The Geomagnetic Dipole
To first approximation, Earth's magnetic field resembles that of a geocentric tilted dipole, with the magnetic axis offset about 11.5° from the rotation axis. This dipolar field component accounts for about 90% of the surface field energy, with the remainder in higher-order multipole terms that arise from complex core flow patterns and crustal magnetization.
The Geodynamo
Convection in Earth's liquid iron outer core, driven by heat loss to the mantle and latent heat from inner core crystallization, generates the geomagnetic field through electromagnetic induction. The magnetic Reynolds number Rm = μ₀σvL must exceed a critical value (~40) for self-sustaining dynamo action, requiring flow speeds of ~10-20 km/year in the core.
Field Geometry and Variations
The dipole field strength varies from ~25 µT at the equator to ~65 µT at the poles, following the characteristic 1/r³ distance dependence. Secular variation — slow changes over decades to centuries — reflects evolving core flow. The South Atlantic Anomaly, a region of anomalously weak field, has been growing and may indicate the early stages of field reorganization.
Paleomagnetic Record
Ferromagnetic minerals in volcanic rocks and sediments record the ambient field direction and intensity at the time of formation. This paleomagnetic archive reveals that Earth's field has reversed polarity hundreds of times over geologic history, with the current normal polarity (Brunhes epoch) lasting 780,000 years. The time-averaged paleomagnetic field aligns with the geographic axis, confirming the intimate connection between Earth's rotation and the dynamo process.