QST January 1946

Listening in on the Stars

Doppler Whistles from Meteor Trails

BY OSWALD G. VILLARD, JR., * W6QYT, EX-W1DMV

While peaking up that communications receiver for reopening on the DX bands, why not take a little time out to make some observations on an interesting, but little known, phenomenon? Few hams know that meteors can be spotted without leaving the operating room. Most of us have done a little night stargazing during our lives, for reasons of romance or profound astronomical speculation, and are consequently familiar with the shooting stars or meteors which flash silently overhead on summer nights like falling Fourth-of-July rockets.

Yet few are aware that it is possible to "hear" these visitors from outer space during the course of their brief plunge through the heavens, and even to estimate their speed without recourse to telescopes, photographic plates, or maps of the solar system.

How is this done, you ask? All that is required is a good short-wave receiver. For it turns out that falling meteors leave trails of ionized gas behind them which affect the propagation of short radio waves. These waves, in fact, are reflected from ionized meteor trails much as they are reflected from the regular layers of the ionosphere. Under certain conditions, when the regular layers are not heavily enough ionized to reflect short-wave signals from one point to another relatively close at hand, sporadic "sky-wave" transmission may still take place over this path because meteor trails are capable of reflecting signals when the regular layers are not. Since the meteor trails are relatively small in size when compared with the whole ionosphere, the amount of reflected energy is usually small and is not readily noticeable in everyday practice. However, when the short-wave listener knows how and where to look, the energy reflected by meteor trails easily can be recognized and identified.

Since a meteor trail is not static, but effectively moves across space with the same velocity as the meteor itself, the signal reflected by the front edge of the trail reaches the listener via a path whose length is rapidly varying. The effect is roughly the same as if the radio transmitter were located on the meteor itself. As a result, the speed of the meteor's travel produces a "Doppler shift" in the frequency of the radio signal just as the motion of an approaching locomotive changes the apparent pitch of its whistle. Now if part of the signal from a fixed transmitter received at a given location consists of energy reflected from a moving meteor trail and part consists of energy reaching the receiver via a path of constant length, the difference in "pitch", or frequency, of the two components will produce a beat note which can be easily detected. The purpose of this article is to outline the conditions under which these Doppler beats can be heard in an ordinary short-wave receiver, and to describe their sound and other characteristics.

Identifying Meteor-Trail Reflections To hear the Doppler beats which are the radio "sounds" made by meteors in the course of their brief plunges through our atmosphere, it is necessary to find and tune in a powerful short-wave broadcasting station operating on a frequency such that you are within its skip zone. The station should preferably be in the 11-, 15-, or 18-mega-cycle band, and may be anywhere up to several hundred miles away, depending on ionosphere conditions. The main requirement is that you should not be receiving a direct, ionosphere-propagated ray - the signal must be very weak and fading, and may have a characteristic "hollow" sound.

Listen closely to this signal, and if your receiver has an "S" meter, keep an eye on the indicated strength. If conditions are right, you will notice from time to time irregular bursts of strength which may send the "S" meter up to fairly high values. These bursts will not last long - perhaps ten or twenty seconds - and the signal will soon fade down to the average low level. Some of the bursts may be quite weak, and may cause only a slight flick of the "S" meter.

At this point it should he noticed that during the first part of these bursts, especially the weaker ones, certain noises are audible on top of the

received program. These sounds are short "whistles" super-imposed on the regular modulation. The pitch may be high or low - anything from a low grunt to a high "pweet." The whistles will last only two or three seconds, their duration being usually much less than that of the bursts of signal.

The whistles will, in general, vary in pitch. They may change from a high to a low tone - a sort of "pweeough" - or they may do the reverse. The pitch may even be constant. It will be unusual if any two whistles sound exactly alike.

On some occasions they and the burst may come thick and fast - two or three a minute; on others they may be less frequent. They are usually more noticeable during the early morning and early evening hours when long-distance transmission is best.

Fig. l - How signals are scattered back into the skip zone.

The whistles are the Doppler "sounds" of meteorites rushing into the earth's ionosphere from the vast regions of outer space. The exact pitch heard is dependent on the speed of the meteor, the radio frequency, and the direction of the meteor's track with respect to the transmitter and receiver.

Shooting Stars That meteors have had an influence on conditions in the ionosphere, and hence on radio transmission, has been known for some time. There is some reason to believe that nighttime "E" region ionization is largely due to the many tens of thousands of fall meteors which are intercepted by the earth's upper atmosphere each day. J. A. Pierce has shown (see Bibliography 1) the connection between certain brief fades in normal radio transmission and the ionized trails caused by meteors. In the same paper he also demonstrated that energy reflected from meteor trails gives rise to brief bursts of signal received from transmitters within the skip zone. In a later paper (see Bibliography 2) he showed that the ionized regions caused by meteor trails could be detected and measured by means of regular ionosphere sounding equipment. Not long after that the chain of events was completed by two Indian scientists who first reported the Doppler "whistles" caused by reflections from the moving meteor trails themselves (see Bibliography 3).

Shooting stars, of course, are really small particles of matter from outer space which bombard the earth's outer atmosphere at tremendous speeds. Collision of these objects with the molecules of gas comprising the atmosphere releases an enormous amount of heat and quickly causes the particles to incandesce. The radiation thus given off causes intense ionization along the path followed by the meteor. This ionization rapidly diffuses outward after passage of the particles, thus at first increasing the area of the disturbance, but decreasing its intensity; eventually the ionization is dissipated, the positive and negative ions recombine, and the reflecting region disappears.

The weight of the matter composing the meteor is remarkably small in comparison with the dimensions of the ionized trail left behind after its passage. Considering ionization of an intensity sufficient to reflect a 10-megacycle signal, Pierce estimates that a meteor of average speed and weighing only one quarter of a gram would be sufficient to ionize a cylinder of the earth's upper atmosphere 1 kilometer in diameter and 100 kilometers long.

Fig. 2 - A moving meteor trail reflects signals to a receiver inside the skip zone.

Why the Meteors Whistle

Let us consider a radio transmitter and receiver located far enough apart - say 30 to 60 miles - so that direct ground-wave transmission is negligible. The operating frequency is such that the receiver is within the skip zone. Under these conditions, the only energy from the transmitter detectable at the receiver which has been transmitted via the regular ionosphere layers will be a very weak signal scattered back to the receiver from irregularities on the earth's surface or in the ionosphere at points outside the area which constitutes the skip zone (see Fig. 1). The paths traversed by these scatter signals are, of course, substantially constant in length, whatever their lengths may happen to bee. The average strength of the scatter signals is also approximately constant, Since the signals consist of components returned from many irregularities at different distances. These components may fade individually; however, since they are many in number the net effect is a somewhat wavery signal whose average strength is roughly constant.

At this point let us suppose that a meteorite arrives at the ionosphere oriented in such a direction that it is headed straight for both the transmitter and receiver. If the particle is a large one the ionization in its trail will be intense enough to reflect a radio signal of the frequency under consideration, even when that ionization has diffused over an area large enough to send back a considerable amount of energy from the transmitter to the receiver.

To the scatter signal at the receiver, then, is added a signal of varying path length reflected from the meteor trail. Each time this path length changes by one wavelength at the frequency of operation the phase of the two signals at the receiver will have changed through 360 degrees, and the amplitude of the resultant will have gone from complete addition to complete cancellation and back again, or through some equivalent change depending on their relative phases at the start of the cycle. So far as a receiver can tell, this change in the effective amplitude of the resultant signal is the same as a modulation on the original scatter signal.

An average speed for meteors during the course of their plunge is 40 kilometers per second, which is 40,000 + 20, or 2000 wavelengths per second at 20 meters. Because each half wavelength of meteor travel represents one full wavelength of path-length change for the reflected signal, a meteor moving at 2000 wavelengths per second will cause 4000 complete cancellations and additions per second of the two signal components at the receiver; in other words, the resultant signal will be modulated at 4000 cycles per second. This is the Doppler tone which is heard.

In actual practice, the pitch of the tone may vary all the way from a very low value to something of the order of 4000 cycles. It is known that meteors enter the earth's atmosphere (or more strictly speaking, ionosphere) along more or less random paths. If the path of the meteor is at right angles to a line drawn from the meteor to the transmitter and receiver; no tone can result because there is no change in the length of the path followed by the energy reflected from the meteor trail. However, if there is a component of velocity either toward or away from the transmitter and receiver, a tone will result. Consideration of the various geometrical possibilities will show that the tone can be either steady or varying in pitch, depending on the actual path of the meteor.

The number of powerful short-wave broadcasters has greatly multiplied during the war and has made easier the job of selecting the proper conditions to "hear" the meteors. In Cambridge, Mass., the effect is particularly noticeable on stations in the New York City area broadcasting to Europe. Hearing the Doppler tones is greatly simplified if the receiver is equipped with a good a.v.c. circuit, which prevents overload during the bursts of signal caused by reflections from the meteor trails.

This method of detecting meteors is, in effect, a practical example of the principle of radar. Instead of microwaves, ordinary short waves are used, and instead of aircraft, meteor trails are detected. But the principle is much the same.

Knowing the pitch of the Doppler tone and the radio frequency used, one can calculate the component of the meteors' speed in the direction of the transmitter and receiver. However, since thc exact path followed by the meteor is seldom known, its true speed cannot be measured in this way. But by listening over a long period of time and noting the pitch of the highest tone heard, a good estimate can be made of the speed at which meteors travel.

1 J. A. Pierce, Proc. I.R.E., Vol. 26, page 892 (1935) "Abnormal Ionization In the E Region of the Ionosphere."
2 J. A. Pierce, Physical review, Vol.59, April (1941) "A Note on Ionization by Meteor..."
3 Chamanlal and Venkatamaran, Electrotechnics, (Bangalore. India), Vol.14, pp.2840, November (1941)," Whistling Meteors - A Doppler Effect Produced by Meteors Entering the Ionosphere."