http://secchi.nrl.navy.mil/wiki/pmwiki.php?n=Main.FAQ
" Q:What is causing the artifact near Mercury in 20111201_232901_tbh1A.png? Why is it rectangular? A: In these HI-1 images, a daily median is used as the best near-real-time method to get CME enhancement. This results in dark spots from planets such as Mercury. When we derive the background, we do an interpolation between two daily median images. Since we make these images the day we receive them, we do not have a daily median for the next day, just the previous day. When the interpolation is done between the previous day and the current day and there is a feature like a planet, this introduces dark (negative) artifacts in the background where the planet was on the previous day, which then show up as bright areas in the enhanced image. Therefore, if (when) we re-generate these images, the bright artifact will go away because we have a daily median from the next day. Following is an illustration of the daily median images. The monthly background has been subtracted to bring out the planet artifact. "
