Ok thank you, makes more sense, I thought it was the day-side

Yes, I was also confused when I first saw it – how could the aurora be visible?! The bright sliver of atmosphere in the lower right is, of course, backlit by the sun which is itself eclipsed by Earth. It's the near-full moon that provides most of the illumination here. Besides both auroral rings you can also see airglow, city lights, and lightning flashes, it's a marvellous photo.