  The photospheric response to solar flares, also known as coronal back
reaction, is often observed as sudden flare-induced changes in vector magnetic
field and sunspot motions. However, it remains obscure whether evolving flare
ribbons, the flare signature closest to the photosphere, are accompanied by
changes in vector magnetic field therein. Here we explore the relationship
between the dynamics of flare ribbons in the chromosphere and variations of
magnetic fields in the underlying photosphere, using high-resolution off-band
H-alpha images and near-infrared vector magnetograms of the M6.5 flare on 2015
June 22 observed with the 1.6 m Goode Solar Telescope. We find that changes of
photospheric fields occur at the arrival of the flare ribbon front, thus
propagating analogously to flare ribbons. In general, the horizontal field
increases and the field lines become more inclined to the surface. When ribbons
sweep through regions that undergo a rotational motion, the fields transiently
turn more vertical with decreased horizontal field and inclination angle, and
then restore and/or become more horizontal than before the ribbon arrival. The
ribbon propagation decelerates near the sunspot rotation center, where the
vertical field becomes permanently enhanced. Similar magnetic field changes are
discernible in magnetograms from the Helioseismic and Magnetic Imager (HMI),
and an inward collapse of coronal magnetic fields is inferred from the time
sequence of non-linear force-free field models extrapolated from HMI
magnetograms. We conclude that photospheric fields respond nearly
instantaneously to magnetic reconnection in the corona.
