Instability of twisted magnetar magnetospheres

We present 3D 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 us to formulate an instability criterion. The rearrangement of magnetic field lines as a consequence of this instability triggers the dissipation of up to 30 per cent of the magnetospheric energy on a thin layer above the magnetar surface. During this process, we predict an increase of the mechanical stresses on to 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.We acknowledge the support from the grants AYA2015-66899-C2-1-P and PROMETEO-II-2014-069. JM acknowledges a Ph.D. grant of the Studienstiftung des Deutschen Volkes. PC acknowledges the Ramon y Cajal funding (RYC-2015-19074) supporting his research. We acknowledge the partial support of the PHAROS COST Action CA16214 and GWverse COST Action CA16104