Twilight phenomenon is produced when exhaust particles from missile or rocket propellant left in the vapor trail of a launch vehicle condense, freeze, and then expand in the less dense upper atmosphere.
Super cool to watch these. I'm around 100 miles from Vandenberg (California west coast launch facility), and I have roof access where I live. The second stage separation happens almost directly over my head and I have a great view of the Falcon 9 boosters doing their boost back burn.