Abstract Title: *
Rectangular torus dynamo model and magnetic fields in the outer rings of galaxies
Authors: *
E.A.Mikhailov, A.D.Khokhryakova
Institution: *
M.V.Lomonosov Moscow State University
Preferred format: *
Oral
Abstract Text: *
The existence of the magnetic fields in the main parts of spiral galaxies has been proved using
both observations and theoretical models. Their generation is described by the dynamo theory.
The magnetic field growth is connected with differential rotation and alpha-effect [1]. The
equations for the magnetic field are complicated, so different two-dimensional models are often
used. For the main part of the galaxy the dynamo mechanism is usually described using the
no-z approximation. It takes into account that the galaxy disc is very thin, so the ratio between
the half-thickness and the radius of the galaxy can be taken as a small parameter, some of the
derivatives may be replaced by algebraic expressions and it is possible to consider only the
components that lie in the equatorial plane [2]. Some of the galaxies have so-called outer rings,
and we can assume that the magnetic fields exist there, too. First estimates were made taking
the no-z approximation [3], but for the rings the thickness and the radial lengthscale are quite
comparable. So we should use the model that takes into account z-component. In earlier works
the model for the magnetic field in round torus was constructed [4]. However, it is much better to
describe the outer ring where the height has the same order as the width, but it has smaller
value. We have constructed the model for the rectangular torus dynamo that can describe the
magnetic field generation in the outer ring better than no-z model and the ring torus dynamo
model. The critical values of the kinematic parameters that are connected with the possibility of
the field growth have been obtained. We have shown that the field can have both the
quadupolar and dipolar symmetry. Typical configurations of the field are presented.
References
1. T. Arshakian, R. Beck, M. Krause, and D. Sokoloff, Astron. and Astrophys., 494, 21, 2009.
2. D.Moss, Mon. Not. R. Astr. Soc., 275, 191, 1995.
3. D. Moss, E. Mikhailov, O.Silâ€™chenko, D.Sokoloff, C.Horrellou, and R.Beck, Astron. Astrophys., 592, A44, 2016.
4. E.A.Mikhailov, Astron. Rep., 61, 739, 2017.