The definition of astronomical twilight is the geometrical condition that the centre of the Sun is between 12 and 18 degrees below the observer’s horizon. In more practical terms it also means that when the Sun is lower than 18 degrees, the atmosphere above you is no longer illuminated directly by sunlight; the Sun is setting for another observer about 2000 km away. However, even if the Sun is more than 18 degrees below your horizon, the sky in your location could still be slightly illuminated by the brighter twilight sky in the direction of the hidden Sun. If locations 2000 km away were also in astronomical twilight, then the indirect illumination would be negligible, and your sky would be as dark as it can get. This would presumably require the Sun to be around 36 degrees below your horizon. Do you have any timings or measurements of how the sky quality changes, and when the effect you’ve seen ends?
British Astronomical Association
Supporting amateur astronomers since 1890