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  • Cited by 33
  • Print publication year: 2007
  • Online publication date: August 2009

13 - The solar dynamo and the tachocline

Summary

The tachocline is believed to play a crucial role in the dynamo that maintains magnetic activity in the Sun. We first review the observational properties of the 11-year activity cycle and the 22-year magnetic cycle, as well as of the recurrent grand minima, with a characteristic 200-year timescale, that are revealed by proxy records. Then we discuss dynamo mechanisms, including differential rotation (the ω-effect), the net effect of gyrotropic motions (the α-effect) and flux transport by both large-scale motions (e.g. meridional flows) and small-scale processes (e.g. turbulent transport). Next we consider the location of the solar dynamo, comparing models with dynamo action distributed throughout the convection zone, located near the surface or (most likely) concentrated near the interface between the convective and radiative zones. Local pockets of strong field can then escape from the vicinity of the tachocline and emerge through the photosphere as active regions. The nonlinear back-reaction of the magnetic field affects transport coefficients (both α and the turbulent diffusivity β) and also drives the zonal flows that are observed. Furthermore, it provides a mechanism for the modulation associated with grand minima. We conclude with our picture of the relationship between convection, differential rotation and the dynamo in the tachocline.

Observations

The Sun exhibits cyclic magnetic activity, as do other slowly rotating stars with deep convective envelopes. This activity is manifested in the sunspot cycle, which has an average period of 11 years, as shown in Figure 13.1.