Diffusive magnetic images of upwelling patterns in the core
A new technique for imaging convection in the outer core is presented, which is based on mean field electrodynamics. The method assumes a frozen magnetic flux balance for the global-scale part of the fluid velocity in the outer core and a diffusive magnetic flux balance for smaller-scale parts of the fluid velocity. The diffusive flux balance implies that local highs and lows in the magnetic field correspond approximately to fluid downwellings and upwellings, respectively. Diffusive flux produces good images of the pattern of upwellings and downwellings near the outer boundary in numerical dynamos, except near the equator. The amplitudes of the upwellings in the diffusive magnetic images correspond to the upwelling amplitudes below the Ekman boundary layer in the numerical dynamos. We have applied this technique to the Oersted initial 2000 core field model and to time-averaged models of the historical geomagnetic field on the core-mantle boundary. It indicates that high-latitude, high magnetic flux density patches correspond to downwellings in the outer core.