  The magnetic field of magnetars may be twisted compared with that of normal
pulsars. Previous works mainly discussed magnetic energy release in the closed
field line regions of magnetars. For a twisted magnetic field, the field lines
will inflate in the radial direction. Similar to normal pulsars, the idea of
light cylinder radius is introduced. More field lines will cross the light
cylinder and become open for a twisted magnetic field. Therefore, magnetars may
have a large polar cap, which may correspond to the hot spot during outburst.
Particle flow in the open field line regions will result in the untwisting of
the magnetic field. Magnetic energy release in the open field line regions can
be calculated. The model calculations can catch the general trend of magnetar
outburst: decreasing X-ray luminosity, shrinking hot spot etc. For magnetic
energy release in the open field line regions, the geometry will be the same
for different outburst in one magnetar.
