Astronomy Picture of the Day . ... When photographing picturesque Crater Lake in Oregon , USA last month, the background sky lit up with auroras of unusual colors . ... Typically, it is known, the lowest auroras appear green . ... The next highest auroras -- at about 200 kilometers up -- appear red , and are also emitted by resettling atmospheric oxygen. Some of the highest auroras visible -- as high as 500 kilometers up -- appear blue , and are caused by sunlight-scattering nitrogen ions. ...