A physical model of the galactic large-scale magnetic field

We present a simple, yet realistic and flexible model for large-scale magnetic fields in astrophysical discs (in particular, spiral galaxies and accretion discs) obtained as an approximate solution of the averaged induction equation in the turbulent disc and halo. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields of the model are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields separately can have either dipolar or quadrupolar symmetry. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. Although the range of magnetic configurations accessible with the model is not restricted to mean-field dynamo solutions, the model can be used for a simple simulation of a time-dependent magnetic field generated by dynamo action. The model is implemented as a publicly available software package GALMAG which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model's magnetic field.

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