Variation of the position angles of (green in thetop and red in the bottom), θ_fil (black), and (blue) along crests 3 (top) and 4 (bottom), from south to north. Only pixels along the crests are considered. The dotted horizontal lines show an angle of 90°. The vertical red line in the top panel is the position of source III (cf., Fig. 10). The horizontal black segments on the top right hand of the panels indicate the 14′′ (0.09 pc) beam size of the data.