  A magnetar's magnetosphere gradually evolves by the injection of energy and
helicity from the interior. Axisymmetric static solutions for a relativistic
force-free magnetosphere with a power-law current model are numerically
obtained. They provide information about the configurations in which the stored
energy is large. The energy along a sequence of equilibria increases and
becomes sufficient to open the magnetic field. A magnetic flux rope, in which a
large amount of toroidal field is confined, is formed in the vicinity of the
star, for states exceeding the open field energy. These states are
energetically metastable, and the excess energy may be ejected as a magnetar
outburst.
