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Thursday, June 14, 2012
2:00 PM - 3:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Postdoc Seminar

Astrophysical Dynamos and a possible explanation of Galactic magnetic fields

Jonathan Graham
T-3 and CNLS

I will introduce turbulent dynamo theory for astrophysical (flow dominated as opposed to magnetically dominated plasma confinement) dynamos. A key ingredient in this theory is the role of kinetic helicity in the production of large-scale magnetic fields. Galaxies have very little helicity, <10%, in their turbulent driving and paradoxically exhibit very large-scale (on the order of kilo-parsecs) magnetic structures. I will present the results of numerical experiments that suggest scale separation (the ratio between the system size and the forcing scale) reduces the required helicity to generate large-scale fields. Classical mean field theory does not adequately predict our results. We propose that injected kinetic helicity must overcome the Lorentz force feedback from a contemporaneous small-scale dynamo to produce large-scale field. The resulting theory fits the data well and predicts that the required helicity drops as scale separation to the negative third power for large separations. This potentially reconciles the disparity between a pile up of small-scale magnetic energy in non-helical dynamo simulations and Galactic observations.

Host: Kipton Barros, T-4 and CNLS