Open AccessGalactic dynamos supported by magnetic helicity fluxes
Author(s) -
Sur Sharanya,
Shukurov Anvar,
Subramanian Kandaswamy
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2007.11662.x
Subject(s) - physics , dynamo , induction equation , astrophysics , equipartition theorem , magnetic helicity , magnetic field , magnetic reynolds number , differential rotation , dynamo theory , solar dynamo , helicity , magnetic energy , magnetohydrodynamics , magnetic flux , quantum electrodynamics , quantum mechanics , stars , magnetization
We present a simple semi‐analytical model of non‐linear, 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 non‐linear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where non‐linear 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. We argue that the latter is significant if the galactic differential rotation is strong enough: in our model, for R ω ≲−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 ≃1 μG 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.