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It is shown that expressions for the global Lorentz force associated with aflaring active region derived by Fisher {\it et al.} (2012) can be used toestimate the Lorentz force changes for strong fields in large structures overphotospheric subdomains within active regions. Gary's~(2001) model for thestratified solar atmosphere is used to demonstrate that in large-scalestructures with typical horizontal magnetic length scale $\gg 300$~km and withstrong magnetic fields ($\ge 1$~kG at the $\tau =1$ opacity layer at 5000~\AA),the Lorentz force acting on the photosphere may be approximated by a surfaceintegral based on photospheric boundary data alone. These conditions cover manyof the sunspot fields and major neutral lines that have been studied usingFisher {\it et al.}'s (2012) expressions over the past few years. The methodgives a reasonable estimate of flare-related Lorentz force changes based onphotospheric magnetogram observations provided that the Lorentz force changesassociated with the flare have a lasting effect on the observed fields, and arenot immediately erased by post-flare equilibration processes.

Estimating Flare-related Photospheric Lorentz Force Vector Changes within Active Regions