Sketch of the basic model. Upper panel: situation in which the plasma thread, represented by the dark rectangle of length l_t, is located at the tube apex. This configuration is in an equilibrium that can be stable or unstable. Lower panel: new equilibrium of the thread at a lower height. The curved magnetic field is uniform and directed along the tube axis. The gravity force is pointing downwards and is projected along the field line according to Eq. (1). The footpoints are supposed to be anchored at the base of the corona, represented by the shaded grey area.