Geomagnetic field

The field is maintained by the geodynamo in Earth’s liquid outer core. Heat escaping from the core, along with compositional buoyancy released as the inner core gradually crystallizes, drives convection in electrically conducting iron alloy; Earth’s rotation organizes that motion through the Coriolis effect.² Moving conductive fluid generates electric currents, and those currents generate magnetic fields that reinforce one another. The outer core lies between the solid inner core and the rocky mantle, so the field is produced deep beneath the surface rather than by permanent magnetization of the crust. Numerical dynamo models reproduce broad features such as polarity reversals and irregular secular variation, although the detailed flow cannot be observed directly.³

The geomagnetic field is approximately dipolar, but its actual shape includes substantial nondipole structure and changes continuously. A compass aligns approximately with the horizontal component of the field, while the magnetic inclination becomes steeper toward the magnetic poles; the angle between geographic and magnetic directions also varies by place and date. Slow changes generated inside the core are called secular variation. They alter magnetic declination, weaken or strengthen regional components, and require periodic updates to navigation models such as the World Magnetic Model.¹ Faster disturbances arise when solar-wind energy reaches near-Earth space, producing geomagnetic storms and rapid fluctuations that are distinct from the long-term core-generated field.

The field carves out a magnetosphere that deflects much of the solar wind and guides charged particles along magnetic-field lines. Its interaction with the solar wind forms a compressed dayside and an extended nightside magnetotail; energy released during magnetic reconnection can drive auroras and geomagnetic storms.² These disturbances can interfere with radio communication, satellite operations, navigation, and electric-power transmission, while measurements of magnetic activity support space-weather forecasting.¹ The field also provides a reference for geophysical surveying: magnetic anomalies in rocks, especially crustal rocks retaining remanent magnetization, reveal structure in the lithosphere. Such crustal signals are local additions to, not the source of, the global core dynamo field.

Several less visible features make the geomagnetic field scientifically valuable. Its intensity and direction are recorded in fired archaeological materials and volcanic rocks, allowing researchers to reconstruct past field behavior; ocean-floor magnetic stripes preserve a history of polarity reversals associated with seafloor spreading.² Reversals are global reorganizations in which magnetic north and south exchange, but they do not occur on a fixed schedule and unfold over periods much longer than human lifetimes. Shorter-lived excursions can produce large directional changes without completing a reversal. Satellite missions such as ESA’s Swarm separate signals from the core, crust, oceans, ionosphere, and magnetosphere, improving models of both deep-Earth processes and near-Earth space.⁴ The field is therefore both a protective space environment and a remote probe of the inaccessible core.

Geodynamo

The self-sustaining process by which motion of electrically conducting fluid in Earth’s outer core generates the magnetic field.

Secular variation

Slow change in the geomagnetic field caused mainly by evolving fluid motion in the outer core.

Magnetosphere

The region around Earth dominated by its magnetic field and shaped by interaction with the solar wind.

Magnetic declination

The angle between geographic north and magnetic north at a particular location and time.

Magnetic reversal

A large-scale transition in which the predominant geomagnetic polarity changes direction.