Yukiko Yokoyama

A model of the geomagnetic 60-year variation

A model of the geomagnetic 60-year variation is proposed. The 60-year variation is regarded as a magnetohydrodynamic oscillation originating in a thin layer at the top of the core. Two types of magnetic field and two types of fluid motion are mainly considered in the layer, which are poloidal magnetic mode (1, 0), toroidal magnetic mode (2, 0), toroidal velocity mode (1, 0), and poloidal velocity mode (2, 0). When the magnetic force and the Coriolis force are dynamically dominant, these four fields make a closed system and cause magnetohydrodynamic oscillations in the layer. Steady magnetic fields of higher modes generated in the deeper part of the core induce fluctuating fields when they pass through the layer. The induced magnetic fields have a common period and two different types of phase. Observed amplitudes of these fields are mostly explained when the maximum velocity in the layer is 10−4 m/s.