I used to use 800ASA,
24mm f2.8, somewhere between about 5 seconds and a minute.
Points to note are:-
a) To some extent there is no "correct" exposure. If you give a different exposure you get a different result, but both can be good. i.e give a good variety of exposures.
b) In Norway/Finland you may have problems with battery life. Batteries generally don't like low temperatures and cameras use power to hold the shutter open and the mirror up, so long exposures can be difficult. Either (1) use a remote power pack inside your clothing, or (2) use several batteries, and swap them over often so that the battery pack itself doesn't cool off too much before being put back in an inside pocket. (2) is probably cheaper if you use generic batteries from 7dayshop or somewhere. Don't try to keep the whole camera warm under your clothes - it will get condensation on it.
c) Aurorae can be quite mobile, and if you give too long an exposure the detail can get blurred out.
d) The above film-based exposure times will allow for reciprocity failure. Film sensitivity depends on exposure time, so 1/100s at f2 doesn't give the same result as 2.5s at f32. Digital will not necessarily want the same exposure times. It's still a starting point for experimentation though.
