Earth’s geomagnetic field plays a key navigational role in everything from smartphones to satellites. But predicting the behavior of the geodynamo that produces the planet’s magnetic field is complicated by a phenomenon known as a geomagnetic jerk: an abrupt change in the core that unpredictably accelerates the evolution of Earth’s magnetic field.
A new model that simulates conditions in the planet’s molten core is shedding some much needed light on the events leading up to a geomagnetic jerk, a breakthrough that may lead to more accurate magnetic field projections.
“One of the main problems when trying to forecast the evolution of the geomagnetic field on timescales from a few years to a few decades is the occurrence of geomagnetic jerks,” says Julien Aubert, a fluid dynamicist at the Paris Institute of Earth Physics and lead author of the new study, published in Nature Geoscience.
First identified in 1978, geomagnetic jerks seem to happen every few years; the last major event was in 2016.
“Satellites give us a clear picture of when these jerks occur, but we don’t yet know why they happen or how to predict them,” Aubert says.
The Earth’s magnetic field is generated through convection in its molten iron core as it cools over geologic time. The core is in constant flux as buoyant blobs of hotter material rise and cooler zones sink. This convective motion produces variations in the magnetic field on century-long timescales.
Jerks also cause changes in the magnetic field but on much shorter timescales, Aubert says. Allowing for these differing timescales was one of the main challenges in designing the model.
“Numerical simulations don’t like when they have to deal with a large disparity in timescales. Rendering the slow convective motions alongside the jerks was a challenge,” Aubert says. “Our simulation is the first of its kind that can properly capture both the slow and fast dynamics of the core.”
The new simulations revealed the conditions leading up to a jerklike event: In the core, blobs of molten material rapidly mobilize, moving outward through the layers toward the outer core. These buoyancy events radiate powerful magnetic waves that travel along magnetic field lines outward, rapidly altering the flow at the surface of the core and producing the geomagnetic jerk.
“This series of events had been hypothesized, but this is the first time that we were able to reproduce it with a physics-based model,” Aubert says.
“This model is a great step forward in being able to predict how and when these events may occur by giving us a clearer idea of the physics and behaviors in the Earth’s core that might generate a jerk,” says William Brown, a geophysicist at the British Geological Survey in Edinburgh, U.K., who was not involved in the new study. More work will need to be done to refine the models and integrate the simulations with field observations to actually make predictions about Earth’s magnetic field, “but this study represents the biggest leap forward we’ve had in some time.”
—Mary Caperton Morton (@theblondecoyote), Science Writer
Morton, M. C. (2019), New model shines spotlight on geomagnetic jerks, Eos, 100, https://doi.org/10.1029/2019EO122443. Published on 29 April 2019.
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