I don't know the details, but it's the exhaust gases from the rocket spreading (in near-vacuum they can spread many kilometers) and being lit up by sunlight. Th

I don't know the details, but it's the exhaust gases from the rocket spreading (in near-vacuum they can spread many kilometers) and being lit up by sunlight. There was a similar plume over San Fransisco from a Trident missile test a while ago: https://www.theverge.com/2015/11/12/9720728/san-francisco-missile-test-ufo https://www.theverge.com/2015/11/12/9720728/san-francisco-mi... I'm guessing the bulb-shape from the first stage is because the gasses spread further at higher altitude.