International Geomagnetic Reference Field

The International Geomagnetic Reference Field (IGRF) 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 (IAGA) since 1965.

The IGRF model covers a significant time span, and so is useful for interpreting historical data. (This is unlike the World Magnetic Model, which is intended for navigation in the next few years.) It is updated at 5-year intervals, reflecting the most accurate measurements available at that time. The current 12th edition of the IGRF model was released in 2015 and is valid from 1900 until 2020. For the interval from 1945 to 2010, 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 ${\vec {B}}(r,\phi ,\theta ,t)$ as a gradient of a magnetic scalar potential $V(r,\phi ,\theta ,t)$ ${\vec {B}}(r,\phi ,\theta ,t)=-\nabla V(r,\phi ,\theta ,t)$ The magnetic scalar potential model consists of the Gauss coefficients which define a spherical harmonic expansion of $V$ $V(r,\phi ,\theta ,t)=a\sum _{\ell =1}^{L}\sum _{m=0}^{\ell }\left({\frac {a}{r}}\right)^{\ell +1}\left(g_{\ell }^{m}(t)\cos m\phi +h_{\ell }^{m}(t)\sin m\phi \right)P_{\ell }^{m}\left(\cos \theta \right)$ where $r$ is radial distance from the Earth's center, $L$ is the maximum degree of the expansion, $\phi$ is East longitude, $\theta$ is colatitude (the polar angle), $a$ is the Earth's radius, $g_{\ell }^{m}$ and $h_{\ell }^{m}$ are Gauss coefficients, and $P_{\ell }^{m}\left(\cos \theta \right)$ are the Schmidt normalized associated Legendre functions of degree $l$ and order $m$ . The Gauss coefficients are assumed to vary linearly over the time interval specified by the model.