Nordita Astrophysics seminars

How much helicity is needed to drive large-scale dynamos?

by Simon Candelaresi (Nordita)

122:026 ()


Magnetic field generation on scales large compared with the scale of the turbulent eddies is known to be possible via the so-called alpha effect when the turbulence is helical and if the domain is large enough for the alpha effect to dominate over turbulent diffusion. Using three-dimensional turbulence simulations, we show that the energy of the resulting mean magnetic field of the saturated state increases linearly with the product of normalized helicity and the ratio of domain scale to eddy scale, provided this product exceeds a critical value around unity. This implies that large-scale dynamo action commences when the normalized helicity is larger than the inverse scale ratio. Recent findings of a smaller minimal helicity may be due to the onset of small-scale dynamo action at large magnetic Reynolds numbers. The onset of large-scale dynamo action is difficult to establish when the kinetic helicity is small.