The Parker instability in axisymmetric filaments: Final equilibria with longitudinal magnetic field

Abstract: We study the final equilibrium states of the Parker instability arising from an initially unstable cylindrical equilibrium configuration of gas in the presence of a radial gravitational field and a longitudinal magnetic field. The aim of this work is to compare the properties of the nonlinear final equilibria with those found in a system with Cartesian geometry. Maps of the density and magnetic field lines, when the strength of the gravitational field is constant, are given in both geometries. In the axisymmetric model, the magnetic field tends to expand in radius, forming magnetic arcades, while knots of gas are formed because the plasma drains radially and strangulates the magnetic field lines, leading to the formation of magnetic bottlenecks. We find that the magnetic buoyancy and the drainage of gas along field lines are less efficient under axial symmetry than in a Cartesian atmosphere. As a consequence, the column density enhancement arising in gas condensations in the axially-symmetric model is smaller than in Cartesian geometry. The magnetic-to-gas pressure ratio in the final state takes more extreme values in the Cartesian model. Models with non-uniform radial gravity are also discussed.