On the earth's core ‐ mantle interface

Recent work on the magnetohydrodynamics of the liquid core of the Earth and on the interaction between core motions and the overlying solid mantle has yielded a number of novel results. A new type of magneto‐hydrodynamic wave has been discovered through an attempt to explain the geomagnetic secular variation; the nature of the horizontal stresses at the core‐mantle interface that are implied by the ‘decade’ fluctuations in the length of the day has been elucidated; and evidence pointing to a previously unsuspected correlation between the Earth's gravitational and magnetic fields has been found. Global‐scale undulations in level of the core‐mantle interface that are so shallow (a kilometre or so) as to be unresolvable by contemporary seismological techniques are strongly implied by some of these results.