There doesn’t appear to be a mountain of material on the cause of the offset in the magnetic field of planetary bodies. I’d assume it is a feature of the fluid dynamics involved coupled with the EM laws involved. Whatever the reality, the cause of magnetic pole offset might have applications in engineering and seems like a relevant question to seek a comprehensive answer to. I wouldn’t know where to start looking with EM stuff, so just about any info is a start?

Riff-in-Thyme said:

There doesn’t appear to be a mountain of material on the cause of the offset in the magnetic field of planetary bodies. I’d assume it is a feature of the fluid dynamics involved coupled with the EM laws involved. Whatever the reality, the cause of magnetic pole offset might have applications in engineering and seems like a relevant question to seek a comprehensive answer to. I wouldn’t know where to start looking with EM stuff, so just about any info is a start?

I’m not completely sure what you are asking. By “planetary bodies” do you mean Earth or do you mean Uranus/Neptune? The difference there is that the magnetic offset for Uranus/Neptune is huge, whereas that for Earth is quite small.

I take it that you’re not talking about compass deviations, which is another matter entirely. I’ve just been reading the book “Compass” (can’t see it on the web right now) which covers all the successive attempts by the British to cope with the compass deviations, particularly those due to iron in ships.

As for dynamo action, I’ve never really understood how the feedback process works. There have been physical experiments at the University of Minnesota . There have been many computer models simulating for example the flipping of the magnetic poles.

For the Earth, magnetic fields other than the dipole field – the quadrupole, octopole, etc. fields play a large role in stabilizing the magnetic field during times of pole reversal.

For Uranus/Neptune, I think of it myself as being analogous to Jupiter’s big red spot. For both planets there is a circulation in a relatively thing spherical shell of conducting liquid. A large cyclonic circulation persisting in those convection regions would generate a dipole magnetic field perpendicular to the axis of circulation, but this axis could be considerably offset from the axis of the planet.

This link gives a kiddies introduction to the numerical simulation of geomagnetic reversal.
http://seismo.berkeley.edu/~rallen/eps122/lectures/L07.pdf

It didn’t help me to understand much, but it may help you.

On the other hand, I find the following link to be too technical.
http://www1.maths.leeds.ac.uk/~rh/PAPERS/1993pepi.pdf
Here the parameter ‘alpha’ is a parameterization of the small-scale non-axisymmetric flow responsible for the deviation between the magnetic pole and the rotation pole.

> the cause of magnetic pole offset might have applications in engineering

The engineering applications so far have been associated with “magnetohydrodynamics”.

For example, here is a Youtube of a boat powered by magnetohydrodynamics:
http://www.youtube.com/watch?v=h0guQCD3QUA

Other applications have been in spacecraft propulsion, plasma containment for fusion reaction (the Tokamak etc.) Have quick look through the Wikipedia article:
http://en.wikipedia.org/wiki/Magnetohydrodynamics

mollwollfumble said:

Riff-in-Thyme said:

I’m not completely sure what you are asking. By “planetary bodies” do you mean Earth or do you mean Uranus/Neptune? The difference there is that the magnetic offset for Uranus/Neptune is huge, whereas that for Earth is quite small.

Just mean any body that generates a magnetic field really. I’m trying to understand why the magnetic pole is offset from the axis of rotation. The offset found withUranus/Neptune only seems exaggerated by a greater liquid depth and convection speed.

mollwollfumble said:

> the cause of magnetic pole offset might have applications in engineering

The engineering applications so far have been associated with “magnetohydrodynamics”.

For example, here is a Youtube of a boat powered by magnetohydrodynamics:
http://www.youtube.com/watch?v=h0guQCD3QUA

Other applications have been in spacecraft propulsion, plasma containment for fusion reaction (the Tokamak etc.) Have quick look through the Wikipedia article:
http://en.wikipedia.org/wiki/Magnetohydrodynamics

I realise magnetohydrodynamics is already used. If polar offset is a rule, then it should be one that can be taken advantage of if understood.

Riff-in-Thyme said:

If polar offset is a rule, then it should be one that can be taken advantage of if understood.

Run it backwards and magnetic fields could be used to brake (or break) an electric motor. Many electric motors operate on the dynamo principle.

mollwollfumble said:

Riff-in-Thyme said:

If polar offset is a rule, then it should be one that can be taken advantage of if understood.

Run it backwards and magnetic fields could be used to brake (or break) an electric motor. Many electric motors operate on the dynamo principle.

But the geo-dynamo mechanism hasn’t been simulated?

mollwollfumble said:

Riff-in-Thyme said:

If polar offset is a rule, then it should be one that can be taken advantage of if understood.

Run it backwards and magnetic fields could be used to brake (or break) an electric motor. Many electric motors operate on the dynamo principle.

The offset of poles does suggest that some form of braking is involved. Would it be a feature promoted by the presence of a “solid” core in the process?

Riff-in-Thyme said:

But the geo-dynamo mechanism hasn’t been simulated?

It’s been simulated both numerically and physically. I’ve been trying to find the physical experiments on the web. Ah, got it, University of Maryland (not Minnesota).

News article:
http://unknownskywalker.tumblr.com/post/13887593892/dynamo-maker-ready-to-roll-two-3-metre-tall
The home page doesn’t say much:
http://complex.umd.edu/

So far as I can gather, the three-metre diameter spherical device has been run up to top speed and has been filled with liquid sodium, but has not yet generated a self-sustaining magnetic dynamo. I don’t know why. IIRC, smaller models did.

mollwollfumble said:

Riff-in-Thyme said:

But the geo-dynamo mechanism hasn’t been simulated?

It’s been simulated both numerically and physically. I’ve been trying to find the physical experiments on the web. Ah, got it, University of Maryland (not Minnesota).

News article:
http://unknownskywalker.tumblr.com/post/13887593892/dynamo-maker-ready-to-roll-two-3-metre-tall
The home page doesn’t say much:
http://complex.umd.edu/

So far as I can gather, the three-metre diameter spherical device has been run up to top speed and has been filled with liquid sodium, but has not yet generated a self-sustaining magnetic dynamo. I don’t know why. IIRC, smaller models did.

Succesfully simulated would be good. ;)

Is the size of that dynamo necessary? I would have thought something smaller would at least be easier to make adjustments to.

Riff-in-Thyme said:

Is the size of that dynamo necessary? I would have thought something smaller would at least be easier to make adjustments to.

I work with small-scale fluid dynamic experiments. Getting them to successfully match what is happening in full-scale can be difficult, and depends on both “boundary conditions” and “non-dimensional numbers”.

A list of the non-dimensional numbers applicable to fluid mechanics is given in http://www.ipplm.pl/bib/nrl/fluid.pdf
For magnetohydrodynamics it is necessary to get the ratios of magnetic force : inertial force : dissipative force : gravitational force : J x B force correct as much as possible, as well as ratios involving heat conduction rate : convective heat transport : radiative heat transport and buoyancy correct. This isn’t easy, and making the models bigger almost always helps.

Boundary conditions become less of a constraint as the models get bigger.

mollwollfumble said:

Riff-in-Thyme said:

Boundary conditions become less of a constraint as the models get bigger.

I guess that should have been obvious.

wasn’t there some experiment with a spherical chamber and “other stuff” to try to replicate the earths dynamo? sorta remember seeing something in a doco not that long ago.

Boris said:

wasn’t there some experiment with a spherical chamber and “other stuff” to try to replicate the earths dynamo? sorta remember seeing something in a doco not that long ago.

Probably the University of Maryland one that we’re talking about.

mollwollfumble said:

Boundary conditions become less of a constraint as the models get bigger.

Would it be logical to assume that exactly where each boundary is set becomes important to the result. For instance, it has been pointed out in other discussions that there is no known way to simulate mass. Mass may be involved with the acceleration difference between lightning and artificially generated arcs. Why would it not be important within the geo-dynamo mechanism? Having two points involved in a system with significantly different FoR’s might be kinda important?

Riff-in-Thyme said:

mollwollfumble said:

Boundary conditions become less of a constraint as the models get bigger.

Would it be logical to assume that exactly where each boundary is set becomes important to the result. For instance, it has been pointed out in other discussions that there is no known way to simulate mass. Mass may be involved with the acceleration difference between lightning and artificially generated arcs. Why would it not be important within the geo-dynamo mechanism? Having two points involved in a system with significantly different FoR’s might be kinda important?

Not sure I understand the question. For simulating the geo-dynamo natural convection is crucial. In the Earth that is driven by heat-induced density changes and gravity. In the model it is driven by heat-induced density changes and centripetal force, with gravity and unwanted extra. In order to have centripetal force in the model greatly exceeding the force of gravity it has to be spun fast, and this helps to accelerate natural convection as well. In even the best model the direction of centripetal force (towards the axis) is going to differ from the direction of gravity on the Earth (towards the centre).

mollwollfumble said:

Riff-in-Thyme said:

mollwollfumble said:

Boundary conditions become less of a constraint as the models get bigger.

Would it be logical to assume that exactly where each boundary is set becomes important to the result. For instance, it has been pointed out in other discussions that there is no known way to simulate mass. Mass may be involved with the acceleration difference between lightning and artificially generated arcs. Why would it not be important within the geo-dynamo mechanism? Having two points involved in a system with significantly different FoR’s might be kinda important?

Not sure I understand the question. For simulating the geo-dynamo natural convection is crucial. In the Earth that is driven by heat-induced density changes and gravity. In the model it is driven by heat-induced density changes and centripetal force, with gravity and unwanted extra. In order to have centripetal force in the model greatly exceeding the force of gravity it has to be spun fast, and this helps to accelerate natural convection as well. In even the best model the direction of centripetal force (towards the axis) is going to differ from the direction of gravity on the Earth (towards the centre).

You have provided a good example of what I am referring to. The balance obtained in the simulation cannot be considered a reproduction of the original. Also, the presence of bearing linkages that directly connect key components whose relative position is maintained in the reality without mechanical intrusion to the system seems to thwart separations that might be crucial to operation of the system.

One area the simulation might be way off is in the assumption that the EM field is produced entirely in the convection zone. On the scale of planetary magnetospheres and greater, why would the dramatic difference in particle stasis between the core and the convection zones not be a component of the dynamo here?

Riff-in-Thyme said:

One area the simulation might be way off is in the assumption that the EM field is produced entirely in the convection zone. On the scale of planetary magnetospheres and greater, why would the dramatic difference in particle stasis between the core and the convection zones not be a component of the dynamo here?

Most abherational is the idea that the effects of gravity can be simulated by centrifugal forces.