GRIMM: The GFZ Reference Internal Magnetic Model based on vector satellite and observatory data

In this paper the new GFZ Reference Internal Magnetic Model (GRIMM) is presented. The model has been derived from nearly 6 yr of CHAMP satellite data and 5 yr of observatory hourly means. At high latitudes, full vector satellite data are used at all local times which allows a separation between, on one hand, the fields generated by ionosphere and field aligned currents, and, on the other hand, the fields generated in the Earth's core and lithosphere. This selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution. The modelled static core field, secular variation and lithospheric field are all in good agreement with previously published magnetic field models. Order five B-splines are used to model the variation in time of the core field. The energy in the secular acceleration has, therefore, a smooth behaviour in time and increases continuously from 2003.5. Mapping the field acceleration from 2001.5 to 2005.5 reveals its rapid and complex evolution over this time period at the Earth's surface. Due to the applied regularization technique, the acceleration energy in spherical harmonics 6–11 is significantly larger than for other models and we show that such a spectrum is acceptable.