Propagation via Ionosphere

The highest layers of the Earth's atmosphere are called the ionosphere, because they contain plasma, which is ionized gas (free electrons and ions). The ionosphere extends from 60 to 1,000 km. Below 60 km the ionization is insignificant because the solar ionizing radiation is getting weaker due to absorption in the higher layers, and because recombination of plasma is fast due to high density of molecules. Above 1,000 km the density of molecules is too low for a significant phenomenon. It is possible to distinguish different layers in the ionosphere, as shown in Figure 10.15; they are called D, E, F1, and F2 layers. The electron density and the height of these layers depend on the solar activity, on the time of day and season, and on the geographical location. During night the D layer nearly disappears, and the F1 and F2 layers merge together. The highest electron density is about 1012 electrons/m3, and it can be found at daytime at the altitude of about 250 to 400 km in the F2 layer.

Let us consider radio wave propagation in plasma. The electric field having a strength E affects (accelerates) a charge q by force qE. If there are N charges in a unit volume, the current density in case of a sinusoidal field is (F = ma = mdv/dt = mj^v)

109 1010 1011 1012 1013

Figure 10.15 Electron density in the ionosphere versus altitude.

109 1010 1011 1012 1013

Figure 10.15 Electron density in the ionosphere versus altitude.

jcm where v is the velocity of the charge and m is its mass. Maxwell's IV equation, (2.21), can now be written as

The charges cause a decrease of the permittivity of medium. Because the electron rest mass is only 1/1,836 of that of a hydrogen ion, the electrons determine the permittivity. The relative permittivity of plasma is

where e is the electron charge and m e is the electron mass, and

is the plasma frequency. The plasma frequency in hertz is fp « 9VN (10.23)

where N is the number of electrons in one cubic meter; that is, the plasma

12 3

frequency is 9 MHz for an electron density of 10 /m .

This treatment does not take into account collisions of electrons with neutral molecules, which causes attenuation. Furthermore, the Earth's magnetic field affects the electron motion, which leads to a direction depending on er; that is, plasma is an anisotropic and nonreciprocal medium and er is a tensor. The nonreciprocity causes Faraday rotation (see Section 6.2). The electric field of a linearly polarized wave may rotate, causing wrong polarization in reception. Therefore, in radio systems utilizing the ionosphere, a circular polarization is preferred.

A wave can propagate in plasma only if its frequency is higher than the plasma frequency. Otherwise er is negative and the wave will be totally reflected. At frequencies well above the plasma frequency, er ~ 1, and the effect of plasma on radio wave propagation is negligible. In practice, there is no need to take the ionosphere into account at VHF and higher frequencies.

A wave propagating vertically into the ionosphere will be reflected at altitude where er = 0, that is, fp = f. If the wave approaches the ionosphere in an angle <0, as shown in Figure 10.16, reflection takes place at a height where fp = f cos <0 (10.24)

Via the ionosphere it is possible to obtain a radio hop to a distance of 4,000 km just with one reflection. Also, longer hops are possible if the wave reflects from the ground back to the ionosphere. In a given radio link the frequency must be higher than the lowest usable frequency (LUF) but lower than the maximum usable frequency (MUF). The LUF and MUF depend on the temporary characteristics of the ionosphere.

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