What is a coronagraph?
The above imagery shows the surroundings of the Sun as seen with a coronagraph. A coronagraph is an instrument that blocks the glaring light of the Sun using a kind of “spoon”, which is the circular blue disk you can see occulting the solar disk (white circle in the middle). By doing so, it provides a kind of permanent total solar eclipse which allows us to see the area close to the Sun. This would otherwise be impossible, because the brightness of the Sun would overwhelm the immediate surroundings of our star.
Mercury before and after May 9
Because of the occulting disk, the LASCO coronagraphs are obviously not be able to show the Mercury transit. But because of its wide field of view, they show Mercury as a tiny bright dot approaching the Sun in the 3-4 days ahead of the transit. After the event, they show Mercury moving away from the Sun. As Mercury approaches the Sun, it is gradually less well visible, as Mercury moves gradually closer to the Sun-Earth line. Indeed, as seen from Earth, we see less and less of the planet’s surface illuminated by the Sun, and more and more of Mercury’s unlit “backside”. After the transit, the reverse is true and Mercury becomes better visible again.
How does Mercury move?
SOHO is situated in a fixed point between the Sun and the Earth. Its speed and orbit around the Sun are similar to those of the Earth. Mercury is much closer to the Sun than SOHO and the Earth and thus moves much faster: a Mercury year (an orbit around the Sun) takes only 88 days. Because on May 9 Mercury is situated between the Sun and the Earth, we see Mercury moving from “left” to “right”. If Mercury would be situated behind the Sun as seen from the Earth/LASCO, the planet would move from “right” to “left”.
Stars and more
The background stars also seem to move from “left” to “right”, but at lower pace than Mercury. In fact, this illustrates the movement of SOHO around the Sun. Mercury will distinguish itself from these real stars, as its position compared to these background stars will change noticeably. The other occasional dots and stripes that may appear in view are no stars, but impacts of high-energy particles slamming onto the detector's camera. Occasionally, one may also observe a whitish cloud emanating from the Sun, which may be visible for many hours. This is a coronal mass ejection (CME), that is, the ejection of a cloud of charged particles following violent activity on the solar surface.
Some of the aforementioned features can be seen in the above gif which was made during the most recent inferior conjunction of 14 January 2016. Mercury is indicated by an arrow. Also two CMEs can be seen moving towards the upper left. The clip will be on display until about 9 May, when we will start putting SOHO’s most recent coronagraphic imagery online.