Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra

Abstract

Magnetic field amplification in astrophysics ultimately requires an understanding of MHD turbulence. Kinetic helicity has long been known to be important for large-scale field growth in forced MHD turbulence and has been recently demonstrated numerically to be asymptotically consistent with slow mean field dynamo action in a periodic box. Here we show numerically that the magnetic spectrum at and below the forcing scale is also strongly influenced by kinetic helicity. We identify a critical value, fh, crit, above which the magnetic spectrum develops maxima at a wavenumber of 1 scale and at the forcing scale. For fh, crit. the field peaks only at the resistive scale. Kinetic helicity may thus be important not only for generating a large-scale field, but also for establishing observed peaks in magnetic spectra at the forcing scale. The turbulent Galactic disk provides an example where both large-scale (greater than the supernova forcing scale) fields and small-scale (less than or equal to forcing scale, with peak at forcing scale) fields are observed. We discuss this and the potential application to the protogalaxy, but we also emphasize the limitations in applying our results to these systems.