(a) Spatial distribution of F1 and F2. The shadow behind F1 represents its projection onto the plane of the sky. (b) Top view of the spatial distribution of F1 and F2. The angle θ represents the angle between the line-of-sight velocity v and the approaching velocity v_r, and d denotes the unit distance perpendicular to the line-of-sight plane. d_err indicates the distance difference between F1 and F2 from Earth, and d₀ is the distance difference perpendicular to the line of sight of the two ends of the two filaments. d₀ is also marked in Fig. 7.