The acoustic and magnetic solar cycle

Abstract

AbstractThe frequency variation of global modes provides information about the solar structure and the rotation rate of the solar interior. The solar‐cycle variation of the rotation rate, the so‐called torsional oscillation, extends throughout the convection zone. Alternating bands of faster‐ and slower‐than‐average rotation move from mid‐latitudes toward the equator during the solar cycle. The variation of the meridional flow, measured with local helioseismic techniques, is of similar amplitude. The zonal‐ and the meridional‐flow variation related to a new cycle are noticeable years before magnetic activity of the new cycle is present at the surface. The sound‐speed variations show a small, but significant variation with the solar cycle near the base of the convection zone, which might be interpreted as a change in magnetic field strength. The mode characteristics, such as acoustic radius and frequency shifts, provide information about changes in the near‐surface layers, where acoustic modes are excited (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)