Abstract

Fluid motions in the Earth’s outer core are dominated by the so-called magnetostrophic balance, in which Coriolis, pressure-gradient, Lorentz and buoyancy forces are all important, but inertial and viscous effects are extremely small. I will show that in general this balance leads to singularities on the tangent cylinder, the cylinder parallel to the rotation axis and touching the inner core. These singularities can be avoided either by restoring viscous effects, however small, or by having the Lorentz force adjust to satisfy a hierarchy of constraints on the tangent cylinder. We suggest that most of the time the internal magnetic field will indeed adjust to satisfy at least the leading one or two of these constraints, and that occasional breakdowns in satisfying these constraints may be a mechanism for triggering torsional oscillations emanating from the tangent cylinder, as indicated by recent observational work.