Evolution of the 2003 October 28 flare observed by the USO Hα telescope for four different time steps covering the start, maximum, and decay phase of the flare (left panels). In the right panels, the total flare ribbon areas detected up to the respective recording time of the image are overplotted, separately in blue (red) for the positive (negative) magnetic polarities. In the bottom panels, we outline the flare PIL (white contour) derived from the corresponding pre-flare MDI LOS magnetic map shown in the right bottom panel. The narrow yellow rectangle in the bottom panels represents an example of one direction perpendicular to the local PIL segment (red line) along which the flare ribbons separation motion and E_c(r, t) were derived.