  We have used Transition Region and Coronal Explorer (TRACE) 1600 \AA images
and Global Oscillation Network Group (GONG) magnetograms to compare ultraviolet
(UV) emissions from the chromosphere to longitudinal magnetic field changes in
the photosphere during four X-class solar flares. An abrupt, significant, and
persistent change in the magnetic field occurred across more than ten pixels in
the GONG magnetograms for each flare. These magnetic changes lagged the GOES
flare start times in all cases, showing that they were consequences and not
causes of the flares. Ultraviolet emissions were spatially coincident with the
field changes. The UV emissions tended to lag the GOES start times for the
flares, and led the changes in the magnetic field in all pixels except one. The
UV emissions led the photospheric field changes by 4 minutes on average with
the longest lead being 9 minutes, however, the UV emissions continued for tens
of minutes, and more than an hour in some cases, after the field changes were
complete. The observations are consistent with the picture in which an
Alfv\'{e}n wave from the field reconnection site in the corona propagates field
changes outward in all directions near the onset of the impulsive phase,
including downwards through the chromosphere and into the photosphere, causing
the photospheric field changes, whereas the chromosphere emits in the UV in the
form of flare kernels, ribbons and sequential chromospheric brightenings during
all phases of the flare.
