Velocity and magnetic fields in the Earth's core estimated from the geomagnetic field

Core-mantle relative motion and coupling

“…The phenomenon can also be evaluated using Stokes' theorem that vorticity integrated over a closed surface is equal to circulation around its periphery. Magnetic field anomalies carried by these larger vortices will be seen as magnetic vortices moving slowly westward with anticyclonic rotation, as seen in the charts of Voorhies (1995) and Matsushima (1995). Their figures, if steady state, infer a quasi‐steady state of vorticity generation and its dissipation by viscosity and possible magnetic field generation.…”

“… Flow patterns at or near the CMB estimated from geomagnetic field models—top drawing from Voorhies (1995), bottom drawing from Matsushima (1995). Each is an estimate of surface flow at the core–mantle boundary using a ‘frozen field’ assumption that horizontal components of magnetic field at the CMB represent similar flows of conducting fluid (see references for details).…”

“…’ We did not predict that vorticity might coalesce flux mostly into several large gyres in each hemisphere, although that result is consistent with vortex theory. The estimates of Voorhies (1995) and of Matsushima (1995) use the property that a magnetic anomaly will remain fixed to the same particles of any fluid with high electrical conductivity. Fluid vortices in a nearly inviscid fluid have a similar property of remaining with the same fluid particles.…”

Secular Variations of the Planetary‐Scale Geomagnetic Anomalies

“…This conception leads to an estimation of the characteristic velocity in the core motion, ∼ 4 × 10 −4 m/s. This velocity, in turn, is used to calculate dimensionless numbers in geodynamo theory, which determine the principal features of the core fluid motion [16,20,21] .…”

“…If the Earth's core is regarded as a perfect conductor, the diffusion term in the magnetic induction equation, which describes the dynamic behaviours in the outer core, can be considered as neglectful compared with the magnetoadvection term, and the frozen-flux hypothesis is adopted for the outer core fluid. This assumption makes it possible to deduce the fluid flow at the CMB from the magnetic field and its secular variations at the CMB [7,16] . In order to resolve the inherent nonuniqueness in using the geomagnetic secular variation to map the core flow, additional assumptions must be made about the nature of the flow.…”