The Ionoshere The term ionosphere refer to the ionized region that exists in the upper atmosphere, and that has an important influence on the propagation of radio waves. This region extends from a lower limit of 50 to 80 km, upward as far as there is atmosphere to be ionized. The ionized results principally from the action of ultraviolet radiation from the sun, but the processes by which it is produced and maintained are obscured by the lack of precise knowledge of the composition, temperature, etc., of the atmosphere at the levels involved. It can be said, however, that the atmospheric pressure in the ionosphere is extremely small, being little if any greater than that often found in a vacumm tube. Because of this, collisions between electrons and ions are relatively infrequent even in the lower part of the ionosphere; so recombination takes place only very slowly. The ionization produced in the daytime is also carried over into the night by some means, the details of which are at present uncertain. Because of the variation in chemical composition of the air with height, and because different gases differ in their ability to absorb solar radiation of different frequencies, there is a tendency for the ionization in the ionosphere to become stratified, so that the curve of electron density as a function of height commonly has several maxima, as shown in Fig.27. There are two semipermanent “Layers” of this character, the E and the F or F₂ layers, with a third designated as F₁ usually present in the daytime. The height of a particular layer and the maximum electron density in the layer will vary at different times of the day and of the year as the result of variations in the composition and temperature of the air at different heights, and of the radiation received from the sun. The distribution of ionization with height in a typical case is shown schematically in Fig. 27, in which three distinct layers are shown corresponding to typical daytime conditions. It will be noted that the ionization does not drop to zero between the layers, but merely has a value les than the maxima on either side.

A radio wave entering the ionosphere from the earth has a tendency to be bent earthward, and if the conditions are favorable, the bending will be sufficient to cause the wave to return to earth as shown in Fig. 27. Upon striking the earth it will then be again reflected upward as shown. This action makes it possible to carry on radio communication over great distances in spite of the earth’s curvature. The wave that reaches these distant points as a result of the action of the ionosphere is termed the sky wave.

Propagation of a Radio Wave in an Ionized Medium. Fundamental Mechanisms Involved.—When a radio wave enters the ionosphere the electric field of the wave exerts a force upon the electrons, setting them in vibration at the frequency of the wave. Under conditions where the earth’s magnetic field has negligible effect, this vibration is along a line parallel to the electric field of the wave. The velocity of vibration lags 90^o behind the electric field, and is inversely proportional to the frequency. Since a moving electron represents a current, each electrons set in vibration by the radio wave acts as a miniature parasitic antenna that absorbs energy from the wave and then reradiates this energy in a different phase. The net effect, after the phase difference between the original and reradiated fields is taken into account, is to bend the wave path away from the regions of electron density toward regions of lower density. The magnitude of this effect varies with the amplitude of the electron vibration, and therefore becomes increasingly great as the wave frequency is lowered.

The electrons in the ionosphere exist in the presence of the earth’s magnetic field. Such a magnetic field exerts a force on moving electron that magnetic field at right angles to the direction of motion. The direction of this force is at right angles to the direction of motion of the electron, and to component of the magnetic field producing the deflecting force. The effect of the earth’s magnetic field at the higher radio frequencies is to cause each electron to vibrate in an elliptical path, as show at a or b in Fig.28, with the major axis of the ellipse lying in the direction of the electric field of the wave.

The ratio of minor axis to major axis increases as the velocity with which the electron vibrates becomes greater, i.e., as the frequency is reduced. This trend continues until at a frequency termed the gyro frequency and having a value of approximately 1.4 mc, the electron vibrates in a spiral path as shown at c, in which the velocity becomes increasingly great. At still lower frequencies the electrons vibrate in loops as shown at d and e, commonly making several loops during each half cycle of the radio wave.

It will be noted that in all cases where the magnetic field of the earth has an influence, the effect is to cause the vibrating electrons to have some motion at right angles to the direction of vibration that would exist in the absence of magnetic field. The polarization of the vibration that would exist in the absence of a magnetic field. The polarization of the fields reradiated by the vibrating electrons will hence differ from that of the passing wave, causing the polarization of the resultant wave to be affected by the presence of the earth’s magnetic field. This effect is maximum at the gyro frequency, but is present to some extent even for waves of much higher and much lower frequencies.

The magnetic field, in addition to affecting the polarization, also cause a wave to be split into two components, which follow different paths, which have different phase velocities, differ in polarization effects, and suffer different attenuations. This is double refraction. An electron set in vibration by a passing radio wave will from time to time collide with a gas molecule. In such a collision, the kinetic energy that the electron has acquired from the radio wave is partly transferred to the gas molecule, and partly reradiated in the form of a disordered radio wave that contributes nothing to the transmission. The net result is therefore an absorbed depends upon the gas pressure (i.e., upon the likelihood of the electron colliding with a gas molecule) and upon the average velocity of the vibrating electron (i.e., upon the average energy lost per collision).