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Home » Physics Homework Help » Quantum Physics » Earth Atmosphere
Earth Atmosphere
The gaseous envelope surrounding the earth is called earth’s atmosphere. The earth atmosphere mainly consist of nitrogen 78%, oxygen 21% and with a little portion of argon, carbon dioxide, water vapour, hydrocarbons, sulphur compounds and dust particles. The density of the atmospheric air goes on decreasing as we go up. The electrical conductivity of the atmospheric air increases as we go up. The earth atmosphere has no sharp boundary. It has been divided into various regions as given below.

1. Troposphere: it extends upto a height of 10 km. the atmospheric air in this region has maximum density which varies from 1 kg/m3 at the surface of earth to 0.1 kg/m3 at the top of this layer. The electrical conductivity of this region is least as compared to other regions of earth’s atmosphere.

2. Stratosphere: it extends from 10 km to 50 km from the surface of earth. The density of air of this region varies from 0.1 kg/m3 to 10-3 kg/m3. There is an ozone layer in this region in between 30 km to 50 km from the surface of earth, which absorbs a large portion of ultraviolet radiations radiated by sun or coming from outer space.

3. Mesosphere: it extends from 50 km to 65 km from the surface of earth. The density of air in this layer varies from 10-3 kg/m3 to 10-5 kg/m3. The temperature of this region falls from 280 K to 180 K with height.

4. Ionosphere: it extends from 65 km to 400 km from the surface of the earth. The density of this region varies from 10-5 kg/m3 to 10-10 kg/m3. In this region temperature increases with height from 180 K to 700 k, that is why, it is called thermosphere. Ionosphere is the outermost part of the earth’s atmosphere. It is composed of ionised of ionised matter (i.e. electrons and positive ions) which plays an important role in space communication. The value of refractive index of ionosphere is less than one.

There are four main layers in earth’s atmosphere having high density of electrons and positive ions, produced due to ionization by the high energy particles coming from sun, stars or cosmos. These layers play their effective role in space communication. These layers are D, E, F1 and F2.

(i) D-layer is at virtual height of 65 km from surface of earth and having electron density ≈ 109 m-3. The extent of ionization of D layer depends upon the altitude of sun. This layer disappears at night. This layer reflects very low frequency (VLF) and low frequency (LF) electromagnetic waves but absorbs medium frequency (MF) and high frequency (HF) electromagnetic waves to a certain degree.

(ii) E-layer is at a virtual height of 100 km, from the surface of earth, having electron density ≈ 1011 m-3. The critical frequency of this layer is about 4 MHz. this layer helps to MF surface-wave propagation a little but reflects some high frequency (HF) waves in day time. This layer does not exist at night.

(iii) F1-layer is at virtual height of 180 km from the surface of earth, having electron density ≈ 5 × 1011 m-3. The critical frequency for this layer is 5 MHz. it absorbs some of the high frequency waves but most of the high frequency waves pass through it and they get reflected from layer F2. This F1 layer merges in F2 layer at night.

(iv) F2-layer is at a vertical height of about 300 km in night time and about 250 to 400 km in day time. The electron density of this layer is ≈ 8 × 1011 m-3. The critical frequency of this layer reflects back the electromagnetic waves of frequency upto 30 MHz but cannot reflect back the electromagnetic waves of frequency 40 MHz or more.

Thus earth’s atmosphere helps in the propagation of electromagnetic waves from one place to another place, upto 30 MHz frequency.

In radiowave communication between two places, the electromagnetic waves are radiated out by the transmitter antenna at one place which travel through the space and reach the receiving antenna at the other place. The strength of the electromagnetic waves goes on decreasing as we move away from the transmitter antenna. There are certain factors which influence the propagation of the electromagnetic waves and path they follow. Our earth’s atmosphere plays an important role in the propagation of electromagnetic waves from one place to another place on the surface of earth.

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