IGRF Health Warning, Errors, and Limitations

The International Geomagnetic Reference Field (IGRF) was introduced by the International Association of Geomagnetism and Aeronomy (IAGA) in 1968 in response to the demand for a standard spherical harmonic representation of the Earth's main field. The model is updated at 5-year intervals, the latest being the 13th generation. IGRF is produced and released by IAGA Working Group V-MOD (formerly V-8).  It has achieved worldwide acceptance as a standard and has proved valuable for many applications. However, it can produce inaccurate results when used incorrectly.

Introduction

The Earth's magnetic field crudely resembles that of a central dipole. On the Earth's surface, the field varies from being horizontal and of magnitude about 30, 000 nanoteslas (nT) near the equator to vertical and about 60,000 nT near the poles; the root mean square (rms) magnitude of the vector over the surface is about 45,000 nT. The internal geomagnetic field also varies on a time-scale of months and longer, in an unpredictable manner. This so-called secular variation (SV) has a complicated spatial pattern, with a global rms magnitude of about 80 nT/year. Consequently, any numerical model of the geomagnetic field has to have coefficients that vary with time.

The IGRF is intended to model the surface level effects of the part of Earth’s magnetic field that originates below the surface. At any one epoch, the IGRF specifies the numerical coefficients of a truncated spherical harmonic series. The model is truncated at n=10 with 120 coefficients up until the year 2000, but after 2000 the truncation is at n=13, with 195 coefficients. The model is specified every five years for epochs 1900.0, 1905.0 etc.