Galactic dynamos supported by magnetic helicity fluxes

Abstract

We present a simple semi-analytical model of nonlinear, mean-field galactic dynamos and use it to study the effects of various magnetic helicity fluxes. The dynamo equations are reduced using the `no-$z$' approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity flux. We discuss the implications and interplay of two types of magnetic helicity flux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising from anisotropy of turbulence as suggested by Vishniac & Cho(2001). We argue that the latter is significant if the galactic differential rotation is strong enough: in our model, for $\Rw\la-10$ in terms of the corresponding turbulent magnetic Reynolds number. We confirm that the intensity of gas outflow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic field supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity flux can boost magnetic field further to achieve energy equipartition with turbulence. For stronger outflows that may occur in starburst galaxies, the Vishniac-Cho flux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic field of at least $\simeq1\mkG$ to be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.Comment: Accepted for publication in MNRA

The large scale magnetic field structure of the spiral galaxy NGC 5775

“…A galactic wind may generally play an important role in the amplification of the largescale magnetic field by the mean-field dynamo, as it may solve the helicity problem of dynamo action (e.g. Sur et al 2007). …”

Magnetic helicity fluxes in interface and flux transport dynamos

“…Nevertheless this constraint may be lifted if the system is able to rid itself of small-scale helicity through at least one of several ways such as open boundaries, advective, diffusive and shear-driven fluxes (Shukurov et al 2006;Zhang et al 2006;Sur et al 2007;Käpylä et al 2008;Brandenburg et al 2009;Guerrero et al 2010). Even though the helicity constraint in direct numerical simulations (DNS) of dynamos with strong shear have been clearly identified, the results can be matched with mean field models with a weaker algebraic quenching than α 2 dynamos .…”

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

“…Recall that Shukurov et al (2006) in Figure 13 considered an advective flux term that is another candidate flux term that emerges from mean field theory. Sur et al (2007) also assessed the role of the Vishniac-Cho flux semi-analyticially in the galactic context and found it can be helpful if the mean magnetic field is above a threshold value to begin with. Vishniac & Shapovalov (2014) considered an isotropically forced periodic box with linear shear, (without the coriolis force) and find that the Vishniac-Cho flux is dominant.…”

“…a dynamo for which the toroidal field growth is dominated by shear from differential rotation), the quenching is such that the field starts to grow and then decays rapidly (e.g. Shukurov et al 2006, Sur et al 2007). That helicity fluxes might alleviate quenching and sustain the EMF (e.g.…”