Do you do much physics?

say you have a linearly decreasing B that decreases from B0 to zero in 1 ms. You are looking at a 100 Ghz wave in this plasma. Over the period of 1 wave (what do you know, time entered into it), B changes by 1 part in 10^5. A change of .001% in the period of the wave is generally considered small.

Heck, over 100 waves, the change in B is 0.1%. Still a small change.

That really is how it is done. Look in any plasma physics book and read the discussion of frozen-in lines.

[quote=rcglinsk;1258071]What if the question is not time dependent? If I measure the change in magnetic flux at points A and B for five seconds, and if the magnetic diffusion time is much longer than five seconds, then I should be able to work backward with the frozen in field line assumption to find some characteristics of point C five seconds ago with a high level of accuracy. [quote]

In this statement you have explicitly set the time frame in which you are working, and thereby made the question time dependent. Also, I think that buried in your statement is the assumption that the field that you measured was essentially constant for the five seconds, which is also a quantification of time dependence. My point is that when one makes approximations, that the conditions under which the approximation is valid need to be made explicit, and if the approximation is made under some sort of assumption regarding dynamics then the evaluation of the dependence on time is part of that exercise. An assumption of a frozen magnetic field with no qualifiers is at best misleading. Alfven, who invented the notion had many qualms about the applicatioin of it too broadly. Handwaving during mathematical derivations can lead to false conclusions, and dividing by zero can be subtle -- even Einstein blew that one on occasion.

[quote] I would be very confused if frozen in field assumption based conversions from a measured variable to a derived variable were built into any long term plasma model, as no matter what diffusion time for the magnetic lines existed the long time scale of the model will far exceed it.[quote]

I'm not sure what you are trying to say here.

Exactly!

Is not "a small change" a qualifier?

You seem to be tripping up on generic qualifiers. The frozen in condition requires a small timescale compared to the diffusion time of the magnetic field. What is 'small' is determined when a specific problem is worked.

Alfven was right when he got worried about too broad application, but it was more because people started getting lazy about how 'small' the small changes in the field were. This was partly due to the fact that you had to do most of your work by hand, and partly due to the expansion of knowledge at the time. That lesson is now taught to new plasma physicists. Well, at least it was taught to this new plasma physicist

In the last bit rcglinsk is trying to say that frozen-in cannot be used on a long time scale. Say like stellar evolution. No matter how slow the diffusion is, there will be enough time for the field to diffuse away.

Electric fields have been looked for. If there was a charge on the sun there would be a field that has never been seen.

[quote=korjik;1258118]Do you do much physics? Nice hypothetical, but not germane to the original estimate and coclusion that E + V X B = 0 with no further qualifications. This is the first mention of a 100 GHz wave or a linearly decreasing B field over a spanof 1ms. Other assumptions would lead to other conclusions. I really don't care how it is done in some textbook if the fundamental mathematics is as you represent it. Non-rigorous symbol manipulations have led to erroneous conclusions before. So, can you come up with a justification for your estimate and back it up rigorously? All that I am looking for is solid mathematics and justified approximatioins. Rigor will convince me. Bluster and attempts at intimidation will not.

Well, this is a usual discussion. There are various things that need to be taken into consideration:

1. Ohm's law give us that E + v x B = η j
2. The conductivity of a plasma is very high, I once calculated the resistance of an accretion disk (in this paper) just to give you an estimate and found that is was on the order of 9.1 x 10^-27 s cm^-1 in cgs units. 1 Ohm ~ 1/9 10^-11 s cm^-1 so this leads to 8 x 10^-15 Ohm, and this is for a rather high density plasma.
3. Similarly one can calculate the resistivity of other plasmas and find that they are high. This means that the diffusion time scale of the magnetic field is long.
4. Now, how about experiments? One of the things that spacecraft in the Earth's magnetosphere measure it magnetic and electric fields and plasma data. The interesting question is always, what kind of electric fields are the spacecraft measuring, and therefore one compares the measured electric field with the measured v x B. This gives a handle on whether one just measures a motional electric field or if there is more to it. Interestingly, in the magnetotail and in the solar wind most of the time it is found that both values are equal within the accuracy that we can measure them. In the paper by Keika et al we do exactly such a thing and we show it in Fig. 6.
5. I basically do not care that Alfvén at one point decided that he did not like the frozen in field anymore, measurements show that it is a very good approximation for a lot of problems in space physics. Even great thinkers can be wrong now and then, see e.g. the cosmological constant in Einstein's work, to obtain a static universe.
6. The limit on the current density is important, because too strong currents develop various kinds of instabilities, which are or are not observed in the space plasma under consideration. At the same time, for very strong currents there has to be a driver and electromotive force working, which needs to be specified.
7. So in all, during processes observed in space physics, we can basically say that 90% of the time the frozen in field condition is valid. And rest assured that the space physicist are highly aware of the fact that frozen-in has an expiration date written on it, please use before end of XXX

__________________
************************************************** *************************
Optimism does not change the laws of physics. (T'Pol)
A good scientist has freed himself of concepts and keeps his mind open to what is. (Dao De Jing 27)
************************************************** *************************
Martin ( http://www.geocities.com/DrMartinV )

As far as we know, there is no charge on the Sun. If there would be a charge, the the solar wind would quickly transport any such net charge away from the Sun.

The problem is not the theory but the application. The EU wants to do away with gravity as the dominating force on larger scales, because the electromagnetic force is 10³⁹ stronger. That this only holds for two free electrons they will easily forget. Electrodynamics is importent in space physics and astrophysics, but not everything is electric.

The answer you got on thunderbolts on reconnection goes back to the theory of exploding double layers by Hannes Alfvén. These, however, have never been observed, although it seemed like an interesting idea when he proposed it. (actually I guess you will find a paper of mine if you search for exploding double layers).

The easiest way of understanding reconnection is through the magnetic field picture that I drew. Yes, you can also try to set up a current system representation of it all. One of the "controversies" in magnetospheric physics is the problem of substorms, are they an inward-out process or an outward-in? If the latter (which most scientists believe) there is reconnection in the tail at a distance of say 25 Earth radii. If the former (which some scientists believe) then there can be a process called current-disruption (T. Lui is the main proponent for this) which can also rearrange the magnetic field. The latter looks a bit like what Thunderbolt writes (but then Lui's work is mathematically founded, whereas Thunder only uses handwaving).

I am not sure what paper Thunder linked to, but he himself has never ever presented any real math on any of the subjects that he discusses on his website.

__________________
************************************************** *************************
Optimism does not change the laws of physics. (T'Pol)
A good scientist has freed himself of concepts and keeps his mind open to what is. (Dao De Jing 27)
************************************************** *************************
Martin ( http://www.geocities.com/DrMartinV )

0. Please understand that I am not disputing the notion of frozen magnetic fields in its entirety. I am trying to understand the conditions under which the approximation holds by understanding in detail the derivation.
1. This is a nit, but I believe that the usual form for Ohm's law in this setting is E + v/c x B = η j. EDIT: My error, the difference is that I have been using Gaussian unts.
2,3. I agree that the conductivity of the materials in question is high.
4. I will take your word for the experimental evidence. I can't get access to the Keika paper. But my interest is more in understanding from a theoretical viewpoint, what the assumptions and limitations are. The experimental evidence would simply confirm that those assumptions are valid in the situation in which the measuremens were conducted. I do not doubt that the approximations are being applied in a valid domain.
5. I basically agree. What is important is what can be derived from basic principles. However, Alfven's thoughts may be a guide to understanding the underlying assumptions and the domain of validity.
6. I agree that a limit on current density is important. But what is the source of that limit and can it be quantified ? And it agree that currents require a driver, but it seems to me that the existence or lack of a driver is part of the set of assumptions on which the approximation is based, and requires consideration. Is there a unstated condition that rules out a driver ?

I think that there may be. If one goes through the discussion on magnetohydrodynamics in Jackson's book or in Landau and Lif****z and tries to reproduce it starting from Maxwell's equations you find that if one assumes that the partial of E with respect to time is esentially 0 (quasi static electrical field) then one comes up with the equation (pleae excuse lack of symbol capability)
partial of B wrt t = curl(VXB) + (c^2/4*pi*sigma)Laplacian(B)
From this one concludes that for times short with respecto to (4*pi*sigma*L^2/c^2) where L is a characteristic length relatec to spatial variation in B that the second term in the equation for partial of B wrt t vanishes and on econlcudes that the B field through a loop moving at velocity V is constant and that the magnetic field is "frozen" in the plasma. One can phrase this in terms of a magnetic Reynolds number. I can understand that argument. But it leaves open the question of verifying the assumptions that the E field varies slowly in time and the variations of the B field in space are known well enough to define the diffusion time. So, my question has evolved to how does one know the conditions of the assumptions well enough for application of the approximations ? This is perhaps where I come back to Alfven's caution agaiinst using the notion of frozen fields in light plasma -- and that begs the question of precisely what constitutes a light plasma.

The argument then goes on to describe the "E X B" drift using the equation
E + v/c x B =0
which I think still relies on a cap on J combined with high conductivity.

7. I don't particularly doubt your assertion that the notion of frozen magnetic fields applies 90% of the time. But I am trying ot figure out what the conditions are that justify that statement, and what prevails in the other 10%. Also, I suspect that the expiration date is not so clearly printed, so I am trying to understand how to determine what that date actually is.

I think my list of conditions to be verified is now down to: 1) a quasi static E field, 2) spatial variation in the B field consistent with a large magnetic diffusion time constant and 3) a quantifieable cap on current density appropriate to the problem. It seems to me that these items need to be described in terms that can be readily verified in any specific problem to which they are applied.

Where is the math not solid and the approximations not justified?

If you do not understand my explanations, you need to get a plasma physics book and do the problems yourself. This forum is too limited in its ability to display math to make it possible to do the full derivations here.

As to E+vxB=0 there are alot of further qualifications. It is assumed that nJ is small because in most cases nJ is small (n=eta for this). The assumption is based on measurement. It isnt dogma that requires the assumption, but practice. There will be times when nJ ends up not being small and then you have to use different methods.

You seem to think that E+vxB=0 is a dogma that is prayed to. It isnt. It is a handy shortcut that applies in a set of circumstances. Plasma physics is pretty much all handy shortcuts and approximations because going back to the general form is generally unsolvable.

I cannot explain it any better here. Mathmatical rigor is not a property of physics. Plasma physics even less so. If you are looking for 'apply equation x to get the exact answer', you wont find it. If you want to learn how the physics is done, go get yourself a book and do it. That is really the only way you will learn it.

P.s. watch your quotes and unquotes when posting please. Sometimes hard to see what is your and not.

(emphasis added)

I don't know anything about your formal training in science (specifically physics), rcglinsk, but there's an extremely important point lurking in the part of your post that I'm quoting, and it deserves to be addressed as clearly and as forcefully as possible.

First, though, a caveat: an idea, a hypothesis, a model, or a theory (or any combo of same) in astrophysics, space physics, etc can be considered on its own merits irrespective of who is a proponent of said idea (etc), and even independent of whether you, me, or anyone else regards it as part of 'EU', 'mainstream', or whatever. Nay, not can, but should.

So, what is this "EU"? Is it a subset of "PC" (Plasma Cosmology)? or overlaps somewhat? And what sort of website is this thunderblots? How do any of these things relate to astronomy, astrophysics, plasma physics, or even science??

ANSWER: as a consistent, coherent approach to understanding the nature of the universe (beyond the Earth's atmosphere), "EU", "PC" (and synonyms) are 'pure woo', to quote from the JREF forum.

Why?

Because those who proclaim themselves to be proponents explicitly and implicitly reject one of the most fundamental aspects of physics (and science in general), consistency.

Even a cursory study of materials claimed to be 'plasma cosmology', or 'electric universe' in their approach turns up examples of the following:

* internal inconsistency: within the same idea/hypothesis/model/theory, mutually inconsistent aspects are presented, with nary a mention that (often quite serious) inconsistencies are there

* inconsistency with well-established theories: curiously, the well-established theories include parts of plasma physics (!); generally however, "EU" and "PC" materials contain inconsistencies with just about every major area of modern physics (though GR seems to be a particular fave)

* the crown jewel, inconsistency with good observations and experimental results.

Of course, any and all these kinds of inconsistencies can be found in any forefront research ... their existence is one of the strongest motivations for doing research! However, for EU/PU/PC/etc proponents, rare it is indeed that any inconsistency is even recognised, much less acknowledged as a reason for concern or a spur for developing a research program, a new experiment, making new observations, and so on.

(As a historical aside, I note that Alfvén at least put together some intellectual scaffolding around his astrophysics/cosmology ideas (etc), so that consistency could be shown to be maintained, if only barely - the 'actualistic' vs 'prophetic' philosophical foundation. However, it seems none of the contemporary EU/PC proponents are even aware of just how thoroughly their advocacy rips this fine Alfvénic philosophical point to shreds)

It gets worse.

At least one of the "EU/PC" leading lights seems to be quite relaxed about academic fraud, and one of the very few vehicles where any EU/PC/etc materials gets published seems to have dropped its standards wrt scientific papers.

I don't know about you rcglinsk, but I'd've expected the EU/PC community to be horrified about this, and addressed it vigorously. After all, nothing hurts a scientist's work more than a scandal about academic fraud; even if only one such can be shown to have happened, the person's scinetific career almost always comes to a screeching halt and the entire body of their work henceforth ignored.

And how did the EU/PC community respond to this apparent case of academic fraud? I'll leave you to answer that for yourself.

Some resources:
* New research results from the "Stardust" mission - a BAUT thread containing details of Wallace Thornhill's apparent academic fraud (just one; I have recently come across other material which may also turn out to be intellectually fraudulent)

* Plasma Cosmology - Woo or not - a JREF forum thread exposing many examples of inconsistency (as above) as well as the apparent complete lack of concern about such inconsistencies.

So, the net for me: present your (ATM) idea, hypothesis, model, or theory any way you wish. However, the moment you (positively) associate it with EU/PC/PU/etc, I feel you have declared your idea to be pseudo-science (at best) and anti-science (at worst), whether you intended to do so or not; if you did not so intend, you had better backpedal very, very hard.

Ditto, wrt Wallace Thornhill and the websites he owns and manages.

That, in a nutshell, is the intellectual conflict.

Now back to your scheduled programme.

Shame on you Nereid for continuing to spin this accusation again, which certainly has no relevance to this thread.

I have already provided an explanation to you, that the poster list went to press before their poster presentation was accepted. That Thornhill was present at ICOPS 2006 can readily be checked in the Technical Session for Thu 8 June 2006.

You are quite welcome to disagree with Thornhill et al, but even scientists generally assume good faith, especially when an explanation has been provided.

You may not like the explanation, and you may not even accept it. But I wonder whether you would be so quick to condemn if you didn't hide behind a pseudonym. I certainly wouldn't accuse other of fraud.

__________________
Ian Tresman
plasma-universe.com

Hello Nereid, seems ages since I have seen you post in ATM.

__________________
'The eye can only see what the mind is prepared to accept'

I think these people found it.

http://www.srl.caltech.edu/ACE/ACENews/ACENews56.html

The 90° PA depletions produce the appearance of counterstreaming electron beams (along the magnetic field) even though no enhancement in particle flux occurs opposite to the strahl. On open field lines this counterstreaming can be confused with counterstreaming associated with the closed field lines that commonly thread coronal mass ejections in the solar wind. The difference is that in the case of depletions there is no enhancement in the electron flux counter to the strahl, whereas there are such flux enhancements on field lines connected to the Sun at both ends.

I suppose that paragraph doesn't make the most sense if you haven't read about the experiment they ran...

Anyway, you've got electrons streaming back toward the sun in solar wind events, you've got arcs (if we want to call them that) from comets going out toward the sun, you have the continual acceleration away from the sun of solar wind (which the latest evidence there shows should be net positive), and you've got the satellite motion deviations (the far out probes that are going slower than they're supposed to, with the constant small acceleration toward the sun). There are now observed currents between the sun and Earth that cause the auroas, and there is a current between jupiter and Io. After a while it seems that the idea of a charged sun/plantets might be something to investigate, as it would help make sense of a whole lot of observations.