In Earth’s Magnetic Pas de Deux, the Inner Core Leads

The phrase “geologic time” conjures images of continents moving at centimeters per year, or of mountains forming and eroding over hundreds of millions of years. Yet, some geologic processes evolve at a rate that allows significant changes to take place on the scale of a human lifetime. Earth’s magnetic field changes on a time scale of decades—presumably mirroring changes in the planet’s electrically conducting, fluid outer core, the motion of which is thought to generate and sustain the geomagnetic field. Last year, Gary Glatzmaier (Los Alamos National Laboratory) and Paul Roberts (University of California, Los Angeles) generated the most realistic model yet of this “geodynamo,” and predicted that the rotation rate of Earth’s inner core evolves on a scale of centuries rather than eons. Moreover, in their simulation the magnetic field and the flow of the fluid outer core caused the solid inner core nearly always to spin more rapidly than the mantle and crust—the result of magnetic coupling between the inner core and a jet‐stream–like layer of rapidly rotating outer‐core fluid at the inner‐core boundary (See PHYSICS TODAY, January, page 17).