The International Geomagnetic Reference Field (IGRF) model is an 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 core field without external sources. The model employs ordinary spherical harmonics expansion of the scalar potential in geocentric coordinates. The IGRF model coefficients are based on multiple data sources including geomagnetic measurements from observatories, ships, aircrafts and satellites.

The International Geomagnetic Reference Field (IGRF) model is the empiric 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 thru 2000 and prelimenary sets for 2005 and for extrapolating from 2005 to 2010.

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 NSSDC 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 program will first request the names of the input and output files. The working directory must not contain files with the name of the output file.

Input file format:

Year Alt Lat Lon

This means year for which calculation is proceed, altitude above sea level, geographic longitude and latitude. Example:

1990 500 -25.36 175.5 

The year had be integer number from 1945 to 2010, altitude - integer positive number in km, latitude - number from -90 to +90 in degrees, longitude - number from 0 to 360 in degrees.

After calculating the file will be created.

Example:

Here

• YEAR, ALT, LON, LAT are the same as in input file;
• B - geomagnetic field in point of observation, Gs;
• B-north, B-east, B-down -components of geomagnetic vector north, east and down (i.e., -B_θ, B_φ, -B_r, Gs.);
• B0 - magnetic field strength in the top of field line, that pass through observation point;
• B/B0 - relation B/B0;
• L - McIlwein L parameter;
• MLONG - magnetic longitude, deg;
• MLAT - magnetic latitude, deg;
• Other parameters are dipole magnetic moment of the Earth (DIMO) and magnetic coordinates (DIP, DEC) - declination and inclination.