Flux-transport and mean-field dynamo theories of solar cycles

Abstract

Abstract. We point out the difficulties in carrying out direct numerical simulation of the solar dynamo problem and argue that kinematic mean-field models are our best theoretical tools at present for explaining various aspects of the solar cycle in detail. The most promising kinematic mean-field model is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock-Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. Depending on whether the diffusivity is high or low, either the diffusivity or the meridional circulation provides the main transport mechanism for the poloidal field to reach the bottom of the convection zone from the top. We point out that the high-diffusivity flux transport dynamo model is consistent with various aspects of observational data. The irregularities of the solar cycle are primarily produced by fluctuations in the Babcock-Leighton mechanism and in the meridional circulation. We summarize recent work on the fluctuations of meridional circulation in the flux transport dynamo, leading to explanations of such things as the Waldmeier effect.

Can we detect the stellar differential rotation of WASP-7 through the Rossiter–McLaughlin observations?

“…For some cases, usually evolved stars, an anti-solar differential rotation was measured (Kóvári et al 2017;Kővári et al 2015;Harutyunyan et al 2016). Differential rotation is a key ingredient in stellar dynamo models, because it contributes significantly to the generation and maintanance of the stellar magnetic field (Choudhuri 2013). The origin of stellar dynamos may be traced to the coupling between the stellar rotation, convection and conduction happening in the stellar plasma.…”

“…In a second moment, the field lines are twisted due to an effect known as α-effect and they are unrolled, until the field returns to be poloidal. The overall stellar activity cycle strongly depends on the differential rotation pattern of the star (Choudhuri 2013), though the underlying mechanisms which generate and sustain this non-rigid rotation are still poorly understood. Measuring this property for a large sample of stars (different spectral types and ages) provides a better understanding of the formation, and maintenance, of stellar magnetic activity and activity cycles as a whole.…”