In November 2013 the European Space Agency (ESA) sent three satellites into space, which have since been making precise measurements of Earth’s magnetic field. For this continues to hold scientific mysteries: as an example, the causal mechanism for the magnetic field reversals remains unclear to this day. One possible mechanism has now been identified by ETH scientists Andrew Jackson and Andrey Sheyko together with Chris Finlay of the Technical University of Denmark, based on simulations performed on the ‘Piz Daint’ supercomputer. Their results were published in the scientific journal ‘Nature’.
Simulations, seismic measurements and the physical properties of minerals – from those we know the composition of Earth’s deep interior – have so far been the only avenues for researching the emergence of our planet’s magnetic field. Because the magnetism acts as a shield against cosmic radiation as well as a navigation system for birds and other animal species, this makes a particularly interesting research topic.
Earth’s molten core in motion
According to present-day knowledge, the so-called ‘geodynamo’ is very likely run by processes taking place in Earth’s inner and outer core. Whereas the inner core is solid and composed mostly of iron and nickel, the outer core is liquid and contains lighter elements as well. The liquid is so hot that the metals in it are no longer magnetic, although they are still able to conduct electricity and heat. Because heavier elements in the outer core tend to sink inward and solidify there, the lighter elements are forced upward.
This process, together with temperature differentials at the boundaries between the inner and outer core and the core and the Earth’s mantle, is thought to cause circulating convection currents in the molten core. These are simultaneously affected by a Coriolis force resulting from the Earth’s rotation. The Coriolis force produces eddies in the molten metal that run perpendicular to the convection currents, which themselves move parallel to Earth’s rotational axis. This induces an electrical current that ultimately gives rise to a dipole magnetic field (north and south poles) as well as weaker, multipolar components.
Two material properties are key to the magnetic field and its reversal: One of these is the viscosity of Earth’s core, which determines how rapidly the core convection currents come to rest. The other is electrical conductivity, which determines the magnetic field’s rate of decay. In previous simulations, both these factors took place at the same tempo.
“In our simulation, we let the magnetic field decay twenty times faster than the convection currents in the molten core”, says ETH professor Andrew Jackson, a co-author of the study. In this way the scientists reduced the value of the coefficient setting the ratio between the two material properties, and approached Earth-like conditions more closely than in earlier simulations.