The movements within Earth’s molten core have been resisting efforts to understand them for a long time, but agreement between a mathematical and physical model has led two physicists to claim they are getting close. Since these motions are the driver of the Earth’s magnetic field, understanding them and why the field changes could turn out to be very important.
The Earth’s magnetic field is considered vital for life, or at least any life more advanced than bacteria. Similar fields are one of the major things we look for when seeking habitable planets. Yet we do not understand it well at all. Most physicists do not consider current explanations of why the magnetic poles wander, and sometimes flip, to be satisfactory, which of course means predictions have little value. Although debate remains about whether a reversal in the field’s direction would be as disastrous as some fear, it would certainly be nice to get some warning.
Since the field is driven by movements within the core, similar to the convection in a pot of boiling water, we need to understand these flows. However, the flows themselves are influenced by the magnetic field they have created, as well as by the Earth’s rotation and internal structure, challenging our capacity to understand what is going on.
One step forward was the concept of a tangent cylinder, but even this has not been sufficient to model movements accurately. Professor Alban Pothérat and Dr Kélig Aujogue of Coventry University think recognizing a previously overlooked feature of tangent cylinders could be the step we need. A physical model of the Earth’s layers gives them confidence they are right.
A schematic of the inner Earth, with the inner and outer cores marked in circles, the imaginary tangent cylinder, and the flows within the outer field, both into and within the cylinder.
Image credit: Pothérat et al./APS 2024
The tangent cylinder is an imaginary structure that runs parallel to the axis around which Earth spins, and touches the points where the solid inner and liquid outer core meet at the equator. Consequently, it separates the polar and the equatorial parts of the outer core, shaping the flows of materials within each.
Models of the Earth’s tangent cylinder and those of other planets relied on the Taylor-Proudman Constraint, which posits that in a rotating core flows occur perpendicular to the axis of rotation, making a series of concentric cylinders. In this view, convection occurs in the polar section of the liquid core, and in its equatorial portion, but does not cross between.
Pothérat and Aujogue write, “Here we establish and verify experimentally that magnetic fields aligned with rotation incur flows into [tangent cylinders]” and these are linked to flows along the cylinder by magnetic fields.
The Little Earth Experiment as seen from above, showing the concentric cylinider flows.
Image credit: Pothérat et al./APS 2024
The authors mathematically modeled the interactions between the Lorentz and Coriolis forces. They then used the Little Earth Experiment (LEE) to test their conclusions. The LEE uses a hemisphere filled with sulfuric acid and a magnetic field of 10 Teslas rotating around a vertical axis. This is then placed in a cylindrical tank of cold water while a heating element drives convection. Observations from the LEE were consistent with the author’s expectations.
The study is published in Physical Review Letters.
