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Magnetic flux emergence and solar dynamo models

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If you have a question about this talk, please contact Dr. Aurélien Crida.

We present the first 3D MHD study in spherical geometry of the non-linear dynamical evolution of magnetic flux tubes in a turbulent rotating convection zone. These numerical simulations use the anelastic spherical harmonic (ASH) code. We seek to understand the mechanism of emergence of strong toroidal fields through a turbulent layer from the base of the solar convection zone to the surface as active regions, with a particular focus on the effects of self-consistently generated mean flows.

Weak field cases indicate that downflows and upflows control the rising velocity of particular regions of the rope and could in principle favour the emergence of flux through Ω-loop structures. For these cases, we focus on the orientation of bipolar regions and find that sufficiently arched structures are able to create bipolar regions with a predominantly East-West orientation. Meridional circulation seems to determine the trajectory of the magnetic rope when the field strength has been significantly reduced near the top of the domain. Local field emergence also feeds back on the horizontal flows thus perturbing the meridional circulation via Maxwell stresses. Finally differential rotation makes it more difficult for tubes introduced at low latitudes to emerge at the surface.

We reintroduce these 3D results in 2D mean-field Babcock-Leighton flux-transport dynamo models, and in particular the time-delay caused by the emergence of toroidal structures from the base of the CZ to the surface. We find that these time delays introduce a strong modulation of the solar cycle amplitude, even when strong and thus rapidly rising flux tubes are considered. This modulated activity and the resulting butterfly diagram are then more compatible with observations than the standard Babcock-Leighton model.

This talk is part of the DAMTP Astro Lunch series.

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