By user52469


2017-12-09 08:04:27 8 Comments

From my knowledge of magnetism, if a magnet is heated to a certain temperature, it loses its ability to generate a magnetic field. If this is indeed the case, then why does the Earth's core, which is at a whopping 6000 °C — as hot as the sun's surface, generate a strong magnetic field?

2 comments

@leftaroundabout 2017-12-09 11:40:42

The crucial part is that earth's outer core is fluid, and that it's conductive. That the material happens to be iron which we know as ferromagnetic is actually rather unimportant, because the geomagnetic field is not created as a superposition of atomic spins like in a permanent magnet. Rather, it's generated via Ampère's law from macroscopic currents, like in an electromagnet made from a wound copper coil with a current going though it. (It is an electromagnet, really.)

The reason there are such currents is that the liquid is in convective motion, probably fuelled by thermal transport from the radioactive decay in the inner core. When a conducting liquid moves, it “pulls any magnetic field line with it”. Starting from a small background field, if the flow is complex and rapid enough, this tends to amplify over time.

This dynamo effect can be described theoretically, numerically, or with laboratory-scale experiments using liquid sodium (sodium being nonmagnetic, but a good conductor and easy to melt). It is not hindered by high temperatures (rather, the high temperatures are often necessary to ensure flow and/or conductance). And it takes place not only on Earth, but also on many other objects:

  • The Sun's dynamo uses the plasma (hydrogen/helium), i.e. the fluid is not a metal at all, nor a liquid, but an ionised gas. This is again driven by convection.
  • The gas giants Jupiter and Saturn have dynamos that apparently consist of hydrogen too, but because of the comparatively low temperatures but still immense pressure it's likely in a metallic state.
  • The ice giants Neptune and Uranus have unusually tilted and irregular magnetic fields. It is assumed that this is due to a dynamo not in the core region like for earth, but in a shell rather higher up in the planet's structure. It probably consists of a mixture of hot, pressurised, liquid water, ammonia and methane, which has enough dissolved ions to be a good conductor.
  • Rocky planets and moons often have earth-like dynamos, most notably Mercury and Ganymede.

Remanent magnetism, the kind that we know from permanent magnets and which only works below the Curie temperature, is only important on rocky planets which don't have a dynamo. The most prominent example is Mars, but this magnetic field is much weaker than the aforementioned dynamo-generated ones.

@Alchimista 2017-12-09 12:37:26

Where are the moving charges that original induced a magnetic field come from? It is supposed/accepted that this steam from ions in the mantle?

@leftaroundabout 2017-12-09 13:05:22

@Alchimista I don't understand your question.

@Alchimista 2017-12-09 13:13:17

Molten iron is a neutral conductor. For a sustained dynamo there should be at charges moving to induced a magnetic field at first. Then a moving conductor in that field will have induced current. This induces a magnetic field and so on. ....but there should be a start at first. Cannot be the magnetic properties of iron as T are too high.

@Alchimista 2017-12-09 13:14:32

Using your words, what was the cause for a " small background field" ?

@leftaroundabout 2017-12-09 13:21:54

@Alchimista ah. Well, I don't think it's known what the original background field was. In doubt it could just be the Sun's. ...Which may seem to kick the stone down the road – but really it doesn't, because in a plasma, the mass difference between electrons and ions means magnetic fields can arise purely from kinetics, you just need sufficiently violent movement. Mostly though, the point is that you don't need to start out with any noteworthy field, because the positive feedback will blow it up exponentially up to a certain strength.

@Alchimista 2017-12-09 13:24:04

Ok . I take it as you said that field could have been very "small ". Might really be that of Sun.......

@Džuris 2017-12-09 16:57:04

@Alchimista It is widely believed that almost any flow of a conducting liquid will give rise to self-excitation of a magnetic field provided that, first, the flow topology is not too simple (e.g. a flow in one direction or a purely rotational flow) and, second, the so-called magnetic Reynolds number is large enough. Here is a review article in quite simple language by the authors of the first "spin metal until magnetic field appears" experiment.

@thermomagnetic condensed boson 2017-12-09 20:40:35

"That the material happens to be iron which we know as ferromagnetic is actually rather unimportant", I would even say that liquid iron is paramagnetic, it's not even ferromagnetic.

@leftaroundabout 2017-12-09 21:20:39

@no_choice99 sure. It can't possibly be ferromagnetic above the Curie temperature (which is of course lower than the melting point), as the OP already stated.

@PlasmaHH 2017-12-10 19:35:34

It would be so much cooler if this effect was caused by the gold core of the earth...

@potential-at-infinity 2017-12-09 09:11:18

The core of the Earth isn't a giant bar magnet in the sense that the underlying principles are different. A bar magnet gets its magnetic field from ferromagnetism while Earth's magnetic field is due to the presence of electric currents in the core.

Since the temperature of the core is so hot, the metal atoms are unable to hold on to their electrons and hence are in the form of ions. These ions and electrons are in motion in the core which forms current loops. The individual currents produce magnetic fields which add up to form the magnetic field around the Earth.

@Pieter 2017-12-09 10:46:41

It is just molten iron. No need to talk about "unable to hold on to their electrons" - that is true in any piece of iron.

@leftaroundabout 2017-12-09 11:06:29

Yes, you don't need that bit about ionisation: metals don't have a band gap, so the electrons can be lifted into the conductance band at arbitrarily low temperature. It's possible to operate a dynamo working like Earth's at temperatures much lower than 6000 K, you just need to make sure the metal is liquid (e.g. Sodium 98°C). doi.org/10.1103/PhysRevLett.98.044502

@leftaroundabout 2017-12-09 11:41:15

(Comment moved to separate answer)

@Ehouarn Perret 2017-12-10 06:30:24

I would not say it's molten iron, since the pressure is so freaking high in the inner core, it's mostly solid.

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