Differential Rotation and Magnetism across the HR Diagram

A 3D Babcock-Leighton dynamo model with spots

Not scheduled

1m

132:028 (Nordita)

Seminar

Speaker

Mark Miesch (High Altitude Observatory, National Center for Atmospheric Research, Boulder)

Description

One of the leading paradigms to account for the 11-year solar activity cycle is the Babcock-Leighton (BL) dynamo model, whereby the principle source of mean poloidal field is attributed to the destabilization, rise, emergence, and dispersal of buoyant magnetic flux structures. Though the existence of sunspots and bipolar active regions is central to the operation of a BL dynamo, their presence is only implicit. In practical dynamo models of the solar cycle, the complex physical processes that underlie the BL mechanism are represented as an axisymmetric, nonlocal, poloidal source term typically formulated as a near-surface alpha-effect that operates on the deep toroidal field. Here we present a new class of 3D BL dynamo models that more faithfully captures the essence of the BL mechanism. It is a mean-field dynamo model but it does not rely on a prescribed alpha-effect. Instead, sunspot pairs (with tilts in accordance with Joy's law) are explicitly introduced by modifying the 3D near-surface magnetic field in response to strong toroidal fields in the lower convection zone. The subsequent evolution of these spot pairs due to imposed mean flows and turbulent diffusion produces a mean poloidal field component as envisioned in the original work of Babcock and Leighton. This evolution alone is enough to sustain the dynamo and produce magnetic cycles. The model can be viewed as a unification of 2D (radius/latitude) BL dynamo models with 2D (latitude/longitude) surface flux transport models. I will discuss the construction of the model, initial results, and future plans. I will also briefly summarize complentary efforts currently underway to clarify the input physics that underly this and other mean-field models of solar and stellar dynamos.