examples where the consequences can be seen?

Cheers.
Tim give a good example of where you would approximate out the change in magnetic field and call it frozen.

I was taught that you need to make sure that a term is small before you approximate it out. that applies to any term, magnetic or not, electric or not, gravitic or not.

As for an example, I am starting on looking at plasma detachment in the plume of the VASIMR. The plasma should drag the field lines a bit, until it detaches. That is a little bit frozen and a little bit not, just like what Tim says above

16. ## The rift

Originally Posted by VanderL
But isn't the crux of Alfven's later work to show that MHD is only applicable in a small minority of cases (the solar interior) and nowhere else?
MHD and "frozen in" are by no means synonymous, and you should not confuse them. Yes, that was the crux of Alfven's later work. That's what I was talking about when I said, "I think you will find that a rift deveolped between Alfven & the astrophysical community". Having developed MHD, Alfven then tried to abaondon it. He was wrong. MHD is applicable almost everywhere, and almost everyone except Alfven saw it. Modern plasma astrophysicists use MHD appropriately, and realize that the closed current models of Alfven are not in fact generally applicable, and are in fact generally wrong (but there are exceptions to every rule, which is why I also said, "One has to study these things on a case by case basis").

But we draw afar from the electric sun. We are engaged in a discussion of whether or not Alfven was right or wrong, which is a far more general discussion of plasma physics.

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Originally Posted by Tim Thompson
I never suggested that Alfven did not invent magnetohydrodynamics. I have a few of Alfven's books, I have studied MHD, and I have even met Alfven myself. I am well aware of his role in the creation of MHD (although I will also point out that it is unfair to single out Alfven, and not even mention the contributions of people like Parker & Cowling, and others). But what I said was true nevertheless. Alfven did not remain loyal to MHD, but went off on his own. And that's a fact.
Fair enough. Thank you for clarifying this.

And thanks also to Ian for providing excellent references.

I think these varying interpretations of plasma behaviours are critical to our discussion here?

18. ## ideal and non-ideal

Originally Posted by iantresman
Have I misunderstood the following: ...
I will second Korjik. Yes, you have misunderstood all of it, primarily because you are trying to understand science "by exegesis", by deconstructing the text, rather than understanding the fundamentals behind the text. Like I said, Alfven never in his life said that the field was not frozen in. but he did say that it was an approximation, and you can see for yourself that he said this repeatedly. Alfven was reacting to the rapid expansion of the theory of ideal MHD, which treats the plasma as having zero resistivity, and permanently frozen magnetic fields. He felt that too many scientists failed to realize the extent to which this was an approximation, and were too ready to apply ideal MHD to real situations, where it might well be inapplicable.

Personally, I think he was too arrogant on this point, presuming to understand what everybody else was thinking. But ideal MHD was always understood as an approximation. Who thinks that "ideal" ever applies to the real world? By concentrating on ideal MHD, scientists were able to bootstrap their understanding of real (non ideal) MHD that much quicker. Now we see, in text's like Dieter Biskamp's Nonlinear Magnetohydrodynamics (Cambridge Monographs on Plasma Physics, 1993), or Priest & Forbes' Magnetic Reconnection (Cambridge University Press, 2000), that the fully non-ideal treatment of MHD is commonplace in plasma astrophysics.

But, again, we grow far from the electric sun. What about the electric sun? What is the electric sun?

19. Originally Posted by Tim Thompson
Having developed MHD, Alfven then tried to abaondon it. He was wrong. MHD is applicable almost everywhere, and almost everyone except Alfven saw it.
• I can find nothing in any of Alfvén's writings which even hints at him abandoning MHD
• That "MHD is applicable almost everywhere" I am sure that Alfvén would disagree with, and this would be the crux of the difference between him and astronomers. Alfvén even quantified the conditions in different types of plasmas where MHD would be applicable or not.
• I am quite sure that everyone will agree that MHD is indeed an approximation... and there are times when "frozen-in" magnetic fields and magnetic merging/reconnection are accepted.

Regards,
Ian Tresman

20. ## not an approximation

Originally Posted by iantresman
1. I can find nothing in any of Alfvén's writings which even hints at him abandoning MHD
2. That "MHD is applicable almost everywhere" I am sure that Alfvén would disagree with, and this would be the crux of the difference between him and astronomers. Alfvén even quantified the conditions in different types of plasmas where MHD would be applicable or not.
3. I am quite sure that everyone will agree that MHD is indeed an approximation... and there are times when "frozen-in" magnetic fields and magnetic merging/reconnection are accepted.
(1) I don't see how you can say this and be self consistent. We all know that Alfven did not believe that MHD was the correct way to handle plasma physics. His entire edifice of plasma cosmology is built on a system of circuit models intended to replace MHD as the proper theoretical treatment of cosmic plasma physics. If he actually trusted MHD, there would not be a "plasma cosmology".

(2) "Astronomers" is an inappropriate word. The people who disagree with Alfven are all plasma physicists, engaged in the professional area of plasma astrophysics. They might be called "astronomers" in some vague, general sense, but most of them are actually not so well trained or educated in astronomy. They are primarily physicists & applied mathematicians.

(3) I think that most agree that there are times when reconnection & frozen fields are "accepted", certainly in the plasma astrophysics game. But MHD is no more an "approximation" than is physics in general an "approximation". There are approximations within the general scope of MHD, such as ideal MHD, or the "frozen field approximation". But MHD itself is not an approximation. It is the apparently correct application of electromagnetism & fluid dynamics to the problems of plasma physics. But it is exceedingly complex, so methods of approximation are often used (as is the case with any complex endeavor) to simplify problems and reach approximate solutions.

But once again we are far afield from the electric sun.

21. Originally Posted by iantresman
• I can find nothing in any of Alfvén's writings which even hints at him abandoning MHD
• That "MHD is applicable almost everywhere" I am sure that Alfvén would disagree with, and this would be the crux of the difference between him and astronomers. Alfvén even quantified the conditions in different types of plasmas where MHD would be applicable or not.
• I am quite sure that everyone will agree that MHD is indeed an approximation... and there are times when "frozen-in" magnetic fields and magnetic merging/reconnection are accepted.

Regards,
Ian Tresman
It is all Oh So Simple with respect to MHD and Frozen in. For the latter Tim already gave the explanation, but let me summarize for completeness.

When is MHD valid?
MHD is an approximation of the plasma equations where it is assumed that the plasma can be seen as some kind of fluid (that is why the "hydro" is there in MHD) and it basically neglects the single particle motion of the plasma. The single particle motion is the gyration motion of the charged particle around the magnetic field. So this approximation is only valid when:
1. We look at time scales that are longer than the longest gyration period in the plasma;
2. We look at spatial scales that are larger than the largest gyro radius in the plasma.
This is basically stated in the first chapter of every book on MHD. Now you just have to take the characteristics of the plasma that you are looking at and see what the larges gyro radius and period is, and then look at what kind of phenomenon you want to look at. An example:
Looking at the solar wind (taken from Goedbloed & Poedts "Principles of MHD", Table 8.1) proton temperature 5 104 K and magnetic field strength 6 nT. Proton gyro radius is L = 0.947 T1/2(K) B-1(nT) km = 35 km. Proton gyro frequency is F = 0.01525 B(nT) Hz = 0.0915 Hz. So if we look at the plasma at spatial scales larger than 35 km and at time scales larger than 10 seconds MHD is applicable.

When is the magnetic field frozen in?
Why reinvent the wheel ...
Originally Posted by Tim Thompson
You miss Alfven's point entirely. He showed that the resistivity of the plasma sets a characteristic time scale for magnetic field drift through the plasma. So, let us say that the drift time scale is 109 years in some plasma. And now let us say that I want to know what's happening in that plasma, in a process that takes 104 years. The time scale I am interested in is very short compared to the drift time scale, so I can safely assume that the magnetic field is forzen into the plasma, over the time scale of my study, with no ill consequences. But, if the drift time scale of that plasma were, say 103 years, then I would be badly mistaken in assuming that the magnetic field were forzen in, because the drift time scale is short compared to my time scale of interest.
A slight addition for the equation addicted, the drift (in text books diffusion) time scale is dependend on the conductivity and a length scale. The magnetic diffusivity eta = 1 / (mu0 sigma) where mu0 is the magnetic permeability of vacuum (4 pi 10-7) and sigma is the conductivity of the plasma. From the induction equation one can then find that the time for a magnetic field to diffuse over length L is given by T = L2/eta, now you just have to calculate the conductivity of the plasma. I let that to the reader for a rainy sunday afternoon.

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[Moderator note: this thread was created by splitting posts from the ATM thread, The Electric Sun (so you will see some ATM ideas and references).

The posts here are not, primarily, on the ATM idea, the electric Sun, but refer mostly to basic plasma physics, MHD (magneto-hydro dynamics), and the history of its development (especially the role of the contributions of Alfv&#233;n).

As there seems to be quite a bit of confusion, among some BAUT members, regarding plasma physics, I think this thread may help to clarify it, as well as provide an opportunity for BAUT members to ask questions about this area of modern physics.]

(repeat of info contained in OP).

23. ## Books

Well, it seems that the "discussion" went nowhere, once this thread was created by Nereid. Plasma physics is a topic that arises in some ATM ideas, but is not amenable to general or simplistic discussion, so it is not surprising that this thread did not advance. The topic is both physically & mathematically intense, and usually specific problems have to have specific answers tailored for them, and cannot be understood by means of the prosey approach that is typical of boards like this.

So, with that in mind, and anticipating that this thread will continue to remain silent, I would like to take the opportunity to list the few plasma physics books I have handy in my library, as possible resources for those who might wish to seek them out as references. They too are physically & mathematically intense, but books can still be read, and pondered over, and even those with a minimal relevant education can get something out of them. I present them in no particular order.

Plasma Astrophysics, Toshiki Tajima & Kazunari Shibata, Westview Press, 2002
Originally published by Addison-Wesley, 1997. As the title implies, this is a book which specializes in the applications of plasma physics to astrophysical problems. Topics include turbulence, structure formation & filaments, dynamo theory, MHD & magnetic reconnection, MHD instabilities & jets, intergalactic plasma, and the origin of cosmological magnetic fields.

The Physics of Plasmas, Boyd & Sanderson, Cambridge University Press, 2003.
A general textbook on plasma physics, not directed towards astrophysics in particular. Topics include particle transport in magnetized plasmas, ideal & resistive MHD, plasma waves, collisional & collisionless plasmas, plasma radiation, and non-linear plasma physics.

Magnetic Reconnection: MHD Theory and Applications, Priest & Forbes, Cambridge University Press, 2000.
As the title implies, the book deals with magnetohydrodynamics, and in particular with the phenomena of magentic reconnection. Topics include current sheet formation, magnetic annihilation, steady & unsteady reconnection, and applications in laboratories, solar physics, magnetospheric physics, astrophysics, and particle acceleration.

Nonlinear Magnetohydrodynamics, Deiter Biskamp, Cambridge Monographs on Plasma Physics, 1993 & 1997.
Topics primarily include instabilities, turbulence, disruptive processes, pinches, and solar flares.

Space Physics - An Introduction to Plasmas and Particles in the Heliosphere and Magnetospheres, May-Britt Kallenrode, Springer 2001 (2nd ed).
Deals exclusively with solar system plasma physics. Topics include MHD, plasma waves and kinetic theory, solar wind & heliosphere, solar, terrestial & planetary magnetospheres, solar-terrestrial interactions, and instrumentation.

And of some historical interest ...

Origins of Magnetospheric Physics, James A. Van Allen, Smisthonian Institution Press, 1983.
Van Allen was himself one of the pioneers of the study of solar system plasmas, and one of the early scientists to realize that space was filled with plasma. He recounts the history of how that science develops, from the beginning of the 20th century, up to the era of Earth orbiting satellites. A good way to find out who did what.

There are a lot more books & papers & journals, I just put down what I have handy. I find these books readable, current & authoritative. If you want to learn something real about plasma physics, you could do worse than start here.

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## Alfven against the mainstream

The new sticky thread at the top of 'Against the Mainstream' suggests that Alfven's plasma physics are very much in the mainstream. This is disingenuous. While some of his work is now accepted by the mainstream -- which is positive progress, granted -- it is VERY IMPORTANT to realise that many of his ideas are still very much outside it. The mainstream is yet to catch up with many of them. This situation will probably change, but it will take time.

For example, Alfven proposed the following hypotheses as a corollary to his lab work.

1. In 1937 Alfven proposed that our galaxy contained a large-scale magnetic field and that charged particles moved in spiral orbits within it, owing to forces exerted by the field. Plasma carried the electrical currents which create the magnetic field.

2. "In order to understand the phenomena in a certain plasma region, it is necessary to map not only the magnetic but also the electric field and the electric currents." Hannes Alfven

3. "Gravitational systems are the ashes of prior electrical systems." Hannes Alfven

To summarise, it seems unfortunate that the implications of his work for cosmology remain largely unrecognised!

Neried. Clearly your post to which I refer needs updating in the light of these facts.

25. Originally Posted by PhantomWolf
The basic law is "A moving charge creates a magnetic field."

But not all magnetic fields are created by moving charges.

Magnetic fields are created by the spin of the charge and when you align the spins on the electrons in an atom in a material you will produce a magnetic field. Electrons or protons moving in a uniform motion will align the spins, but you can align the electrons without a current, iron is very good at doing it, though other materials will do it as well.

The moral of this is while:

Current = Magnetic Field
Magnetic Field ≠ Current
Maybe a too simple thought of me, but isn't an electron with spin a moving charge? Or has electron-spin really nothing to do with motion?

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Originally Posted by P.Asmah
The new sticky thread at the top of 'Against the Mainstream' suggests that Alfven's plasma physics are very much in the mainstream. This is disingenuous. While some of his work is now accepted by the mainstream -- which is positive progress, granted -- it is VERY IMPORTANT to realise that many of his ideas are still very much outside it. The mainstream is yet to catch up with many of them. This situation will probably change, but it will take time.

For example, Alfven proposed the following hypotheses as a corollary to his lab work.

1. In 1937 Alfven proposed that our galaxy contained a large-scale magnetic field and that charged particles moved in spiral orbits within it, owing to forces exerted by the field. Plasma carried the electrical currents which create the magnetic field.

2. "In order to understand the phenomena in a certain plasma region, it is necessary to map not only the magnetic but also the electric field and the electric currents." Hannes Alfven

3. "Gravitational systems are the ashes of prior electrical systems." Hannes Alfven

To summarise, it seems unfortunate that the implications of his work for cosmology remain largely unrecognised!

Neried. Clearly your post to which I refer needs updating in the light of these facts.
If you wish to start a new ATM thread, on Alfvén ATM hypotheses, you are welcome to do so.

Please bear in mind that BAUT is an internet discussion forum focussed on astronomy, astrophysics, cosmology, and space science, and is avowedly scientific in its approach.

With regard to "Alfven's plasma physics", and this thread in the General Science section, this means those parts of his work, on plasma physics, which are in the mainstream today.

As you agree, I'm sure, we do not accept discussion of Newton's ideas concerning alchemy here in the General Science section of BAUT, because those ideas are most definitely against the mainstream. Similarly wrt various (a)ether theories, no matter what the record of the work of 19th century physicists who promoted these theories is.

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Professores: Thank you all for this most informative discussion. Makes one realize that Gravitation is merely some form of magnetics.
And Further. Vermuch enjoyed the high level discussion.
Thanks
Houssein Afghani

28. Originally Posted by Housseinafghani
Professores: Thank you all for this most informative discussion. Makes one realize that Gravitation is merely some form of magnetics.
And Further. Vermuch enjoyed the high level discussion.
Thanks
Houssein Afghani
Sorry, Houssein, but gravitation has nothing to do with magnetics, and it beats me how you got to this conclusion reading this thread.

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I seem to remember a Japanese naval vessel that used MHD. It didn't seem to go very fast.

30. Originally Posted by publiusr
I seem to remember a Japanese naval vessel that used MHD. It didn't seem to go very fast.
The Magnetohydrodynamic drive. I suspect that it's advantage is that it is quiet.

Which reminds me of another non-mechanical plasma drive, the Hall Effect thruster, based on double layers. [ref]

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