PSAS/ EarthMagneticFieldModel

Modeling the Earth's Magnetic Field


Earth's magnetic field consists of:

And to a lesser extent:

The IGRF and WMM model the Main field where as the EMM also includes the Crustal field.

WMM applicable between 1 km below sea level to 850 km above.

The models are implemented through (Schmidt semi-normalized) spherical harmonic expansions of empirically determined spherical harmonic coefficients. These coefficients, along with secular variation coefficients for the spherical harmonic coefficients, are released and updated in five year periods known as epochs.

Spherical harmonics description goes here.

The IGRF gives up to degree and order 13 coefficients and the WMM gives up to 12, totaling 208 and 180 in number. The EMM provides up to degree and order 720 coefficients, thats over 500,000 total. This allows the EMM to model wavelengths down to 52 km for a resolution of 26 km but at a cost of much longer calculation time.

Our Implementation

Currently we are using the WMM but ideally we would like to use the EMM. To do so would require improvements to the code currently committed to PSAS as it begins to underflow at around degree 404 or 405.

Switching between IGRF and WMM should be easy, it is just a matter of interchanging the coefficient files.

Another thing to look at is implementing a (inverse) Fast Spherical Harmonic Transform, the spherical harmonics equivalent to the (inverse) Fast Fourier Transform.