Dynamo Redux
Bada BUMP, bada BUMP, bada BUMP BUMP BUMP
You must forgive my slow nature. Threads & posts move far faster around here than I can. Does anyone out there have a life?
Moving right along ...
All interesting but unless I'm mistaken there are no electric field measurements. There are some calculations. It seems as though they are so sure sure of their theory that there are no sensors for E fields!! ...
I don't actually know if they measure electric fields directly, but your complaint is irrelevant to the point. They call it
electromagnetism for a reason; you don't get
electro- without
magneto- and vice-versa, and they are not independent of one another. So, unless you are now prepared to claim that Maxwell's classical electromagnetism is wrong, there is no need to measure electric fields, because they will reveal themselves in the magnetic field measurements.
The exception of course is that a
static electric field will not generate a magnetic field. However, if there is a static electric field, it will induce an electric current in the conducting medium of the experiment. And that means the device will begin generating magnetic fields, as all electric currents do. And that means that the device will begin working before it is even turned on. I think in this case we can give our experimentalist colleagues the benefit of the doubt: They will probably notice that the device has started working without waiting for them to hit the "on" switch.
And we know moving electrons (in the sodium) are responsible for the magnetic field observed after the seed field is removed.
Electrons in the conduction band. They are moved mechanically, not with an applied electric field. But their motion results in an induced magnetic field, and that of course is the whole point. The conduction band electrons
by themselves constitute a flow of negatively charged particles, a classical electric current. But all of those conduction band electrons are matched by protons in the sodium nuclei, so that any macroscopic volume of the fluid will be charge neutral, carrying the same number of positive protons as negative electrons. So looking from the outside we don't see a classical electric current, but rather the flow of an electrically conducting, charge neutral fluid.
Notice how their experiments involved vertically aligned columns? And helical flow....
Not all of them, but so what? It is not the goal of the experiment to simulate the specifics of a given astrophysical or geophysical system, but rather to verify experimentally that the theoretical expectations are physically valid. That was in fact done as the experimental results are consistent with theoretical predictions.
Here are some simulations. How do they compare?
The Glatzmaier-Roberts model.
ttp://www.es.ucsc.edu/~glatz/geodynamo.html
In principle, they compare nicely. Both the cylindrical geometry experiments and the spherical geometry simulations by Glatzmaier & Roberts (
Glatzmaier & Roberts, 1995;
Roberts & Glatzmaier, 2000) rely on the same theoretical principle: A flowing electrically conducting fluid will generate a magnetic field via the dynamo principle. However, the
Nonlinear Dynamics Laboratory at the University of Maryland has several spherical geometry dynamo experiments designed to simulate the geodynamo, and I know the results have been compared to the Glatzmaier & Roberts models (i.e.,
Shew, et al., 2001), but I don't know the current status of the comparison. That's a question of interest, so I will follow up on that.
I doubt that the magnetic actually follows the simulated field lines. They way they bend??.. Unless your saying that the gradient follows the vector in some fashion?? No. You need a coherent current flow to produce the observed field.
Actually, no, you don't need a coherent flow. Actually, it has been known for a long time now that coherent flows will not produce a self sustaining dynamo (
Cowling, 1933 proved that no axially symmetric current flow can sustain an axially symmetric magnetic field by dynamo action). Incoherent & turbulent flow is what you want, because that guarantees a dynamo. And yes, the predicted & observed magnetic fields agree.
Both the solenoid diagram and the simulations are correct. Your assumption that they actually have something to do with each other is the wrong part. The solenoid generates a truly axial field, but Earth's magnetic field is far from truly axial; it is bent, and has numerous large & small scale deformations that are typical of incoherent flow in the dynamo. The geomagnetic field does not look like the solenoidal field except as a crude, first approximation.
Do you see what kind of current flow makes the kind of magnetic field that is in the sun and earth? You would have to have a strictly toroidal liquid metal flow to achieve the kind of field they are looking for.
Quite wrong, a strictly toroidal flow will never do. Aside from violating Cowling's Theorem, it would produce a strictly axial field, which would not match the observed field for either the Sun or Earth (or any other celestial body for that matter).
Now if you had an electric current flowing under the crust along the equator, say in the metal layer of the earth, then you would have your magnetic field of the correct shape and the poles would be able to float around as observed.
Entirely wrong, and unphysical to boot. If you had such a current you would get a magnetic field remarkably different from the Earth's observed magnetic field. For one thing, the axis of the axial field generated by that current flow would align with the rotational axis of Earth, which is very much not the case for Earth's magnetic field, which is not even really axial at all. Furthermore, such a current flow could never duplicate the myriad large & small scale time variable structure in Earth's magnetic field.
But the whole idea is
unphysical, which means such a current simply cannot flow at all without doing grave injustice to the various laws of physics. Consider, for instance that we are used to electric currents that flow in wires. You have to remember that the wire constrains the current. Why would any current, unconstrained by the boundaries of wires, flow along nicely in a line under the equator (or any other confined path)? Such a current would be unconstrained and would spread out widely. And besides, there is no "metal layer" between the crust & mantle. The only "metal layer" will be the liquid outer core, which is a good deal deeper into Earth than that, between the mantle and Earth's solid inner core.
But we can so ask: Why would such a current flow at all? Current don't just flow in order to have something to do. They flow because they are pushed either by mechanical forces or an electromagnetic field. What is the force that would push your equatorial current along? Would it be an equatorial electric field? And where would that come from? Electric fields usually come from charge separation, but charge separation usually comes from electric fields, which come from charge separation, which comes from electric fields ... and so forth. I trust you see the problem here? That's why I say that your current is
unphysical.
The beauty of dynamo theory is that it will give you a magnetic field without charge separation or classical electric currents. An even greater beauty of dynamo theory is that it is not "just a theory" as some like to say (not me I might add), but a matter of real physics in the real world.
