March 1940 QST
Reading through this article reminds me of studying for the amateur radio exams. In fact, the information presented in this 1940 QST piece does not seem to be lacking anything that contemporary discussions include. My point is that a great amount of knowledge had already been amassed about earth's upper atmosphere a mere four decades after the first transatlantic radio communications were accomplished by Marconi on December 12, 1901 from Poldhu in Cornwall, England, to Newfoundland, Canada. Considering that at the time no instrumented sounding rockets had been launched into the extreme upper layers (F1 & F2, beginning at around 120 mi | 200 km), a lot had been discerned about characteristics as they pertain to radio communications. Balloons were used for direct measurements at lower altitudes, but inferences based on observed behavior of long distance communications provided most of the related knowledge.
As time goes on, the unremitting efforts of physicists in probing the upper atmosphere bear the fruit of continual additions to our knowledge of the things that happen to the signals We launch so hopefully into space. It's helpful, now and then, to take stock of that knowledge. The accompanying article, extracted from the Bureau of Standards' Letter Circular LC-575, is an up-to-date summary of known ionosphere effects. "Must" reading for the chap who wants to keep well informed.
In the high atmosphere, above about 50 kilometers (30 miles), the air particles are separated so far that collisions between them are far less frequent than in the lower atmosphere, and when an air particle is ionized by ultraviolet radiation from the sun it remains ionized for a considerable time. Therefore at any given time a large proportion of the air particles are in an ionized condition. This does not occur much below about 50 kilometers (30 miles), because the ionizing radiations from the sun are largely absorbed in the higher regions of the atmosphere. Likewise there is not very great ionization density above about 400 kilometers (250 miles), because the air is so rare at such heights that there are not enough atoms to provide for great ionization density. The region in which the ionization is great, great enough in fact to affect radio wave transmission, is thus between 50 and 400 kilometers (30 and 250 miles) above the earth's surface, and this region is called the "ionosphere".
The ionization in the ionosphere is not uniformly distributed with altitude but is stratified, and there are certain definite layers in which the ionization density is such as to reflect radio waves. These layers do not remain always at the same height but vary diurnally, seasonally, and otherwise, in both height and ionization density. There may be a considerable number of such layers at a given time. There are two principal ones, called the "E" and "F" layers. The E layer is at a height of 90 to 140 kilometers at different times, usually about no kilometers. The term F-layer is ordinarily reserved for the other layer as it exists at night; in the daytime during most of the year it divides into two layers which are called the "F1" and "F2". The night F layer is at a height of about 180 to 400 kilometers. The F1 layer exists in the daytime, at a height of about 140 to 250 kilometers. The F2 layer also exists in the daytime, at a height of about 250 to 350 or more kilometers in the summer, and about 150 kilometers in the winter day. (The "virtual" heights, defined later, are somewhat higher than these values.) The fourth layer, which is semi-permanent, is the "D" layer; it exists only in the daytime, and its height is of the order of 50 to 90 kilometers. Little has been done on the determination of the quantitative characteristics of the D layer, its effects being largely inferred rather than directly observed. Existing knowledge covers mainly the E, F, F1, and F2 layers.
The structure of the ionosphere may be visualized in an elementary way from Fig. 1, which is for a typical summer daytime condition, the E, F1, and F2 layers all being present. This diagram is drawn to scale, so the angles of reflection of radio waves from the layers may be estimated correctly. The three layers are shown as mere thin lines, for simplicity. The layers have in fact a certain thickness, and the density of ionization varies somewhat in this thickness. At the right of the diagram is a rough illustration of a possible distribution of ionization density with height.
Dotted lines indicate two of many possible paths of radio waves from a transmitter to a receiver as transmitted by reflection from the ionosphere layers. This picture, simple as it is, does in fact represent the basic mechanism of radio wave transmission over long distances. When we consider the variations of ionization and height of the layers with time, and the effects of the ionization upon the received intensity and the limits of transmissible frequency at any particular time, the picture loses its simplicity. However, most of the phenomena of long-distance radio transmission are completely explainable in terms of the ionosphere.
The principal ionosphere characteristics which control or determine long-distance radio transmission are the height and the ionization density of each of the ionosphere layers. Since each layer has a certain thickness it is necessary to define the sense in which the term "height" is used. When a ray or train of waves is reflected by a layer, it is slowed down as soon as it starts to penetrate into the layer. The process of reflection thus goes on from the point at which the waves enter the layer until they have been fully turned down and leave the layer. This is true whether the waves travel vertically or obliquely to the ionosphere. It is illustrated for the oblique case in Fig. 2. The waves follow a curved path in the layer until they emerge at the same vertical angle at which they entered. The time of transmission along the actual path BCD in the ionized layer is the same as would be required for transmission along the path BED if there were no ionized particles present. The height h from the ground to E, the intersection of the two projected straight parts of the path, is called the "virtual height" of the layer. This is the important quantity in all measurements and applications.
The virtual height of a layer is measured by transmitting a radio signal from A, and receiving at F both the signal transmitted along the ground and the echo, or signal reflected by the ionosphere, and measuring the difference in time of arrival of the two. The signal is a special, very short pulse in order that the two may be separated in an oscillograph, as the time differences are mere thousandths of a second. The difference between the distance (AE + EF) and AF is found by multiplying the measured time difference by the velocity of light. From this and the known distance AF, the virtual height h is calculated. It is usual to make AF zero by transmitting the signal vertically upward and receiving it at the same place (and it is for this case that the term "virtual height" rigorously applies). The virtual height varies somewhat with frequency.
The effectiveness of the ions in reflecting the waves back to earth depends on the number of ions present in a unit of volume, i.e., the ionization density The higher the frequency, the greater is the density of ionization required to reflect the waves back to earth. It has been shown that, for electron ionization, the relation (for the ordinary ray, explained below) is
N = 0.0124f2
where N is the number of electrons per cubic centimeter and 1 is the highest frequency, in kilocycles per second, at which waves sent vertically upward are reflected back to earth. Waves of all frequencies higher than this pass on through the ionized layer and are not reflected back to earth, while waves of all lower frequencies are reflected. This frequency is called the "critical frequency," and measurement of it is, with the equation just given, a means of measuring the maximum ionization density in an ionized layer. (Waves of higher frequencies than the critical are sometimes reflected by another mechanism - see discussion of "Sporadic E" later.)
Measurements of critical frequency are usually made by means of vertical or nearly vertical transmission (that is, with the transmitter and receiver not far apart). The process is to measure the virtual height, by the method described above, repeating the determination at successively increasing frequencies until the waves are no longer received back from the layer. The highest frequency at which waves sent vertically upward are received back from the layer is the critical frequency of that layer. Typical results of such measurements are illustrated in Figs. 3, 4 and 5, for different times of year, day and night. They show critical frequencies as sharp increases in virtual height.
In Fig 3, starting at a frequency below 2000 kc. the virtual height is found (in this example) to be about 110 kilometers, and remains at about this height until about 3300 kc. The critical frequency of the E layer at the time of this measurement is thus 3300 kc.; i.e., this is the highest frequency at which vertically-incident waves are reflected back to earth. All such waves of higher frequency penetrate through the E layer and go on up to a higher layer, the F2. The F2 layer has a greater ionization density and so it reflects back waves of frequency greater than 3300 kc. It is not until frequencies greater than 11,500 kc. are used that the F2 layer fails to reflect them, in the case illustrated.
Near the critical frequency the waves are excessively retarded in the ionized layer, which accounts for the rise of the curve at the critical frequency. At the right of the curve appear two critical frequencies for the F2 layer. This is an indication of double refraction of the waves due to the earth's magnetic field, two components of different polarization being produced. One is called the "ordinary" wave and the other the "extraordinary" wave. The symbols o and x, respectively, are used for these components. The critical frequency of a layer n is represented by the symbol fn, and to such symbol the o or x is added as a superscript. Thus the critical frequencies of the F2 layer for the ordinary and extraordinary waves are indicated by the respective symbols, SYMBOL HERE and SYMBOL HERE.
In the case of the E layer, the ordinary wave usually predominates and the extraordinary wave is so weak it does not affect radio reception. At Washington the critical frequency for the extraordinary wave is about 750 kc. higher than for the ordinary wave (for frequencies of 4000 kc. or higher). The difference in frequency is proportional to the intensity of the earth's magnetic field at the place of reflection, and is therefore different at different places on the earth. In reporting results of measurements of critical frequency it is now customary to give the values for the ordinary wave; practice varied in the past.
Besides the virtual heights and critical frequencies, the absorption of the energy of radio waves by the ionosphere is an important factor in limiting radio transmission. This absorption exists because the ions set in motion by the radio waves collide with air molecules and dissipate as heat the energy they have taken from the radio waves. Consequently the energy thus absorbed from the radio waves is greater, the greater the distance of penetration of waves into the ionized layer and the greater the density of ions and air molecules in the layer, and hence the greater the number of collisions between ions and air molecules. Absorption is especially great in the daytime, and it occurs chiefly in the low ionosphere, in the D or E layers. It also occurs in the high ionosphere, near critical frequencies. Much of the low-layer absorption disappears with the decrease of low-layer ionization at night. Higher frequencies are less affected by absorption than are lower frequencies, for waves passing through the same ionized layers.
Regular Variations of Ionosphere Characteristics
There are three principal types of variation of critical frequencies which are fairly regular with time. These are diurnal variations, seasonal variations, and year-to-year variations with the sunspot cycle.
The diurnal and seasonal variations of the critical frequencies of the normal E layer are particularly regular. The critical frequencies vary with the altitude of the sun, being highest when the sun is most nearly overhead. Thus the diurnal maximum of the E critical frequency (fE) is at local noon, and the seasonal maximum is in mid-summer. At night this layer usually does not reflect waves of frequencies higher than about 1000 kc.
The diurnal and seasonal variations of the critical frequencies of the F2 layer at Washington are quite different from those of the E layer. The winter F2 critical frequencies exceed any regular critical frequency found during the summer. In the winter a broad diurnal maximum occurs in the daytime, centered around 1:00 P.M. local time. In the summer a broader diurnal maximum centers about sunset. During the night the winter critical frequencies are usually lower than the corresponding summer values. Thus, the highest F2-layer critical frequencies occur during the winter day, and the lowest F-layer critical frequencies occur during the winter night; the summer day and night values are between.
The F2 virtual heights are much lower during a winter day than during a summer day. The F virtual heights at night are about the same in winter as in summer.
The seasonal effects in the ionosphere synchronize with the sun's seasonal position, not lagging a month or two as do the seasons of weather. Winter conditions in the F2 layer obtain during a period of several months from about October to March, and summer conditions for a period of several months from about April to September. Around the equinoxes, there is a transition period of a month or two in which the change occurs between winter and summer conditions; in these transition periods the ionosphere characteristics (except for the effects of sporadic E) fluctuate between winter and summer conditions and are thus more erratic than during the rest of the year.
There are important changes in ionosphere characteristics in the 11-year sunspot cycle. From the sunspot minimum in 1933 to the sunspot maximum in 1937 the F- and F2-1ayer critical frequencies doubled (for most hours of the day), and the E-layer critical frequencies became 1.25 times as great. A consequence is that the best radio frequencies for long-distance transmission were approximately twice as great in 1937 as in 1933 (except for summer daytime, when they were about 1.5 times as great). In about 1944 they will return to minimum values.
The condition of the ionosphere varies somewhat with latitude. For all latitudes of continental United States the differences from the Washington values appear to be negligible, but the values in Alaska and in the Canal Zone are somewhat different.
Applications to Radio Transmission
From the vertical-incidence critical frequencies and virtual heights of the ionosphere layers, at any given time, it is possible to calculate the upper limit of radio frequency that can be transmitted over any distance. The calculated values of maximum usable frequency are found to agree with direct observation of radio transmission over such distances.
When radio waves are transmitted along the earth over any distance, they strike the ionosphere obliquely (Fig. 1). Such obliquely incident waves can be reflected back down with lower ionization densities than can vertically incident waves. It results that the larger the angle of incidence (angle of wave path with the vertical), i.e., the greater the transmission distance, the higher is the upper limit of frequency of waves that can be reflected from a layer of given ionization density or critical frequency. This upper limit of frequency, for transmission via an ionosphere layer, for a particular time and transmission distance, is called the "maximum usable frequency." It may be calculated roughly, to a first approximation, by multiplying the critical frequency of the layer by the secant of the angle of incidence.
The accurate calculation of maximum usable frequencies from vertical-incidence critical frequencies is quite complicated. For convenience, typical values for the conversion are given in Table 1.
To obtain the maximum usable frequency for transmission over a given distance by way of a given layer, multiply the critical frequency by the ratio given in the table. Where blanks appear in the table, and for distances over 3500 kilometers, the distance is too great for one-hop transmission, i.e., transmission over such distances requires multiple reflection from the ionosphere with intervening reflection from the ground.
The distance at which a given frequency is the maximum usable frequency is also the minimum distance over which that frequency is receivable. This minimum distance for any frequency is called the skip distance; at any less distance it is impossible to receive on that or higher frequencies, except for sporadic or scattered reflections.
The highest maximum usable frequencies (in north temperate latitudes) occur during the winter day and the lowest during the winter night. The summer values for both night and day lie between these two extremes except as modified by sporadic-E or scattered reflections.
Maximum Usable Frequencies over Long Paths
Since the local time of day, and hence the ionosphere characteristics, may vary a large amount throughout a long transmission path, it is necessary to consider what part of the path determines the conditions of transmission. For single-hop transmission (transmission by a single reflection from the ionosphere), it is the region half-way between the transmitter and receiver whose conditions determine the transmission, because it is there that the reflection from the ionosphere takes place. In multi-hop transmission, when the radio waves are reflected from the ionosphere, then from the ground, then back to the ionosphere, etc., the determining conditions are in the middle of each hop.
The maximum possible distance of transmission between the transmitter and receiver whose conditions determine the transmission, because it is there that the reflection from the ionosphere takes place. In multi-hop transmission, when the radio waves are reflected from the ionosphere, then from the ground, then back to the ionosphere, etc., the determining conditions are in the middle of each hop.
The maximum possible distance of transmission by a single hop is limited by the geometry of the earth's surface and the layer, and also by absorption or other limitation at the ground of those waves which are nearly tangential to the earth's surface. It is found in practice that the minimum angle with the ground of the radio waves transmitted or received (over land) averages about 3 1/2 degrees. From these considerations the geometry indicates that the maximum distance along the earth by a single hop is ordinarily about 3500 kilometers for the F2 layer, and about 1700 kilometers for the E layer. Single-hop transmission may sometimes be possible at greater distances than these while at the same time multi-hop transmission over the same path may be more efficient.
Because of the variation of ionosphere characteristics with longitude, different frequencies may be necessary for transmission in different directions from a given place. For example, around sunset in winter lower frequencies are used in transmitting eastward than in transmitting westward from the same location. This does not mean, however, that different frequencies would be necessary or desirable in opposite directions over the same path.
For very long paths in which widely different longitudes (i.e., times of day) are involved, it sometimes happens that the waves travel different parts of the way by different layers. For such cases, it is necessary to take account of the heights of the different layers to determine the lengths of the several hops.
Effects of Ionosphere Irregularities
The primary effects of the ionosphere on radio wave propagation are those already described, which are due to the normal or regular characteristics of the ionosphere. The modes of variation of those characteristics have been shown to be of a regular and fairly predictable nature. There are some other ionosphere phenomena which are irregular in their times of occurrence, and make radio phenomena in general much less predictable. Five types of such phenomena have been identified: sporadic-E-layer reflections, scattered reflections, sudden ionosphere disturbances, prolonged periods of low-layer absorption, and ionosphere storms. While all five are irregular in time, the first two are primarily due to irregularities in space.
It sometimes happens that waves are reflected by the E layer on frequencies higher than that at which the E-layer waves normally disappear and the reflection of waves by higher layers begins. Thus, in the example shown in Fig. 3 waves may sometimes be reflected at the E-layer height of 110 kilometers on frequencies of 4 or 5 or more megacycles. These reflections are due to a process different from the normal reflection in the ionized layer; the process is probably one of reflection from a sharp boundary of stratified ionization. The existence of these "sporadic E" reflections necessitates a redefinition of the term "critical frequency," previously defined as the highest frequency at which waves sent vertically upward are received back from the layer. When sporadic E reflections occur they may be received simultaneously with reflections from higher layers; for example, vertical-incidence reflections might be received at 8 Mc. from both the E and the F2 layers. The E-layer critical frequency, more precisely defined, is the value (3300 kc. in the example shown in Fig. 3) at which the observed virtual height shows a sudden rise to large values as the frequency is increased. Except for the occasional occurrence of sporadic E reflections, all waves of higher frequency pass through the E layer and are not reflected by it.
Sporadic E leads to interesting results in long-distance radio transmission. It accounts for long-distance transmission up to higher frequencies than by any other means. Strong vertical-incidence reflections by sporadic E sometimes occur at frequencies up to about 12 Mc. By reason of the large angles of incidence possible with the E layer, this makes occasional long-distance communication possible on frequencies as high as 60 Mc. Such communication is generally for only a short time and for restricted localities. Sporadic E is thus patchy or sporadic in both geographical distribution and time.
Sporadic E occurs most commonly in the summer, May to August, particularly in the morning and evening, but may occur any time of day or night. It occurs occasionally at all seasons, particularly in the evening. It occurs more at high latitudes than in equatorial regions.
An irregular type of reflection from the ionosphere occurs at all seasons and is prevalent both day and night. These reflections are most noticeable within the skip zone, or at frequencies higher than those nominally receivable from the regular layers. Like sporadic E, they occur at frequencies which may exceed the F2 critical frequencies, but are unlike sporadic E in that they are complex and jumpy thus causing signal distortion; they occur and disappear fitfully, and are almost useless for communication purposes. Some types are of very weak intensity. The scattered reflections are characterized by very great virtual heights, usually somewhere from 400 to 1500 kilometers. Their occurrence was for a time thought to indicate the existence of another layer above the F2 layer which might be called the G layer. It is now, however, thought that they are of several types, and that some of them are due to complex reflections from small, ephemeral, scattered patches or "clouds" of ionization in or between the normal ionosphere layers, and thence to one or more layers and ground by single or multiple reflection.
Sudden Ionosphere Disturbances
The most startling of all the irregularities of the ionosphere and of radio wave transmission is the sudden type of disturbance manifested by a radio fadeout. This phenomenon is the result of a burst of ionizing radiation from a bright chromospheric eruption on the sun, causing a sudden abnormal increase in the ionization in the D layer (below the E layer), frequently with resultant disturbances in terrestrial magnetism and earth currents as well as radio transmission. The radio effect is the sudden cessation of radio transmission on frequencies above about 1500 kc.
The drop of the radio signals to zero usually occurs within a minute. The effects occur simultaneously throughout the hemisphere illuminated by the sun, and do not occur at night. The effects last from about ten minutes to an hour or more, the occurrences of greater intensity in general producing effects of longer duration. The effects are more intense, and last longer, the lower the frequency in the high-frequency range (i.e., from about 1500 kc. up). It is consequently sometimes possible to continue communication during a radio fadeout by raising the working frequency.
The radio, magnetic, and other effects are markedly different from other types of changes in these quantities. The effects are most intense in that region of the earth where the sun's radiation is perpendicular, i.e., greater at noon than at other times of day and greater in equatorial than in higher latitudes.
Taking due account of the variation of the effects with frequency and distance, varying effects in differing directions can be explained. Reception in the United States from stations in the southern hemisphere usually exhibits greater effects than reception from other directions (because of passing the equatorial regions). Similarly, when the disturbance occurs at a time when it is morning at the receiving point the effects are usually greater in reception from the east than from the west, and vice versa for the afternoon (because of passing the region where it is noon). A radio fade-out sometimes occurs when it is night at the receiving point, but only when the path of the waves is somewhere in daylight.
Prolonged Periods of Low-layer Absorption
This phenomenon is similar to the sudden ionosphere disturbance in its effects and characteristics except that its beginning as well as recovery is gradual and it has a longer time duration, commonly several hours. The intensity diminution is in general not as severe as in the more intense fadeouts, but sometimes the intensities fall to zero.
The low-layer absorption effect appears to be due to increased ionization in the D layer (below the E layer), exactly as for the sudden ionosphere disturbances. The increased ionization is caused by an abnormally great outpouring of ultraviolet light from the sun, but in this case it is not so sudden as in the eruptions which cause the sudden ionosphere disturbances. The variation of the efects with frequency, and other characteristics, are .the same as for the sudden ionosphere disturbances.
Both phenomena occur at all seasons, but the prolonged periods of low-layer absorption have been found to occur in a group of several weeks' duration at periods of high sunspot activity, the groups being separated by more or less quiet periods of several months. They frequently but not always occur during periods when sudden ionosphere disturbances are numerous.
An ionosphere storm is a period of disturbance in the ionosphere in which there are great anomalies of critical frequencies, virtual heights, and absorption. Radio transmission is poor (except for the low frequencies, below 500 kc., which are sometimes improved). An ionosphere storm usually lasts one or two days, and occurs both day and night. It is usually accompanied by a magnetic storm (i.e., a period of unusual fluctuation of terrestrial magnetic intensity). During the first few hours of very severe ionosphere storms the ionosphere is turbulent, stratification is destroyed, and radio wave propagation erratic. During the later stages of very severe storms and during the whole of more moderate storms, the upper part of the ionosphere, principally the F2 layer, is expanded and diffused. The critical frequencies are much lower than normal and the virtual heights much greater, and therefore the maximum usable frequencies are much lower than normal. It is often necessary to lower the working frequency in order to maintain communication during one of these storms. There is also increased absorption of radio waves during an ionosphere storm. Ionosphere storms are most severe in auroral latitudes .and decrease in intensity as the equator is approached. Ionosphere storms occur approximately simultaneously over wide geographical areas. The condition of the ionosphere is much less uniform from point to point than on undisturbed days.
Posted December 9, 2015