The upper part of stratosphere contains a zone called the ionosphere. Here, we have a large concentration of free ions and therefore high ionization levels. This layer refracts radio waves back to Earth.
The refraction is maximum at roughly between 100 to 160 Kilometre over the earth’s surface. Another high ionisation layer is found at 250 kilometres over the earth’s surface.
These layers are called the Kennely, Heavy Side and Appleton layers. Kennely or D layer is the lowest layer, which is found at an average height of 75 km over earth.
Heavyside or E layer is found at an average height of 125 km over earth. Appleton or F layer is the highest layer, which is found at an average height of 225 km over earth.
D layer forms at sunrise and disappears at sunset. E layer reduces in altitude at sunrise and increases in altitude after sunset. F layer splits into two at sunrise and rejoins at sunset. You would read more about these in your Radio Navigation lessons.