It looks (from the published curves) that New Portra 400 has at least as much speed as Portra 800. So you could expose P400 at 800 and probably be quite happy. Since you're scanning it, C41 gives you a lot more latitude than you would have with a chrome, especially if the dynamic range is over-expanded by push processing in order to bring the speed up.
Provia 400X reportedly pushes very well, certainly to 800. You gain some contrast of course but I'm not sure that's a problem for northern lights - just make sure you don't have any artificially-lit structures appearing in-frame to blow out.
10 days isn't much but I bet you could shoot a roll of each at 800 in an afternoon and have them developed/scanned well before you needed to leave. Only you can judge the level at which grain is or isn't "acceptable" for your purposes...