COMPETITION AMONG NONLINEAR EFFECTS IN TEARING INSTABILITY SATURATION

The nonlinear evolution of a tearing instability in a sheet pinch in a slab geometry is numerically studied. It is found that, when the whole unstable spectrum is excited, to a small but finite level, a new nonlinear effect, an inverse energy cascade, occurs. The inverse cascade corresponds, in the physical space, to a coalescence process. The nonlinear development of the instability is then considerably modified with respect to the case in which only one unstable mode is present. In particular, the inverse cascade is responsible for the saturation of the single tearing-unstable modes. However, the nonlinear effects associated to the coalescence do not slow the growth of the instability, as a whole. On the contrary, the perturbation energy exponentially also grows during the nonlinear stage, up to the saturation of the instability. Moreover, a recurrent behavior is found after the saturation, corresponding to the creation of localized current sheets, where new tearing instabilities develop. The competition among the different nonlinear effects is discussed.