Reflection and Refraction by the ionosphere

The region of the earth’s atmosphere, where the gases are ionized by the intense radiations from the sun is known as the ionosphere. The electrons and ions are not distributed evenly in the ionosphere, so the ionosphere is divided into different layers. At greater altitudes, the radiations from the sun are very intense but the concentration of gas molecules is very low. Hence at these altitudes, the ionization density is very small. The region around these greater altitudes, atmosphere makes a layer of electrons and ions. As the altitude decreases, the concentration of gas molecules and atmospheric pressure increases. Hence the ionization density increases till it becomes maximum, below this height, the atmospheric pressure increases but the intensity of the solar radiations decreases because most of the radiations have been used in process of ionization at higher altitudes. Therefore, the concentration of ionization density falls from maximum. Each layer of the ionosphere has different ion or electron density. The ionization density of each layer varies with the time of the day, season and altitude. For every layer, there is a certain frequency, above which an electromagnetic wave incident on it will not be reflected back to the earth. Thus, the layers of the ionosphere are characterized in terms of their critical frequencies and heights. At night the two uppermost layers have a permanent existence, while the lowermost layer disappears at night. Long distance radio communication is possible due to the reflection of the radio waves by the uppermost layers. The lowermost layer exists only during the day and it only reflects back the low frequency waves. The presence of this layer during the day decreases the intensity of the high frequency waves reflected by the uppermost layers. That is why the reception of high frequency waves is far better at night than during the day time.

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