IGRF

The International Geomagnetic Reference Field (IGRF) model is the empirical representation of the Earth's magnetic field recommended for scientific use by a special Working Group of the International Association of Geomagnetism and Aeronomy (IAGA). The IGRF model represents the main (core) field without external sources. The model employs the usual spherical harmonics expansion of the scalar potential in geocentric coordinates. The IGRF model coefficients are based on all available data sources including geomagnetic measurements from observatories, ships, aircrafts and satellites.

The IGRF model consists of sets of coefficients for a global representation of the Earth magnetic field for the years 1945, 1950, 1955, etc. There are definitive coefficient sets (DGRF##.DAT) for which no further revisions are anticipated and IGRF##.DAT and IGRF##S.DAT for which future updates are expected. IGRF##S.DAT provides the first time derivatives of the coefficients for extrapolation into the future.

• The 10th generation of the IGRF model (IGRF10) consists of definitive coefficients sets for 1945 through 2000, preliminary sets for 2005, and secular variations extrapolating from 2005 to 2010.
• The 11th generation of the IGRF model (IGRF11) consists of definitive coefficients sets for through 2005, preliminary sets for 2010 and extrapolations from 2010 to 2015.
• The 12th generation of the IGRF model (IGRF12) consists of definitive coefficients sets through 2010, preliminary sets for 2015 and extrapolations from 2015 to 2020.
• The 13th generation includes definite coefficients sets through 2015, preliminary sets for 2020 and extrapolations through 2025.

In combination with the IGRF coefficient sets different subroutines have been used to determine the components of the magnetic field vector and the L-value at a given location. The CCMC version uses the subroutines FELDG (magnetic field vector) and SHELLG (L shell) developed by G. Kluge at the European Space Operations Center (ESOC). His use of inverse cartesian co- ordinates simplifies the computation. The IGRF subroutines were developed by A. Zunde of the U.S. Geological Survey (USGS). The program BILCAL produces tables of the geomagnetic field strength, vector components (B-abs., B-north, B-east, B-down, declination, inclination), equatorial/minimum field strength (B0), dipole moment, and L-value in latitude, longitude (geodetic), altitude, or year (decimal).

The IGRF model is revised and updated every 5 years. Results obtained for the last 5-year interpolation time period and the 5-year extrapolation period of a previous version are altered slightly through definite values replacing preliminary values and new preliminary coefficients replacing secular variations.

The model only takes the epoch time (year, month, day).

Magnetic field strength, azimuth and inclination, field line traces.

Citation to include: International Geomagnetic Reference Field: the 13th generation, Alken, P., Thébault, E., Beggan, C.D. et al. International Geomagnetic Reference Field: the thirteenth generation. Earth Planets Space 73, 49 (2021). doi: 10.1186/s40623-020-01288-x