Simulations of the Polar Field Reversals during Cycle 22

Abstract

The revised Mount Wilson synoptic magnetic data for the period September 1987 through March 1996 are used as the basis of numerical simulations of the evolution of both the northern and southern polar magnetic fields during the reversal and declining phases of cycle 22. The simulations are based on numerical solutions of the flux-transport equation which involve, as parameters, the maximum meridional flow speed, v0, and the supergranule diffusivity, κ. By matching characteristics of the observed and simulated fields, such as the observed reversal times, the evolution of the net flux above 60 °, and the migration of the polar crown, empirical values of these parameters, i.e., v0=11 m s−1,κ=600 km2 s−1, may be determined. Further, the observed decrease in the mean net flux above 60 ° during the late declining phase of cycle 22 can be simulated only by increasing the diffusivity to 900 km2 s−1. However, direct observations of the supergranule velocities yield values of the diffusivity of order 200 km2 s−1, and we show that the inclusion of a pattern of emerging bipoles in the simulations can increase the diffusion of these fields and that, together with a more realistic value of the diffusivity, it is possible to reproduce qualitatively the features of the observed polar field reversals.