Weird Shift of Earth's Magnetic Field Explained

Earth's magnetic field shields the planet from charged particles streaming from the sun, keeping it from becoming a barren, Mars-like rock. For more than 300 years, scientists have recorded a westward-drifting feature in the field that models have been unable to explain.By relying on insights gleaned from previous work, as well as data collected over nearly four centuries, an international team of scientists has been able to provide a model that accounts for the western drift of the magnetic field on one side of the planet.

People have tried various configurations regarding the state of the core-mantle alignment," lead author Julien Aubert, of the Université Paris Diderot in France, told SPACE.com in an email."The ingredients were here, but they were never put in the right configuration, in particular for reproducing the geomagnetic westward drift."

Driven by temperature and gravity

The magnetic field that encases the planet is caused by interactions deep inside Earth's core. The inner core is solid, while the outer core features flowing liquid iron, which generates currents that in turn lead to magnetic fields.

The field surrounding the Earth changes over time, with shifts occurring most prominently in low latitudes in the Western Hemisphere. The fast-moving magnetic patches that occur near the equator drift approximately 10 miles (20 kilometers) per year. These changes are driven by intense regions of activity in the core, the cause of which scientists have been at a loss to explain.

Relying on more than 400 years of data, including more than a decade's worth of continuous global satellite observations, Aubert's team was able to create the first model to explain the westward drift.

Gravity aligns the inner core and the mantle, the layer between the core and the outer crust, forcing liquid metal in the outer core into enormous rotating vortexes known as gyres. These gyres can reach as large as 1,700 miles (2,700 km) deep within the mantle, while smaller gyres occur closer to the surface. As these gyres are concentrated at low latitudes, core convection expels them and pushes them westward, the model shows.

At the same time, the cooling Earth is causing the outer core to slowly solidify. Over the course of a few billion years, the entire core will become solid iron.

The team concluded that the core was not growing evenly, but instead seemed to be cooling at different rates, with the solid material changing differentially across the core.

"Inner core growth is thought to be fastest below Indonesia because of the influence from Earth's mantle," Aubert said.

This broken-up cooling causes an uneven buoyancy release from the outer core, which distorts the gyre as it reaches the core's surface. These shifts create westward-drifting patches along the equator in the Western Hemisphere.