  We present three-dimensional force-free electrodynamics simulations of
magnetar magnetospheres that demonstrate the instability of certain degenerate,
high energy equilibrium solutions of the Grad-Shafranov equation. This result
indicates the existence of an unstable branch of twisted magnetospheric
solutions and allows to formulate an instability criterion. The rearrangement
of magnetic field lines as a consequence of this instability triggers the
dissipation of up to 30% of the magnetospheric energy on a thin layer above the
magnetar surface. During this process, we predict an increase of the mechanical
stresses onto the stellar crust, which can potentially result in a global
mechanical failure of a significant fraction of it. We find that the estimated
energy release and the emission properties are compatible with the observed
giant flare events. The newly identified instability is a candidate for
recurrent energy dissipation, which could explain part of the phenomenology
observed in magnetars.
