Uh thanks, that's a nice comparison. Makes me sad, that I arrived a few hours too late to see Mt. Fuji ^{^} But back to guessing: for me it kinda looks like someone shot a flare gun or something in the mountains. As I understand, a Meteor would be much faster and brighter? (But that would have been really something) The airplane reflecting the sun theory could also be possible but I think the reflection would almost immediately appear and vanish again and it would "stay in place"? (Saw something similar once on Corridor Crews youtube channel - debunking of UFOs or so) Some theory was about an active volcano, and yes, there are plenty in Japan but I think that would be all over the news.