  We consider the three-dimensional instability of a layer of horizontal
magnetic field in a polytropic atmosphere where, contrary to previous studies,
the field lines in the initial state are not unidirectional. We show that if
the twist is initially concentrated inside the unstable layer, the
modifications of the instability reported by several authors (see e.g. Cattaneo
et al. (1990)) are only observed when the calculation is restricted to two
dimensions. In three dimensions, the usual interchange instability occurs, in
the direction fixed by the field lines at the interface between the layer and
the field-free region. We therefore introduce a new configuration: the
instability now develops in a weakly magnetised atmosphere where the direction
of the field can vary with respect to the direction of the strong unstable
field below, the twist being now concentrated at the upper interface. Both
linear stability analysis and non-linear direct numerical simulations are used
to study this configuration. We show that from the small-scale interchange
instability, large-scale twisted coherent magnetic structures are spontaneously
formed, with possible implications to the formation of active regions from a
deep-seated solar magnetic field.
