Formation of an arc-shaped magnetic field morphology around filamentary molecular clouds, as described by Inoue et al. (2018). After the interaction between the cloud and shock-front, with the velocity v in the co-moving frame of the shock-front, a filamentary structure is formed (shown in an end-on view on the right). B shows the direction of the initial magnetic field before the collision and the morphology of the magnetic field after the collision.