Joint Center for Heliophysics/Astroplasmas Seminar

What type of dynamo is the Sun running? By "type of dynamo" we are referring to how the Sun generates, organizes, and transports magnetic flux in such a way as to sustain its 11-year activity cycle (22-year magnetic cycle). For over half a century, the responses to this question have centered on two general paradigms (though each has a host of specific variations). Both paradigms attribute the generation of organized toroidal magnetic flux primarily to the Omega-effect, which converts poloidal flux to toroidal flux by means of differential rotation and amplifies it. The principle distinguishing factor between the two dynamo paradigms is the means by which the mean poloidal field is generated (a necessary ingredient in a viable dynamo). In convective dynamos, it is generated by turbulent convection, made helical by the joint influence of rotation and stratification. In Babcock-Leighton (BL) dynamos, mean poloidal field is generated by the buoyant destabilization, rise, and dispersal of magnetic flux structures that begin their lives as coherent toroidal flux tubes and that are manifested in solar observations as sunspots and related bipolar magnetic regions. A particularly promising variation on the BL paradigm is the concept of a Flux-Transport Dynamo (FTD) model. Loosely, FTD models are BL models in which the global meridional circulation plays an important role in regulating the period of the magnetic cycle. So, we can restate the opening question as; Is the Sun running a convective dynamo or a flux-transport dynamo? What if we were to ask the same question about other stars? Would we get the same answer? Are these paradigms mutually excusive? I will review some recent progress in modeling both convective and BL/FT models and I will discuss some of the outstanding challenges. Highlights from our recent work on convective dynamos include a simulation with a regular cycle and a "grand minimum", the emergence of order with increasing turbulent intensity, and the role of magnetic helicity in magnetic self-organization. Highlights on the FTD front include a novel 3D FTD model and the implications of recent helioseismic results on multi-cell meridional circulation patterns and anomalously low convective amplitudes. Finally, I will argue that progress in understanding these two paradigms can only come from their unification; how does a convective dynamo produce sunspots?