Hi, I think it is the phenomenon called "Noctilucent cloud".
They can be seen during twilights and they are the highest clouds in the earth's atmosphere, around 80 Km high. It is a recent discovery and they are still not very well understood.
But I am not 100% sure sorry, I am not a meteorologist, but I briefly studied them during my bachelor.
Cheers
RE: What phenomenon is this?