The Petrozavodsk phenomenon (1977)

[Manti]

 https://en.wikipedia.org/wiki/Petrozavodsk_phenomenon

It is unexplained to this day.Could just be a magnetic phenomenon like the northern lights but higher up in the atmosphere. Or a space debris collision.If it was related to any military activity, I would think we would know by now, the documents becoming all declassified at the fall of the Soviet Union.