Not Much Helicity is Needed to Drive Large Scale Dynamos

Jonathan Pietarila Graham, jpietarilagraham@mailaps.org, Los Alamos National Laboratory, USA
Eric Blackman,University of Rochester, USA
Pablo Mininni,NCAR, USA
Annick Pouquet, NCAR, USA

Abstract
Understanding the in situ amplification of large scale magnetic fields in turbulent astrophysical rotators has been a core subject of dynamo theory. When turbulent velocities are helical, large scale dynamos that substantially amplify fields on scales that exceed the turbulent forcing scale arise, but the minimum sufficient fractional kinetic helicity f_hC has not been previously well quantified. Using direct numerical simulations for a simple helical dynamo, we show that f_hC decreases as the ratio of forcing to large scale wave numbers k_F/k_min increases. From the condition that a large scale helical dynamo must overcome the backreaction from any non-helical field on the large scales, we develop a theory that can explain the simulations. For k_F/k_min >= 8 we find f_hC < 3%, implying that very small helicity fractions strongly influence magnetic spectra for even moderate scale separation.