What is Ionosphere?

What is Ionosphere?

The ionized region of the earth’s upper atmosphere extending from about 40 km to the height of a few earth radii above the surface of the earth, is referred to as the ionosphere. The ionosphere is made up of electrons, and positive and negative ions in the background of neutral particles of the atmosphere. The concentration of the neutral particles is much greater than that of the charged species, so that the ionosphere is a weakly ionized medium.

 The direct interactions of the atmospheric constituents, such as oxygen and nitrogen, with X-ray and ultraviolet radiation from the sun produce electrons and positive ions simultaneously in the ionosphere. A few electrons that are so produced, adhere to the neighbouring neutral particles to create negative ions. Also some electrons recombined with neighbouring positive ions to reproduce the original neutral particles. In this way the generation and the loss of electrons and ions in the ionosphere occur simultaneously.

In equilibrium, the concentration of electrons and negative ions, taken together, equals that of the positive ions, so that the ionosphere is electrically neutral.

In view of their low mass and high mobility, the electrons in the ionosphere greatly influence the propagation of radio waves through the ionosphere. The variation of electron concentration with height in the ionosphere must be known in this context. The stratum or a region of the ionosphere where the electron concentration attains a maximum value at a particular height and drops off on either side, is termed a layer. The maximum electrons concentration in a layer is called the peak electron concentration of the layer.

 The height at which the peak electrons concentration is reached, is called the peak height of the layer. The  different  ionospheric layers occurring generally during day and night. Mainly four ionospheric layers exist during day time,. The lowest and the highest layers are referred to as the D and F₂ layers, respectively. The two intermediated layers are called the E and F₁ layers. The E layer is above the D layer and the F₁ layer is below the F₂ layer. The electron concentration DIMINISHES RAPIDLY WITH HEIGHT ABOVE THE F₂ LAYER. At night, the D layer practically disappears, and F₁ and F₂ layers coalesce to produce a single layer, known as the F layer. Compared to the daytime values, the electron concentration at all heights in the E and F layers at night  decreasing considerably, as shown.

 The peak electron concentration and the peak height for E,F₁ and F₂ layers change from time to time in a day ( diurnal variation), from month to month over a year(seasonal variation), and with the activity of the sun( sun-spot cycle variation). These variations are regular and predictable. Therefore, these layers are termed the regular layers. The variation of the F₂ layer is greater than that of the E and F₁ layers. The E, F₁ and F₂ layers are exploited to reflect the radio waves back to the earth in sky wave propagation. The D layer of the ionosphere attenuates the radio waves travelling through it. The strength of the radio waves reflect from the ionosphere is thus greater at night when the D layer is almost absent. Consequently, the reception of the sky wave is better at night than at day.