  We present the first simulations of evolving, strongly twisted magnetar
magnetospheres. Slow shearing of the magnetar crust is seen to lead to a series
of magnetospheric expansion and reconnection events, corresponding to X-ray
flares and bursts. The axisymmetric simulations include rotation of the neutron
star and the magnetic wind through the light cylinder. We study how the
increasing twist affects the spindown rate of the star, finding that a dramatic
increase in spindown occurs. Particularly spectacular are explosive events
caused by the sudden opening of large amounts of overtwisted magnetic flux,
which may be associated with the observed giant flares. These events are
accompanied by a short period of ultra-strong spindown, resulting in an abrupt
increase in spin period, such as was observed in the giant flare of SGR
1900+14.
