This is surely consistent with a positive lightning strike?
Positive lighting is many more times more powerful than a standard negative strike but far rarer accounting for 5% of all strikes. It comes from the very top of the cloud sometimes striking through clear air many miles from the storm itself.
According to the Wikipedia article on lightning an average negative strkie carries 30 KiloApms of current whereas a positive strike averages around 300 with the strongest carrying 500 KiloAmps.
I have yet to establish whether modern aircraft can withstand the very strongest positive strikes (i.e those around the 500 KA mark).
Does anyone know what the maximum current a modern aircraft can withstand is?