This evolution creates a twist in the magnetosphere supported by currents - until a critical maximum twist is reached ($\varphi_{\rm crit} \sim 1$rad) beyond which no magnetospheric equilibrium solutions exist [@Akgun2017]. The stability of the magnetosphere close to this critical point is the subject of this paper.

At the same time as the crustal magnetic field evolves, other processes in the magnetosphere can also contribute to the evolution. The untwisting of the magnetosphere on timescales of $\sim1$yr [@Beloborodov2009 and discussion in section \[sec:secular\]], may be a competing action to the twisting process described above.

Although the velocity of the footprints is typically very slow, numerical simulations of the magnetothermal evolution of magnetars including the magnetosphere show that, close to the critical point, it can be as fast as $v_{\varphi}\sim 1$kmyr$^{-1}$ [see @Akgun2017] in the most optimistic scenario. Therefore, close to the critical twist, the magnetosphere twists slowly ($\dot\varphi_{\rm max,crit} \,\lesssim\, 0.1$radyr$^{-1}$), evolving on timescales $\,\gtrsim\, 10$yrs. In the best case scenario, this timescale is comparable to the untwisting timescale ($\sim1$yr) and, hence, parts of the magnetosphere could sustain a significant twist. This timescale is still much longer than the dynamical timescale of the system (see below). Therefore, in our study 