  Persistent high-energy emission of magnetars is produced by a plasma corona
around the neutron star, with total energy output of ~10^{36}erg/s. The corona
forms as a result of occasional starquakes that twist the external magnetic
field of the star and induce electric currents in the closed magnetosphere.
Once twisted, the magnetosphere cannot untwist immediately because of its
self-induction. The self-induction electric field lifts particles from the
stellar surface, accelerates them, and initiates avalanches of pair creation in
the magnetosphere. The created plasma corona maintains the electric current
demanded by curl(B) and regulates the self-induction e.m.f. by screening. This
corona persists in dynamic equilibrium: it is continually lost to the stellar
surface on the light-crossing time and replenished with new particles. In
essence, the twisted magnetosphere acts as an accelerator that converts the
toroidal field energy to particle kinetic energy. The voltage along the
magnetic field lines is maintained near threshold for ignition of pair
production, in the regime of self-organized criticality. The voltage is found
to be about 1 GeV which is in agreement with the observed dissipation rate
\~10^{36}erg/s. The coronal particles impact the solid crust, knock out
protons, and regulate the column density of the hydrostatic atmosphere of the
star. The transition layer between the atmosphere and the corona is the likely
source of the observed 100-keV emission from magnetars. The corona also emits
curvature radiation up to 10^{14}Hz and can supply the observed IR-optical
luminosity.
