Electromagnetic Waves – Layers of Earth’s Atmosphere and the D-Layer

Different types of electromagnetic waves are produced when there is a periodic change of electric or magnetic field. Depending upon the occurrence of periodic changes and the power generated, different rays of different wavelengths are produced in the electromagnetic spectrum.

A variety of electromagnetic waves travel from across space to Earth. These waves are emitted by various objects floating across the universe. These waves include ultraviolet rays, X-rays and gamma rays which can harm the living beings on Earth. However, some of the electromagnetic waves never reach the surface of the Earth as they are absorbed by the Earth’s atmosphere. Other types of electromagnetic waves, like magnetic nanometer waves, have different frequencies, oscillations and wavelengths, which makes them travel through the Earth’s atmosphere without being absorbed. 

Electromagnetic waves of different frequencies have different sources and effects on matter. Let’s take a look at them.

Types of electromagnetic waves

• Gamma rays-

Gamma Rays have the shortest wavelengths; no longer than 0.01 nanometer. They are harmful to humans as they carry a lot of energy.

Gamma radiation is generated inside stars as well as during other phenomena occurring in the universe, such as the merging of stellar corpses and the death of stars. Gamma Rays are absorbed by the Earth’s atmosphere.

• X-rays-

X-rays have wavelengths ranging between 0.01 nanometers and 20 nanometers. They can penetrate soft tissues of the human body but not bones. This characteristic of X-rays is the reason why they are used for generating images of bones inside the human body.

• Ultraviolet rays-

Ultraviolet rays, commonly known as UV rays, fall between X-rays and visible light on the electromagnetic spectrum. Ultraviolet light is called ‘black light’ because it cannot be seen by the human eye. These rays are very harmful to the living beings on Earth but they are absorbed by the ozone layer of the Earth’s atmosphere.

• Visible light- 

These electromagnetic rays have wavelengths ranging between 400 and 700 nanometers. They are called so because they are visible to human eyes.

• Infrared rays- 

These rays have a wavelength between visible light and radio waves.

• Microwave-

Microwaves have wavelengths varying from 1 millimetre to 1 metre. They are used in short-wave communication and microwave ovens. They are easily absorbed by water vapours, which makes them useful in heating food. Microwaves are smaller in comparison to radio waves.

• Radio waves-

Radio Waves have wavelengths ranging from a metre to hundreds of metres. It can be said that all electromagnetic waves longer than microwaves are radio waves. Some types of radio waves are radar waves (used in radar guns), AM radio waves, etc. 

Now, let us find out about some layers in the Earth’s atmosphere and their relation to electromagnetic waves.

The D layer of the ionosphere

The Earth’s atmosphere has different layers, namely Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere. These layers protect the Earth from electromagnetic waves, solar radiation, meteors, and asteroids.

Ionosphere

• The Ionosphere region spans from 50 km to 965 km above the Earth’s surface. It extends from the Thermosphere to some parts of the mesosphere and exosphere. 
• This region has numerous electrically charged particles of atoms and molecules, which are known as ions and electrons.
• Ionisation in this sphere is caused by solar radiation on atoms and molecules of air. The amount of ionisation is directly proportional to the amount of solar radiation.
• The presence of electrically charged particles is responsible for radio propagation to distant places on Earth.

Layers of ionosphere

The Ionosphere is further divided into different layers, namely D, E, and F depending upon the characteristics and altitude. These layers affect radio communications in different ways.

D layer of the ionosphere

This region is the lowest ionospheric region and lies between 60 km to 90 km above the Earth’s surface. The ionisation in this layer is present only during the day and starts fading as the sunsets. It is because the ionisation is sustained by the sun and ionisation stops after dusk as the source of ionisation is not available.

Causes of ionisation

Ionisation in the D layer of the Ionosphere is caused due to radiation, known as Lyman-alpha radiation, at a wavelength of 121.6 nanometers. It ionises nitric oxide gas present in the atmosphere. Hard X-rays are also responsible for some of the ionisation process, especially during noon when the sun is at its peak.

Impact on radio signals

• The D layer of the Ionosphere is attributed to the depletion of signals, especially in the Low Frequency and Medium Frequency regions of the radio spectrum.
• This depletion is caused when vibrating electrons collide with molecules, which results in a small loss of energy. 
• But with millions of electron collisions, the amount of energy loss becomes significantly high and results in the reduction of signals.
• The decrease in signals happening in this layer largely depends on the frequency, the number of gas molecules present and the level of ionisation.
• At night, due to low ionisation, the layer has very little effect on most of the radio communication signals.

Factors affecting radio signals

Frequency 

The reduction in signals is inversely proportional to the square of the frequency, i.e., increasing the frequency by two-fold reduces the signal reduction level by a factor of four. This prevents low-frequency signals from reaching higher regions, except at night.

Number of gas molecules 

Reduction in signals is directly proportional to the number of gas molecules. When there is a greater number of gas molecules, it results in a high number of collisions and ultimately more depletion.

Level of ionisation 

Reduction in signals is directly proportional to the level of ionisation. This means the reduction is higher when the level of ionisation is higher.

E Region

This region lies between 90 km and 160 km above the Earth’s surface. Soft X-ray and far-ultraviolet solar radiation are responsible for the ionisation of molecular oxygen in this layer. In the E region, ionisation happens at night but less than during the daytime.

F Region

This region spans from 160 km to 500 km above the Earth’s surface. Among all the layers of the Ionosphere, this region has the highest density of free electrons. Extreme ultraviolet radiation of 10-100 nanometres is attributed to the ionisation of atomic oxygen in this layer.

Conclusion

Electromagnetic waves of different wavelengths are produced when there is a synchronised oscillation of electric and magnetic fields. Most of the electromagnetic waves that travel towards Earth are dangerous. However, they are absorbed by different layers of the Earth’s atmosphere.

The Ionosphere has numerous electrically charged particles, which helps the radio propagation to distant places on the Earth. The D layer of the Ionosphere depletes the radio signals when millions of electrons collide, resulting in energy loss and attenuation of signals.