  Sunspots are concentrations of magnetic field visible on the solar surface
(photosphere). It was considered implausible that solar flares, as resulted
from magnetic reconnection in the tenuous corona, would cause a direct
perturbation of the dense photosphere involving bulk motion. Here we report the
sudden flare-induced rotation of a sunspot using the unprecedented
spatiotemporal resolution of the 1.6 m New Solar Telescope, supplemented by
magnetic data from the Solar Dynamics Observatory. It is clearly observed that
the rotation is non-uniform over the sunspot: as the flare ribbon sweeps
across, its different portions accelerate (up to 50 deg per hr) at different
times corresponding to peaks of flare hard X-ray emission. The rotation may be
driven by the surface Lorentz-force change due to the back reaction of coronal
magnetic restructuring and is accompanied by a downward Poynting flux. These
results have direct consequences for our understanding of energy and momentum
transportation in the flare-related phenomena.
