Two-dimensional correlation analysis. Panel (a) shows the correlation of L_b and the footpoint distance. Panel (b) displays the correlation of L_b and the propagation velocity. The solid green line was obtained from grouped mean data by linear fitting. Panel (c) presents the correlation of L_b and the brightening time. Panel (d) presents the correlation of the brightening and intensity decreasing time and footpoint distance. The pink dots represent the brightening time and the blue dots represent the intensity decreasing time.