Dark, narrow, straight lines move around within images from Interface Region Imaging Spectrograph (IRIS) in such a way that they appear intentional and systematic, and not just some imaging artifact as discussed in this SDO question and this SOHO question.
What causes this line?
above: screen shots from YouTube videos of IRIS imagery.
below: The full YouTube videos of IRIS imagery.
above: Photograph of IRIS from the detailed article about the IRIS' optical instrumentation at Spaceflight 101. Photo credit LMSAL.
above: Details of IRIS' Spectrograph and Slit-Jaw Imager from the article about the IRIS' optical instrumentation at Spaceflight 101. Image credit NASA/LMSAL.