Do not expect to see the Northern Lights during August, even up north in the interior near Fairbanks... I guess it's possible, but highly unlikely, and if they are visible at all during that time of year it'll probably only be a small bit of green along the horizon, not the multi-colored dynamic display you might be expecting or hoping for. August is, however, a great time to be in Alaska for fishing and sight-seeing. I don't think there's any reason to go any further north than Denali, and maybe I'm biased, but I'd spend more of my time on the Kenai Peninsula and less time up north. Let me know if you need any assistance with fishing trips.
Mark
Fishology