Understanding Ionospheric Layers: D, E, F1, and F2

This article explains the different layers of the ionosphere, namely the D, E, F, F1, and F2 layers. Radio frequency waves travel through the medium above the Earth, and understanding these layers is crucial for understanding radio wave propagation.

Figure 1 shows a simplified structure of the atmosphere above the Earth, which includes the troposphere, stratosphere, and ionosphere.

• The troposphere is the layer closest to the Earth’s surface, extending up to about 15 kilometers.
• The stratosphere lies above the troposphere, reaching up to approximately 90 kilometers. This layer plays a vital role in absorbing a significant portion of the sun’s radiation.

The absorption of solar radiant energy in the stratosphere heats the atmosphere and leads to ionization, creating free electrons, positive ions, and negative ions. This ionized layer is known as the ionosphere. Various types of radiation, including UV radiation, alpha particles (α), beta particles (Β), cosmic rays, and meteors, interact within this layer. These interactions result in distinct physical properties at different altitudes within the atmosphere.

The Ionospheric Layers

Let’s examine the characteristics of each layer:

D Layer

• Located between 50 and 90 kilometers above the Earth.
• Attenuates HF (High Frequency) radio waves during the daytime.
• Ionization in this layer largely disappears at night.

E Layer

• Positioned above the D layer, with maximum density reaching up to 110 kilometers.
• Primarily influenced by UV light from the sun.
• Tends to decay uniformly at night.
• Sporadic E (Es) Layer: Forms unpredictably, often in cloud-like formations of varying sizes. Its occurrence can’t be easily predicted. It can appear during both day and night.

F Layer (Divided into F1 and F2)

The F layer exists above the E layer and is further divided into the F1 and F2 layers.

F1 Layer

• Located above the E layer, reaching a maximum density up to 220 kilometers.
• Behaves similarly to the E region and generally follows Chapman’s law.
• Critical frequency ranges from 5 MHz to 7 MHz around noon.
• Electron density varies from 2 x 10⁵ to 4.5 x 10⁵ electrons per cubic centimeter.
• Most HF waves penetrate the F1 layer, while some are reflected.
• Exhibits a significant absorption capability for HF waves.
• Density is typically lower in winter compared to summer.

F2 Layer

• Situated above the F1 layer, with maximum density occurring around 250 kilometers.
• Located at altitudes between 250 and 400 kilometers.
• Critical frequency ranges from 5 MHz to 12 MHz, generally higher at lower-altitude stations.
• Electron density varies from 3 x 10⁵ to 2 x 10⁶ electrons per cubic centimeter.
• Forms due to ionization caused by UV radiation, X-rays, and other radiations.
• Does not strictly obey Chapman’s law.
• The most crucial reflecting layer for high-frequency radio waves.
• Ionization levels in the F2 layer are affected by the Earth’s magnetic field, atmospheric conditions, and geomagnetic disturbances.