The velocity field at the Earth’s core–mantle boundary

Aiming to recover the fluid flow at the Earth’s Core–Mantle Boundary (CMB) and the possible relation with geomagnetic jerks, we use the global “gufm1” model which makes use of the expansion of each element of the geomagnetic field in spherical harmonics, covering the last 400 years. Under the frozen–flux approximation, the induction equation for the radial component of the geomagnetic field (Br) is employed due to the continuity of this component through the CMB. The velocity field at the CMB is essentially two-dimensional and is separated into toroidal and poloidal parts. Then these parts as well as the radial component of geomagnetic field and its secular variation are expanded in spherical harmonics series and are substituted in the induction equation. By involving the Elsasser and Gaunt integrals, a system of algebraic equations is obtained. Solving this system of equations we recover the spherical harmonics coefficients of the two parts of the velocity field. The jerks, sharp change in the trend of secular variation of the geomagnetic field, are thought to be related to sharp changes in the velocity field at the CMB. There are jerks for which such changes are obtained, while in other cases this relation is not confirmed. Thus jerks may not be related to the velocity field at the CMB or small-scale velocity field (not studied here) may have its contribution.