A jolt from the deep
Deep below Earth's surface, the planet's molten outer core churns with liquid iron, generating the magnetic field that protects life from solar radiation. This field is not static; it constantly shifts and drifts. But sometimes, it does something far more abrupt. Without warning, the field's steady change in momentum lurches. This event is a "geomagnetic jerk," a rapid, V-shaped change in the magnetic field's acceleration that occurs over just a few months.
First formally described in 1978, these events have been identified in magnetic observatory data reaching back to 1901. Major global jerks were recorded around 1969, 1978, 1991, and 1999, with a rapid series of events occurring in 2008, 2011, 2014, and 2017. While the term "jerk" comes from physics—where it means a rate of change of acceleration—it aptly describes the sudden and unpredictable nature of these planetary-scale jolts. They occur at irregular intervals, roughly every three to twelve years, and their unpredictability complicates efforts to forecast the behavior of our planet's magnetic shield.
The physical location for this global phenomenon is tied to the Institut de Physique du Globe de Paris (IPGP), a world-leading research center for Earth sciences. Scientists at IPGP have been central to understanding these events, developing computer simulations that model the core's complex dynamics to show their origin.
Waves from the core
Geomagnetic jerks originate roughly 2,900 kilometers down, at the boundary of the molten iron outer core. For decades, their precise cause remained a puzzle. Recent supercomputer simulations, some requiring the equivalent of 4 million hours of calculation, have provided a compelling explanation.
The leading model suggests that jerks are caused by buoyant blobs of molten material that slowly rise through the outer core. As these blobs approach the top of the core over a period of about 25 years, they generate powerful hydromagnetic waves, also known as Alfvén waves. These waves travel along the existing magnetic field lines toward the core's surface, where they become focused and amplified. This process creates a sudden, sharp disturbance in the flow of liquid metal, and the effect rapidly propagates to the surface, where we observe it as a geomagnetic jerk.
Not all jerks are global. Some are regional events, detected on one continent but not another. For instance, a jerk was measured over North America in 1949 but was absent from European records. This variability shows the complex nature of the geodynamo deep within the Earth.