THE SOLAR SURFACE TOROIDAL MAGNETIC FIELD

摘要

The solar cycle of magnetic activity is thought to be a consequence of a dynamo process in which a dipole field produces a toroidal field from differential rotation (called the Ω-effect) and a twisting process produces a dipole field from the toroidal field (called the α-mechanism). These two magnetic field components are alternately destroyed and recreated in a cycle that lasts in total 22 years. Although the dipole field of the Sun has long been observed and studied, the toroidal field has never before been detected or measured. Our analysis uses solar rotation to yield meridional and east-west components of velocity and magnetic field vectors from the observed line-of-sight projection of the field. Our analysis of 18.5 yr of data from the 150 foot solar tower telescope on Mount Wilson using this method reveals for the first time a clear signal of a reversing toroidal magnetic field on the solar surface with strength comparable to that of the well-observed dipole component of the global magnetic field. The meridional velocities show a zone of convergence near latitudes of 60° during much of the observed period. Such flow convergence implies the subsidence of the toroidally magnetized fluid in this zone. If the toroidal field occupies the bulk of the polar regions of the Sun's convective envelope, then there is enough magnetic flux to reverse and rebuild the toroidal field at the convective-radiative interface known as the tachocline that is at the inner boundary of the Sun's convective envelope. These two steps—the creation of a toroidal field at high latitudes and a mechanism to reverse the tachocline toroidal field-are parts of the dynamo process that are prominent in models but have not previously had direct observational support.