Models

There are several magnetic models in the Geomagnetic Data Library. These were either created at Goddard or at other agencies. These models usually consist of a series of spherical harmonic coefficients to a particular degree and order. Several models also include first derivative terms to describe the secular variation of the field.

The International Association of Geomagnetism and Aeronomy (IAGA) of the International Union of Geodesy and Geophysics (IUGG) developed a uniform basis for reporting main-field calculations. The recommended form is the International Geophysical Reference Field (IGRF). These models represent the main field without external sources and are formed every 5 years starting with 1945. Secular variation coefficients are included to calculate the field for intervening years.

The Definitive International Reference Field (DGRF) for each epoch is an updated version of the IGRF that replaces each model once the data available is considered complete. Several of the IGRF and DGRF models can be found on CD:MODELS001.

Other main field models of interest also included on CD:MODELS001:

GSFC 1900-1995 Spline models (also on CD:GSFCS95XX01)

GSFC Sq models (also on CD:GSFCSQSB01)

Defense Meteorological Satellite Program (DMSP) 12 and 13 GSFC project models 1996

Strategic Defense Initiative (SDI) GSFC project models 1991

Models prepared by Joseph Cain

GSFC, IZMIRAN, and US/UK IGRF-95 Candidate models

Jeremy Bloxham 1690-1840 and 1840-1990 Spline models

Also found on this CD are listings of several observatory bias values that were applied in the modeling process. These X, Y, Z values in units of nT were used to account for the crustal signal measured by the world-wide distribution of observatories.

Since 1990 the magnetics group at the Goddard Space Flight Center has been using the Newfit software to produce main and external field models. This collection of FORTRAN programs uses a gauss iterative least squares estimator with constraints to estimate the Earth's magnetic field based on observed data. Typical output from this program is a set of magnetic field parameters that describe spatial and temporal aspects of the field. `ARC' parameters refer to those model parameters sensed by a particular measurement source (i.e. observatory biases) while `COMMON' parameters refer to thoses model parameters sensed by all measurement sources (i.e. spherical harmonic coefficients). Several files are output from the Newfit software:

 
		UCOR   		Contains the correlation matrix for the final model

		UCOV   		Contains the covariance matrix for the final model

		UINF   		Contains the normal equations for the final model

		UNED   		Contains the final model and the specific characteristics of the model

Of the four files above, the first three are general for any fitting algorithm and the last file (UNED) is specific to the application (in this case magnetic field modeling).