Alpenglow is caused by the scattered light at sunset and sunrise.
It is nothing more than the reflection of the afterglow on the mountains.
The alpenglow is increased by the purple rim at the upper part of the twilight
wedge rising in the east after sunset.
The alpenglow is especially colourful when it rained short before sunset, so that the rocky slopes are still wet and shine in the light of the setting sun. Snow-covered slopes also increase the intensity of the alpenglow.