1st, the canon EF-S 10-22mm is an excellent lens, I've been using it for about 2 yrs. It is definitely a specialized lens that gives you the equivalent of 16-35mm on full-frame. This is an ultra to standard wide angle zoom and nothing else, it has little reach, but if you shoot landscapes, shoot in tight limited spaces and like to experiment making highly distorted images, it might be for you.
Once you get beyond 24mm the level of distortion can get pretty strange if you don't shoot perfectly level or at a parallel angle with your subject. Both horizontal and vertical lines are affected; curving buildings, trees, etc. A good article/discussion on using ultra wide lenses can be found at the link below.
How to Use Ultra-Wide Lenses
http://www.kenrockwell.com/tech/how-to- ... lenses.htm
Consider renting before buying the 10-22mm. Call your local camera shop and rent it for half or a full day, perhaps over a weekend. Shoot everything you can, explore how ultra wide works. Also, DXO Labs makes a great piece of software that helps correct distortion and I highly recommend it if you buy the 10-22mm; you can also correct lens distortion in Photoshop.
DxO Optics Pro 6
Good luck shooting the Aurora...it looks amazing/beautiful.
PS. My everyday digital kit consists of a 40D, 10-22mm, 50mm f1.4 and 70-200mm zooms.
I've not really needed a mid level zoom, usually reverting to primes for the middle ground, but the new EF-S 15-85mm looks pretty nice and will get you 24-135mm coverage; more reach. And 24mm is plenty wide enough for shooting landscapes or wide expansive skies...like the Aorora!
You might look at Gordon's review of the 15-85mm, its very informative and you get IS with this lens; the 10-22mm is non IS.
Canon EF-S 15-85mm IS USM
http://www.cameralabs.com/reviews/Canon ... 5mm_IS_USM