  The no-hair theorem of general relativity states that isolated black holes
are characterized by three parameters: mass, spin, and charge. In this Letter
we consider Kerr black holes endowed with highly magnetized plasma-filled
magnetospheres. Using general relativistic kinetic plasma and resistive
magnetohydrodynamics simulations, we show that a dipole magnetic field on the
event horizon opens into a split-monopole and reconnects in a plasmoid-unstable
current-sheet. The no-hair theorem is satisfied, in the sense that all
components of the stress-energy tensor decay exponentially in time. We measure
the decay time of magnetic flux on the event horizon for plasmoid-dominated
reconnection in collisionless and collisional plasma. The reconnecting
magnetosphere should be a powerful source of hard X-ray emission when the
magnetic field is strong.
