GEOMAGNETIC POLARITY REVERSALS, TRANSITION FIELD STRUCTURE, AND CONVECTION IN THE OUTER CORE

Convection in the outer core is dominated by rotation effects. Theory indicates that convection in a rapidly rotating sphere of conducting fluid produces transversely isotropic magnetohydrodynamic induction, with an axis of symmetry parallel to the rotation vector. We have examined the long-term behaviour of azimuthally averaged core magnetic fields maintained by transversely isotropic induction, using numerical calculations of spherical mean-field dynamos. The inductive effect of core convection is represented parametrically by a nonlinear, anisotropic α effect with magntiude Rα concentrated near the inner core boundary to simulate convection driven by inner core crystallization. Duration of polarity epochs is extremely sensitive to Rα. The change from a frequent reversal region to an infrequent reversal regime (superchron) requires only a small increase in Rα. The model predicts a positive correlation between polarity epoch duration and dipole field intensity. -from Authors