Imaging core flow from geomagnetic secular variation: Consequences for core-mantle interactions and geomagnetic dipole moment changes

Flow in the fluid outer core just below the core-mantle boundary is inferred from geomagnetic secular variation data, assuming frozen magnetic flux, tangential geostrophy, and a new physical assumption termed helical flow. Helical flow, in which tangential divergence correlates with radial vorticity, removes non-uniqueness in the inversion of the magnetic induction equation. My flow solutions using geomagnetic field models from the 2000 Oersted and 1980 Magsat satellites resemble previous flow models, but contain more flow along contours of radial magnetic field. I invert geomagnetic secular variation between 1895-1985 to isolate the time-average and time-dependent parts of the flow. The most prominent flow structure is a large anti-cyclonic vortex in the southern hemisphere. Time-average zonal flow outside the inner core tangent cylinder is generally westward in the southern hemisphere but nearly zero in the northern. Westward polar vortices occur inside the tangent cylinder. Mantle driving seems responsible for the mid-latitude asymmetry in the zonal core flow; core driving is responsible for the flow at high latitudes. Changes in the core's angular momentum calculated from my time-dependent core flow agree well with decade-scale length-of-day measurements. I fit the time-dependent flow to a torsional oscillations model with periods 88 and 48 years. I test the quality of my core flow imaging method by inverting synthetic magnetic secular variation data from numerical dynamo models, and find that my method delineates most large-scale flow features. The correlation coefficient is large for a dynamo case with large-scale flow and magnetic field pattern, but degrades substantially in more complex cases. Including tangential magnetic diffusion improves flow recovery; however, unmodeled radial diffusion and data truncation effects cause severe artifacts. Finally, I combine geomagnetic secular variation data, time-dependent core flow, and dipole moment time-evolution equations to identify mechanisms of geomagnetic dipole moment change between 1895-1985. Meridional advection and radial magnetic diffusion are comparable and account for essentially all the observed moment decrease. Between 1895-1965, effects of tangential advection and radial diffusion on the equatorial moment cancel, allowing the geomagnetic tilt to remain nearly constant. Since 1970, the two mechanisms have both reduced the equatorial moment, causing the tilt decrease.