force-free simulations of twisted magnetospheres (reviewing details on maintaining the force-free regime in appendix \[sec:ff\_breakdown\]). The observed rapid dissipation of electromagnetic energy through the magnetar crust is interpreted and related to observable quantities, such as luminosity estimates, shear stresses on the stellar crust, and opacity models, in section \[sec:discussion\]. Along this paper we use Gaussian units in CGS, except for section \[sec:ff\_equations\] in which we use Heaviside-Lorentz with geometrised units ($G=c=M_\odot=1$). For convenience we express current densities in A m$^{-2}$ and voltages in V, instead of the corresponding CGS units.

Physics of magnetars {#sec:physics}
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The basic structure of the magnetar interior is a (likely) fluid core of $\sim 10$ km radius, amounting for most of the mass of the object, surrounded by a solid crust of about $1$ km size. Outside, there is a tenuous, co-rotating magnetosphere connected to the NS by magnetic field lines (threading the central object) that extend up to the light cylinder at distances larger than $10^5$ km. We start by discussing some basic properties of the different parts of the magnetosphere relevant for the interpretations and models presented later in this work.

Currents supporting the magnetosphere {#sec:current_magnetsophere}
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For the typical rotation periods of 