(a) HMI intensity image, with the same FOV as that of Figure 2, showing the slit position indicated by the white arrow, “S.” (b) Time slice from HMI intensity images for the S slit depicted in (a) showing the penumbra decay from the outer penumbra to the inner umbra. The dashed red line, the linear fitting to the outer edge of the decaying penumbra, indicates an average speed of 1.4 ﹩\mathrm{km}\;{{\rm{s}}}^{-1}﹩. (c) The horizontal LF changes, ﹩\delta {{\boldsymbol{F}}}_{h}﹩, in the flare are denoted by the short arrows aligned to their direction with lengths proportional to their magnitude. The vertical field components are plotted as grayscale background and isogauss contours with levels of ±500, 1000, and 1500 G. The FOV, indicated by the dashed box in (a), is 30″ × 30″. (d) The magnitude changes of ﹩\delta {{\boldsymbol{F}}}_{h}﹩ as a function of time in an area centered on the penumbral decaying region indicated by the green boxes in (a) and (c), which are computed from the ﹩| \delta {{\boldsymbol{F}}}_{h}| ﹩ integrated over the area. Time profile of the ﹩{\text{}}{GOES}﹩ 1–8 Å SXR flux is overplotted as the gray curves in a arbitrary unit to fit the panels, and the pre-flare main PIL is plotted as the dashed orange curve in (c). In (b) and (d), the horizontal red bars indicate the flare duration, and the blue ones indicate the duration of the penumbra decay.