The Geomagnetic Field and Secular Variation

The Earth's main magnetic field is, to first order, that of a tilted geocentric dipole, but it departs from this in detail and drifts measurably from year to year in a slow change called secular variation.

Definition

The geomagnetic field is the magnetic field observed at and above the Earth's surface, dominated by an internal geocentric dipole and described mathematically by a spherical-harmonic expansion, whose slow temporal changes of internal origin constitute secular variation.

Scope

This topic covers the description and time evolution of the Earth's main magnetic field: its representation by a spherical-harmonic potential dominated by a geocentric dipole, the field elements (declination, inclination, intensity), and reference models such as the International Geomagnetic Reference Field. It treats secular variation, including westward drift and geomagnetic jerks, and the separation of internal from external field sources. The emphasis is on the present field and its decadal changes rather than its deep-time record.

Core questions

• How is the main field described by a geocentric dipole and spherical harmonics?
• What are the field elements, and how are they mapped in reference models?
• What is secular variation, and how fast does the field change?
• How are internal and external contributions to the field separated?

Key concepts

• Geocentric axial dipole and non-dipole field
• Field elements: declination, inclination, intensity
• Spherical-harmonic (Gauss) representation
• Secular variation, westward drift, and geomagnetic jerks
• International Geomagnetic Reference Field

Key theories

Spherical-harmonic representation of the field

Gauss showed that a source-free magnetic field can be written as the gradient of a potential expanded in spherical harmonics, allowing the main field to be separated into a dominant dipole, non-dipole terms, and contributions of internal versus external origin.

Secular variation from core flow

The slow drift of the field, including its westward motion and abrupt jerks, reflects changing fluid flow at the top of the liquid core, providing a surface window onto core dynamics on decadal timescales.

Mechanisms

The main field originates in electric currents in the liquid core; at and above the surface it is current-free and so derivable from a scalar potential, whose spherical-harmonic coefficients change slowly as the underlying core flow evolves, producing the measured secular variation while crustal and external (ionospheric and magnetospheric) sources add shorter-wavelength and more rapid contributions.

Clinical relevance

Accurate models of the field and its secular variation are essential for magnetic compass navigation, directional drilling, satellite and spacecraft operations, and as a continuously updated reference for magnetic surveys.

History

Halley mapped declination over the Atlantic around 1700, Gauss in the 1830s introduced the spherical-harmonic analysis and first separated internal from external sources, and the modern International Geomagnetic Reference Field, now informed by satellite missions, provides regularly updated global models of the field and its variation.

Key figures

• Carl Friedrich Gauss
• Edmond Halley
• George Backus

Seminal works

• backus1996
• merrill1996
• alken2021

Frequently asked questions

Why does a compass not point to true north?

A compass aligns with the local horizontal magnetic field, whose direction (declination) differs from geographic north because the geomagnetic field is not a perfect axial dipole and varies from place to place; reference field models give the declination needed to correct a compass.

What is secular variation?

Secular variation is the slow, year-to-year change in the Earth's main magnetic field, such as the drift of magnetic features and the gradual weakening of the dipole, driven by evolving fluid motion in the liquid outer core.