Geir Øye (site, aurora gallery) imaged this display at Ørsta, Norway on December 27, 2005. The tall curtains start with a green base at ~90-100 km (56-62 mile) which changes to red above ~150 km (93 miles). Both colours are from oxygen atoms excited by electrons accelerated downwards from the magnetotail.  Atoms losing their excitation energy with red emission do so very slowly and only get a chance radiate at very high altitudes where the pressures are extremely low and deactivation by collisions is even slower. Image ©Geir �ye, shown with permission.