I took some night photos at Natural Bridges Natural Monument in Utah, United States. This is a very dark site which is Class 2 on the Bortle scale. See the Dark sky map. I took the photos mid-May, well after the end of Astronomical Twilight (22:07) and before the start of Astronomical Twilight (04:23). To my surprise, there appears to be light pollution on both photos. I don't know what this might be, as the nearest artificial sources of light are very far away. The sources of light are stationary (at least over 15 minutes). Is it really light pollution in both cases?
Both photos taken with Sony A6000 with a 30 second shutter speed. Lens is a Samyang 12 mm F/2 NCS (35-mm equivalent 18 mm focal length). Photos taken wide open at F/2. For the sake of uploading, I resized all photos from 4000×3000 pixels to 1600×1069 pixels.
Some airplanes are unmistakable, but there's also two yellow glows near the horizon. 52 km to the southeast is Bluff, UT with a population of 320. 52 km to the south-southeast is Mexican Hat, UT with a population of 31. Would such small places at such large distances show up so clearly?
A faint glow is visible near the horizon. It's much clearer in the star trail version:
In the final two photos, we can clearly see the effect of the blinking lights of airplanes. But even more prominent, in particular in the star trail photo, is the band of yellow light near the horizon. Is this band light pollution, camera effect, or a real phenomenon such as airglow? I would be surprised if light pollution was so prominent more than 100 km from any significant lightsource in northerly direction.