Filament or the on-disk counterpart of the 2015 prominence. Panel a: same as Fig. 1a, but plotted for the observations obtained on 2015 February 9. The filament is visible as a dark elongated structure. The white box marks a section of the filament, identified by a solid white line, that crosses the central meridian. Panel b: zoom into the region marked by the white box in panel a. Here the white line a–b has the same meaning as in panel a. The shorter dotted lines (perpendicular to the solid line) cover the width of the filament. Panel c: intensity variation along slit a–b showing a dip at the location of the filament. The dotted vertical lines mark the width of the filament (same as in panel b), which is 8″ (corresponding to about 6 Mm). See Appendix E.