geomagnetism

Geomagnetism is the scientific study of Earth's magnetic field and its sources, structure, and temporal variation. It encompasses the generation of the field inside the planet, the magnetic properties of the crust and mantle, and the influence of the solar wind and space environment on the near-Earth field.

The dominant source is the geodynamo in the liquid outer core, where convection of metallic iron and nickel conducts electricity, creating a self-sustaining magnetic field. Remanent magnetization in rocks and mineral grains in the crust and upper mantle preserves past field directions. External fields arise from currents in the ionosphere and magnetosphere, and from the solar wind.

The field observed at the Earth's surface resembles a tilted dipole with a roughly 11-degree inclination relative to the rotation axis, but it includes significant regional crustal anomalies. The field strength ranges around 25 to 65 microteslas at the surface, varying with location and time. A second, non-dipolar component from crustal magnetization and deep-seated heterogeneities is present.

The field changes gradually over years to decades (secular variation) and occasionally undergoes polarity reversals on timescales of hundreds of thousands of years. Paleomagnetism, the study of preserved magnetization in rocks, provides records of past field directions and intensities used to infer plate movements.

Measurements come from ground-based magnetic observatories, ships and aircraft surveys, and satellite missions such as Ørsted, CHAMP, and Swarm. Data are used for navigation, mineral exploration, and to probe the interior of the planet through techniques like geomagnetic secular variation modeling and magnetotellurics.

Geomagnetism also encompasses the interaction of the magnetic field with the solar wind, producing space weather phenomena such as auroras and magnetospheric storms, which can affect satellites and power systems.