This series of geomagnetic main field models has been prepared in the framework of the definition of the International Geomagnetic Magnetic Field Model (IGRF-13 -- https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html). A detailed description of the model is freely available in:
Ropp, G., Lesur, V., Baerenzung, J., and Holschneider, M. (2020), Sequential modelling of the earth’s core magnetic field, Earth, Planets and Space, 72(1) :153, doi :10.1186/s40623-020- 01230-1.

In this series, each model is decomposed in spherical harmonics of degrees 1 to 18. The Gauss coefficients corresponding to this decomposition are available in the file: MCM_2000.01_to_2019.10_V1.shc, whereas their estimated standard variations are provided in the file MCM_2000.01_to_2019.10_V1.std. The series of models is made of 80 snapshots approximatively at 3-month interval. The reference date for each snapshot are given on the first line of the file. The following lines are associated with one Gauss coefficient each with its degree and order indicated in the first two columns. The next columns correspond to the Gauss coefficient values at the reference dates.