Rapid core field variations during the satellite era: Investigations using stochastic process based field models

We present a new ensemble of time-dependent magnetic field models constructed from satellite and observatory data spanning 1997-2013 that are compatible with prior information concerning the temporal spectrum of core field variations. These models allow sharper field changes compared to traditional regularization methods based on minimizing the square of second or third time derivative. We invert satellite and observatory data directly by adopting the external field and crustal field modelling framework of the CHAOS model, but apply the stochastic process method of Gillet et al. (2013) to the core field. We report spherical harmonic spectra, comparisons to observatory monthly means, and maps of the radial field at the core-mantle boundary, from the resulting ensemble of core field models. We find that inter-annual fluctuations in the external field (for example related to high solar-driven activity, most prominent at mid-to-high latitudes) can pollute our core field models if not not handled correctly. We discuss the nature of this troublesome external signal and describe strategies to handle it. Finally, coming back to the question of rapid core field variations, we give an example of how physical hypotheses can be tested by asking questions of the entire ensemble of core field models, rather than by interpreting any single model.