Helioseismic Tests of Dynamo Models

Abstract

Helioseismology provides important input and test data for dynamo theories of solar activity by measuring variations of the internal structure and dynamics of the Sun with the activity cycle. Recent results from the GONG network and MDI/SOHO space experiment obtained in 1996-2000 cover the period of transition from the ‘old’ solar cycle 22 to the ‘new’ cycle 23. These data have revealed correlated variations of zones of generation of the solar magnetic fields and zonal shear flows in the convection zone. An attempt is made to detect solar-cycle variations in the tachocline region at the base of the convection zone, which is believed to be the main cite of the solar dynamo. However, the current results are controversial. By comparing the internal rotation profile with the rotation rates of the ‘old’ and ‘new’ magnetic fluxes it has been suggested that both fluxes were generated in a low-latitude zone of the tachocline. Zonal flows and structures migrating towards the equator and probably associated with dynamo waves have been detected in the solar interior. They show a curious sudden displacement towards higher latitudes in the second-half of 1999. This is not explained by the current dynamo theories and indicates a complex dynamical behavior of the solar dynamo. Recently developed methods of local helioseismology have allowed us to investigate processes of formation of active regions and sunspots. In particular, converging vortex flows have been found beneath a sunspot in agreement with Parker’s theory.