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 13th edition of the IGRF model (IGRF-13) was released in December 2019 and is valid from 1900 until 2025. For the interval from 1945 to 2015, it is "definitive" (a "DGRF"), 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
Magnetic scalar potential, ''ψ'', is a quantity in classical electromagnetism analogous to electric potential. It is used to specify the magnetic H-field in cases when there are no free currents, in a manner analogous to using the electric ...
:
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 (the polar angle),
is the Earth's radius,
and
are Gauss coefficients, and
are the Schmidt quasi-normalized
associated Legendre functions of degree
and order
:
where
:
is the normalization coefficient for the Schmidt quasi-normalized formulation.
The
term usually present in the associated Legendre functions is omitted in this formulation and is instead accounted for in the Gauss coefficients.
The Gauss coefficients are modelled as a piecewise-linear function of time with a 5-year step size.
See also
*
Geomagnetic latitude
*
Magsat
*
Ørsted (satellite)
*
CHAMP (satellite)
Challenging Minisatellite Payload (CHAMP) was a German satellite launched July 15, 2000 from Plesetsk, Russia and was used for atmospheric and ionospheric research, as well as other geoscientific applications, such as GPS radio occultation, ...
References
External links
IGRF Model Description by IAGAIGRF Health Warning, Errors, and Limitations
{{Geophysics-stub
Geomagnetism
Magnetic field of the Earth