The force-free twisted magnetosphere of a neutron star – II. Degeneracies of the Grad–Shafranov equation

We extend our previous study of equilibrium solutions of non-rotating force-free magnetospheres of neutron stars. We show that multiple solutions exist for the same sets of parameters, implying that the solutions are degenerate. We are able to obtain configurations with disconnected field lines, however, in nearly all cases these correspond to degenerate higher energy solutions. We carry out a wide parametric search in order to understand the properties of the solutions. We confirm our previous results that the lower energy solutions have up to similar to 25 per cent more energy than the vacuum case, helicity of the order of similar to 5 (in some defined units), maximum twist of similar to 1.5 rad and a dipole strength that is up to similar to 40 per cent larger than the vacuum dipole. Including the degenerate higher energy solutions allows for larger theoretical limits of up to similar to 80 per cent more energy with respect to the vacuum case, helicity of the order of similar to 8 and a dipole strength that can now be up to four times that of the vacuum dipole, while the twist can be significantly larger and even diverge for configurations with disconnected domains. The higher energy solutions are probably unstable, therefore, it is unlikely that such magnetospheres exist under normal conditions in magnetars and high magnetic field pulsars.