Fluid flow near the surface of Earth's outer core

Maps of the fluid flow at the core surface are important for a number of reasons: foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models; from the flow, the force balance at the top of the core can, at least in part, be deduced; the flow can provide short-term predictions of the secular variation; the flow is important in understanding changes in the length of day; and constraints on lateral temperature variations and topography at the core-mantle boundary may be derived. Unlike the case of mantle convection, only very small lateral variations in core density are required to drive the flow; these density variations are too small (by several orders of magnitude) to be imaged seismically, so instead we use the geomagnetic secular variation to infer the flow. Despite considerable recent progress in mapping the core flow, substantial differences exist between maps produced by different researchers. Here we examine the possible underlying reasons for these differences, paying particular attention to the inherent problems of nonuniqueness. We focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications.