General information of Hα fibrils and their nearby filament. (a) NVST Hα image and (b) SDO/HMI LOS magnetogram. The red, green, cyan, and blue dashed lines outline the fibrils L1, L2, L3, and L4, respectively. The pink dotted lines represent the filament. The red rectangle in (a) shows the FOVs of Figures 2, 4, 6(a)–(f), and 7. The N1, N2, and N3, and P1 and P2 in (b) separately denote the negative and positive magnetic fields. See Section 3 for details.