The Ionosphere is the upper portion of the atmosphere. The ultra violet radiation from the sun will ionize the upper layer of the atmosphere. In this layer free electrons and positive and negative ions are present and hence this layer of ions is known as ionosphere.
There are four layers: D, E, F1 and F2.
1. D Layer: It is lowest layer at an height of 70 kms with thickness 10 km. The ionization density is maximum at noon and disappears at night.
2. E Layer: It is the next layer at an height of 100 kms with thickness 25 km. The layer disappears at night due to recombination of ions and molecules.
3. F1Layer: It is the next layer at an height of 180 kms with thickness 20 km. It provides more absorption for HF waves.
4. F2Layer: It is the next layer at an height of 250-400 kms with thickness 200 km. It is having highest electron density of all layers, due to this F2 layer remains present at night time.
In this propagation, the transmitted signal transmits into the upper atmosphere where it is bent i.e reflected back to earth. This bending of the signal takes place due to the presence of the ionosphere layer.
Its Frequency Range is from 3 MHz to 30 MHz