The subsun is located just as deeply below the horizon as the sun is above it. It is a pure reflection halo. Therefore, the subsun is white. It is the most common halo below the horizon and is often seen from aircraft above cirrus or ice clouds.
The subsun is formed by external reflection on the upper base face or by internal reflection on the lower base face of
platelets that float almost horizontally. The ice platelets act like a very calm water surface, reflecting the sun. The subsun can also be created by doubly oriented column crystals. However, these crystals are rare. Since the ice crystals must be below the observer, the subsun can only be seen from an aircraft, from mountains or in ice fog. The more the platelets tilt about the horizontal position, the more the subsun is stretched vertically. With strong tilting it turns into a
lower light pillar below the horizon.
