May 2, 2019: Earth’s magnetic field is notoriously inconstant. The north pole itself has been wandering across the Arctic for centuries. Currently, it is racing from northern Canada toward Siberia on an unpredictable path that has prompted hurried updates to world magnetic models. And then there are the “geomagnetic jerks.” Every 3 to 12 years, Earth’s magnetic field suddenly accelerates in one direction or another, a phenomenon that has puzzled scientists since it was recognized in the late 1970s.
Above: The rate of change in vertical magnetic fields at the Honolulu observatory (blue) and in Earth’s orbit (red). Sudden changes in the slope indicate geomagnetic jerks. [More]
The most recent jerk occurred in 2017 following a rapid-fire sequence of similar disturbances in 2008, 2011, and 2014. There is evidence for jerks going all the way back to 1901. Some are global, felt everywhere, while others are regional, spanning single continents or less. The unpredictability of jerks has complicated efforts to forecast geomagnetism.
A new study may solve the mystery. In a paper published on April 22nd in Nature Geoscience, Julien Aubert (Paris Institute of Earth Physics) and Christopher C. Finlay (Technical University of Denmark), describe how they created a computer model for geomagnetic jerks based on the physics of hydrodynamic waves in Earth’s molten core. According to their model, jerks originate in rising blobs of metal that form deep inside our planet. These slow-moving blobs can take 25 years to rise to the top of the convection zone. As they buoy upwards, the blobs disrupt the normal flow of magnetic field-generating currents and, in turn, cause jerks. The model successfully reproduces the form and timing of recent events.
Above: A computer simulation of molten blobs floating up from Earth’s core. Credit: Aubert and Finlay, Nature Geoscience (2018)
Geomagnetic jerks are just one aspect of Earth’s magnetic variability. Globally, Earth’s magnetic field has weakened by more than 10% since the 19th century with an even faster decline in the 2000s. Satellite data show the changes are uneven. According to CHAMP, Ørsted, and SWARM, the field has recently weakened by about 3.5% at high latitudes over North America, while it has strengthened about 2% over Asia. The region where the field is at its weakest – the South Atlantic Anomaly – has moved steadily westward and weakened further by about 2%.
At present, no one can predict these changes. However, Aubert and Finley’s successful model of jerks suggests that a deeper understanding may be within reach. You can read their original research here.
Note: The name “jerk” was borrowed from dynamics, where it means the rate of change of the acceleration of a body–that is, the third derivative of its position with respect to time. Geomagnetic jerks are therefore the first derivative of magnetic acceleration.