Symmetry and Magnetic Fields

Ethan Vishniac (University of Saskatchewan)

September 18, 2014

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Abstract: I will explore the role of magnetic helicity, the intrinsic “twistiness” of a magnetic field, in generating large scale magnetic fields in stars and galaxies. I show that there is a large scale flux of magnetic helicity in differentially rotating fluids (like stars) even in the absence of a large scale magnetic field. In an inhomogeneous system, like a galaxy or a star, this leads to the large scale separation of magnetic helicity, which becomes the dominant driver of the dynamo process. We give a qualitative account of the growth of a large scale magnetic field in a realistic system with no initial large scale field. It begins with an incoherent dynamo and a magnetic helicity flux operating independently of any large scale field and ends with a transition from linear growth to saturation due to turbulent mixing. Applying this to the growth of large scale magnetic fields in galactic disks we find that the time required to reach saturation is a few rotational periods. A qualitative application to stellar dynamos convective stars reproduces the scaling laws for magnetic field amplitudes seen in slow and rapid rotators.