New hypothesis explains why our magnetic field is slowly drifting westward

Since humans started tracking Earth’s magnetic field for the last four centuries or so, it has steadily drifted westward. Experts have wondered about the reason for this movement, but a study presents a possible reason for the drift.

The study, titled “Could hydrodynamic Rossby waves explain the westward drift?” was published in the journal Proceedings of the Royal Society A, led by O.P. Bardsley, a doctoral student at the University of Cambridge. Bardsley is the author of a new study on the Rossby wave hypothesis, which posited that unusual waves from Earth’s outer core could be the reason for the westward drift.

The slow waves, or Rossby waves, float in rotating fluids. The waves were named after Carl-Gustaf Arvid Rossby, an American meteorologist who first identified the waves and explained their movement.

Also called “planetary waves,” Rossby waves are present in various large and rotating bodies like the atmosphere and the oceans on Earth. Even Jupiter and the Sun have Rossby waves. Since Earth’s outer core is also a rotating fluid, Rossby waves also flow within it.

However, while atmospheric and oceanic Rossby waves have crests that move westward against Earth’s eastward rotation, Bardsley noted that the Rossby waves in the core are “a bit like turning atmospheric Rossby waves inside out.” The waves have crests that constantly move east.

Rossby waves and Earth’s magnetic field

As the magnetic iron in the planet’s core rotates, it generates Earth’s geomagnetic field. The geomagnetic field has a crucial function for all life on the planet: it shields Earth from solar radiation. If Earth loses this magnetic field, the planet’s surface would perish as charged particles from the sun tears away at Earth’s atmosphere.

As Bardsley was studying the waves within the planet’s core, he realized that some of these waves could finally shed light on a mystery surrounding Earth’s magnetic field. For over 400 years, scientists have measured magnetic declination, or “the difference between true north and the point where a compass needle points.”

The magnetic field is linked to several little local anomalies and it makes a compass needle move around a bit (which depends on where you’re standing), unlike true north.

Data from the study showed that in the last four centuries, the anomalies uncovered by these declination measurements usually drifted westward. Bardsley explained that the westward drift mainly manifests as “a series of blobs over the Atlantic near the equator.” The “blobs” can drift at about 10.5 miles (17 km) yearly.

Most of the theories that have tried to explain the drift are often about the dynamics of the outer core. Bardsley shared that based on the most popular hypothesis, the outer core has a gyre similar to the atmosphere’s jet stream.

The gyre drifts to the west, and it also drags the planet’s magnetic field as it moves. But Bardsley said there’s one problem: there’s no concrete reason for the existence of this gyre. While he’s not disregarding the gyre, Bardsley believes that the lack of direct evidence means that other reasons are still valid.

One possible reason is that Rossby waves could be behind “the weirdness of the magnetic field on Earth’s surface.” This seems strange, especially since Rossby waves within the core feature eastward-moving crests, instead of the westward-moving drift. Bardsley added that crests of waves don’t always indicate the total energy movement of the waves.

Bardsley added that it’s possible that there are a group of waves with crests going east but with most of the energy headed towards the west.

A similar thing can happen with water waves since their crests usually head in the same direction as the bulk of their energy. But the energy doesn’t always travel at the same speed.

While surface measurements of the geomagnetic field determine the bulk of energy movement, Bardsley shared that they don’t always capture “all the wiggly little details.” It’s possible that Rossby waves with a large-scale tendency to move energy to the west could be causing the westward drift observed over the Atlantic Ocean. The small-scale details, such as eastward-moving crests, would be difficult to observe. (Related: The magnetic field created by our oceans’ currents finally mapped in unprecedented detail.)

The westward drift of Earth’s magnetic field and the Rossby wave hypothesis are not connected to an otherwise popular question concerning the magnetic field: will it flip?

In Earth’s recorded history, magnetic north and magnetic south have regularly swapped places. Bardsley noted that this isn’t something to worry about since it often occurs after at least 10,000 years. But the process is linked to an increase in the number of recorded anomalies and a weakening of the magnetic field in between the North and South poles.

If a field is weakened, it may let more solar particles through, and this can disrupt electric grids and result in issues with navigational systems. Scientists are unsure if the weakening of the magnetic field over the last couple of hundred years heralds an upcoming reversal or if it’s just a sign of a “recoverable wobble.”

Learn more about other findings on Earth’s magnetic field at

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