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36. Introduction
In Section 3, the general behavior of radio waves of various frequencies in the ionosphere was described and explained in the light of the most recent studies in the field. Investigations of peculiar and anomalous propagation of vhf and uhf signals are being carried on continuously throughout the world and are bringing to light some rather remarkable phenomena. This Section is concerned with those effects, other than forward scattering in the ionosphere, which promise to influence the operation of long-range uhf and vhf communications circuits, radar systems, radio astronomy, and radio navigational systems.
37. The Effect of the Aurora Borealis
The aurora borealis, long a scientific mystery, is now believed to be the visual effect of varying ionization in the upper atmosphere.
The apparent correlation between the appearance of the aurora and the occurrence of radar echoes has been under investigation for some time. It now appears that the visual aurora does produce scattering and reflection of radar signals under certain conditions: if the aurora is at a low elevation, the correlation is very significant.
Little success has been realized in attempting to obtain radar echoes from high elevation aurora.
This effect, known as azimuthal aspect sensitivity, has been measured as precisely as possible; signals in the order of 100 mhz show no echoes when the elevation is greater than 15 degrees with respect to the radar antenna, while lower frequency signals in the region of 30 mhz exhibit little echo effect when the angle is greater than 20 degrees. In addition to the azimuthal effect, the studies have demonstrated that echoes are much stronger with the antenna oriented toward the north -- even in the most northerly countries, such as Norway -- than when the echo search is made toward the south. Thus, aurora ionization by echo methods is most readily detected when the radar is directed toward the magnetic north pole at low angles of elevation. The agreement among many of the workers is that radar echoes are due to scattering from ionized areas aligned along the earth's magnetic lines of force.
The strength of radar echoes from the aurora has been shown to depend upon the time of day and upon the season of the year.
Very little echoing is observable during the noon hours with maxima occurring around midnight; although the data presently avail able from various sources are not sufficient for precise estimates, it is quite definite that the strongest radar echoes are obtained at the time of the year when the visual aurora are usually brightest; that is, during the equinoctial months.
38. The Effect of Meteors upon Wave Propagation
When a meteor flashes into the upper layers of the air it forms an ionized trail about two miles in length. Under special conditions, this trail is capable of reflecting a large enough proportion of the incident radio energy to make it detectable by radio means. Early investigations into short-lived bursts of reflected energy within the normal skip zone showed that these transient echoes were of meteor-trail origin.
Fig. 23. Reflection from a meteor trail; (A) as obtained with radar equipment.
(B) As used in communications.
It is fairly well established that these trails are markedly aspect sensitive. This term is used to indicate a condition in which the angle of incidence of the radio waves at the reflective boundary is an important factor in determining the strength of the returned signals. In the case of a radar system, the close proximity of the transmitter and receiver establishes a perpendicular aspect sensitivity; the strongest meteor-trail reflection occurs when the radar waves are incident on the trail at an angle of 90 degrees (Fig. 23) . For a communications application in which there is a large distance between the transmitter and receiver, the angle of incidence must equal the angle of reflection toward the receiver.
Meteor trails are predominantly found between (approximately) 37 miles and 50 miles above the surface of the earth, the greatest occurrence being at about 43 miles. Echoes are of short duration, usually of the order of a half-second. During certain meteor showers, specifically the Perseid meteors, the echo duration may be extended to as much as 20 seconds. It has been found that the echoes may often be analyzed to determine the velocity of the meteor quite precisely; much study has yet to be done in this field, however, before the technique is well enough developed for general astronomical purposes.
39. The Faraday Effect
The first radar moon echoes obtained in 1946 by the U. S. Army marked the beginning of many such attempts, most of which have been successful. Most workers report two distinct types of fading.
(1) One type of fading, having a period of a few seconds, is attributed to the libration (an oscillatory motion, like that of a balance) of the moon itself.
(2) The second type, with a period of approximately one hour, is most intense a few hours after sunrise. This points to ionospheric effects. On the other hand, there is some evidence that it is due to the so-called "Faraday Effect," in which the plane of polarization of a wave is rotated in the presence of the earth's magnetic field. Verification of this theory has been reported recently; transmitters using perpendicularly oriented dipoles have received moon echoes with fading periods that agree closely with those predicted by the theory.
40. Ionospheric Absorption of Radio Waves
The refractive effect of the ionosphere has been discussed in previous Sections and has been shown to result from the variations in electrical density in the successive regions of this portion of the atmosphere. A second phenomenon involves energy losses as a radio wave passes into the ionosphere. Basic theory of transmission through ionized air predicts that very little loss will take place in the uhf region in this manner. Since there has been insufficient experimentation at the ultra high frequencies, the evidence uncovered in tests with normal high frequencies has been extrapolated to the uhf range by some authorities. These conclusions are summarized below.
The ionosphere contains "free" electrons. As the field of the passing wave acts upon these electrons, they move about in response to the alternations of polarity, the amplitude of the oscillation depending directly upon the strength of the field and inversely upon the mass of the particle. Such oscillation results in re-radiation of energy at the same frequency, so that very little of the total energy of the wave is lost; The conversion from field to kinetic and back to field energy is quite efficient. In other words, the fact that oscillation occurs does not account, in itself, for ionospheric absorption.
The only charged particles in the ionosphere that are involved in this process are the electrons; positive ions are not affected be cause of their relatively great inertial mass, a characteristic which insures that they will remain quiescent even when immersed in an alternating field. Oscillating electrons can, however, collide with neighboring gas molecules and positive ions, their kinetic energy being sufficient to set the latter in motion. This conversion does produce a net field loss, since a part of the field energy is converted into kinetic energy of massive particles (i.e., into heat). Thus, the degree of absorption is determined to a great extent by the frequency of these electron-molecule-ion collisions.
Experimental evidence points to the D region of the ionosphere as the site of the absorptive action. At low and middle latitudes of the Earth, the degree and time of occurrence are largely under solar control; at high latitudes, localized effects are associated with aurora, magnetic, and ionospheric disturbances, rather than with the sun. Although it is still the prevailing view that streams of charged particles originating in the sun are responsible for most of the 0-layer ionization, the details of the mechanism are still far from clear.
41. Effect of the Ionosphere on Radio Waves of Extra-Terrestrial Origin.
Although most of the interest in ionospheric effects centers around communications circuits using the uh£ range, the comparatively new science of radio astronomy is very much concerned with signals of extra-terrestrial origin. Radio astronomy shows great future promise, not only as a supplement to optical astronomy, but as a field in itself. For this reason, the effect of the ionosphere on the so-called "Janssen signals" from space is of prime importance in that it makes necessary the correction of coordinates of "radio stars" as observed on radio telescopes.
Ionospheric refraction. The signals from radio stars have been found to deviate to a greater degree than that predicted by standard ionospheric refractive theory. Much theoretical work has been carried on in recent years to help explain the discrepancies be tween the predicted and the experimental radio paths. Theories based upon increasing curvature of the upper F2 region and upon greatly increased electron density in the ionosphere during the summer, particularly in the layer of the F2 region immediately above the point of maximum ionization, are currently under investigation.
"Twinkling" or radio stars. The radio signal stream from localized regions in outer space is by no means constant in intensity.
These rapid and unpredictable variations -- now commonly referred to as scintillation or twinkling -- were once thought to be inherent in the sources themselves. Later work by separated groups of observers demonstrated that the scintillations observed from various points on the earth simultaneously could not be correlated with each other. In this sense, the radio observations are similar to optical twinkling -- an effect due to the earth's atmosphere. In the radio case, however, the disturbance originates in the ionosphere rather than the troposphere.
The anticipated results of proposed experiments on radio star scintillation may be summarized somewhat as follows: scintillations will begin to appear when the received frequency falls be low about 3000 me, the intensity depending upon the angle of elevation of the source above the horizon and the latitude of the observer. As the frequency is decreased below 100 mhz, the deviation from standard received intensity may be as much as 50 percent at low angles of elevation, the intensity falling to a minimum as the source reaches the zenith (directly overhead). Also, scintillation effects will be more pronounced at high latitudes, because ionospheric disturbances are of greater intensity in the polar regions.
42. QUIZ
(1) Under what conditions does the aurora borealis influence transmission?
(2) Explain "azimuthal aspect sensitivity."
(3) What is meant hy "aspect sensitive" in relationship to meteors?
(4) What are the two types of fading associated with the "Faraday Effect"?
(5) List the pertinent conclusions that have been drawn with respect to the ionospheric absorption of radio waves; to ionospheric refraction; to "twinkling" of radio stars.