corona
“The aurora may appear as rays shooting out in all directions from a single point in the sky. This dramatic form occurs when you are directly beneath the swirls and folds of an active curtain. The rays are actually hundreds of miles long and perspective makes them appear to converge.” (via University of Alaska Fairbanks)
arc
“At its least active, the auroral curtain forms diffuse, glowing streaks hanging quietly in the sky. This form has no distinct structure.” (via University of Alaska Fairbanks)
columnal/pillars
The auroral curtain sometimes appears to touch a distant mountain top or even rise like smoke. This illusion occurs because you are seeing a several-hundred mile long aurora near the horizon where perspective gives the illusion that it is touching the ground. (via University of Alaska Fairbanks)