From scientists to everyone: the IGRF Model

In IAGA there is a group called V-MOD: Geomagnetic Field Modeling, part of Division V. The aim of this group is "to promote and coordinate international efforts to model and analyze the internal geomagnetic field and its secular variation on both global and regional scales". 

The main result of their efforts is the IGRF model. IGRF stands for International Geomagnetic Reference Field, and is a standard mathematical description of the Earth's main magnetic field and its temporal variation, called secular variation. IGRF is widely used in a multitude of different studies, such as the Earth's interior, the crust, the ionosphere and magnetosphere. It is also used in satellite attitude determination and control systems and other applications requiring orientation information. 

The model is the product of a collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world. It is a voluntary work for the benefit of all. Each time a new model is to be obtained, different teams of scientists come together and propose different candidate models. The final model is usually a mean or median of all candidate models and some kind of weighting scheme may also be applied. The overall process is described in papers for each one of the candidate models, the evaluation process and finally a paper describing the final model. 

Map of the declination at 2020.0 as given by IGRF-13 (Alken et al., 2021)

The Earth's field changes continuously and in order to account for temporal changes on timescales of a few years, the IGRF is regularly revised, typically every 5 years. The years for which coefficients are provided are called model epochs. The coefficients of a certain epoch represent a snapshot of the geomagnetic field at that time, and can be labeled either as a Definitive Geomagnetic Reference Model (DGRF, which are unlikely to be improved in future IGRF revisions) or as an IGRF. 

The last generation, IGRF-13, was finalized in December 2019 by a task force of group V-MOD. It provides a DGRF model for epoch 2015.0, an IGRF model for epoch 2020.0, and a predictive IGRF secular variation model for the 5-year time interval 2020.0 to 2025.0. The main field coefficients describe the spatial variation of the field to a maximum spherical harmonic degree and order of 13, while the secular variation extend to a maximum degree and order of 8. 

Satellite data, such as the one provided by the ESA Swarm mission and the ground observatory network were crucial to the latest IGRF generations. Data from other satellite missions were also used. 

The coefficients to calculate the model may be found in the designated paper (Alken et al, 2021) and also online in digital form along with the software to compute magnetic field components at different times and spatial locations. 

Where to find the model and other information:

- model IGRF-13 paper: https://doi.org/10.1186/s40623-020-01288-x 

- model issue, with candidate models' papers and evaluation process paper: https://earth-planets-space.springeropen.com/igrf13 

- working group V-MOD webpage and model: https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

 

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After a master in Geophysics from Portugal, Diana Saturnino got a PhD in Geomagnetism and continued working on the subject for a few more years in Denmark and France. Now she's looking for different adventures. She can be contacted via e-mail here.

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