Non-linear force-free magnetic field lines (VCA-NLFFF) above the sunspot at 16:50 UT. The blue image background represents the SDO/HMI magnetogram with the sunspot (in white) and magnetic field lines computed in the line-of-sight plane (left column) and rotated to the north of 90 degrees (right column). Vertical black solid lines show the X = −62″ slit position. The magnetic field lines (red curves) were computed with parameters: ngrid = 120, 150, and 150 & bmin = 150, 180, and 180 in the upper, middle, and bottom panels, respectively.