  The activity of magnetars is believed to be powered by colossal magnetic
energy reservoirs. We sketch an evolutionary picture in which internal field
evolution in magnetars generates a twisted corona, form which energy may be
released suddenly in a single giant flare, or more gradually through smaller
outbursts and persistent emission. Given the ages of magnetars and the energy
of their giant flares, we suggest that their evolution is driven by a novel
mechanism: magnetic flux transport/decay due to persistent plastic flow in the
crust, which would invalidate the common assumption that the crustal lattice is
static and evolves only under Hall drift and Ohmic decay. We estimate the field
strength required to induce plastic flow as a function of crustal depth, and
the viscosity of the plastic phase. The star's superconducting core may also
play a role in magnetar field evolution, depending on the star's spindown
history and how rotational vortices and magnetic fluxtubes interact.
