Reversals of the Solar Source Surface Magnetic Field and of the Planets

Abstract

The solar magnetic field on a surface of 2.5 solar radii, known as the source surface, can be approximated by a dipole field. This dipolar field rotates by 180° meridionally throughout a sunspot cycle, although the polar unipolar fields on the photosphere do not show such a shift across the equator. It is of interest to assume that the solar source surface corresponds to the surface of the magnetized planets and that the photosphere corresponds to the core surface. An advantage of the solar situation is that one can directly observe the photospheric magnetic fields which correspond to the magnetic fields of the core surface. We examine why the dipolar field on the source surface is inclined with respect to the rotation axis and why the inclination angle changes from 0° to 180° (or 180° to 0°) during a sunspot cycle. We assume that the main dipole is axially aligned with the rotation axis, because the unipolar fields in the polar regions do not shift across the equator. It can be shown that the inclination and its change arise from the growth and decay of a few dipolar sources oriented in an east-west direction near the equator. The combined field of the axially aligned dipole and the equatorial dipoles provides an inclined dipole on the source surface. The equatorial dipoles are identified as large-scale weak dipolar fields which contain active regions. The rotation of the dipole on the source surface arises from a relative change of strength of the equatorial dipoles and the axial dipole. On the basis of the above study of the solar situation, we suggest that the inclination and eccentricity of the dipole axis of the magnetized planets (including the earth) arise from the growth and decay of equatorial dipoles near the core surface. The reversal of the earth’s dipole field may be explained in a way similar to the reversal of the dipole field on the source surface.