International Geomagnetic Reference Field


The International Geomagnetic Reference Field is a standard mathematical description of the large-scale structure of the Earth's main magnetic field and its secular variation. It was created by fitting parameters of a mathematical model of the magnetic field to measured magnetic field data from surveys, observatories and satellites across the globe. The IGRF has been produced and updated under the direction of the International Association of Geomagnetism and Aeronomy since 1965.
The IGRF model covers a significant time span, and so is useful for interpreting historical data. It is updated at 5-year intervals, reflecting the most accurate measurements available at that time. The current 13th edition of the IGRF model was released in December 2019 and is valid from 1900 until 2025. For the interval from 1945 to 2015, it is "definitive", meaning that future updates are unlikely to improve the model in any significant way.

Spherical Harmonics

The IGRF models the geomagnetic field as a gradient of a magnetic scalar potential
The magnetic scalar potential model consists of the Gauss coefficients which define a spherical harmonic expansion of
where is radial distance from the Earth's center, is the maximum degree of the expansion, is East longitude,
is colatitude, is the Earth's radius,
and are Gauss coefficients, and
are the Schmidt normalized associated Legendre functions of degree and order. The Gauss coefficients are assumed to vary linearly over the time interval specified by the model.