What Is Geomagnetism?

Geomagnetism is the scientific study of Earth's magnetic field: its origin, structure, spatial distribution, temporal variations, and interactions with the surrounding space environment. The field is generated primarily in Earth's outer core by the dynamo action of electrically conducting, convecting liquid iron, producing a broadly dipolar field that extends from deep in the interior through the crust and into the near-Earth space environment known as the magnetosphere. Geomagnetism draws on geophysics, plasma physics, and electromagnetic theory, and its applications span navigation, mineral exploration, space weather monitoring, and the study of Earth's deep interior.

The geomagnetic field is characterized at any location by three components: intensity, inclination (the angle of the field vector below the horizontal), and declination (the angle between geographic north and magnetic north). These parameters vary continuously across Earth's surface and change over time due to processes both in the core and in the ionosphere. The USGS Introduction to Geomagnetism provides a systematic overview of these components and the observational network used to track them.

Earth's Magnetic Field Structure

Earth's field approximates a geocentric axial dipole, with magnetic poles that are offset from the geographic poles by roughly 11 degrees and that migrate slowly over decades. At Earth's surface, the field ranges in intensity from approximately 25,000 nanoteslas near the equator to over 60,000 nanoteslas near the magnetic poles. At depth, the core field is far stronger and more complex, with higher-degree multipole components that are attenuated before reaching the surface. Secular variation, the slow change of the field on timescales of years to centuries, reflects changing patterns of convection in the outer core. Paleomagnetic studies, which recover ancient field directions from the remanent magnetization in volcanic and sedimentary rocks, show that the field has reversed its polarity hundreds of times over Earth's history, with the most recent reversal approximately 780,000 years ago.

Electromagnetic Induction and Magnetic Variations

The geomagnetic field is not static. External sources, primarily electric currents in the ionosphere driven by solar radiation, and in the magnetosphere driven by the solar wind, produce time-varying magnetic signals at Earth's surface. These external variations induce secondary currents in the conductive interior, a process governed by Faraday's law of electromagnetic induction. The induced currents in turn generate their own magnetic fields, which superimpose on the primary field. Short-period variations with periods of seconds to hours reflect solar activity, geomagnetic storms, and ionospheric dynamics. Longer-period signals with periods of days to years carry information about the electrical conductivity structure of Earth's mantle. The USGS Geomagnetism Program's network of 14 magnetic observatories continuously records these variations, providing the data needed to separate core, crustal, and external-field contributions.

Magnetosphere

The magnetosphere is the region of near-Earth space in which the geomagnetic field dominates over the solar wind plasma pressure, forming a teardrop-shaped cavity extending roughly 10 Earth radii toward the Sun and hundreds of radii in the anti-sunward tail. The magnetosphere shields Earth's surface from the bulk of the solar wind particle flux and acts as a reservoir of trapped energetic particles in the Van Allen radiation belts. Solar wind disturbances, particularly coronal mass ejections, compress and energize the magnetosphere, producing geomagnetic storms and substorms with measurable effects on the surface field. Research compiled by NOAA's Space Weather Prediction Center describes how magnetospheric dynamics translate into the surface field fluctuations that are monitored by the geomagnetic observatory network.

Applications

Geomagnetism has applications in a wide range of fields, including:

• Magnetic compass navigation and declination correction
• Aeromagnetic and marine magnetic surveys for mineral and hydrocarbon exploration
• Space weather monitoring and geomagnetic storm impact mitigation
• Paleomagnetic dating and geologic time scale calibration
• Satellite attitude determination and orbit modeling
• Deep Earth conductivity profiling and mantle studies