Panel a: HMI magnetogram of the selected area taken on January 26, 2016 at 16:30 UT. Panels b–j: distribution of the decay index n of the potential magnetic field at different heights above the area shown in first figure (panel a). Areas where n > 1 are tinted with gray, while regions with n < 1 are white. Red lines indicate the position of the polarity inversion lines (PIL). The green contour shows the position of the filament taken from the co-aligned BBSO Hα filtergram. The torus instability in the zone of the PIL occurs for decay-index values n > 1.