  We analyze high-cadence vector magnetograms (135~s) and flare-ribbon
observations of 37 flares from the Solar Dynamics Observatory to understand the
spatial and temporal properties of changes in the photospheric vector magnetic
field and their relationship to footpoints of reconnected fields. Confirming
previous studies, we find that the largest permanent changes in the horizontal
field component lie near the polarity inversion line, whereas changes in the
vertical field are less pronounced and are distributed throughout the active
region. We find that pixels swept up by ribbons do not always exhibit permanent
changes in the field. However, when they do, ribbon emission typically occurs
several minutes before the start of field changes. The changes in the
properties of the field show no relation to the size of active regions, but are
strongly related to the flare-ribbon properties such as ribbon magnetic flux
and ribbon area. For the first time, we find that the duration of permanent
changes in the field is strongly coupled with the duration of the flare,
lasting on average 29\% of the duration of the GOES flare. Our results suggest
that changes in photospheric magnetic fields are caused by a combination of two
scenarios: contraction of flare loops driven by magnetic reconnection and
coronal implosion.
