A mechanically driven dynamo?

Delegates at the SEDI 2016 meeting - a prize to anyone who spots me...

Delegates at the SEDI 2016 meeting – a prize to anyone who spots me…

 

All the standard text books say that Earth’s magnetic field is driven by convection. But what if it isn’t? I recently attended the SEDI (Studies of Earth’s Deep Interior) meeting in Nantes, France and was impressed by the work of Benjamin Favier and others looking into possible alternatives to the standard model of providing the power needed to drive the electric currents in Earth’s Outer Core that are responsible for our planetary magnetic field. These alternatives fall under the umbrella of “mechanical driving processes” and, as far as my limited understanding in this highly specialised field goes, the process can most easily thought of as “stirring” the fluid the outer core much as you would tea in a cup by doing work against resisting forces. However, clearly there is no tea-spoon involved. Rather, at the risk of pushing the analogy too far, the cup is being jiggled from the outside and it is the fluid’s interactions with the “container” (in this case the rest of Earth) as it sloshes about that is producing the flows.

So, what is doing the jiggling? We have already known the answer to this for a long time: interaction between the gravitational fields of Earth, the sun and the Moon cause variations in Earth’s shape and rotation. Because of the core’s elliptical shape, these variations are efficient means of converting gravitational energy into mechanical work (flows in the liquid).

The talk by Benjamin Favier (page 34 of this document) specifically focused on tidal forcing and on librations (small oscillations in Earth’s orbit). In it, he showed that these processes are very efficient at producing small scale flows within the outer core because of its very low viscosity and very high rotation rate. Together, he argued, they could easily account for the magnetic field we see today.  So, do they? After all, to paraphrase a comment made after the talk – “We KNOW the Earth is subject to such forcing, but we don’t KNOW  that the core is convecting”. Well, the jury is still a long way out. Benjamin confidently predicted the first fully self-consistent numerical mechanically forced dyanmos within a few years. The real challenge then will be to look at the magnetic behaviour they produce and compare it to that observed in the real Earth. The “Mechanical Forcing Brigade” (my name) face something of an uphill battle for the recent Earth because the convection-driven numerical dynamos are already doing a remarkably good job at replicating observed behaviour. My interest lies much further back in time. Before the inner core nucleated, it was much harder to generate buoyancy driven flow in the outer core; perhaps mechanical forcing had a (primary? Sole?) role to play in generating the magnetic field back then? Only comparisons between the next generation of dynamo simulations and newly updated palaeomagnetic records will tell us. I look forward to being part of that…

 

About AndyB

Andy Biggin is a researcher interested in the behaviour of the ancient geomagnetic field and what this can tell us about the Earth's interior. He has been based at the the University of Liverpool's Geomagnetism Lab since 2009.
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