Sensitivity of a Babcock-Leighton Flux-transport Dynamo to Magnetic Diffusivity Profiles

Details Sensitivity of a Babcock-Leighton Flux-transport Dynamo to Magnetic Diffusivity Profiles Sensitivity of a Babcock-Leighton Flux-transport Dynamo to Magnetic Diffusivity Profiles

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009spd....41.0809z&link_type=abstract

American Astronomical Society, SPD meeting #41, #8.09

Astronomy and Astrophysics

Astronomy

Scientific paper

We studied the influence of various magnetic diffusivity profiles on the evolution of poloidal and toroidal magnetic fields in a flux transport dynamo model for the Sun. The diffusivity (magnetic permeability/resistivity) is the least known ingredient in kinematic,
flux-transport dynamo models. We mathematically constructed various theoretical profiles of the depth-dependent diffusivity, based on constraints from mixing length theory and turbulence, and on comparisons of observed solar poloidal field evolution with results from flux-transport dynamo modeling. We then studied the effect of each profile in the cyclic evolution of dynamo fields in the Sun, by solving the mean-field dynamo equations. We compared our theoretical time-latitude diagrams from model output, with solar synoptic maps. Finally, we investigated how the toroidal and poloidal field structures inside the convection zone are influenced by different diffusivity profiles. Based on these simulations, we study which depth dependent diffusivity profiles may be most relevant for magnetic flux evolution in the Sun.
We found, surprisingly, that the solar dynamo depends relatively weakly on the *shape* of the magnetic diffusivity profile; and, for a fixed location of diffusivity gradient, the evolution of magnetic field and dynamo are slightly sensitive to the actual *slope* of the gradient. Most interestingly, we find that the actual *location* of a given diffusivity gradient can have profound influences. In particular, without a gradient close enough to the tachocline, insufficient toroidal field may be carried up by the meridional circulation to sustain normal dynamo cycles.
We gratefully acknowledge the use of Mausumi Dikpati's dynamo model, and critical guidance and helpful feedback from Dr. Dikpati and Peter Gilman (HAO/NCAR, Boulder CO, 80301).
This work was supported by NASA grants NNH05AB521, NNH06AD51I, the NCAR Director's opportunity fund, the HAO/NCAR Visitor Program, and TESC's Sponsored Research program.
The National Center for Atmospheric Research is sponsored by the NSF.

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