The investigation of whistlers and related phenomena is a key element in studies of very-low-frequency propagation, satellite communication, the outer ionosphere, and solar-terrestrial relationships. This comprehensive text presents a history of the study of the phenomena and includes all the elements necessary for the calculation of the characteristics of whistlers and whistler-mode signals.
An introduction and brief history are followed by a summary of the theory of whistlers and a detailed explanation of the calculation of their characteristics. Succeeding chapters offer a complete atlas of a variety of whistlers, including those observed in satellites and those generated by nuclear explosions; the results of satellite observation of whistler-mode propagation; the method of reducing whistler data and obtaining electron density information; a full atlas of the various kinds of emissions; and an outline and comparison of the theories of generation of emissions.
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Whistlers and Related Ionospheric Phenomena
By Robert A. Helliwell
Dover Publications, Inc.Copyright © 1993 Robert A. Helliwell
All rights reserved.
Among the many accidental discoveries of science are whistlers, which, with related phenomena, comprise a group of complex and fascinating natural events that can be heard on very low frequencies with the simplest of audio-frequency equipment.
Whistlers are remarkable bursts of very-low-frequency (VLF) electromagnetic energy produced by ordinary lightning discharges. These bursts travel into the ionosphere, where their interaction with free electrons forces them to follow approximately the lines of force of the earth's magnetic field. Traveling many earth radii beyond the earth's surface, they bring back information about the distribution of ionization in the outer atmosphere and form the basis for a new and novel means of communication.
Noises similar to whistlers, called VLF emissions, appear to originate within the earth's ionosphere, possibly on streams of charge that flow in from the sun or are trapped in the earth's magnetic field. They, too, carry information about sun-earth relationships, but the interpretation of this information is a problem that remains to be solved.
To many first-time observers these phenomena seem almost unbelievable. To others they suggest supernatural origins. During the early phases of research on whistlers at Stanford University, the subject was of great interest to newspaper reporters. Articles entitled "Voices from Outer Space" stimulated a substantial flow of fan mail from many parts of the world. One correspondent, gratified to learn that research in this field had at last begun at Stanford, wrote at length about his own investigations during which, he said, he was able to hear without benefit of any special equipment the weird sounds described in the newspaper report. Moreover, he stated, he had identified the producers of these strange sounds as the people on Mars. Others found close connections between the whistler phenomena and flying saucers. One contributor reported that she had heard whistlers on a three-quarter-ton Admiral air conditioner. As the occult aspects of the subject faded away, ordinary scientific curiosity began to produce information that has resulted in a fairly complete and understandable picture of whistlers.
Nature and occurrence of whistlers. Whistlers are radio signals in the audio-frequency range that "whistle." Usually a whistler begins at a high frequency and in the course of about one second drops in frequency to a lower limit of about 1000 cycles per second. Some whistlers are very pure gliding tones; others sound "swishy"—much like air escaping from a punctured balloon tire. Some whistlers are very short, lasting a fraction of a second; others are long, lasting two or three seconds. Often whistlers occur in groups. In one type of group the whistler appears to echo several times with an equal time lapse between different members of the train of echoes. In each whistler of the group the rate of decrease of frequency is less than that in the preceding whistler. These groups are called "echo trains." Sometimes two or more distinct, similar whistlers appear to overlap in time; these are called multiple whistlers. The amplitude of whistlers is greatest at a frequency usually near 5000 cps, but sometimes as high as 15,000 cps. On rare occasions whistlers have been observed to sweep all the way from 35,000 down to 300 cps.
Many whistlers are preceded by a sharp impulse that usually sounds like a click in the reproducer. These impulses, called "atmospherics," or sometimes "spherics" for short, are produced by strokes of lightning which may be many thousands of miles away. The radiation from the lightning stroke travels at approximately the speed of light in the space between the earth and the lower edge of the ionosphere, called the earth–ionosphere waveguide. At times when the reflection efficiency of the ionosphere is high this radiation may echo back and forth between the boundaries of the waveguide many times before disappearing into the background noise. Then the received disturbance consists of a series of impulses, which produces a faintly musical or chirping sound. This particular type of atmospheric is usually called a "tweek."
During a period of high whistler activity, there is usually no uncertainty about the relation between whistlers and the sharp clicks preceding them. However, many whistlers appear without an associated sharp click. These latter whistlers are believed to originate in lightning flashes in the opposite hemisphere of the earth, which explains why the atmospheric from the source is often not an identifiable event at the receiver. Occasionally, however, these atmospherics are strong enough to be clearly identified in recordings made in the opposite hemisphere.
The variable occurrence of whistlers is understood in broad terms. Whistlers tend to be more common during the night than during the day, mainly because of the relatively high absorption in the daytime ionosphere, and they are more frequent at locations and times where lightning storms are common, or at points magnetically conjugate to regions of lightning activity, i.e., points that have a common magnetic field line with the active regions but that lie in the opposite hemisphere. As a result of the dependence of whistlers on lightning as well as on propagation factors, the day-to-day variation in whistler occurrence is great. Many days may pass without the observation of a single whistler. On other occasions whistlers may occur at rates exceeding one per second.
Synoptic data on the occurrence of whistlers show that whistler activity tends to be greatest at middle latitudes, reaching a maximum in the vicinity of 50 degrees geomagnetic latitude. At the geomagnetic equator whistlers are virtually unknown, and in polar regions their rate is significantly lower than in middle latitudes.
Recordings made simultaneously at spaced stations show that a whistler may spread over an area typically about 1000 km in diameter. On occasion very strong whistlers have been detected at stations spaced as much as 7000 km apart.
From these observations we can characterize the whistler as a local phenomenon that is concentrated at middle latitudes and that shows marked variations in occurrence even from day to day.
Methods of observation. Man has no sense that enables him to detect radio waves of ordinary intensity. (Very strong radio waves, however, can produce a noticeable or even dangerous increase in body temperature.) For the detection of whistlers it is necessary simply to employ a transducer that converts electromagnetic waves to sound waves. Perhaps because of this simplicity of observation, whistlers were observed early in the history of radio. One method of detection is to listen to a telephone receiver connected to a long, rural telephone line or to a submarine cable. The telephone line or cable acts as an antenna, and the telephone receiver converts the weak electrical currents into sound waves. An ordinary high-fidelity audio amplifier connected to 50 feet or so of wire makes another excellent detector of whistlers. It is also possible to detect whistlers by inserting metallic probes in the earth at some distance from each other and connecting these to a high-gain audio amplifier. The earth-probe circuit acts like a loop antenna in picking up electromagnetic waves.
The basic requirement for the detection of whistlers is that a voltage be induced in an electrical circuit by the very-low-frequency electromagnetic waves of the whistler. This voltage must be amplified and converted into a form suitable for observation. Whistlers may be reproduced with earphones or a loudspeaker for direct detection by ear, they may be recorded on magnetic tape for later reproduction, or they may be displayed directly on an oscilloscope or chart recorder for visual observation.
Modern methods of observation are basically the same as those used originally, except that antennas are smaller, frequency ranges are wider, and accurate timing is provided. A typical whistler antenna consists of a single-turn loop of copper wire in the shape of a delta with an elevation of about thirty feet. This loop is connected through a transformer to a high-gain, low-noise, wideband audio amplifier. The output of the amplifier is recorded on a conventional magnetic-tape recorder together with time marks from a local clock or from a radio time-standard station. Networks of whistler stations of this general design are scattered over the surface of the earth to provide data on the geographic variations of whistlers and related phenomena.
Sources of whistlers. As we have stated, whistlers and lightning are closely associated. A few visual observations of lightning and aural observations of the associated whistlers have been made, but most of the data have been obtained from radio recordings. Relatively little is known about the spectra of lightning discharges that precede whistlers. It appears that any strong lightning discharge can excite a whistler. However, there is also evidence that many whistlers originate in unusually intense lightning discharges with peaks in their energy spectra in the vicinity of 5 kc/s. From direction-finder studies it is clear that the causative lightning discharges can be located thousands of kilometers from the receiver or from the receiver's conjugate point. Intense electromagnetic impulses that can excite ordinary whistlers are also produced by nuclear bombs.
Dispersion. Energy from a lightning discharge enters the ionosphere and is guided by the lines of force of the earth's magnetic field into the opposite hemisphere. As the radio waves travel along this path they are dispersed. This means that the different frequency components of the wave travel with different velocities. Usually the high frequencies travel faster than the low. Since the causative impulse from the lightning discharge excites all frequencies simultaneously, the signal at the end of the path consists of a gliding tone in which the high frequencies arrive first. The length of the path and the velocity differences are such that the energy is stretched out over a period of about one second.
The waveform of a whistler is sketched in Fig. 1-1a with all frequencies divided by 400 to give better definition. The actual variation of frequency (f) with time is shown in Fig. 1-1b, and is called the dynamic spectrum of the whistler. By plotting 1/[square root of f] versus t, one obtains a straight line as in Fig. 1-1c; the reciprocal of the slope of this line is called the dispersion D and is equal to the time of propagation multiplied by the square root of the frequency (see p. 32 for further discussion). This law describes most whistlers at middle latitudes and at low frequencies rather closely. The dispersion of a whistler depends on the length of the path over which it travels and on the electron density along the path. Hence at high latitudes, where the paths are very long, dispersion is high, and conversely at low latitudes dispersion is low. When electron density is high, as it is during the years near sunspot maximum, the dispersion is also high. During magnetic storms the electron density in the whistler medium is lowered and the dispersions are correspondingly reduced. In satellites, where the path can be very much shorter than that observed between points on the ground, the dispersion can be very small indeed. In fact, whistlers observed from satellites at heights of 1000 to 2000 km are so short that it is difficult to distinguish them from ordinary atmospherics.
Since dispersion is closely related to electron density, it becomes an important quantity in the study of the variations of electron density in the ionosphere and magnetosphere.
Paths of propagation. Perhaps the most interesting and puzzling feature of whistler propagation is the path of propagation. Although the law of dispersion described above has been known for many years, the path followed by a whistler was discovered comparatively recently. Even now the picture is not complete because the actual path has not been observed experimentally but has only been inferred from the experimental data.
From the discrete traces of multipath whistlers, from the precise integral relationship of the members of an echo train, and from related data, it has been concluded that the paths must be fixed in the ionosphere. This conclusion has led to the hypothesis of a field-aligned enhancement of ionization that acts as a waveguide or "duct" to trap the whistler energy and produce discrete traces. Figure 1-2 shows such a path beginning at the earth's surface (at point A) and following a typical dipole line of force of the earth's magnetic field to point B in the opposite hemisphere. The diagram is drawn roughly to scale, so that the bottom edge of the ionosphere (labeled E layer) is shown properly in relation to the length of the field-line path. Energy from a lightning source near the earth's surface travels in the earth–ionosphere waveguide and enters the ionosphere continuously along the lower surface. Wave components that enter the ionosphere at the location of a duct are then trapped and conveyed to the opposite hemisphere along the same line of force, where they emerge from the ionosphere and enter the earth–ionosphere waveguide.
Wave components entering elsewhere through the lower boundary also follow curved paths of propagation, but these paths do not coincide exactly with the lines of force of the earth's magnetic field. Furthermore, this second type of wave, upon arriving at the lower boundary of the ionosphere in the opposite hemisphere, does not readily cross the boundary, and so is not easily detected on the ground. However, a satellite-borne receiver can pick up this type of wave. Because of the relatively small number of ducts present at one time, most of the whistlers observed by a satellite-borne receiver will probably not have followed field-aligned paths.
Let us now return to the properties of the ducted signals. Because of dispersion, the impulse entering at point A (Fig. 1-2) is gradually lengthened as it travels until it becomes a gliding tone, as shown in Fig. 1-1. If we assume that the energy per unit frequency interval is constant, and that there are no energy losses along the path, the amplitude of the wave will decrease as the frequency decreases, since the spread in time per unit frequency interval increases as the frequency is reduced. Hence for this case the leading edge of the whistler shows the strongest amplitude, and the envelope of the whistler gradually decreases in strength with decreasing frequency.
The inset diagrams of Fig. 1-2 show the dynamic spectra of the whistlers observed at receivers near the source and near the conjugate point. The first whistler observed is one that has passed once over the path and is picked up in the opposite hemisphere. This is called a "one-hop" or "short" whistler and is labeled 1 in the lower inset diagram. Since the exit boundary of the magnetospheric path is sharp, some of the energy is reflected. This energy travels back along the same path and becomes further dispersed, until it emerges from the "two-hop" path. The curve of frequency versus time for the two-hop whistler is labeled 2 in the upper inset diagram. At each frequency the delay is twice that observed in the one-hop whistler on the lower diagram. This echoing process continues, resulting in trains of whistlers at the two ends of the path. One can see from the diagram that the ratio between the delays of the members of the echo train will be 1, 3, 5, and so forth at the one-hop end of the path, and 2, 4, 6, and so forth at the source end of the path.
Although only one path is shown in Fig. 1-2 for purposes of explanation, the ionosphere often contains a number of such paths, which can be excited approximately simultaneously by the signal radiated from a lightning source. Because the lengths of these paths are different, and because the distribution of electron density and magnetic-field strength along them is different, the delays will be different.
Often when multiple ducts are present some whistlers appear to result from propagation over a combination of these ducts. These "mixed-path" whistlers are not well understood; it is probable that conditions for coupling between the ends of the ducts are fairly good on certain occasions. A possible explanation is that the ducts terminate well above the F layer, permitting energy to spread out, be reflected from the lower boundary, and enter any other ducts that are present.
Excerpted from Whistlers and Related Ionospheric Phenomena by Robert A. Helliwell. Copyright © 1993 Robert A. Helliwell. Excerpted by permission of Dover Publications, Inc..
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Table of Contents
3. Theory of Whistlers
4. Characteristics and Occurrence of Whistlers
5. Fixed-Frequency Whistler-Mode Experiments
6. Electron-Density Measurements Using Whistlers
7. VLF Emissions