Main steps of the extraction algorithm applied to the image described in Fig. 1. The images are inverted for visualisation purposes. Panel A: White top-hat operation. The small, bright features of the image (here highlighted in black) smaller than 200 pixels (the size of the SE used in this operation) are isolated. Panel B: Fixed threshold. The image is binarised. Panel C: Small object removal function. Noise smaller than 5 pixels (the SE size) is eliminated, retaining only the structures of interest. Panel D: Labelling. Each coronal off-limb structure is labelled with a unique number and colour.