Fascinating observation and photo documents.
Cool, Andrew, great details. Here's even an English source about that launch
https://www.untvweb.com/news/china-l...-3-satellites/
And this one explains the booster 3 stages of the rocket
https://www.nasaspaceflight.com/2018...long-march-3b/
"The first two stages, as well as the four strap-on boosters, use hypergolic (N2O4/UDMH) propellant while the third stage uses cryogenic (LOX/LH2) propellant."
On the side: that the circle stayed in shape over the whole time you saw it means that no debris accompanied the "explosion" which could have deformed the exhaust cloud at the very beginning?
Do you remember this one from Oct 2017 in Russia? In the spaceweather.com article there's a link to the photographer's VK-page, the Russian Facebook. Beautiful captures of the blast, with an aurora, too.
http://spaceweather.com/archive.php?...h=10&year=2017
You can click through the whole sequence.
https://vk.com/alex_626?z=photo23062...hotos230626868