Planetary Magnetic Feild Inversion

LURCH

OK, I realize this may be more geophysics then "geology", but I beg for leniency. I have recently been exchanging e-mails with Dr. Daniel Lathrop of the University of Maryland. He is currently involved in a project attempting to model the earth's magnetic field and, in particular, polar inversions of that field. I have long held a suspicion that once the earth's magnetic field has destabilized beyond a certain critical threshold (when it is ready to flip), it is solar max that triggers that flip. His response made it clear that I did not adequately communicate the idea. I will soon be writing a follow up e-mail in an attempt to clarify, but in the meanwhile I had hopes that we might discuss the idea here.

For starters, have I grossly over estimated the force of the Suns magnetic field at this distance?

Ivan Seeking

I didn't think that the source of the earth's field was well understood. What do you think is the exact mechanism for this influence?

Zero

Doesn't magnetism work by the inverse-square law? If so, it would be easy to overestimate the effects of solar radiation, on account of the vast distance, and the tendency of radiation to dissipate rather quickly over distance.












Then again, I am the General board mentor..I may in fact have no idea what I am talking about.

Ivan Seeking

I think the local influence of the sun's field compared as to the earth's is about 1%. I can probably find this if no one knows for sure. This is mentioned in the context of tracking historic solar intensity - which it seems goes as the solar magnetic field strength - in the residual magnetic fields frozen in rocks.

LURCH

That's true, we don't understand it very well. That is Dr Lathrop's project; to model the field and find out how it's generated. He is currently working on models that assume a dynamo effect for the cause, partly because dynamoes are known to reverse polarity periodically.

His first models generated no magnetic field at all. So the other question I e-mailed to him was whether radiative decay was taken into account in the computer models. But I didn't state the thought clearly, and he responded that they didn't use radioactive materials in the laboratory model because of the strict regulations for handling such materials. The misscomunication was unfortunate, but it was encouraging to hear that they're being carefull!

zoobyshoe

Lurch,

I don't know what the "solar max"
is and would be interested in
finding out.

As for the earths magnetic field,
(you probably recall my question
about the magnetic field of stars
in the other thread) I was reading
about the Thompson Effect and this
struck me as a suspect that ought
to be brought in for questioning.
The Thompson Effect relies on the
voltage that is created whenever
there is a temperature different-
ial in a piece of metal.

To the extent there is a tempera-
ture differential between the
hot molten core of the earth and
the cooler outer skin, there
ought to be a voltage whereever
concuction of electricity can
occur.

I have no idea how much iron there
may be down there or if it would
be evenly distributed enough
around the interior, but it seems
to me this would be a good place
to look for the exiting voltage
of his self-exiting dynamo.

Please excuse me if he's already
thought of this.
Excuse me also for babling on
about it when I have no idea how
it may contribute to the flip of
the earth's magnetic field.

-zooby

LURCH

Hey Zooby-"zoot zoot!"

Some good thoughts there. Also brought to my attention that I've not provided any introduction to Dr Lathrop's work. He's got a "DYNAMO's" werbsite for anyone who'd like to follow along as I have been. His models, as you'll see, are mostly liquid sodium, but that should not make much difference in the answer to your queries. I don't see any detailed account of wether the Thompson Effect has been considered. I'll ask him in my next e-mail. I think I'll also ask him to join us here at the Forums, so he doesn't have to keep getting e-mails from total strangers (that's got to be kinda creepy).[8)]

As for Solar Max; it is the period of maximum Solar activity leading up to and during the inversion of the Sun's magnetic field , which occurs once every elleven years.

zoobyshoe

Lurch,

Glad you found that interesting.

For clarity's sake I want to say
that the voltage gradient created
by a temperature gradient in a
conductor is not, itself, called
the Thompson Effect. There doesn't
seem to be a name for this effect.

The temperature/voltage gradient
is a contributing factor to the
Thompson Effect.

The Thompson Effect is part of a
closely related trio of thermo-
electric effects: The Seebeck
Effect, The Peltier Effect, and
The Thompson Effect.

The unnamed sub effect of the
Thompson Effect is described in my
encyclopedia like this:

"That a temperature gradient pro-
duces an electric field can be ex-
plained as follows. Heating up one
end of the metal bar increases
the thermal velocities of the ele-
ctrons at that end. Consequently,
they migrate toward the cooler end, creating an electric imbal-
ance and therefore an electric field."

-zoob

P.S. Thanks for explaining the
solar max. You are saying the
sun's magnetic field flips every
11 years? That's intense.

I will check Dr. Lathrop's site.
Liquid sodium? Hot stuff.

dhris

Sorry if I'm reviving an old thread, but I just found this one from a link left by Lurch in the other forum. I am interested to know, Lurch, how you think the solar max would trigger a reversal? I'm not saying I think it's wrong, I'm just wondering what you think the mechanism would be for that?

In the last couple of years, there have been numerical simulations of the Earth's core (simulations of the coupled Navier-Stokes and induction equations) in which stable magnetic fields have been maintained by turbulent fluid flow. They have even observed some reversals without considering any outside influences. Reversals are actually a generic feature of chaotic generation of magnetic field, since the equations are invariant under reversal of the field polarity. Of course, no numerical model is even close to simulating the actual Earth since putting Earth-like parameters into the code leads to some serious numerical problems.

dhris

dhris

And as for Dr Lathrop's experiments, something like what he is proposing really should work if the leading theory (what I consider to be the leading theory anyway) of the Earth's field generation is correct. The problem so far is most likely that his devices are just too small (I'm not really familiar with his work though, so that's just a guess). You need a certain magnetic Reynolds number before dynamo action is even possible.

The magnetic Reynolds number is defined by R=vL/η, where v is the velocity scale, L is the size of the system, and η is the magnetic diffusivity. Either you need to attain really high fluid velocities, or the system needs to be large. I would be interested in seeing the Reynolds numbers he actually achieved for the smaller systems and what he expects for the 3m system but I can't find it anywhere on his site. Perhaps you could ask him?

dhris

zoobyshoe

dhris

What lies at the heart of the Lathrop experiment, then, seems to be the fact that "stable magnetic fields have been maintained by turbulent fluid flows", as you said, in "numerical simulations."

What concerns/confuses me is how the dynamics of the classic Faraday unipolar dynamo translate into this model of the earth as a dynamo.

(You created a link earlier to something you refered to as a "two disc" dynamo but my little web tv system is not capable of accessing that site.) Tesla, it seems, was briefly intrigued by the realization that a disc of charged segments rotated relative to any conductor would induce current in that conductor, and sketched out what he thought would be the best configuration for a dynamo working on this principle. The term "two disc dynamo" puts me in mind of this. (Tesla just wanted to document his thinking about this, I guess. He states he didn't think it was efficient enough to have any practical application.)

Tesla's electrostatic dynamo has all the elements of the Faraday dynamo: magnetic field cuts conductor which induces current giving rise to stronger magnetic field which cuts conductor, etc.

I'm confused about what is doing what in the liquid sodium models.

-Zooby

dhris

Well, that, and the fact that there are some theoretical results that show that turbulence with certain properties will be good at regenerating magnetic field. For example, turbulence that produces helical fluid motions, like what is seen in rotating convection, can be quite effective at regenerating field (probably the motivation for his propellers in that one experiment).

Well, the fluid plays the part of both the conducting disc AND the wire. Essentially, the effect of a conducting fluid on an magnetic field is to transport the magnetic field lines along with the fluid (a result known as Alfven's theorem). For example, in the limit of infinite conductivity, if an initial magnetic field vector connects two fluid elements at one time, it will connect the same two elements at all later times. This means that if the fluid elements move apart from each other (through straining or differential rotation), then the magnetic field vector connecting them will grow in magnitude, resulting in amplification. No fluid has infinite conductivity obviously, but the result is still approximately true when the conductivity is high. In turn, of course, stretching field lines like that means that the fluid has done some work, and so it loses energy (which is gained by the magnetic field).

Of course, in a turbulent situation, the fluid motion is unpredictable and it's hard to imagine any coherent amplification resulting. But it turns out that helical motions (essentially little tornados), even if they are somewhat randomly scattered throughout the core can produce an overall current in the fluid that leads to a large-scale field. The Earth is currently believed to be what's called an alpha-omega dynamo, which means that the primary sources of field regeneration are differential rotation and helical motions inside the core.

dhris

zoobyshoe

Why is this refered to as a
"theorem" rather than an "effect"? I don't understand this. What is "the limit of infinite conductivity" ?
I'm not sure what the word "straining" means in this context. This brings me to a big question I had that I haven't brought up yet. At the tempertures we're talking about, isn't the conductivity almost nil?

I did a quick search on alfvens theorem and found that this all comes under the heading of magnetohydrodynamics. You, dhris, are the only person I've noticed here speaking about it (course, I don't read every post). What is this all about, and what success have they had in generating magnetic fields in the way you described above (it's called "regenerating"?).

Thanks for your time and effort. It's very much clearer.

-Zooby

dhris

Well, I'm not too sure what relegates something to a mere "theorem" rather than an "effect", but I would assume in this case it's because there's no real physical effect being discovered. The only physics is the fact that moving a conductor in a magnetic field produces currents in the conductor. Alfven's theorem provides us with a convenient way to visualize the effects of fluid motion on the magnetic field lines.

Well, I'm basically just referring to the stretching of distances between fluid elements (something that is obviously not possible in a rigid conductor).

I just mean we take the conductivity to be infinite in the equations. Alfven's theorem is strictly true only in this regime, but it is still approximately true for finite conductivity.

No, the outer core has a fairly high conductivity, believed to be something like 10^5 (ohm-m)^-1, whereas saturated salt water at room temperature has a conductivity of around 20 (ohm-m)^-1.

[QUOTE] I did a quick search on alfvens theorem and found that this all comes under the heading of magnetohydrodynamics. You, dhris, are the only person I've noticed here speaking about it (course, I don't read every post). [\QUOTE]

Magnetohydrodynamics is essentially just fluid mechanics in the presence of a magnetic field, ie the Navier-Stokes and Maxwell's equations. Unfortunately the actual practice is not so simple. The resulting system of equations is nonlinear, resulting in chaotic behaviour, meaning that exact solutions are only possible after making some serious approximations.

Depends what you mean by "success". It has only recently become possible to solve the full equations on computer (1997 was the first real result I believe). These simulations feature turbulent convective flow, and stable magnetic fields (I believe one code actually has reversals). However, even in those simulations, it is still not possible to use Earth-like parameters.

It is called "regenerating" because that's what dynamos do. They amplify and reinforce an already existing field. No external source of field is necessary (except for whatever caused the initial field). The question becomes one of determining what types of fluid flow are most advantageous for reinforcing the field, and how these flows are obtained.

Of course, as you and others have pointed out, there is quite a lot we don't know about planetary magnetic fields. What I have described is what seems to be considered the most likely candidate for the presence of our magnetic field. I'm sure there are many scientists with different ideas. I've even heard of one guy who thinks the field is maintained by the fluid motion in the oceans, and not the core at all!

dhris

LURCH

I asked, and he sent back this reply:
Apearently he has the 3m working already. What is the difference between the Re and the Rm?

dhris

Interesting. Thanks for asking him. I would like to see what the results are for the 3m system. Hopefully he puts it on his site soon!
They are both dimensionless measures of the velocity. But Re is a measure of the velocity compared to the viscosity and is relevant for determining if the flow is turbulent. The magnetic Reynolds number measures the velocity compared to the magnetic diffusivity (related to the rate of decay of magnetic field) and is relevant for determining the possibility of dynamo action.

dhris

LURCH

Well, great minds think alike (and coincidentally, so do ours [6)] ); I e-mailed him to ask for those results just before coming to the Forums today.