A unified large/small-scale dynamo in helical turbulence

Abstract

We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong smallscale dynamo action. During the kinematic stage, the unified large/small-scale dynamo grows fields with a shape-invariant eigenfunction, with most power peaked at small scales or large k, as in Subramanian & Brandenburg (2014). Nevertheless, the largescale field can be clearly detected as an excess power at small k in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength B, relative to the total rms field B rms , decreases with increasing magnetic Reynolds number, Re M . However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the small-scale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the Re M -dependent quenching of B/B rms . These results are obtained for 1024 3 DNS with magnetic Prandtl numbers of Pr M = 0.1 and 10. For Pr M = 0.1, B/B rms grows from about 0.04 to about 0.4 at saturation, aided in the final stages by helicity dissipation. For Pr M = 10, B/B rms grows from much less than 0.01 to values of the order the 0.2. Our results confirm that there is a unified large/small-scale dynamo in helical turbulence.

Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution

“…As discussed further below, it is also possible that we see evidence of Kazantsev scaling with a k 3/2 subrange at k Mm < 0.03, which would be indicative of a small-scale dynamo (SSD) (Kazantsev 1968). Indeed, astrophysical dynamos operating at high magnetic Reynolds numbers are expected to exhibit a unified version of dynamo action that combines elements of both small-scale and large-scale dynamos (Subramanian 1999;Subramanian & Brandenburg 2014;Bhat et al 2016). On the other hand, the bihelical magnetic field expected from an α effect is usually expected to imply an actual increase in magnetic power at small k; see Figure 3 of Brandenburg (2001).…”

Filament formation in wind–cloud interactions– II. Clouds with turbulent density, velocity, and magnetic fields

“…As a result, enhancement and annihilation of magnetic energy occur in such environments through dynamo action (e.g., see Subramanian 1999;Brandenburg & Subramanian 2005;Federrath et al 2014), and reconnection events (e.g., see Lazarian & Vishniac 1999;Lazarian 2014), respectively. Second, turbulence also plays an essential role in shaping the ISM and influencing the processes occurring in it, such as star formation (e.g., see Mac Low & Klessen 2004;Santos-Lima et al 2010;Santos-Lima, de Gouveia Dal Pino & Lazarian 2012;Kainulainen, Federrath & Henning 2013;Padoan et al 2014;Leão et al 2013;Salim, Federrath & Kewley 2015, Krumholz & Kruijssen 2015, dynamo-regulated growth of magnetic fields (e.g., see Schleicher et al 2013;Santos-Lima et al 2014;Schober et al 2015;Bhat, Subramanian & Brandenburg 2016), and acceleration and diffusion of cosmic rays (e.g., see Yan & Lazarian 2002;Weidl et al 2015).…”

A new constraint on mean-field galactic dynamo theory

“…Both are expected to occur simultaneously in turbulent astronomical bodies such as stars and galaxies. There have been some attempts to understand these in a single unified framework (Subramanian 1999;Subramanian & Brandenburg 2014;Bhat et al 2016). Nevertheless, given sufficient scale-separation, it seems reasonable to treat them as distinct, yet interconnected, entities (Brandenburg & Subramanian 2005a;Brandenburg et al 2012a).…”