Transport properties in the Earth's core

The Earth's magnetic field is one of the key essentials that allow life on Earth in the form we known it, with its active protection against the radiation coming from the Sun as highly energetic charged particles. Indeed, the field itself can be affected by the Sun's activity, with effects extending from spectacular aurora borealis to problems on tele-communications, navigation systems, the operation of artificial satellites, and even extended black-outs. Variations of the Earth's magnetic field are common; since the first measurement by Gauss in 1835 it has shown a relative 5% intensity decay in the past 150 years. On longer time scales of 250,000 years on average the field shows large instabilities that lead to reversals of the North-South orientation of the axis of its main dipolar component. Geomagnetic excursions, characterised by significant changes in the field strength felt at the surface of the Earth by up to 20 % and only a partial temporary (with times scales of thousand or tens of thousand years) re-orientation of the dipolar axis, are also common in the Earth's history, and are thought to be due to transfer of magnetic energy form dipolar to higher order multipolar components of the field, which decay more rapidly with distance from the Earth's core. It was proposed that excursions are due to reversals in the liquid outer core, but not in the solid inner core, which acts as a stabiliser for the field preventing full reversal. Back in 1905 Albert Einstein hailed the origin of the Earth's magnetic field as being one of the greatest unsolved problems facing modern physicists. A century later this is probably still true, with the modelling of the geo-dynamo being one of the most challenging problems in the geo-sciences. Although originally thought to be due to permanently magnetised rocks in the Earth's crust, it is now widely accepted that the geo-magnetic field, present for most of the Earth's history, is actually generated dynamically in the Earth's liquid outer core, thanks to convective motions that couple to the Earth's rotation due to Coriolis forces, and arrange themselves mainly in North-South columns, aligned with the Earth rotation axis. The theory used to describe the behaviour of magnetic field generated by a rotating electrically conductive fluid is called dynamo theory or the geodynamo if referred to the Earth. A fundamental ingredient for a dynamo to work is electricity, or in other words the metallic nature of the core: spiralling metal in a magnetic field generates electric currents, which in turn generate another magnetic field. When this generated magnetic field reinforces the original one, a dynamo which sustains itself is created. In its general formulation, geodynamo theory involves the solution of at least five non-linear equations. These equations can be solved numerically, once the parameters entering their definitions are known. In these equations, and in particular the Ohm's law in a moving conductor and the induction equation, we find the electrical conductivity, while in the heat transport equation we find the thermal conductivity. The latter is particularly important to estimate the amount of heat carried along the core adiabat, as this heat is a base-line that needs to be supplied before anything else can be available to drive the geodynamo. These two conductivities also happen to be among the poorest understood of all the core parameters, being uncertain by factors of 2 or 3. Even a modest change of a factor of 2 over currently accepted values would have a dramatic effect on the heat conducted down the adiabatic gradient, changing many published scenarios for the thermal history of the Earth and its magnetic field. Here we plan to calculate the thermal and the electrical conductivity with accurate quantum mechanics methods, and assess the impact of the new estimates on the interpretation of existing and future geodynamo simulations and models for core evolution.