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I'm interested in getting a basic physical understanding of how Earth's magnetic field is generated. I understand that it's a "dynamo" type of effect, driven by convection currents in the molten outer core. These currents cause charges to move, and this generates the field.

However, what I can't find a good explanation of is why there is a separation of charges in the first place. Presumably, moving neutrally charged molten iron would have the same effect as moving any other neutrally charged thing, i.e. it wouldn't create a field. And presumably, if the fluid wasn't moving then it would become neutral pretty quickly, since molten iron is a good electrical conductor.

So am I right in thinking that the charge separation is the result of positive feedback, in that an intial deviation from neutrality would generate a field, and this would (somehow) cause a greater separation of charges, resulting in a kind of self-maintaining charge separation? Or is there another explanation?

In either case, does anyone know of a good resource that explains the basic principles in physical terms? I know that the interior dynamo is a very complex phenomenon, but I'd like something that gives a good physical picture of how the electromagnetic and fluid dynamical phenomena interact, rather than diving straight into partial differential equations.

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I join the question. I would also like to know what is the natural variability in the magnitude of the magnetic field in 100 years etc. Is it common that it decreases by 10% in 150 years, like in the recent 150 years? –  Luboš Motl May 31 '12 at 13:01
    
@Nathaniel I don't think that you need a charge seperation per se, a current should do something already. –  Bernhard May 31 '12 at 13:35
    
@Bernhard but the Earth has no net charge - so unless something causes charges to separate, there will be no current... –  Nathaniel May 31 '12 at 13:47
    
@Nathaniel Is there net charge in a wire carrying current? No, it is electrons moving around. It is the conductivity of the liquid that is an essential property –  Bernhard May 31 '12 at 13:49
    
@Bernhard my reasoning was as follows: the electrons have to move relative to the protons in the wire. If I just wave a disconnected, uncharged wire around it will not make a field no matter how fast I move it, because the field generated by the moving protons cancels the one from the moving electrons. So if the magnetic field is generated by a moving fluid then that fluid would need to be charged. But from John Rennie's answer it seems that actual currents do play a role - so now I need to understand what drives those currents. –  Nathaniel May 31 '12 at 15:53

2 Answers 2

up vote 3 down vote accepted

According to Wikipedia the magnetic field is indeed the result of feedback. Actually the Wikipedia article is very good so I'm not sure how much there is left to say. The convection currents from the inner core outwards get bent onto spirals by the coriolis effect of Earth's rotation, and this gives a geometry where the magnetic field and electric currents sustain each other.

Re Luboš' comment, I'd have a Google around the NASA web site as they have loads of data about pretty much everything to do with the Earth e.g. http://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ is an article aimed at the general public. There's bound to be raw data on the site somewhere.

The Wikipedia article mentions how hard it is to numerically model the magnetic field generation in the core. There have been a couple of really quite alarming experiments in the last decade trying to model the core. For example see http://www.nature.com/news/dynamo-maker-ready-to-roll-1.9582 - if 13 tons of liquid sodium isn't alarming I don't know what is :-) See http://physicsworld.com/cws/article/news/2007/mar/09/molten-sodium-mimics-earths-magnetic-field-flipping for an earlier experiment that claims to have modelled the field reversals.

For an excellent popular introduction to this see the BBC Horizon programme called "The Core". This is on YouTube, though I'm not sure that's an official upload so how long the programme will stay there I don't know.

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Thanks for this - I read the Wikipedia article a while ago and evidently missed or forgot about the part where it mentions the feedback cycle. But I'd really like to have a better physical understanding of how the magnetic field and the currents sustain one another. As an analogy, the atmosphere is extremely complex, but nevertheless one can start by understanding the notion of a convection cell, and then later learn about the Coriolis force, the effects of latent heat, etc. I'm really hoping for a similarly digestible explanation - but if none is forthcoming I'll accept this answer. –  Nathaniel Jun 1 '12 at 12:01
    
(Part of the problem may just be that I don't have all that much experience with electromagnetism, so talk of things like "frozen-in field lines" on the Wikipedia page all seems a bit mysterious to me.) –  Nathaniel Jun 1 '12 at 12:01
    
It might be a useful analogy to think of an alternator, as used in cars across the globe. Alternators contain no permanent magnets. They work because the electricity they generate is passed through the field coils to generate a magnetic field, and this in turn allows the rotor to generate the electricity. See Wikipedia or just Google for "alternator". I'd guess that the core will be much more complicated because you don't have well defined conductors for the current to flow in. –  John Rennie Jun 1 '12 at 17:07

Actually you don't need to split charges to have a movement of the fluid. To give you an idea, when you have a magnet and bring an iron thing next to it, it "forces" some items of the iron thing to become oriented in the same direction as the magnetic field. Then, it becomes itself a non-permanent magnet (it creates its own magnetic field; depending on the matter, the magnetic field will destroy the magnetic field inside of the thing or be oriented in the same direction). Both the magnet and the iron item remain (even locally) electrically neutral in this process.

Now in the center of the earth the situation is a bit different, since it is liquid. Electrons are free to move in the fluid (because Ni-Fe are not very careful of their outer electrons), but they remain anyway electrically neutral at any time (when an electron leaves his Iron atom, another one takes its place and so on). Thus we have a global mouvement of charges whithout need to split them.

This generates an electrical current that in return creates a magnetic field.

So am I right in thinking that the charge separation is the result of positive feedback, in that an intial deviation from neutrality would generate a field, and this would (somehow) cause a greater separation of charges, resulting in a kind of self-maintaining charge separation? Or is there another explanation?

The correct image would be that a small current creates a small magnetic field, which will have positive feedback on the movement of particles, and so on! But it's not so simple. Actually, that's an open question that interest both mathematicians and physicists (including myself ;) ), and the answer is not so simple to answer theoretically and most of the situations we can imagine lead to an exponentially decreasing magnetic field (but it's not what we observe in the earth, the sun and many other stellar object, so there exists other possibilities!).

Why is that not so simple? Because when an electron moves, it radiates some electromagnetic field, and part of it is "lost" (you can imagine that as the fact that some part of the electromagnetic field of the earth gets out of it, thus by conservation of energy the kernel loses some energy!). Thus we have positive feedback between the magnetic field and the mouvement of the fluid on the one hand, and "lost" due to radiations.

Now how did it begin? The kernel had initially his own mouvement, and we only needed a small pitch (which statistically happens for sure) to launch the full system gradually, and that's it, we're now in that!

If you want to know more on that subject, the keyword is Magnetohydrodynamics. The situation has been put in equations long ago, and there are some very good explanations (far better than my little blabla) on how it works on the internet.

To answer to Luboš Motl question

I would also like to know what is the natural variability in the magnitude of the magnetic field in 100 years etc. Is it common that it decreases by 10% in 150 years, like in the recent 150 years? –

It's even worse than that, actually we have "very often" a phenomena called geomagnetic reversal, that is that the north becomes south and south becomes north. Actually it didn't happen since we invented the compass, but we spotted this phenomena by the fact that some volcanic rock didn't have the same magnetic orientation (cf my first paragraph) when they formed along the ages. This phenomena is also very hard to predict theoretically, although we managed to see it in experiments in the last decade. Watch out, it may happen for real in the next centuries :)

(By the way, it happens faaar more often in the sun - about every ten years -, and this was already observed for long)

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