Hα images observed at the main rise phase (left) and at the main energy release phase peak (right). The local PIL segment is shown by the white line. Multicolor rectangles indicate the directions perpendicular to the local PIL segments, along which we followed the flare ribbon separation to derive the local electric field strength evolution E_c(r, t). Overlaid on the Hα image is the RHESSI HXR 100−200 keV (blue) and the 2.2 MeV γ-ray line (red) contours for 11:06:20–11:09:20 UT (from Hurford et al. 2006). The two HXR FPs lie on the outer borders of the extended conjugated Hα ribbons. We note that one of the two footpoints (rooted in negative polarity) is located outside the 30 positions along the PIL that we are studying.