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We have used Transition Region and Coronal Explorer (TRACE) 1600 \AA imagesand Global Oscillation Network Group (GONG) magnetograms to compare ultraviolet(UV) emissions from the chromosphere to longitudinal magnetic field changes inthe photosphere during four X-class solar flares. An abrupt, significant, andpersistent change in the magnetic field occurred across more than ten pixels inthe GONG magnetograms for each flare. These magnetic changes lagged the GOESflare start times in all cases, showing that they were consequences and notcauses of the flares. Ultraviolet emissions were spatially coincident with thefield changes. The UV emissions tended to lag the GOES start times for theflares, and led the changes in the magnetic field in all pixels except one. TheUV emissions led the photospheric field changes by 4 minutes on average withthe longest lead being 9 minutes, however, the UV emissions continued for tensof minutes, and more than an hour in some cases, after the field changes werecomplete. The observations are consistent with the picture in which anAlfv\'{e}n wave from the field reconnection site in the corona propagates fieldchanges outward in all directions near the onset of the impulsive phase,including downwards through the chromosphere and into the photosphere, causingthe photospheric field changes, whereas the chromosphere emits in the UV in theform of flare kernels, ribbons and sequential chromospheric brightenings duringall phases of the flare.

Abrupt Longitudinal Magnetic Field Changes and Ultraviolet Emissions Accompanying Solar Flares