  Sudden relaxation of the magnetic field in the core of a magnetar produces
mechanical energy primarily in the form of shear waves which propagate to the
surface and enter the magnetosphere as relativistic Alfv\'en waves. Due to a
strong impedance mismatch, shear waves excited in the star suffer many
reflections before exiting the star. If mechanical energy is deposited in the
core and is converted {\em directly} to radiation upon propagation to the
surface, the rise time of the emission is at least seconds to minutes, and
probably minutes to hours for a realistic magnetic field geometry, at odds with
observed rise times of $\lap 10$ ms for both and giant flares. Mechanisms for
both small and giant flares that rely on the sudden relaxation of the magnetic
field of the core are rendered unviable by the impedance mismatch, requiring
the energy that drives these events to be stored in the magnetosphere just
before the flare. ends, unless the waves are quickly damped.
