New software for the World Magnetic Model (WMM)

The WMM is a large spatial-scale representation of the Earth's magnetic field. It consists of a degree and order 12 spherical-harmonic expansion of the magnetic potential of the geomagnetic main field generated in the Earth’s core. Apart from the 168 spherical-harmonic “Gauss” coefficients, the model also has an equal number of spherical-harmonic Secular-Variation (SV) coefficients predicting the temporal evolution of the field over the upcoming five-year epoch. The WMM is produced by the U.S. National Geophysical Data Center (NGDC) in collaboration with the British Geological Survey (BGS). The current WMM is valid up to December 2009. We present the new software for the next-generation WMM (2010-2015). The new software is written in C using a modular approach that enables the reuse of WMM subroutines in different applications. The core functions are optimized for speed and efficiency. The software accurately computes the associated Legendre functions up to degree 2800, making it adaptable for high-degree geomagnetic models. Conversion between heights referenced to mean sea level (MSL) and WGS-84 ellipsoid has been newly implemented using the EGM-96 geoid. We also demonstrate the use of the new WMM software to compute magnetic field elements of the degree-720 Enhanced Magnetic Model (EMM). For computational efficiency, the EMM can also be evaluated using a 3D mesh interpolator.