Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An investigation of magnetic field distortions in accretion discs around
  neutron stars. I. Analysis of the poloidal field component   We report results from calculations investigating stationary magnetic field
configurations in accretion discs around magnetised neutron stars. Our strategy
is to start with a very simple model and then progressively improve it
providing complementary insight into results obtained with large numerical
simulations. In our first model, presented here, we work in the kinematic
approximation and consider the stellar magnetic field as being a dipole aligned
with the stellar rotation axis and perpendicular to the disc plane, while the
flow in the disc is taken to be steady and axisymmetric. The behaviour in the
radial direction is then independent of that in the azimuthal direction. We
investigate the distortion of the field caused by interaction with the disc
matter, solving the induction equation numerically in full 2D. The influence of
turbulent diffusivity and fluid velocity on the poloidal field configuration is
analysed, including discussion of outflows from the top and bottom of the disc.
We find that the distortions increase with increasing magnetic Reynolds number
R_m (calculated using the radial velocity). However, a single global parameter
does not give an adequate description in different parts of the disc and we use
instead a `magnetic distortion function' D_m(r,\theta) (a magnetic Reynolds
number defined locally). Where D_m<<1 (near to the inner edge of the disc)
there is little distortion, but where D_m>1 (most of the rest of the disc),
there is considerable distortion and the field becomes weaker than the dipole
would have been. Between these two regions, there is a transition zone where
the field is amplified and can have a local minimum and maximum. The location
of this zone depends sensitively on the diffusivity. The results depend very
little on the boundary conditions at the top of the disc.

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The categories of this paper are: econ
7
The categories of this paper are: hep-th
1
The categories of this paper are: cs
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The categories of this paper are: eess
36
The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: q-fin
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The categories of this paper are: patt-sol
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The categories of this paper are: supr-con
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The categories of this paper are: gr-qc
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The categories of this paper are: nlin
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The categories of this paper are: math-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: comp-gas
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The categories of this paper are: atom-ph