Where the Earth’s magnetic field comes from

The Earth’s magnetic field may approximate to a simple dipole, but explaining precisely how that dipole is generated and maintained is not simple at all. The field originates deep in the Earth, where temperatures are far too high for any material to maintain a permanent magnetisation; the dynamism that is apparent from the wandering of the magnetic poles with respect to the spin axis (secular variation), and the quasi-periodic flips in field polarity, also suggest that some process is actively generating and maintaining the geomagnetic field. Geophysicists therefore look to the most dynamic region in the planetary depths, the molten outer core, as the source of the force that directs our compass needles.

One of the more important processes going on in the core is the slow growth of the solid inner core at the expense of the outer core, as the centre of the Earth loses heat to the mantle, and ultimately the surface. As the iron crystallises, it releases latent heat, generating thermal buoyancy forces. Additionally, seismic measurements show that whilst the outer core is much less dense than experiments and calculations predict, this deficit is much smaller in the inner core; therefore crystallisation also leaves behind the lighter elements that make up around 10% of the outer core (it’s not entirely certain what these elements are, but sulphur is a popular candidate), generating strong compositional buoyancy forces. Together with a possible contribution from heat generated by radioactive decay, these buoyancy forces drive vigorous convection of the outer core, with flow rates of the order of 10 kilometres per year.

As you may dimly remember from school, the motion of a conductor in a magnetic field generates a current; currents also generate their own magnetic field. The iron in the outer core is a conductor, so its motion in the presence of the existing geomagnetic field induces electric currents, generating new magnetic flux, as well as forces that modify the pattern of convective flow. The key to a stable, self-sustaining geodynamo is that these processes feed back on each other, until the pattern of induced currents – both electrical and convective – generates a magnetic field that has the same, or almost the same, configuration as the input field, reinforcing and maintaining it over geological time. The field induces currents which generate the field. This is no free lunch: Ohm’s law dictates that resistance to the electrical current flow will cause energy to be lost from the system, so without the external input of energy from convection, within a thousand years or so the field would weaken and die.

You can get your head round this idea without developing too much of a headache, even if the ‘geodynamo’ looks nothing like any electrical generator that humans have ever imagined. However, matching this general account to the actual behaviour of the actual magnetic field of the Earth, in all its polar wandering, polarity reversing, glory – is not so easy, not least because many of the physical parameters that control convective flow, such as the viscosity of the outer core, are poorly constrained. Plus, magnetohydrodynamic modelling is as fiendishly complicated as its name suggests. So it was quite a surprise when in almost the first serious attempt, Glatzmaier and Roberts [1] managed to produce a model that not only produced a self-sustaining dipole field that looked a lot like the Earth’s, but also went through periodic spontaneous reversals (click the link to see some animations).

a spontaneous geomagnetic reversal in Glatzmaier and Roberts' model

This behaviour was rather unexpected, because the set-up of the model was governed by what was computationally feasible, meaning that it wasn’t particularly focussed on trying to match the real conditions in the Earth’s core (Glaztmaier and Roberts were aiming more at establishing the validity of the proposed dynamo mechanism in general). But it turned out that they had got one important boundary condition right: the Earth’s rotation, which has a strong influence on the patterns of convection in the outer core. Most significantly, it has a tendency to produce helical convection currents which align with the spin axis (the strong orange lines in the figure below show the spiralling motion that the rotation of the core superimposes on the larger-scale currents, represented by the thicker, fainter lines). This helical flow tangles up the magnetic flux within the core; flux lines (blue) only tend to escape the core at higher latitudes where the helical flows intersect with the core-mantle boundary, giving the field its overall dipolar shape.

schematic representation of convection within the outer core

Thus, it seems that to a first approximation, any spinning, convecting, conductive shell will tend to produce a magnetic field which broadly looks like the Earth’s. Some caution is still required, though; if you remember, the Earth’s field also looks like a bar magnet, so we already know that very different mechanisms can produce similar looking magnetic results. Thus, it’s still hard to say for sure how well this picture actually represents what’s going on in the Earth’s outer core; but this sort of model does give us a useful starting point to direct our studies. For example, these models suggest that one useful place to focus on might be at high latitudes, within the zone where the ‘tangent cylinder’ of the inner core intersects with the surface. If our models are correct, within the tangent cylinder the inner core acts as an impediment to convective flow, reducing the length scale of the circulating currents and increasing the overall variability of the local field. This is something that we can go and look for. Additionally, by comparing the geological record of field behaviour – the rate and character of reversals and secular variation over time – to the behaviour of different models run with different physical properties, we might be able to better constrain parameters like the viscosity and composition of the outer core. And once we properly understand the origin and behaviour of today’s magnetic field, we might have some idea of what is was like in the distant past. Is it reasonable to assume that the field has always been dipolar? That’s certainly a question I’d like to answer – although I think I’ll leave the 4th order differential equations to somebody else.

[1] G.A. Glatzmaier and P.H. Roberts, “A three-dimensional self-consistent computer simulation of a geomagnetic field reversal,” Nature, 377, 203-209 (1995).

Categories: basics, geology, geophysics, palaeomagic, paper reviews

Comments (9)

  1. Kim says:

    Nice explanation – I’ve always had trouble twisting my mind around studies of the magnetic field, but you explained that very nicely. Those animations are great, too – perfect for showing someone what a magnetic field reversal would look like.
    (BTW, I’ve run across a lot of non-geologists who think a magnetic pole reversal involves the Earth physically flipping over, with devastating consequences for everything on the planet. That animation would be a good way to illustrate what’s more likely going on.)

  2. Cherish says:

    Oooh! My favorite! (Literally…this is the stuff I find most compelling about geosciences.) Unfortunately it appears that Glatzmeier has decided to move onto other things with dynamos, leaving the rest of us to figure it out.
    Kim…are you serious?! People really think that?!

  3. Chris,
    I wonder sometimes about the limits of polar wander. What are the presumed limits on how far from the planet’s rotational axis that the magnetic poles can wander? The animation makes it look almost like the “blue pole” crosses the equator, but that doesn’t square with the sense of (relatively minimal) polar wander that most geologists assume.
    I know Joe Kirshvink (Caltech) has suggested some crazy ideas about “true” polar wander… What’s your sense of it, as a paleomagnetician?

  4. Daniel C. Smith says:

    It is hard for me to picture the feedback mechanism that reinforces the magnetic field. Lenz’s law suggests (to me, at least) that it should be a negative feedback, i.e. it should oppose the existing field. What am I missing? Is the answer in how the induced current affects the convection?

  5. Lab Lemming says:

    At some point we should have a geoblogospheric shitfight about when the inner core first crystallized.

  6. Chris Rowan says:

    Kim – if that is so, maybe I should add something to my ‘misconceptions in geology’ list!
    Callan – the “true polar wander” that Joe Kirschvink is discussing is actually related to plate motions rather than the behaviour of the magnetic field – it’s a proposed motion of the entire lithosphere (all the plates) relative to the spin axis, caused by the Earth trying to minimise its moment of inertia (e.g., when high latitude supercontinents cause a mass excess). It’s very hard to spot, if it has happened at all – possibly a worthy subject for a future post?
    It terms of the behaviour of the magnetic field, secular variation can cause the poles to wander some distance (10 degrees or more) away from the poles. There are also the periodic excursions and reversals, where the dipole field weakens and the pole wanders to, and across, the equator (it’s also possible that during these periods that there is more than one “north/south pole” randomly dotted about on the Earth’s surface – you can actually see that in the middle panel of the reversal sequence above). However, all these variations take place over (geologically) rapid timescales – even a full reversal only takes about 4,000 years – and all secular variation averages out to a dipole centred on the spin axis over periods of longer that 5-10,000 years. So if you want to use palaeomag to reconstruct plate motions, these variations are relatively unimportant.
    Daniel – the important thing the realise is that external energy is being fed in to the system to maintain convection – without that, the whole system would collapse.

  7. William says:

    Okay, convection is driven by the various energy releases from stored potential as it cools, so the field strength stays roughly the same since energy loss to the field is balanced by new energy entering.
    Question: how’d it get started? Moving conductors in the absence of a magnetic field doesn’t set up a net current anywhere. Would the motion of tiny random magnetic domains be enough to kickstart this, or do we figure Earth had a magnetic moment inherited as it coalesced from the primordial solar cloud?
    Second question, possibly related: where, exactly, are the currents? The magnetic field is being produced by induced currents, right? So where in Earth’s core could I stick the two probes of an ammeter and measure a movement of net charge?

  8. Joao says:

    Thank you for a wonderful post. I had always wondered how the poles reversed since I first heard of it in highschool (like 20+ years).
    Do you know if these changes in earth’s magnetic field have any implication in the migration of animals (birds, for example)?

  9. Chris Rowan says:

    William: the most obvious source of stray magnetic flux to start the whole thing going would be the sun – or possibly ionized gas clouds in the early planetary nebula.
    I’m not sure I quite understand the question about currents – but (if you could do so) probes placed at any distance apart in the outer core would probably register some sort of current.
    Joao: that’s a good question. Quite how birds and other creatures with magnetoreceptors actually use them is not entirely clear. I’d guess that its as an aid to visual cues (such as the position of the sun). This, plus the slow change in the field from their perspective (it would take several dozen generations for a reversal to happen) may allow them to adapt.