The F-layer persists in earth's atmosphere at night. What are the reasons for it ?

Explanation

The F-layer persists at night in earth's
atmosphere due to following reasons:
1. It is a top most layer in earth's atmosphere
which is highly ionized and inspite of recombi
nation, ionization persists to some degree.
2. The actual air density in the F layer is low
but ionization density in this layer is high so
most of the molecules in this layer are in ionized
state in night.
3. Due to low density of air in this layer, the
molecules have a large free path. The molecular
collision rate is low.