The evolution of the core-surface flow over the last seven thousands years

We analyze the core-surface flow over the last seven thousands years derived from CALS7k.2, a continuous model of the geomagnetic field and its secular variation for the period 5000 BC to 1950 AD. This model relies on indirect observations of the geomagnetic field through the sampling of natural archives (lake sediments, lavas) and archaeological artifacts. An extensive comparison between gufm1, a historical field model spanning the interval 1590 to 1990, and CALS7k. 2 is carried out. While both are similar in their descriptions of the large scale core field, their agreement compares poorly in terms of secular variation. The differences are primarily caused by the different data quality used to build these models. Such a comparison is essential for assessing the reliability of the results of flow inversion. We seek instantaneous core flow solutions that are purely toroidal and tangentially geostrophic for which we can reasonably neglect the effect of magnetic diffusion. Our flow solutions suggest that the core-surface flow undergoes different regimes of zonal flow direction, with epochs of mainly westward flow alternating with epochs of mainly eastward flow. The episodes of eastward and westward flow are robust as they are obtained for purely toroidal and tangentially geostrophic flow. The changes in the large scale zonal flow direction seem to recur with a typical period of between 540 and 1050 years. Variations in thermal and magnetic winds occurring on timescales comparable with the convective overturn of the liquid outer core could account for the observed zonal flow variations.