Field strength (top, in Gauss) and azimuth (bottom, in degrees counted with respect to the oriented E–W solar parallel) after ambiguity resolution in 4893 pixels. The magnetic field was determined within the horizontal field hypothesis. The bottom of each image is parallel to the solar limb. The inclined black thick line delineates on the left-hand side the main body of the prominence for the gradient studies reported in Sect. 3.