I mentioned double layers because they surely would be formed, whether by Buneman instability in plasma currents or between the plasma regions with different conditions (thanks to the strong gradients mentioned previously). That, and the remarks on Weibel instability, are just my attempts to brainstorm the problem, to see where electric fields may arise that could create currents that would generate magnetic fields.

Now, regarding your explanation. Are you saying that, in the absence of an external magnetic field and given the ideal MHD conditions (thus, no electric fields on a macro scale), mechanical movement of locally quasi-neutral plasma can produce a magnetic field on a macro scale?

Leo

Double layers between two plasmas with different characteristics fall into the category of "non current carrying" double layers. For a good explanation go to the wiki page on DLs which you can trust, because I wrote most of it and I got my PhD on DLs.

So, no current flowing, thus no magnetic fields generated.

The current carrying double layers, the ones you would find in a circuit already have to have current flowing which gets forced into creating DLs when e.g. the density drops below a certain level or the drift speed gets too great and the Buneman instability turns on.

Ideal MHD is basically absent in real life space physics, but that as an aside.
Plasma is "chaotic" (one should use the word with some care, because of the strict definitions in science, but for the moment it suffices). This means there is random motion of the ions and electrons in the cloud or disk that you are looking at. This means that sporadically there will be small current loops formed, creating a mini magnetic dipole. Now, if there is motion in the cloud (e.g. like in the Sun there is convection) or in the disk (differential rotation) things can work out that one part of this small dipole gets pulled along at a different speed than the other part, and thus it gets deformed, stretched, etc. (there is no good wiki page on this topic, so you will have to look it up in a book for the details).

So there need not be a seed field, the dynamo action can start from statistical fluctuations and shear flows.

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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)
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Martin ( http://www.geocities.com/DrMartinV )

First of all tusenfem: thanks for the details! As I said, I'm no plasma expert, I just trust what I read in the journals. Second of all, with regards to this comment:

I think in the case of an accretion disk (but not necessarily the solar wind or Earth/Jupiter/Saturn/Titan/etc.'s magnetosphere), the term "chaotic" actually does apply. The system is very nonlinear and I would expect the motion of the plasma has sensitive dependence on initial conditions, a prerequisite for "chaos" in the mathematical sense. It's not so much random motion as it is small instabilities leading to large changes in the later configuration. But please correct me if I'm wrong here.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Yes. I'm quite aware of that. But the location of such double layers can influence the geometry of the magnetic field.

Absolutely. That is why I was surprised to find that the models use MHD and not PIC codes.

Leo

I have no real problem with the word "chaotic" for the accretion disk.

__________________
************************************************** *************************
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 )

Yes. I'm quite aware of it. But the non-current double layers are important too, because their location tells us much about the geometry of the magnetic field.

Absolutely. That is why I was surprised that the models talked about here didn't use PIC codes.

I see what you mean. Although, yes, one has to be careful about termonilogy here. Magnetic fields do often have an "organizing" effect on plasma. What in a neutral fluid results in turbulence may, in plasma, produce some very organized plasma waves.

And thanks for the information!

Leo

In reply to parejkoj's comment:

parejkoj, made this comment. to my comment & to referenced paper that noted that BH mass evolves as (1+z) where as quasar luminosity evolves as (1+z)^3. As noted below if quasars are powered by accretion discs, the quasar luminosity for the same mass flow rate through the disc, is linearly proportional to the black hole mass. Therefore if the BH mass is twice as large the same mass flow will produce twice as luminous a quasar.

I not sure what your point or concern is. You state that quasar luminosity does not appear to correlate with black hole mass. That would be interesting, if there was statical evidence of that. Do you have a reference? What evidence do you have that matter is moving through the accretion disc? Note I provided references of bright quasars without a visible accompanying host galaxy to feed the quasar.

Also, matter can not move through the accretion disc faster than than Eddington limit. At the Eddington limit the quasar photon energy exceeds the pull of the quasar on free electrons.

The following is more details concerning the Eddington limit and quasar luminosity, assuming quasars are powered by accretion disc.

http://rocinante.colorado.edu/~pja/a.../lecture25.pdf

The Luminosity of a quasar is assuming quasars are powered by accretion discs:

Luminosity quasar ≈( η × G x M(black hole) x dm/dt)∕r

Where η typically is estimated to be equal to 10%.

G is the universal gravitational constant.

dm/dt is the rate mass is moving through the accretion disc.

As can be seen from the formula and the basic physics, if the BH is larger matter that falls into the BH via the accretion disc will release more energy.

I have a couple questions for you William.

Can you derive the formula for the "Eddington limit"? Please don't link to a paper discussing it. I want to know if you can derive it (and before you ask, yes, I can). There are several assumptions one has to make in the derivation: do you know what they are?

Because from your post above, it appears that you don't really know what the "Eddington limit" is.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

I don't. We have to find evidence of either pop III stars, or at least diminishing metallicity in the observable past to sustain current theories.

There have been a couple of papers of late that suggest that it will always be impossible to observe pop III stars, that they are inflated beyond the event horizon. This is disturbing, because when you push a hypothetical beyond observational limits, you are eliminating observational tests, without which all speculation about metallicity evolution becomes just that: Speculation, not a viable hypothesis, not a supportable theory.

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jwj

It's ok not to know. We should try harder to find out.

I expect there is evidence for metallicity evolution throughout the forest of our now visible universe. An apparent lack in a few trees does not turn the entire forest of evidence into speculation.

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Everyone is entitled to his own opinion, but not his own facts.

I have been reviewing the observational and theoretical work on Ultra Luminous X-ray objects, ULX. I am following the train of thought concerning the Paradox of Youth stars. (i.e. Could the ULX objects be from a similar mechanism. I am assuming MECO fragmentation, due to charge unbalance, but on a larger scale than the paradox of youth stars.) There are other hypotheses to try to explain what is an ULX and more importantly why ULX are created in certain regions of galaxies and in certain galaxies and not in others.

ULX are more common in galactic centres and around other ULXs. ULXs are also found in compact dwarf galaxies.

The convention hypothesis is that ULX are either super large stars (low metallicity) that can some how radiate at super Eddington levels or that ULX are intermediate black holes that emit via a new type of accretion disc mechanism. The ULX emission is almost all non thermal which is not consistent with a hot accretion disc. (The BH ULX require a cool accretion disc were most of the disc energy is channelled into a jets.) ULX are not strong emitters at radio frequencies, however, which needs to be explained as large quasars with jets are strong radio emitters.

This lecture is a good introduction to the subject (the lecture is mislabelled as large BH, it is about ULX.) At the end of the lecture there is an interesting comment concerning the low metallicity and large stars.

http://video.google.com/videoplay?do...76285684758748


http://en.wikipedia.org/wiki/Ultraluminous_X-ray_source

The following an excerpt from this paper.

http://arxiv.org/abs/astro-ph/0501312