Recently, there were quite a few nights featuring the Northern Lights. As most people probably know, these are caused by energetic charged particles of the solar wind interacting with the Earth's magnetosphere, causing electrons to ionize gases in Earth's atmosphere. These then fluoresce in different wavelengths producing the colours that we associate with the aurora. Oxygen usually produces green, though the colour can be orange-red if the energy levels are high. Nitrogen produces blue or red. When the sun is very active (flares or coronal mass ejections) the auroras can be most spectacular.
Here is a photo a colleague of mine took, at the Elk Island National Park dark sky preserve, about 50 km outside of Edmonton, Alberta, Canada (population approximately one million). I believe the exposure was about 30 seconds. Note that this was taken at about 11:00 p.m., late in March, 2015. Very pretty.
Here's a photo that I took the same night, from my home in Edmonton. I took it at about 2:00 a.m. with my iPad. I had to push the image a lot in the image processing software GIMP. The aurora is the green light about 3/4 of the way up the picture, past the trees. Not so pretty, but kind of spooky.