Comparing Geomagnetic Field Models with a Sensitivity Matrix

Compare how similar two geomagnetic field models are by producing a sensitivity matrix of their Gauss coefficients.

Planetary geomagnetic field models are often written in terms of Gauss coefficients (eg., see here for more information: https://www.ngdc.noaa.gov/geomag/WMM/data/WMMReports/WMM2015_Report.pdf).

This code compares how similar two geomagnetic field models are by producing a sensitivity matrix of their Gauss coefficients. Thus, this function can be used to compare a geomagnetic model you've made to a standard reference field model.

For Earth, standard reference models include:
> the World Magnetic Model (https://www.ngdc.noaa.gov/geomag/WMM/soft.shtml)
> the International Geomagnetic Reference Field (https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html)

There are geomagnetic field models for other planets that also use Gauss coefficients within a spherical harmonic expansion to describe the field. This function will also work for those models.

Please cite using the reference below whenever you use this code to obtain results for published work.