geomagnetisme

Geomagnetisme refers to the study of the Earth's magnetic field, its sources, and its temporal and spatial variations. It encompasses the magnetosphere, the ionosphere, the crust, and their interactions with the solar wind, with practical implications for navigation, space weather, and geology.

The main source of the geomagnetic field is the geodynamo in the planet’s liquid outer core, where convection of conducting iron generates electric currents that sustain a planetary-scale magnetic field. Crustal rocks can also retain magnetization that records past field directions. At large scales the field is roughly dipolar but the orientation and intensity change over time due to core flow, a phenomenon known as secular variation. Reversals and excursions of the field have occurred irregularly over geological timescales.

Observations come from ground-based observatories, airborne surveys, and satellites, which, together with mathematical models, yield global representations of the field. Global models like the International Geomagnetic Reference Field separate the main, long-term field from shorter-lived crustal anomalies, providing estimates of the field at the Earth's surface and in near-Earth space.

The geomagnetic field shapes the magnetosphere, deflecting charged particles from the solar wind and influencing space weather. Interactions with the solar wind can trigger auroras and geomagnetic storms, which can affect satellites, power grids, and navigation systems. Paleomagnetism, using rocks and sediments, reconstructs past field strength and direction, informing studies of plate tectonics, oceanography, and chronology. Understanding geomagnetisme thus links geophysics, geology, and space science.