Their guesses about what's really going on are at least as crazy as the lights themselves. One frightening theory, which has been given some credence by a few tests, is that the valley is actually highly radioactive. Radon piggybacks on dust particles until it gets up in the atmosphere and decays, producing the lights. If this is true, then it's bad news for local residents -- radon generally isn't a "party" element. Luckily there has been little-to-no reported upsurge in local Hulkism.
The fjord trolls are as baffled as anyone.
Stretching even further for an explanation, some scientists think that the valley might actually be one enormous C cell battery. It's been established that one wall of the valley is rich with copper deposits, and the other is rich with zinc, and that's pretty much the basic composition of batteries. All it would need is some kind of acid to connect the two sides and some way of charging, and you'd have the makings for a neat magic trick, like producing sparks in the atmosphere that look like an alien invasion.
Or you can just take some acid yourself and see the same thing.