Non-axisymmetric instability of axisymmetric magnetic fields

Context. The MHD instabilities can generate complex field topologies even if the initial field configuration is a very simple one. Aims. We consider the stability properties of magnetic configurations containing a toroidal and an axial field. In this paper, we concentrate mainly on the behavior of non-axisymmetric perturbations in axisymmetric magnetic configurations. Methods. The stability is treated by a linear analysis of ideal MHD equations. Results. In the presence of an axial field, it is shown that the instability can occur for a wide range of the azimuthal wavenumber m, and its growth rate increases with increasing m. At given m, the growth rate is at its maximum for perturbations with the axial wave-vector that makes the Alfvén frequency approximately vanishing. We argue that the instability of magnetic configurations in the ideal MHD can typically be dominated by perturbations with very short azimuthal and axial wavelengths.