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The ionosphere is the portion of the upper atmosphere, which extends from about 50 to 1000 km above the Earth, where ions and electrons are present in quantities sufficient to affect the propagation of electromagnetic waves.
The major part of the ionization is produced by solar X-ray and ultraviolet radiation and by corpuscular radiation from the Sun. Although the Sun is the largest contributor toward the ionization, cosmic rays make a small contribution; moreover any atmospheric disturbance affects the distribution of the ionization. The ionosphere is a dynamic system controlled by many parameters including acoustic motions of the atmosphere, electromagnetic emissions, and variations in the geomagnetic field. Because of its extreme sensitivity to atmospheric changes, the ionosphere is a very sensitive monitor of atmospheric events.
Figure 1 shows typical day and night profiles of electron density in the ionosphere. It shows several regions in which the electron density increases with height (the D, E, and F regions). The existence of different ionospheric regions is caused by the fact that the atmosphere is a mixture of gases that differ in their susceptibility to ionizing radiation, and thus produce maximum ionization at different altitudes.
The degree of ionization and height of each ionospheric region vary greatly with time (sunspot cycle, seasonally, and diurnally), with geographical location (polar regions, mid-latitudes, and equatorial regions), and with certain solar-related ionospheric disturbances. In Figure 1, R represents the monthly median solar index.

Figure 1 Typical day and night profiles of electron density in the ionosphere Typical day and night profiles of electron density in the ionosphere

The following regions characterizes the ionosphere:

D region. The lowest part of the ionosphere extending from about 50 to 90 km above the Earth is the D region. This region is formed by two different layers: the D layer (with a maximum of ionization at about 65 km) and the C layer (with a maximum of ionization at about 55 km). During quiet conditions this region is present only at daylight hours; however additional D-layers may be produced at any time of day or night by high-energy electrons and protons originating from the sun, and associated with geomagnetic disturbances. The ionization of this region is however very low ranging from 107 to 1010 e/m3 and as a result the HF radio waves aren’t reflected by this region which is then mainly responsible for absorption of high-frequency radio waves.
When a radio wave enters the ionosphere, the free electrons are set into motion by the alternating electric field on the wave. The energy that is transferred from the wave to the free electrons is lost when the electrons collide with a molecule. Therefore the greatest energy loss is in the D region, where the quantity Nn (N is the electron density, and n the collision frequency between electrons and neutral molecules), to which the absorption loss is proportional, reaches its maximum value.

E region. The E region, extending from about 90 to 140 km above the Earth, is mainly a diurnal region. It is characterized by a single maximum of electron density, the E layer, at about 120 km (1011 e/m3) and, like the D region, by the presence of O2+ and NO+ ions.
Similarly to the D layer, the E layer shows a diurnal behavior with a maximum of ionization at local noon. The free electron concentration in this region is in fact strongly dependent on the solar zenith angle with a daily maximum in correspondence of the maximum elevation and a seasonal maximum in summer. Despite to what happens in the D layer, during quiet conditions the E layer may have a residual of ionization at nighttime.
Within this region, due to different mechanisms, highly variable dense and thin layers (whose thickness varies from 0.2 to 2 km), called E sporadic layers, may develop. At mid- and low-latitudes the probability of occurrence of these layers is major during the summer daytime hours, while at high-latitudes the probability of occurrence is more pronounced at nighttime hours.

F region. The F region, extending from about 140 to 500 km above the Earth, is the most important part of the ionosphere in terms of HF communications. During the day there are two separate layers in the F region, the F1 and F2 layers. At night these two layers combine to form a single F layer, usually called F2 layer.
The F1 layer is the lower part of the daytime F layer. It extends from about 140 to 240 km above the Earth, and it exists only during daylight hours, disappearing at night. Maximum density (~1011 e/m3) of the F1 layer occurs shortly after noon, local time, when the sun is directly overhead.
The F2 layer extends from about 245 to 400 km above the Earth. It is present 24 hours a day but varies in altitude with geographical location, solar activity, and local time. The critical frequency for this layer peaks (~1012 e/m3) after local noon and decreases gradually throughout the night. Ions in the lower part of the F-layer are mainly NO+ and are predominantly O+ and H+ in the upper part.
Besides the maximum of the F2 layer, the electron density starts decreasing monotonically and the upper ionosphere slowly merges with the overhanging magnetosphere.

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