electromagnetism - How does Earth's interior dynamo work?

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.

Answer

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.