Theoretical Basis of Stellar Activity Cycles

Numerical simulations of hydromagnetic turbulence in the presence of shear and/or convection have given us new clues as to how the solar dynamo might work. Simulations suggest that here is signiicant large-scale magnetic eld generation at the bottom of the convection zone, where the radial shear is largest. The nature of the dynamo in the simulations seems to be qualitatively similar to an-dynamo. However, the origin of the eeective in the simulations is not thermal convection, but magnetobuoyancy and magnetic shear instabilities. This results in a negative -eeect. The eeciency of the-eeect on the one hand, and losses on the other, should increase as the eld strength increases. It is argued that this could lead to an increasing ratio between cycle and rotation fre-queniesc with increasing eld strength, as is indicated by the stellar cycle data of Baliunas and collaborators. Furthermore, to explain the cycle data for active stars one has to invoke another, as yet unknown, type of magnetic instability for which both and the ux loss are abruptly reduced above a certain eld strength. However, details of this speculation are still unclear.