The event below was found and shared by helioviewer.org and YouTube user “LudzikLegoTechnics” – thanks for that! It’s a wonderful example of a cool loop arcade forming and evolving.

Cool loop arcade seen in SDO AIA 304.

What you’re looking at is a bunch of magnetic field lines, forming into loops of relatively cool plasma (50,000 K), at the limb of the Sun. For comparison, the surface of the Sun is around 4500-6000 K, and natural temperatures on the Earth’s surface are around 200 – 340 K.

There are quite a number of things going on here. Firstly, there are two active regions quite close by each other. Secondly, in the movie below (in AIA 171, which sees plasma at around 600,000 K):

Same loop system seen in SDO AIA 171.

you can see that some field lines appear to open up and a small faint eruption takes off. Finally, the loops form, with bright loop tops; it is likely that magnetic reconnection is taking place here. All in all, this is a very good example of just how complicated the activity on the Sun can be; with so much going on, there’s a lot to learn.