Kinetic helicity needed to drive largescale dynamos
Abstract
Magnetic field generation on scales that are large compared with the scale of the turbulent eddies is known to be possible via the socalled α effect when the turbulence is helical and if the domain is large enough for the α effect to dominate over turbulent diffusion. Using threedimensional 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 of around unity. This implies that largescale dynamo action commences when the normalized helicity is larger than the inverse scale ratio. Our results show that the emergence of smallscale dynamo action does not have any noticeable effect on the largescale dynamo. Recent findings by Pietarila Graham [Phys. Rev. EPLEEE81539375510.1103/PhysRevE.85.066406 85, 066406 (2012)] of a smaller minimal helicity may be an artifact due to the onset of smallscale dynamo action at large magnetic Reynolds numbers. However, the onset of largescale dynamo action is difficult to establish when the kinetic helicity is small. Instead of random forcing, they used an ABC flow with timedependent phases. We show that such dynamos saturate prematurely in a way that is reminiscent of inhomogeneous dynamos with internal magnetic helicity fluxes. Furthermore, even for very low fractional helicities, such dynamos display largescale fields that change direction, which is uncharacteristic of turbulent dynamos.
 Publication:

Physical Review E
 Pub Date:
 April 2013
 DOI:
 10.1103/PhysRevE.87.043104
 arXiv:
 arXiv:1208.4529
 Bibcode:
 2013PhRvE..87d3104C
 Keywords:

 47.65.Md;
 07.55.Db;
 95.30.Qd;
 96.60.Hv;
 Plasma dynamos;
 Generation of magnetic fields;
 magnets;
 Magnetohydrodynamics and plasmas;
 Electric and magnetic fields solar magnetism;
 Astrophysics  Solar and Stellar Astrophysics;
 Physics  Plasma Physics
 EPrint:
 10 pages, 13 figures