PSL - first processes

The extracted feature magnetogram, Bt,i, is also used to derive properties based on the polarity separation line (PSL). Figure 7 summarizes the extraction of feature properties related to PSLs and Table 2 lists the properties derived. Initially, the feature is segmented into its positive and negative components. These components are used to create a positive and negative mask, each of which is dilated by 4 pixels. The two masks are summed and the region of mask overlap becomes the PSL binary mask, MP SL,t,i. The algorithm then thins MP SL,t,i to one pixel (MP SL,thin,t,i) and sums the non-zero pixels to determine the PSL length (LP SL,t,i). Lsg,t,i is obtained by summing only those pixels which have ∇Bt,i > 50 G Mm−1 , where ∇Bt,i is calculated by numerical differentiation using 3-point Lagrangian interpolation.

• Separate each feature mask into a positive and a negative mask
• Dilate each mask by 4 pixels (or the arcsec equivalent)
• Add the two masks together, where they overlap then becomes the PSL binary mask
• Thin(?) the mask two one pixel width (or the arcsec equivalent)
• Sum the non-zero pixels to get PSL length

No source file, test, or entry point is named in the issue. Locate the existing feature-mask extraction flow and its tests, then use the listed PSL mask, thinning, length, and gradient-threshold steps as the acceptance checklist; done means the derived PSL properties are produced and verified.