A simulation of the Earth’s magnetic field
Image: Aubert et al./IPGP/CNRS Photo library

Every several years, magnetic field observatories record quick changes to the position or strength of the planet’s magnetic field, so-called geomagnetic jerks. The cause of these shifts has remained a mystery.

Improvements to ground-based observatories and more magnetic field measurements from satellites have given scientists new insights into the planet’s magnetic behavior. Though we can’t look directly into Earth’s core, these measurements have let scientists build models to help infer what might be going on in there. Most recently, scientists realized that that suddenly released, moving blobs inside the core could explain the geomagnetic jerks.

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“The ability to numerically reproduce jerks offers a new way to probe the physical properties of Earth’s deep interior,” researchers Julien Aubert  from the University of Paris and Christopher Finlay  from the Technical University of Denmark write in a new paper, published in Nature Geoscience.

Earth scientists are pretty sure that the planet generates its own magnetic field thanks to the geodynamo: heat-generated motion of conductive liquid metal inside the core, combined with the planet’s spin. Changes are generally slow—like being on a fancy new train, where you barely perceive the forces of speeding up or slowing down. But sometimes the magnetic field experiences “jerks,” where it unpredictably goes from speeding up to slowing down, similar to the whiplash-inducing jolts you might feel on a rickety old train.

Simulation of matter traveling through the core, potentially causing geomagnetic jerks. (Video: Aubert and Finlay, Nature Geoscience (2018))

These “jerks” occur over the course of just a few years, and have appeared in 2007, 2011, and 2014, according to observed data. To better understand this, the researchers built a simulation of Earth, in which the outer core is represented as a rotating spherical shell filled with an electrically conducting liquid, and the inner core and mantle as solids sandwiching the liquid later. It’s like a spinning jawbreaker with a layer of goo between the center and the outer shell.

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The model revealed that isolated blobs appearing in the liquid could move upward and around the core, creating waves through the liquid’s electrically charged atoms. Once these waves are stopped by the solid mantle, they compress, leading to the temporary disturbances in the magnetic field that cause the geomagnetic jerk. It would take around 25 years from the release of the blob to the observation of a jerk.

This is a model based on what we know of Earth’s interior, and it’s worth taking it (and all models) with a grain of salt. Nor is this the only possible explanation for these jerks—others have suggested they come from a torsional, or twisting, oscillation in the core, though the researchers behind the new paper argue that torsional oscillations are too weak to generate the signals observed in real life.

Still, models are crucial for better understanding how motion deep in our planet manifests here on the surface, and understanding the jerks might help scientists predict them in the future. You might be aware that the magnetic field protects us from solar rays that would otherwise make the planet unlivable. So it’s nice to be able to understand it better.

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