Wave propagation through ionosphere
We receive the electromagnetic energy from the sun which travels as electromagnetic waves through the earth’s atmosphere; we also communicate to and from satellites where the electromagnetic waves travel through the atmosphere. Hence, electromagnetic wave propagation through atmosphere is of significant interest to us. Among the different layers of earth’s atmosphere, ionosphere influences the electromagnetic wave propagation the most.
Ionosphere is the region of earth’s upper atmosphere extending from approximately 50 km to more than 1000 km above the earth. In this region, the constituent gases are ionized, mostly because of ultraviolet radiation from the sun. The positive ions and electrons are free to move under the influence of the fields of a wave incident upon the medium. The positive ions are however heavy compared to electrons and hence comparatively immobile. As the altitude increases, the density of ionosphere increases because of the increase in the amount of radiation. Beyond a certain altitude, the electron density decrease because the density of atmosphere itself reduces with the increase in altitude.
The typical electron density in ionosphere is shown in Fig. 4. This simplistic representation of ionosphere is referred to as the Chapman model. The electron density is found to be maximum at about 400 km. Atmosphere has homogeneous mixture of gases. So, the density profile has multiple peaks.
The regions around the peaks are identified by layers called as D, E and F layers. In sky wave communication, the electromagnetic waves are deflected by layers of ionosphere; the radio waves are guided back to the earth’s surface due to the deflections at different layers of the ionosphere. At noon, the atmosphere is fully ionized and at night only F2 layer survives. A sporadic E layer is formed due to recombinations at night time but only F2 layer is available for sky wave communication at night.
Figure 4: Electron density variation as a function of height in the ionosphere. The densities are larger during the day than at night. The density is maximum at a height of about 400 km (ref:www.astrosurf.com)
Permittivity of ionosphere
When an electric field is incident on ionosphere, the charges in the ionosphere start moving, thus causing a conduction current. If the electric field is oscillating, the conduction current is primarily due to the electrons since the positive charges are heavier and cannot possibly oscillate due to their smaller mobility. So while the electrons move, they collide with the positive ions and the neutral atoms, which can be considered to be at rest. This results in a momentum transfer from the electrons and hence a deceleration. The equation of motion of the electrons can be written as,
It can be observed from eqn (78) that the dielectric constant of plasma is always less than 1. Thus, plasma has a refractive index of less than 1, and it can be imaginary if f < fp. At lower heights, when the electron density, N is small, plasma frequency is relatively low, and hence the f > fp. The refractive index is almost 1. As the height increases, N increases, and consequently the refractive index decreases up to the F2 layer, where the electron density is maximum. Beyond the F2 layer, the refractive index again increases gradually. Thus, the electromagnetic wave successively changes its direction as its propagates through ionosphere, gets completely deflected back into the earth’s atmosphere, thus supporting sky wave communication.