The Ionosphere

Author: J.B. Hoag

That part of the energy radiated from an antenna which travels in an upwards direction is called the sky wave. The fact that field intensities, which are very much larger than can be expected from the ground wave alone, are observed at great distances from a transmitter suggests that the sky wave has been turned back to the earth again. Two scientists, Kennelly and Heaviside, independently and almost simultaneously, proposed that a spherical conducting shell surrounded the earth some miles above it and reflected the sky waves back just as light is reflected from the inner surface of an inverted metal bowl.¹

The Kennelly-Heaviside reflecting layer is now called the ionosphere and is known to consist of several layers or regions, one outside the other, in which the rarefied air has been broken up into electrons and ions, or positively charged gas molecules. The electric field of the sky wave oscillates the small electrons easily back and forth and they, in their turn, set up the electromagnetic field which returns to earth. The ionization or electrification of the air is due in large part to the sun's rays. Thus the location and number of electrons suffers great changes day and night, winter and summer. Hence the sky wave which returns to the earth is also subject to seasonal and diurnal variations. It is subject to changes over the 11-year sunspot cycle. In addition, the ionization of the region is greatly altered whenever unusual changes occur in the earth's magnetic field. Then the layers usually break up and expand, and radio transmission is poor. Sudden disturbances on the sun are accompanied by fadeouts, when the sky wave disappears almost instantly. Also, for unknown reasons, " patches " of intensive ionization sometimes occur in small regions and give rise to unexpectedly long distance transmission.

It is not clear just why there should be layers instead of a continuous distribution of ions and electrons. The lowest layer, called the E region, is about 70 miles above the earth's surface, has its greatest density of ionization around local noon. At night, without the sun's rays, it is but weakly ionized. At this height, the ions and electrons are sufficiently close together to re-combine quickly to form neutral particles.

Above the E region lies the F region, about 175 miles above the earth at night. Here the atmospheric pressure is so low that ions and electrons re-combine slowly. The number of ions reaches a minimum just before sunrise. The layer splits into two layers during the daytime, the lowest of which is called F₁ and the higher F₂. Their ionization is greatest at about local noon.

If a transmitter is used which sends out a sudden, sharp pulse of energy, and the receiver is located nearby, two (or more) pulses will be received. The first pulse to come in is the direct or ground wave, the second is the pulse which traveled up to the ionosphere and back again. By recording the pulses and measuring the extra amount of time needed for the sky pulse, it is possible to calculate the virtual height of the ionosphere. This is the height calculated on the assumption that the wave traveled throughout its entire path at the unabated velocity of light. This is not true in the ionosphere, so that virtual heights are always greater than the actual heights to which the wave rises.

1	The mathematical treatment is essentially the same in the two cases; uses the same concepts and similar equations.
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