Characteristics of different ionospheric layers

The ionosphere is divided meteorologically into different regions or layers and each layer exhibit different characteristics.

The layers of the ionosphere are:

• D-Layer
• E-Layer
• E_s-Layer
• F₁-Layer
• F₂-Layer

1. Characteristics of D-Layer:

• It is the lowest layer of the ionosphere.
• It exists at an average height of 70 km.
• Its thickness is 10 km.
• It exists only in day-time.
• Its ionization properties depend on the altitude of the sun above the horizon.
• It is not a useful layer for HF communication.
• It reflects some VLF and LF waves.
• It absorbs MF and HF waves to some extent.
• Its electron density, N = 400 electrons/cm³.
• Its virtual height is 60 to 80 km.
• Critical frequency of the layer, f_c = 180 kHz.

2. Characteristics of E-Layer:

• It exists next to D-Layer.
• It exists at an average height of 100 km.
• Its thickness is about 25 km.
• It exists only in day-time.
• The ions are recombined into molecules due to the absence of sun at night.
• It reflects some HF waves in day-time.
• It disappears at nights.
• Its electron density, N = 2 × 10⁵ electrons/cm³.
• Its virtual height = 110 km.
• Its critical frequency, f_c ≈ 4 MHz.
• Maximum single-hop range ≈ 2350 Km.

3. Characteristics of E_s-Layer:

• It is a sporadic E-Layer.
• Its appearance is sporadic in nature.
• If at all it appears, it exists in both day and night.
• It is a thin layer.
• Its ionization density is high.
• It appears close to E-Layer.
• If it appears, it provides good reception.
• It is not a dependable layer for communication.

4. Characteristics of F₁-Layer:

• It exists at a height of about 180 km in day-time.
• Its thickness is about 20 km.
• It combines with F₂-Layer during nights.
• HF waves are reflected to some extent.
• It absorbs HF to a considerable extent.
• It passes on some HF waves towards F2-layer.
• It’s virtual height ≈ 180 km.
• Its critical frequency, f_c ≈ 5 MHz.
• Maximum single-hop range ≈ 3,000 km.

5. Characteristics of F₂-Layer:

• It is the most import layer for HF communication.
• Its average height is about 325 km in day-time.
• Its thickness is about 200 km.
• It falls to a height of 300 km at nights as it combines with the F₁-Layer.
• The height of F₂-Layer varies drastically with the time of the day, the average ambient temperature and sunspot cycle.
• It exists at nights also.
• It is the topmost layer of the ionosphere.
• It is highly ionized.
• It offers better HF reflection and hence reception.
• Electron density of F-Layer, N = 2 × 10⁶ electrons/cm³.
• Its virtual height = 300 km in day-time and 350 km in the night.
• Its critical frequency, f_c ≈ 8 MHz in day-time and f_c ≈ 6 MHz at nights.
• Maximum single-hop range ≈ 3,800 km during day-time and 4,100 km at night.