Slices used to analyze the filaments, superimposed on the integrated line intensity, corrected for opacity, Aτ_m ΔV, of the blue filament (blue contours) and the red filament (red contours). The origin of offsets along the filaments is taken at the position of the intersection of the two slices, and increase toward the west (blue filament) and the northwest (red filament) from − 40″ to + 30″. Ticks in the central line of the slices are at intervals of 10″.