Use Velvia 50 at EI64. Take it to the max!
....or wuss out and use Provia 100. I don't think 400X has very nice colour, but more like an Edwardian looking-glass palette. You'd be wanting to accentuate the colours, which are sometimes pale and almost always fleeting. A long exposure will record more of the 'dancers in the dark' than a quick one, but it's all what strikes you as personal preference.
Northern lights and star trails would be a treat!!
I am heading to Norway in the next 3 years for the Northern Lights — research is proceeding.
ha ha....capturing the lights on Velvia 50 would be magical....if only I could do that. 50 or even 64 is way way way too slow for the lights.