Thanks for the replies
If there was something floating around in the mirror box area, would it be visible at all exposures? I remember adjusting the exposure or fill-light in Lightroom for this, and most of my other pictures from that night, and it wasn't until then that the red swirl appeared. Which, come to think of it, makes me uncertain it's not appearing on any of the other pictures, because i didn't adjust all of them, but perhaps around 60 out of 90. I will check those out later!
I don't think it can be Aurora borealis. If it is, it's not like any Aurora Borealis i've ever seen, and we have a lot of that where i live in northern Norway. I think it can be something small that floats by, or it can be the lights of either a plane or a satelite or something, which gets distorted in the atmosphere. I don't think it can be industrial pollution; my little corner of the world is very clean, but you never know. Some sort of cloud or lights reflecting and bouncing around in/on something i definately think it can be. Perhaps i should send the RAW-image to NASA?