The Ionosphere

At the surface of the earth the atmosphere consists of about 76 per cent nitrogen, 23 percent oxygen, and 1 percent argon. At increased heights the oxygen and argon percentages decrease and the nitrogen increases until at about 60 miles the atmosphere is almost entirely nitrogen with but a small amount of oxygen and a trace of helium. After this point is passed the nitrogen decreases very rapidly until at about 100 miles the atmosphere is almost 95 percent helium with but 5 per cent nitrogen.¹⁰

The air pressures at these heights are well described by the following quotation:¹¹

In the vicinity of 50 miles (80 kilometers) it has dropped to approximately 1/160,000 of that at the surface. This pressure is what we would ordinarily call a fair vacuum. At 125 miles (200 kilometers) the pressure is approximately 1/180,000,000 of an atmosphere, which is probably lower than the best vacuum usually obtained in the laboratory.⁽¹⁾

The bending of the electromagnetic waves (rays) and their reflection back toward the earth by the ionosphere requires the presence of free electrons. These are produced by ionization of the rarefied gases just considered by radiations from various sources, chiefly ultraviolet light from the sun and cosmic rays from space, and by the action of electron streams shot out by the sun.¹²

Figure 4. The successive wave fronts of radio waves directed toward the ionosphere are shown by the short heavy lines. The upper portion of the wave front enters the ionosphere first and penetrates more deeply. The upper portion travels at a greater velocity, causing refraction, thereby turning the wave and reflecting it back toward the earth. Some energy absorption occurs in the ionosphere.

The refraction (Fig. 4) in the ionosphere that causes bending of the rays and their reflection back toward the earth is explained as follows:¹²

When an electromagnetic wave traverses this reflecting region the unattached electrons are moved by the influence of the wave and absorb energy from it. On account of their mass and charge and the presence of the earth's magnetic field the electrons in their movements reradiate the absorbed energy slightly out of phase with the passing wave; in doing so they change the effective velocity within the refracting zone. This variation in effective velocity is, of course, the cause of refraction.⁽²⁾

⁽¹⁾ Reprinted by permission, courtesy C. W. Rice, and QST. ⁽²⁾ Reprinted by permission, courtesy of R. A. Heising and the Bell Laboratories Record.

Ionospheric Regions. The ionosphere is considered as being composed of an E region between about 90 and 140 kilometers (56 and 87 miles) above the surface of the earth and an F region between 140 and 400 kilometers (87 and 250 miles) above the surface of the earth.¹

Ionospheric Layers. Measurements of radio waves reflected back to the earth have identified the existence of several ionized layers in the regions of the ionosphere just described.

Figure 5. Typical variations in the virtual heights of the ionosphere layers. The corresponding penetration, or critical, frequencies are shown. (Data from Reference 8.)

The E layer¹ is a "permanent" layer existing (Fig. 5) at about 110 kilometers (70 miles). Although it is permanent, in that it is present day after day, the ionic density of the E layer is not constant. It is strongest during the day, and at night it may almost disappear; also, it is subject to erratic variations.

The F layer¹ is a "permanent" layer at about 300 kilometers (185 miles) that varies as shown in Fig. 5. During all except the winter months, the F layer divides in the daytime into the F₁ and F₂ layers. The F layer also is subject to erratic variations.

A D layer has been found occasionally in the daytime at 50 to 90 kilometers (30 to 56 miles), but relatively it is of little importance.

In addition to the daily and seasonal variations depicted in Fig, 5, the ionosphere and its "layers" are affected by the 11-year sunspot cycle and by the number of sunspots present.¹³ The ionosphere is subject to sudden changes during erratic solar activity and the related phenomena of the aurora borealis and magnetic storms.¹⁴ The word layers was placed in quotation marks because they do not exist as layers in the usual sense. This is indicated at the right in Fig. 6 which shows a typical daytime distribution of ionization.⁸