Magnetic field lines from a PF model of the HMI observed photospheric field for a region encompassing the CH. The background image is the HMI observation, and positive (negative) regions are shown in white (black). The CH is bounded in pink and part of the filament channel in orange. The colour along a field line represents the height above the photosphere and ranges from black (low height) through blue, green, yellow, and red for an increasing height to the limit of the model volume at ∼0.1 R_⊙.