The first sign of the aurora is a pale greenish mist in the Northeast, hazy and faint, like sunset-residue, or a city just over the horizon. It's subtle - you can't really look at the sky and say "here there is greenglow, and here there is not", but there are places where the inkshadow outlines of things are strangely sharp, and it strikes you as odd that you can see every pine needle silhouetted against the midnight sky. Sometimes that's all there is, the sunrise that can't be.
But sometimes, the mist is in twining milky pale horizontal bands that arc lattice-work over eachother, like someone just came along and loosely braided the sky. They drift a little, ghosts against the wind.
Thursday we got the full show; the thin hazy bands resolved into curtains of green fire that arch like a lover and snap like sails in a strong wind. The entire sky becomes a harp of light, with the low, curled hills as sounding board and the shining waterfall of the milky way as its column. You could see each string sparkle into existance as it was played, long narrow spines of green light flaring upward in intricate and strange inhuman scales. Music of the spheres.
I think everyone should see the aurora at least once before they die.
See also: cow's post, here.