Found this on Flickr - Northern Lights on Slide Film. Perhaps e-mailing him, he can tell you what he used.
Beware the use of Portra 400 for this subject matter, being a portrait film, and also designed to work under mixed lighting (think, neutralizes "color casts"), you may very well end up with washed out, muted, "auto white balanced away" colors.

Most slide films dont have these issues (hence, terrible under indoor lighting or mixed lighting), which is a strength in your case. Stick to ektar and 400X. Triple up on the 400x, not thye Portra. As this shot proves, the Northern Lights can be captured very well with slide film. And given it is from 2001, its proibably either E200 pushed (800 max, which 400X does easily), or Provia 400F.
1 roll 400X at 400
1 roll at 800
1 roll at 1600
Your set.