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A search for radio echoes of long delay
.Journal of Atmospheric and Terrestrial Physics, 1952, Vol. 2, pp. 272 to 281. Pergamon Press Ltd. London
A search for radio echoes of long delay K. G. BUDDEN * and G. G. YATES** (Received 20 February 1952) ABSTRACT A description is given of some a t t e m p t s to detect long delayed radio echoes of the type reported by vA~ DER POL [2], STO!~MER [1] and others, with delays between 3 sec and 15 sec. I t has been suggested t h a t such echoes might arise b y reflection from ionised corpuscular streams from the sun. To investigate this, frequencies were used which would penetrate the ionosphere at most times of day, and it was also necessary to use the highest possible t r a n s m i t t e r power. Both these requirements are fulfilled by some commercial short wave transmitters, and we are indebted to Messrs. Cable and Wireless Ltd., who arranged special transmissions from Ongar. No echoes of long delay were observed. Owing to the large number of short wave commercial transmitters now operating, the interference level was probably far higher t h a n it would have bee~ in 1928. Nevertheless, it is possible to estimate the minimum signal power which could be detected by the receiving apparatus. The probable size and density of the clouds of ions emitted from the sun is then examined, and it is concluded t h a t such clouds are not likely to be dense enough to give detectable radio echoes. I t is tentatively suggested t h a t the echoes of long delay heard by previous observers were associated in some way with ionised regions fixed relative to the earth, and t h a t the failure to observe echoes in the present investigation was due to the choice of frequencies which were too high. Some suggestions are made for the planning of future a t t e m p t s to observe long delayed echoes. 1.
INTRODUCTION
Several reports have been made of radio signals received from transmitters on the earth after a time delay of several seconds, (for example, STORMER [ ] ] , VAN DER POL [2]). The delay varies between about .3 and 15 sec, and the echoes are usually very distorted. No completely satisfactory explanation has been put forward, b u t various suggestions have been made. VAN D•R POL [2] and FUCHS [3], [4] have suggested t h a t the echoes m a y be associated with the ionosphere, and have described mechanisms b y which the long delays might occur. STORMER [1], [5] considered that the echoes might arise b y reflection from clouds of ions at great distances from the earth, and PEDERSEN [6] has shown how radio waves might be guided over long paths well outside the earth b y " b e l t s " or " b a n d s " of ions. There is now considerable evidence that ionised clouds are sometimes emitted from the sun towards the earth, and give rise to terrestrial magnetic storms. The experiments described in this paper were planned to investigate whether reflections of radio waves could be obtained from clouds of ions well outside the earth. It was therefore decided to use frequencies which would penetrate the ionosphere, and it was also obviously desirable to use the highest available transmitter power. These requirements are both admirably fulfilled b y commercial short wave transmitters. We are greatly indebted to Messrs. Cable and Wireless, Ltd., who made available two of their transmitters at Ongar, Essex. The exact frequencies used depended mainly on which transmitters could be released from commercial service. The experiments * Cavendish Laboratory, Cambridge. ** Solar Physics Observatory, Cambridge.
272
A search for radio echoes of long delay
were made with the transmitters GMC, frequency 13.455 Me/s, power 30 kw, and GMV, frequency 20-675 Mc/s, power 9 kw. A description of the aerial systems and receivers is given in section 4. No long delayed echoes were observed in any of the experiments. Observations were sometimes difficult to make owing to heavy interference from other commercial short wave transmitters. These were much more numerous than in 1928 and 1929 when STORMER and VAN DER POL made their observations. The sensitivity of the apparatus is examined in sections 6, 7, 8 and 9, and estimates are made of the density and size of the clouds of ions that would be necessary to give the minimum detectable signal. There is very little information regarding the density of the clouds emitted from the sun, but the suggestions that have been made are all very much lower than would be required to give detectable radio echoes. We conclude that the long delayed echoes observed in the past were probably associated with some ionised region close to the earth, and the evidence in favour of this is summarised in section 10. Some suggestions for the planning of future attempts to observe long delayed echoes are made in section 11. 2.
PREVIOUS OBSERVATION OF EC~tOES OF LONG DELAY
The most striking observation of echoes of long delay were probably those of VAN DER POL [2], and STeRMER [1], with signals of frequency 9.55 Mc/~ from the transmitter P C J J at Hilversum. The echoes were observed on several different days in March and October, 1928. The delays between the times of arrival of the main signal and the echo varied between three seconds and fifteen seconds, and the echo signal was usually very distorted. The echoes were again observed in 1934 during a series of tests organised b y the World Radio Relay League, and inaugurated b y Sir EDWARD APPLETON. The observers were British radio amateurs, and the frequencies used were 9.5 Me/s. and 6.7 Mc/s. The echoes had characteristics very similar to those reported b y ~JroRMER and VAN DER POL. Similar echoes have also been reported b y GALLE, TALON and FERRI~ [8]. 3. POSSIBLE EXPLANATIONS An explanation of the occurrence of these echoes was put forward b y VAN DER POL in his original communication to " N a t u r e , " [2]. He suggested that t h e y might be due to a long transit time in a region of the ionosphere where the group velocity is very low. Such a region is well known t o ~ c c u r at the level in the ionosphere where the refractive index of the medium is close to zero. APPLETON [9] pointed out that this explanation is probably untenable, because the waves would be attenuated in the ionosphere to an extent which can be estimated. He showed that after times of the order of the observed delays, the signal amplitudes would be far too small to be observed at all. The explanation would only be a possible one if the delay occurred at a level in the ionosphere far higher than the known levels of the E and F regions, where the frequency of collisions of electrons with gas molecules is very small. This explanation has been examined in more detail b y P~.DERSEN [6], BREIT [10], and b y FtlCHS [3] ,[4], who deduced that the electron collision frequency in the region where the delay occurs must be of the order of 0.5 sec -1. THOMAS [11] estimated that even if the gas pressure is so low that collisions of the electrons with gas 273
K. G. BUDDEN and G. G. YATES
molecules are negligible, there is still an effective collision frequency of the order of 104 sec -1 due to the electrostatic forces between the electrons. Hence, the signals would be attenuated far too heavily to give observable echoes. J o o s [12] also examined the suggestion that the observed delay arose because of a low group velocity somewhere in the path of the waves. He concluded that, quite apart from possible absorption, the dispersion which is necessarily associated with a low group velocity would be so great that it would reduce the signal amplitude far below the minimum detectable level. A second possible explanation is t h a t the echoes are due to the reflection of radio waves from clouds of ions or electrons at some distance from the earth. This explanation was favoured at first by STORMER [1] [13], who suggested that streams of ionised particles shot from the sun towards the earth m a y be responsible for the echoes. Such streams of ions moving towards the earth would have their trajectories modified by the earth's magnetic field, and ST~RMER [13] has shown that t h e y would be distributed in a region bounded on one side by a toroidal surface surrounding the earth. The particles would be excluded from the inside of this toroid, and he suggested that the echoes might be evidence of the reflection of radio waves from its inner surface. The estimated size of the toroid would account for delays of the right order (7 see). A third explanation was given by PrDERSE~¢ [6], who showed how radio waves could be guided along "belts" or " b a n d s " of ions. He suggested that such guided waves could travel over large curved paths starting and ending near the earth. ST~RM~R [14] discussed this suggestion, and reviewed the possibility that such paths were associated with a ring of current round the earth, due to the motion of charged corpuscles. This ring would be in the earth's magnetic equatorial plane, and STORMER showed that, with this explanation,-the conditions for receiving long delayed echoes are most favourable when the line from the earth to the sun is also in this plane. He gives data which show that echoes were in fact observed most frequently at these times. Explanations in terms of m a n y reflections in the ionosphere have been suggested by JA•CO [15] and JELSTRUP [16]. An explanation based on guided waves was suggested by J o N r s c c and MIHUL [17]. ArrLETO~ [9] suggested t h a t the waves might be repeatedly focussed at the antipodes and near the transmitter, so as to give an audible signal after several seconds. The suggestions of ST~RM~.a and PEDEr~SEN have acquired a new interest in view of recent theories to explain the occurrence of terrestrial magnetic storms. It is known that magnetic storms frequently start at times of the order of 20 hrs, after the occurrence of a bright solar eruption. It has therefore been suggested that simultaneously with the occurrence of the eruption, a burst of particles is shot out from the sun, and takes about 20 hrs to reach the earth. There is ample evidence in favour of this theory, (see, for example, CHAPMAN [18]) and it was therefore of special interest to investigate whether long delayed echoes could be heard during the period immediately following a solar flare. ST~RM~.R'S conclusions were based on the calculated trajectories of single charged particles in the earth's magnetic field. More recently CHAPMA~ and FERR~O [19], [20], [21] have shown t h a t the clouds of particles moving from the sun towards the earth are most likely to be electrically neutral, ionised clouds, since clouds of charges 274
A search for radio echoes of long delay
of one sign would be rapidly dispersed b y electrostatic repulsion. The effect on a moving, electrically neutral cloud, of the earth's magnetic field has been discussed b y CHAPMA~ and FERRARO. They showe,: that the particles are deflected in such a w a y that in the cloud there is a hollow sur~ ,,unding the earth. This hollow is somewhat similar to that pictured b y STORMER. At a later stage the streams of ions move into a closed structure surrounding the earth in the form of a current ring, with which is associated the start of the terrestrial magnetic storm. This is similar to the ring described b y SToRMER [5]. It is of interest to consider whether the observed long delayed echoes could originate b y reflection at the inner surface of the hollow, or from the current ring. This is considered further in sections 8 and 9, and we conclude that such reflections would be far too weak to give echoes. A similar conclusion was reached b y DOSTAL [22], who also considered whether the echo amplitude could be increased b y a focussing mechanism due to the concavity of the reflecting surfaces. 4.
EXPERIMENTAL DETAILS
Attempts to observe echoes of long delay were made at Cambridge during 1947, 1948, and 1949. When the experiments were planned, their main interest was thought °to be the possible detection Of reflecting or scattering objects at great distances from the earth. Fairly high frequencies were therefore used so that waves travelling radially outwards from the earth could penetrate the ionosphere. Apart from this, the choice of frequencies was dictated mainly by the availability of the transmitters, which had to be diverted from commercial service for the special transmissions. Two frequencies were used, namely 13.455 Mc/s, (power 30 kw), and 20.675 Mc/s, (power 9 kw). The transmissions were provided b y Messrs. Cable. and Wireless, Ltd., from their transmitting station at Ongar, Essex. The transmitting aerial used for both frequencies was a horizontal dipole approximately half a wavelength long, at about one quarter wavelength above the ground. This arrangement was chosen so as to give maximum radiation in a direction radially outwards from the earth. Unmodulated continuous waves were used, keyed on and off with Morse characters. During the first quarter hour of each transmission, the transmitter sent its call sign and V's continuously, so that the receivers could be tuned. It then sent single call signs at approximately t w e n t y second intervals. In some transmissions the call sign and V's were sent for five minute periods every half hour, so that the receiver adjustments could be checked. Most of the transmissions lasted two hours. The receiver had two stages of radio frequency amplification with V R 91 valves. These were followed b y a mixer valve fed from a local oscillator, which was crystal controlled. The output of the mixer was fed to an I.F. amplifier operatingat 455Kc/s with a band width of about 100 Kc/s. This was followed b y a second mixer valve, fed from a second local oscillator, which could be manually adjusted. The output of the second mixer was fed to a second I.F. amplifier, operating at 130 Kc/s, with a band width of about 2 Kc/s. The output of this was caused to beat with a continuous oscillator, so as to give a note of about 1000 c/s. This was fed to an audio frequency amplifier incorporating a narrow band filter, and thence to a cathode follower valve. The output of this was fed to a pair of headphones, and to the vertically deflecting plates of a cathode ray oscillograph. The overall bandwidth of the 275
K. G. BUDDEI~ and G. G. YATES
receiver w a s about 200 c/s, and was determined b y the narrow band filter in the
audio frequency amplifier. I f an observed echo were returned from a moving object, it would suffer a Doppler change of frequency. An additional unit was therefore added, in later experiments, to allow a search to made on other nearby frequencies. It was fed from the output of the first I.F. amplifier, and consisted of a local oscillator, an I.F. amplifier at 130 Kc/s, and output circuits similar to those in the main receiver. Its output was connected to the vertically deflecting plates of a second cathode ray oscillograph. The two parts of the receiving apparatus will be referred to as the "main receiver ", and the "Doppler receiver". The range of frequencies to which the Doppler receiver could be tuned was determined b y the bandwidth of the first I.F. amplifier, and was about 100 kc/s on the high frequency side of the transmitter frequency. A frequency change o f this amount would be produced b y a reflecting or scattering object having a "line of sight" velocity of about 1100 kin/see towards the earth. The horizontally deflecting plates of the cathode ray oscillographs were connected to a source of time-base voltage which gave a slow linear sweep to the spot, with a duration of about 20 sec, and triggered b y the main signal. The screens of the cathode ray oscillograph were photographed using paper moving slowly and continuously, so that successive ~raverses of the time-base appeared as parallel lines about 7 cm long and 1 cm apart. In the tests on 13.455 Me/s, the receiving aerial was a horizontal half wave dipole at about one quarter wavelength above the ground. In the tests on 20.675Me/s, a few ohselvations were made with this type of aerial, but in most of the experiments the aerial uEed was a vertical Vee aerial, designed to have maximum sensitivity for radiation coming radially inwards to the earth. The polar diagram of this aerial was tested, using a small oscillator in a captive balloon. The diagram was found to have the shape expected, and the aerial had a power gain of approximately 8 to 9. 5.
METHOD OF OBSERVATION AND RESULTS
During a transmission, as soon as the transmitter began sending call signs at 20 sec intervals, the camera was switched on, and an observer listened to the received signals. There was usually only one observer, and he could listen to the output either of the main receiver, or of the Doppler receiver, but not to both. As far as possible, notes were made of any signals received, such as atmospherics, signals from other transmitters, or other man-made interference. The volume of traffic in the frequency band near 13.455 Mc/s was often heavy, and distant transmitters working practically on the same frequency sometimes caused trouble. Another effect of the heavy traffic was that the frequencies to which the Doppler receiver could be tuned were severely limited. The procedure with this receiver was to search near the transmitter frequency for a frequency band which sounded free from signals, and to leave the Doppler receiver tuned to this frequency. After five or ten minutes, the procedure was repeated, and if possible a new frequency was chosen. The displaced frequency used was always higher than the transmitter frequency, because it was thought that the echoes might most probably be due to streams of ions moving towards the earth. In view of the interference and the very limited number of frequencies that could be used, it is probable that the search for echoes with the Doppler receiver was of little value. 276
A search for radio echoes of long delay
The heavy interference observed with the frequency 13.455 Mc/s meant t h a t over 50 % of the signals examined were considered to be spoilt by interference. This was the main reason for changing to the frequency 20.675 Mc/s. At the higher frequency, interference was less, p a r t l y because there were fewer commercial transmitters in the frequency band, and p a r t l y because the frequency was above the maximum usable frequency (M.U.F.) for m a n y transmission paths. Even so, the interference level was still high, and about 20 % of the signals examined were considered to be spoilt by interference. On the higher frequency, a very strong " r o u n d the world" echo was often observed (delay 0.13 see). At first, in a search for echoes which might occur at any time, transmissions were arranged at regular intervals, usually on one day in each fortnight. In each day transmissions were usually made during the intervals 0001-0200 hrs, 0600-0800 hrs, 1200-1400hrs, and 1800-2000hrs. In later experiments the transmissions were specially arranged when reports were received of solar flares or of other phenomena which might be associated with the emission of solar corpuscular streams. In these cases, four transmissions per day were made for a period of three days immediately following the occurrence of the flare. The majority of the observations were made in the years (1947-1949) near the epoch of m a x i m u m sunspots, and were spread fairly uniformly through the seasons. About 100 transmissions, each using about 200 signals, were made on a frequency of 13.455 Mc/s. During 22 of these, both the main receiver and the Doppler receiver were used. 35 two-hour transmissions were made on a frequency of 20.675 Me/s, during all of which both receivers were used. In all the experiments, the total mlmber of signals emitted was about 27 000. I n no case was any signal received which could be considered as an'echo of long delay, or which resembled in any way the echoes described by previous observers. 6.
THE SENSITIVITY OF THE APPARATUS
A theoretical lower limit to the sensitivity of the equipment was set by the thermal noise generated in the input circuit of the receiver with a power equal to 4 k T B, where B is the bandwidth, T is the absolute temperature, and k is BOLTZMANN'S constant. With an overall bandwidth of 200 c/s, and the temperature of the input circuit 300 ° K, the theoretical thermal noise power was thus about 3.2 × 1(I-is w. In actual tests with a signal generator, it was found t h a t an input signal power of 10-1e w gave a signal-to-noise ratio of about 4:1 in amplitude at the outt)ut of the receiver. This corresponds to a noise power level of about 6.2 × 10 -is w. I t was considered t h a t a signal input of about 5 × l0 -17 w was necessary to give a clear response on the photographic record, and t h a t about 6 ×10 -is w would give an audible signal. In the following discussion, it is assumed t h a t the transmitting aerial had a power gain G t ~ 3 in the vertically upward direction. The receiving aerial was assumed {o have an effective cross-sectional area of 3 22/4 ~, when intercepting radiation coming radially inwards to the earth, where 2 is the wavelength. This is twice the value for a Hertzian dipole in free space. The values for a Hertzian dipole and a half wave dipole are roughly the same. A factor 2 is included to allow for reflection at the ground. 20 JATP. Vol. 2.
277
K. G. BUDDEI~" and G. G. YATES 7.
DETECTION OF A PLANE PERFECT REFLECTOR
If the radiation from the transmitter were reflected at a plane perfectly reflecting surface at a distance R from the earth, the power returned to the receiver would be
{ w 0,/4 (2
× {3
(1)
At the greatest distance for detection this would be 10 -17 w. With W : 30 kw, A : 22 m, this would correspond to a distance R ~ 1.5 × l0 s k m . This would give a time delay between the main signal and an echo of 1000 sec. Thus a plane perfect reflector at a distance slightly greater t h a n t h a t of the sun would give the minimum detectable signal. 8.
SHARPLY BOUNDED IONISED REOION
The only way in which a cloud of ions in space could have a reflection coefficient approaching u n i t y is t h a t it should contain a concentration of electrons higher t h a n the critical concentration for the radio frequency used. This critical concentration is given by ;N O : e o m p2/4 ~ e 2 where
e, m : charge and mass of the electron, eo = permittivity of free space, p = angular frequency of the wave.
For waves of frequency 13.5 Me/s, this gives No = 2.3 × 10° cm -3. I t is apparently very difficult to estimate the density of particles in the streams of corpuscles travelling from the sun to the earth. CHAPMANand FERRARO [20], [21] tentatively suggest a figure of 100 to 1 000 electrons per cc, when the stream is near the earth. CHAPMAI~and BARTELS [23] mention a possible figure of 105 particles per ce in the stream when it leaves the sun. Clouds with densities greater t h a n this, and with temperatures of the order of t h a t of the sun's chromosphere would probably diffuse outwards rapidly. I t therefore seems unlikely t h a t electron densities as high as 106 cm -3 would occur in the streams of corpuscles, and so we should expect the stream to have a reflection coefficient very much less t h a n unity. We now consider what signals we should expect to be returned to the earth from clouds having electron densities of the order of 1000 cm -3. These signals would depend upon the shape and size of the cloud, which is difficult to assess, but some idea of the upper limit of the reflecting power m a y be obtained by supposing t h a t the cloud has a sharp plane boundary. We therefore consider a homogeneous sharply bounded medium, having an electron density N ---- 1000 cm -3. The reflection coefficient for waves incident normally on the boundary is @~ (# -- 1)/(# + 1) where /~ is the refractive index of the medium given by # 2 ~ 1 - 4 z N e2/e0 m p2. Since # is very nearly unity, we have with good accuracy ]@] = ~ N e 2 / e o m p 2. This reflection coefficient expresses the ratio of the amplitudes of a reflected wave to an incident wave. The reflection coefficient for power is the square of this, t h a t is, I@12= ~2N2 ed/e~ m~P 4. We must now multiply the expression (1) by this quantity to get the power returned to the receiver. If, as before, we equate the result to 10 -17 w, then the resulting value of R is 6.3 × 104 km. This distance would give a time delay between the main signal and an echo of only 0.42 sec, which is much 278
A search for radio echoes of long delay less t h a n the longest delay times (of the order of 15 sec), report ed b y STORMER and VAN DES POT,. B y a slight extension of the above calculation, we find t h a t the elect r o n density required to give a detectable echo with a delay of 15 sec, is 2.7 × 1Ocm -3. B u t it should be r e m e m b e r e d t h a t the sharply bounded model gives only an upper limit to the reflecting power, and t h a t diffuse clouds would r e t u r n very much less energy to the earth. Hence, to obtain echoes with delays as great as 15 sec, electron densities very much greater t han 2.7 × 104 cm -a would be required. 9. SCATTERING BY CLOUDS OF ELECTRONS As a f u r th er check on the above calculations, we m a y discuss how energy could be r etu r n ed to the ear t h b y scattering from clouds of electrons. CHAPMANand FERRARO [19], [20], [21] suggest t h a t the clouds of ions responsible for magnetic storms are v er y much larger t h a n the earth, and eventually envelop it. A cloud would therefore subtend a large solid angle at the earth. Consider a p a r t of a cloud of electrons which is at a distance R from the earth. If such a cloud were effective in giving an audible echo, the thickness of the region responsible would be much less t h a n the distance travelled b y radio waves in the duration of ORe Morse sign, say ~ sec. We m a y take this as an upper limit for the thickness of t h a t p a r t of the cloud t h a t is effective at a n y moment. We therefore consider a disk shaped region of the cloud on the surface of a sphere of radius R concentric with the earth. The thickness of the cloud is d-= 3 × 104km, and its area is ~rR 2, so t h a t its volume is ~R2d, and it contains ~ R e d × N electrons, where N is the average electron density in the cloud. Now it is well known t h a t the total power scattered b y a free electron is p = 8 ~r e4 Po/3 e2oc 4 m 2, where c is the velocity of light, and P0 is the flux of power in the incident wave. Since the electrons are assumed to be distributed r a n d o m l y in the cloud, tile powers in all the scattered waves must be added. Substitution of Po-- WGt/4."r R2 then shows t h a t the scattered power entering the receiver is
(w¢,/4 Re)× (s e'/3
c'
× (3 e/4
× (1/4 Re) ×
Re
d.
If N = 1000 cm -a, R : 3 × 105 km (corresponding to a delay of 2 see), W := 30kw, G t = 3, th en the received power would be 1-0 × l0 -a° w. This is smaller t h a n the minimum detectable signal b y a factor of 10le. I t therefore seems reasonable to conclude t h a t ionised clouds could not bc detected b y the scattering of radio waves t r a n s m i t t e d from the earth, even if t h e y had electron densities v e r y much greater t h a n those suggested by CHAPMA:N and FERRARO. ~V(~ must not, however, exclude the possibility t h a t such clouds might be detected in the future due to their absorption of radio waves passing through them. For this, it would of course be necessary to use some extra-terrestrial source of radio waves, such as the sun or the stars. There are reasons for believing t h a t the radio echoes of long delay observed in the past are not connected with objects v e r y distant from the earth. These reasons are summarised in the next section. 10. THE ORIGIN OF ECHOES OF LONG DELAY There appear to be three good reasons for supposing that echoes of long delay are associated with some system which is fixed in space relative to the earth. Wc discuss these reasons in turn. 2o*
279
K. G. BUDDEIqand G. G. Y+,T~S 1. There is no detectable difference between the frequencies of the echo signal and the transmitted signal. This was reported very strikingly b y VAN D:ERPOL [2]. We m a y assume that he was listening to a note whose frequency was of the order of 1000 c/s, and that he would readily have detected a change of pitch of less than a whole tone. This would mean a frequency difference of about 100 cycle sec -1. If such a change of frequency were produced b y reflection from a moving object, its velocity would be 1.5 km sec -1. In fact, a velocity of 0.5 km sec -1 would probably have been detectable. It is difficult to see how the motion of particles travelling from the sun to the earth with velocities of the order of 1000 km sec -1 could have their motions modified so that their velocity relative to the earth was consistently reduced to less than 0.5 km see -1. It seems more reasonable to assume that the echoes are associated with some system moving permanently with the earth. 2. Echoes of long delay were observed mainly or wholly during the daytime. GALL:E, TALON and F:ERRI]~ [8] state that t h e y disappeared during a solar eclipse, and reappeared slowly afterwards. This observation suggests that the formation of the echoes depends on the ultra-violet light from the sun, and can be affected b y the shadows of the earth and the moon. 3. The echoes have been heard on frequencies 9.5 and 6.7 Me/s, but not on frequencies 13.5 and 20.7 Mc/s. This suggests that some feature of the echoes m a y be associated with the ionosphere. GALLE, TALON and F:ERRI~ used a frequency of 12 )~[c/s, but their observations were made in Indo-China, that is, near the equator, whereas all other reports of long delayed echoes came from temperate latitudes. We suggest, very tentatively, that long delayed echoes are due to the propagation of guided waves of the t y p e described b y ~)EDERSEN [6] (see section 3), over long curved paths formed b y belts of ions outside the earth but fixed relative to it, and that the successful launching of such waves depends on the state of the ionosphere. ]1. SUGG:ESTIONSFOR FUTURE TESTS In view of the conclusions of the last section, it is suggested that in any further attempts to observe echoes of long delay, it is desirable to use rather lower frequencies than were used in our tests. The frequency chosen should probably be less than 10 Mc/s, and it m a y also be undesirable to use aerials whose directions of maximum transmission or sensitivity are radially outwards from the earth. Better results might be achieved with aerials having horizontal directivity, and if a study could be made as to how the strength of the echoes depends on the directivity of the aerials, this would give some useful information about the nature of the waves responsible for the echoes.
Acknowledgements--The work described in this paper was carried out partly at the Solar Physics Observatory, Cambridge, and partly at the Cavendish Laboratory, Cambridge, where it formed part of a programme of radio research supported by the Department o/ Scienti/ic and Industrial Research. Our grateful thanks are due to Messrs. Cable and Wireless,Ltd, and especially to their Chief Engineer, Mr. SMALE, and to Mr. KEEN and the staff of the transmitting station at Ongar, for arranging the special transmissions. We also wish to thank our assistant, Mr. A. B. RE:EDER, who gave invaluable help with the construction of the receivers, and the taking of the observations. 280
A search for radio echoes of long delay REFERENCES
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A search for radio echoes of long delay K. G. BUDDEN * and G. G. YATES** (Received 20 February 1952) ABSTRACT A description is given of some a t t e m p t s to detect long delayed radio echoes of the type reported by vA~ DER POL [2], STO!~MER [1] and others, with delays between 3 sec and 15 sec. I t has been suggested t h a t such echoes might arise b y reflection from ionised corpuscular streams from the sun. To investigate this, frequencies were used which would penetrate the ionosphere at most times of day, and it was also necessary to use the highest possible t r a n s m i t t e r power. Both these requirements are fulfilled by some commercial short wave transmitters, and we are indebted to Messrs. Cable and Wireless Ltd., who arranged special transmissions from Ongar. No echoes of long delay were observed. Owing to the large number of short wave commercial transmitters now operating, the interference level was probably far higher t h a n it would have bee~ in 1928. Nevertheless, it is possible to estimate the minimum signal power which could be detected by the receiving apparatus. The probable size and density of the clouds of ions emitted from the sun is then examined, and it is concluded t h a t such clouds are not likely to be dense enough to give detectable radio echoes. I t is tentatively suggested t h a t the echoes of long delay heard by previous observers were associated in some way with ionised regions fixed relative to the earth, and t h a t the failure to observe echoes in the present investigation was due to the choice of frequencies which were too high. Some suggestions are made for the planning of future a t t e m p t s to observe long delayed echoes. 1.
INTRODUCTION
Several reports have been made of radio signals received from transmitters on the earth after a time delay of several seconds, (for example, STORMER [ ] ] , VAN DER POL [2]). The delay varies between about .3 and 15 sec, and the echoes are usually very distorted. No completely satisfactory explanation has been put forward, b u t various suggestions have been made. VAN D•R POL [2] and FUCHS [3], [4] have suggested t h a t the echoes m a y be associated with the ionosphere, and have described mechanisms b y which the long delays might occur. STORMER [1], [5] considered that the echoes might arise b y reflection from clouds of ions at great distances from the earth, and PEDERSEN [6] has shown how radio waves might be guided over long paths well outside the earth b y " b e l t s " or " b a n d s " of ions. There is now considerable evidence that ionised clouds are sometimes emitted from the sun towards the earth, and give rise to terrestrial magnetic storms. The experiments described in this paper were planned to investigate whether reflections of radio waves could be obtained from clouds of ions well outside the earth. It was therefore decided to use frequencies which would penetrate the ionosphere, and it was also obviously desirable to use the highest available transmitter power. These requirements are both admirably fulfilled b y commercial short wave transmitters. We are greatly indebted to Messrs. Cable and Wireless, Ltd., who made available two of their transmitters at Ongar, Essex. The exact frequencies used depended mainly on which transmitters could be released from commercial service. The experiments * Cavendish Laboratory, Cambridge. ** Solar Physics Observatory, Cambridge.
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A search for radio echoes of long delay
were made with the transmitters GMC, frequency 13.455 Me/s, power 30 kw, and GMV, frequency 20-675 Mc/s, power 9 kw. A description of the aerial systems and receivers is given in section 4. No long delayed echoes were observed in any of the experiments. Observations were sometimes difficult to make owing to heavy interference from other commercial short wave transmitters. These were much more numerous than in 1928 and 1929 when STORMER and VAN DER POL made their observations. The sensitivity of the apparatus is examined in sections 6, 7, 8 and 9, and estimates are made of the density and size of the clouds of ions that would be necessary to give the minimum detectable signal. There is very little information regarding the density of the clouds emitted from the sun, but the suggestions that have been made are all very much lower than would be required to give detectable radio echoes. We conclude that the long delayed echoes observed in the past were probably associated with some ionised region close to the earth, and the evidence in favour of this is summarised in section 10. Some suggestions for the planning of future attempts to observe long delayed echoes are made in section 11. 2.
PREVIOUS OBSERVATION OF EC~tOES OF LONG DELAY
The most striking observation of echoes of long delay were probably those of VAN DER POL [2], and STeRMER [1], with signals of frequency 9.55 Mc/~ from the transmitter P C J J at Hilversum. The echoes were observed on several different days in March and October, 1928. The delays between the times of arrival of the main signal and the echo varied between three seconds and fifteen seconds, and the echo signal was usually very distorted. The echoes were again observed in 1934 during a series of tests organised b y the World Radio Relay League, and inaugurated b y Sir EDWARD APPLETON. The observers were British radio amateurs, and the frequencies used were 9.5 Me/s. and 6.7 Mc/s. The echoes had characteristics very similar to those reported b y ~JroRMER and VAN DER POL. Similar echoes have also been reported b y GALLE, TALON and FERRI~ [8]. 3. POSSIBLE EXPLANATIONS An explanation of the occurrence of these echoes was put forward b y VAN DER POL in his original communication to " N a t u r e , " [2]. He suggested that t h e y might be due to a long transit time in a region of the ionosphere where the group velocity is very low. Such a region is well known t o ~ c c u r at the level in the ionosphere where the refractive index of the medium is close to zero. APPLETON [9] pointed out that this explanation is probably untenable, because the waves would be attenuated in the ionosphere to an extent which can be estimated. He showed that after times of the order of the observed delays, the signal amplitudes would be far too small to be observed at all. The explanation would only be a possible one if the delay occurred at a level in the ionosphere far higher than the known levels of the E and F regions, where the frequency of collisions of electrons with gas molecules is very small. This explanation has been examined in more detail b y P~.DERSEN [6], BREIT [10], and b y FtlCHS [3] ,[4], who deduced that the electron collision frequency in the region where the delay occurs must be of the order of 0.5 sec -1. THOMAS [11] estimated that even if the gas pressure is so low that collisions of the electrons with gas 273
K. G. BUDDEN and G. G. YATES
molecules are negligible, there is still an effective collision frequency of the order of 104 sec -1 due to the electrostatic forces between the electrons. Hence, the signals would be attenuated far too heavily to give observable echoes. J o o s [12] also examined the suggestion that the observed delay arose because of a low group velocity somewhere in the path of the waves. He concluded that, quite apart from possible absorption, the dispersion which is necessarily associated with a low group velocity would be so great that it would reduce the signal amplitude far below the minimum detectable level. A second possible explanation is t h a t the echoes are due to the reflection of radio waves from clouds of ions or electrons at some distance from the earth. This explanation was favoured at first by STORMER [1] [13], who suggested that streams of ionised particles shot from the sun towards the earth m a y be responsible for the echoes. Such streams of ions moving towards the earth would have their trajectories modified by the earth's magnetic field, and ST~RMER [13] has shown that t h e y would be distributed in a region bounded on one side by a toroidal surface surrounding the earth. The particles would be excluded from the inside of this toroid, and he suggested that the echoes might be evidence of the reflection of radio waves from its inner surface. The estimated size of the toroid would account for delays of the right order (7 see). A third explanation was given by PrDERSE~¢ [6], who showed how radio waves could be guided along "belts" or " b a n d s " of ions. He suggested that such guided waves could travel over large curved paths starting and ending near the earth. ST~RM~R [14] discussed this suggestion, and reviewed the possibility that such paths were associated with a ring of current round the earth, due to the motion of charged corpuscles. This ring would be in the earth's magnetic equatorial plane, and STORMER showed that, with this explanation,-the conditions for receiving long delayed echoes are most favourable when the line from the earth to the sun is also in this plane. He gives data which show that echoes were in fact observed most frequently at these times. Explanations in terms of m a n y reflections in the ionosphere have been suggested by JA•CO [15] and JELSTRUP [16]. An explanation based on guided waves was suggested by J o N r s c c and MIHUL [17]. ArrLETO~ [9] suggested t h a t the waves might be repeatedly focussed at the antipodes and near the transmitter, so as to give an audible signal after several seconds. The suggestions of ST~RM~.a and PEDEr~SEN have acquired a new interest in view of recent theories to explain the occurrence of terrestrial magnetic storms. It is known that magnetic storms frequently start at times of the order of 20 hrs, after the occurrence of a bright solar eruption. It has therefore been suggested that simultaneously with the occurrence of the eruption, a burst of particles is shot out from the sun, and takes about 20 hrs to reach the earth. There is ample evidence in favour of this theory, (see, for example, CHAPMAN [18]) and it was therefore of special interest to investigate whether long delayed echoes could be heard during the period immediately following a solar flare. ST~RM~.R'S conclusions were based on the calculated trajectories of single charged particles in the earth's magnetic field. More recently CHAPMA~ and FERR~O [19], [20], [21] have shown t h a t the clouds of particles moving from the sun towards the earth are most likely to be electrically neutral, ionised clouds, since clouds of charges 274
A search for radio echoes of long delay
of one sign would be rapidly dispersed b y electrostatic repulsion. The effect on a moving, electrically neutral cloud, of the earth's magnetic field has been discussed b y CHAPMA~ and FERRARO. They showe,: that the particles are deflected in such a w a y that in the cloud there is a hollow sur~ ,,unding the earth. This hollow is somewhat similar to that pictured b y STORMER. At a later stage the streams of ions move into a closed structure surrounding the earth in the form of a current ring, with which is associated the start of the terrestrial magnetic storm. This is similar to the ring described b y SToRMER [5]. It is of interest to consider whether the observed long delayed echoes could originate b y reflection at the inner surface of the hollow, or from the current ring. This is considered further in sections 8 and 9, and we conclude that such reflections would be far too weak to give echoes. A similar conclusion was reached b y DOSTAL [22], who also considered whether the echo amplitude could be increased b y a focussing mechanism due to the concavity of the reflecting surfaces. 4.
EXPERIMENTAL DETAILS
Attempts to observe echoes of long delay were made at Cambridge during 1947, 1948, and 1949. When the experiments were planned, their main interest was thought °to be the possible detection Of reflecting or scattering objects at great distances from the earth. Fairly high frequencies were therefore used so that waves travelling radially outwards from the earth could penetrate the ionosphere. Apart from this, the choice of frequencies was dictated mainly by the availability of the transmitters, which had to be diverted from commercial service for the special transmissions. Two frequencies were used, namely 13.455 Mc/s, (power 30 kw), and 20.675 Mc/s, (power 9 kw). The transmissions were provided b y Messrs. Cable. and Wireless, Ltd., from their transmitting station at Ongar, Essex. The transmitting aerial used for both frequencies was a horizontal dipole approximately half a wavelength long, at about one quarter wavelength above the ground. This arrangement was chosen so as to give maximum radiation in a direction radially outwards from the earth. Unmodulated continuous waves were used, keyed on and off with Morse characters. During the first quarter hour of each transmission, the transmitter sent its call sign and V's continuously, so that the receivers could be tuned. It then sent single call signs at approximately t w e n t y second intervals. In some transmissions the call sign and V's were sent for five minute periods every half hour, so that the receiver adjustments could be checked. Most of the transmissions lasted two hours. The receiver had two stages of radio frequency amplification with V R 91 valves. These were followed b y a mixer valve fed from a local oscillator, which was crystal controlled. The output of the mixer was fed to an I.F. amplifier operatingat 455Kc/s with a band width of about 100 Kc/s. This was followed b y a second mixer valve, fed from a second local oscillator, which could be manually adjusted. The output of the second mixer was fed to a second I.F. amplifier, operating at 130 Kc/s, with a band width of about 2 Kc/s. The output of this was caused to beat with a continuous oscillator, so as to give a note of about 1000 c/s. This was fed to an audio frequency amplifier incorporating a narrow band filter, and thence to a cathode follower valve. The output of this was fed to a pair of headphones, and to the vertically deflecting plates of a cathode ray oscillograph. The overall bandwidth of the 275
K. G. BUDDEI~ and G. G. YATES
receiver w a s about 200 c/s, and was determined b y the narrow band filter in the
audio frequency amplifier. I f an observed echo were returned from a moving object, it would suffer a Doppler change of frequency. An additional unit was therefore added, in later experiments, to allow a search to made on other nearby frequencies. It was fed from the output of the first I.F. amplifier, and consisted of a local oscillator, an I.F. amplifier at 130 Kc/s, and output circuits similar to those in the main receiver. Its output was connected to the vertically deflecting plates of a second cathode ray oscillograph. The two parts of the receiving apparatus will be referred to as the "main receiver ", and the "Doppler receiver". The range of frequencies to which the Doppler receiver could be tuned was determined b y the bandwidth of the first I.F. amplifier, and was about 100 kc/s on the high frequency side of the transmitter frequency. A frequency change o f this amount would be produced b y a reflecting or scattering object having a "line of sight" velocity of about 1100 kin/see towards the earth. The horizontally deflecting plates of the cathode ray oscillographs were connected to a source of time-base voltage which gave a slow linear sweep to the spot, with a duration of about 20 sec, and triggered b y the main signal. The screens of the cathode ray oscillograph were photographed using paper moving slowly and continuously, so that successive ~raverses of the time-base appeared as parallel lines about 7 cm long and 1 cm apart. In the tests on 13.455 Me/s, the receiving aerial was a horizontal half wave dipole at about one quarter wavelength above the ground. In the tests on 20.675Me/s, a few ohselvations were made with this type of aerial, but in most of the experiments the aerial uEed was a vertical Vee aerial, designed to have maximum sensitivity for radiation coming radially inwards to the earth. The polar diagram of this aerial was tested, using a small oscillator in a captive balloon. The diagram was found to have the shape expected, and the aerial had a power gain of approximately 8 to 9. 5.
METHOD OF OBSERVATION AND RESULTS
During a transmission, as soon as the transmitter began sending call signs at 20 sec intervals, the camera was switched on, and an observer listened to the received signals. There was usually only one observer, and he could listen to the output either of the main receiver, or of the Doppler receiver, but not to both. As far as possible, notes were made of any signals received, such as atmospherics, signals from other transmitters, or other man-made interference. The volume of traffic in the frequency band near 13.455 Mc/s was often heavy, and distant transmitters working practically on the same frequency sometimes caused trouble. Another effect of the heavy traffic was that the frequencies to which the Doppler receiver could be tuned were severely limited. The procedure with this receiver was to search near the transmitter frequency for a frequency band which sounded free from signals, and to leave the Doppler receiver tuned to this frequency. After five or ten minutes, the procedure was repeated, and if possible a new frequency was chosen. The displaced frequency used was always higher than the transmitter frequency, because it was thought that the echoes might most probably be due to streams of ions moving towards the earth. In view of the interference and the very limited number of frequencies that could be used, it is probable that the search for echoes with the Doppler receiver was of little value. 276
A search for radio echoes of long delay
The heavy interference observed with the frequency 13.455 Mc/s meant t h a t over 50 % of the signals examined were considered to be spoilt by interference. This was the main reason for changing to the frequency 20.675 Mc/s. At the higher frequency, interference was less, p a r t l y because there were fewer commercial transmitters in the frequency band, and p a r t l y because the frequency was above the maximum usable frequency (M.U.F.) for m a n y transmission paths. Even so, the interference level was still high, and about 20 % of the signals examined were considered to be spoilt by interference. On the higher frequency, a very strong " r o u n d the world" echo was often observed (delay 0.13 see). At first, in a search for echoes which might occur at any time, transmissions were arranged at regular intervals, usually on one day in each fortnight. In each day transmissions were usually made during the intervals 0001-0200 hrs, 0600-0800 hrs, 1200-1400hrs, and 1800-2000hrs. In later experiments the transmissions were specially arranged when reports were received of solar flares or of other phenomena which might be associated with the emission of solar corpuscular streams. In these cases, four transmissions per day were made for a period of three days immediately following the occurrence of the flare. The majority of the observations were made in the years (1947-1949) near the epoch of m a x i m u m sunspots, and were spread fairly uniformly through the seasons. About 100 transmissions, each using about 200 signals, were made on a frequency of 13.455 Mc/s. During 22 of these, both the main receiver and the Doppler receiver were used. 35 two-hour transmissions were made on a frequency of 20.675 Me/s, during all of which both receivers were used. In all the experiments, the total mlmber of signals emitted was about 27 000. I n no case was any signal received which could be considered as an'echo of long delay, or which resembled in any way the echoes described by previous observers. 6.
THE SENSITIVITY OF THE APPARATUS
A theoretical lower limit to the sensitivity of the equipment was set by the thermal noise generated in the input circuit of the receiver with a power equal to 4 k T B, where B is the bandwidth, T is the absolute temperature, and k is BOLTZMANN'S constant. With an overall bandwidth of 200 c/s, and the temperature of the input circuit 300 ° K, the theoretical thermal noise power was thus about 3.2 × 1(I-is w. In actual tests with a signal generator, it was found t h a t an input signal power of 10-1e w gave a signal-to-noise ratio of about 4:1 in amplitude at the outt)ut of the receiver. This corresponds to a noise power level of about 6.2 × 10 -is w. I t was considered t h a t a signal input of about 5 × l0 -17 w was necessary to give a clear response on the photographic record, and t h a t about 6 ×10 -is w would give an audible signal. In the following discussion, it is assumed t h a t the transmitting aerial had a power gain G t ~ 3 in the vertically upward direction. The receiving aerial was assumed {o have an effective cross-sectional area of 3 22/4 ~, when intercepting radiation coming radially inwards to the earth, where 2 is the wavelength. This is twice the value for a Hertzian dipole in free space. The values for a Hertzian dipole and a half wave dipole are roughly the same. A factor 2 is included to allow for reflection at the ground. 20 JATP. Vol. 2.
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K. G. BUDDEI~" and G. G. YATES 7.
DETECTION OF A PLANE PERFECT REFLECTOR
If the radiation from the transmitter were reflected at a plane perfectly reflecting surface at a distance R from the earth, the power returned to the receiver would be
{ w 0,/4 (2
× {3
(1)
At the greatest distance for detection this would be 10 -17 w. With W : 30 kw, A : 22 m, this would correspond to a distance R ~ 1.5 × l0 s k m . This would give a time delay between the main signal and an echo of 1000 sec. Thus a plane perfect reflector at a distance slightly greater t h a n t h a t of the sun would give the minimum detectable signal. 8.
SHARPLY BOUNDED IONISED REOION
The only way in which a cloud of ions in space could have a reflection coefficient approaching u n i t y is t h a t it should contain a concentration of electrons higher t h a n the critical concentration for the radio frequency used. This critical concentration is given by ;N O : e o m p2/4 ~ e 2 where
e, m : charge and mass of the electron, eo = permittivity of free space, p = angular frequency of the wave.
For waves of frequency 13.5 Me/s, this gives No = 2.3 × 10° cm -3. I t is apparently very difficult to estimate the density of particles in the streams of corpuscles travelling from the sun to the earth. CHAPMANand FERRARO [20], [21] tentatively suggest a figure of 100 to 1 000 electrons per cc, when the stream is near the earth. CHAPMAI~and BARTELS [23] mention a possible figure of 105 particles per ce in the stream when it leaves the sun. Clouds with densities greater t h a n this, and with temperatures of the order of t h a t of the sun's chromosphere would probably diffuse outwards rapidly. I t therefore seems unlikely t h a t electron densities as high as 106 cm -3 would occur in the streams of corpuscles, and so we should expect the stream to have a reflection coefficient very much less t h a n unity. We now consider what signals we should expect to be returned to the earth from clouds having electron densities of the order of 1000 cm -3. These signals would depend upon the shape and size of the cloud, which is difficult to assess, but some idea of the upper limit of the reflecting power m a y be obtained by supposing t h a t the cloud has a sharp plane boundary. We therefore consider a homogeneous sharply bounded medium, having an electron density N ---- 1000 cm -3. The reflection coefficient for waves incident normally on the boundary is @~ (# -- 1)/(# + 1) where /~ is the refractive index of the medium given by # 2 ~ 1 - 4 z N e2/e0 m p2. Since # is very nearly unity, we have with good accuracy ]@] = ~ N e 2 / e o m p 2. This reflection coefficient expresses the ratio of the amplitudes of a reflected wave to an incident wave. The reflection coefficient for power is the square of this, t h a t is, I@12= ~2N2 ed/e~ m~P 4. We must now multiply the expression (1) by this quantity to get the power returned to the receiver. If, as before, we equate the result to 10 -17 w, then the resulting value of R is 6.3 × 104 km. This distance would give a time delay between the main signal and an echo of only 0.42 sec, which is much 278
A search for radio echoes of long delay less t h a n the longest delay times (of the order of 15 sec), report ed b y STORMER and VAN DES POT,. B y a slight extension of the above calculation, we find t h a t the elect r o n density required to give a detectable echo with a delay of 15 sec, is 2.7 × 1Ocm -3. B u t it should be r e m e m b e r e d t h a t the sharply bounded model gives only an upper limit to the reflecting power, and t h a t diffuse clouds would r e t u r n very much less energy to the earth. Hence, to obtain echoes with delays as great as 15 sec, electron densities very much greater t han 2.7 × 104 cm -a would be required. 9. SCATTERING BY CLOUDS OF ELECTRONS As a f u r th er check on the above calculations, we m a y discuss how energy could be r etu r n ed to the ear t h b y scattering from clouds of electrons. CHAPMANand FERRARO [19], [20], [21] suggest t h a t the clouds of ions responsible for magnetic storms are v er y much larger t h a n the earth, and eventually envelop it. A cloud would therefore subtend a large solid angle at the earth. Consider a p a r t of a cloud of electrons which is at a distance R from the earth. If such a cloud were effective in giving an audible echo, the thickness of the region responsible would be much less t h a n the distance travelled b y radio waves in the duration of ORe Morse sign, say ~ sec. We m a y take this as an upper limit for the thickness of t h a t p a r t of the cloud t h a t is effective at a n y moment. We therefore consider a disk shaped region of the cloud on the surface of a sphere of radius R concentric with the earth. The thickness of the cloud is d-= 3 × 104km, and its area is ~rR 2, so t h a t its volume is ~R2d, and it contains ~ R e d × N electrons, where N is the average electron density in the cloud. Now it is well known t h a t the total power scattered b y a free electron is p = 8 ~r e4 Po/3 e2oc 4 m 2, where c is the velocity of light, and P0 is the flux of power in the incident wave. Since the electrons are assumed to be distributed r a n d o m l y in the cloud, tile powers in all the scattered waves must be added. Substitution of Po-- WGt/4."r R2 then shows t h a t the scattered power entering the receiver is
(w¢,/4 Re)× (s e'/3
c'
× (3 e/4
× (1/4 Re) ×
Re
d.
If N = 1000 cm -a, R : 3 × 105 km (corresponding to a delay of 2 see), W := 30kw, G t = 3, th en the received power would be 1-0 × l0 -a° w. This is smaller t h a n the minimum detectable signal b y a factor of 10le. I t therefore seems reasonable to conclude t h a t ionised clouds could not bc detected b y the scattering of radio waves t r a n s m i t t e d from the earth, even if t h e y had electron densities v e r y much greater t h a n those suggested by CHAPMA:N and FERRARO. ~V(~ must not, however, exclude the possibility t h a t such clouds might be detected in the future due to their absorption of radio waves passing through them. For this, it would of course be necessary to use some extra-terrestrial source of radio waves, such as the sun or the stars. There are reasons for believing t h a t the radio echoes of long delay observed in the past are not connected with objects v e r y distant from the earth. These reasons are summarised in the next section. 10. THE ORIGIN OF ECHOES OF LONG DELAY There appear to be three good reasons for supposing that echoes of long delay are associated with some system which is fixed in space relative to the earth. Wc discuss these reasons in turn. 2o*
279
K. G. BUDDEIqand G. G. Y+,T~S 1. There is no detectable difference between the frequencies of the echo signal and the transmitted signal. This was reported very strikingly b y VAN D:ERPOL [2]. We m a y assume that he was listening to a note whose frequency was of the order of 1000 c/s, and that he would readily have detected a change of pitch of less than a whole tone. This would mean a frequency difference of about 100 cycle sec -1. If such a change of frequency were produced b y reflection from a moving object, its velocity would be 1.5 km sec -1. In fact, a velocity of 0.5 km sec -1 would probably have been detectable. It is difficult to see how the motion of particles travelling from the sun to the earth with velocities of the order of 1000 km sec -1 could have their motions modified so that their velocity relative to the earth was consistently reduced to less than 0.5 km see -1. It seems more reasonable to assume that the echoes are associated with some system moving permanently with the earth. 2. Echoes of long delay were observed mainly or wholly during the daytime. GALL:E, TALON and F:ERRI]~ [8] state that t h e y disappeared during a solar eclipse, and reappeared slowly afterwards. This observation suggests that the formation of the echoes depends on the ultra-violet light from the sun, and can be affected b y the shadows of the earth and the moon. 3. The echoes have been heard on frequencies 9.5 and 6.7 Me/s, but not on frequencies 13.5 and 20.7 Mc/s. This suggests that some feature of the echoes m a y be associated with the ionosphere. GALLE, TALON and F:ERRI~ used a frequency of 12 )~[c/s, but their observations were made in Indo-China, that is, near the equator, whereas all other reports of long delayed echoes came from temperate latitudes. We suggest, very tentatively, that long delayed echoes are due to the propagation of guided waves of the t y p e described b y ~)EDERSEN [6] (see section 3), over long curved paths formed b y belts of ions outside the earth but fixed relative to it, and that the successful launching of such waves depends on the state of the ionosphere. ]1. SUGG:ESTIONSFOR FUTURE TESTS In view of the conclusions of the last section, it is suggested that in any further attempts to observe echoes of long delay, it is desirable to use rather lower frequencies than were used in our tests. The frequency chosen should probably be less than 10 Mc/s, and it m a y also be undesirable to use aerials whose directions of maximum transmission or sensitivity are radially outwards from the earth. Better results might be achieved with aerials having horizontal directivity, and if a study could be made as to how the strength of the echoes depends on the directivity of the aerials, this would give some useful information about the nature of the waves responsible for the echoes.
Acknowledgements--The work described in this paper was carried out partly at the Solar Physics Observatory, Cambridge, and partly at the Cavendish Laboratory, Cambridge, where it formed part of a programme of radio research supported by the Department o/ Scienti/ic and Industrial Research. Our grateful thanks are due to Messrs. Cable and Wireless,Ltd, and especially to their Chief Engineer, Mr. SMALE, and to Mr. KEEN and the staff of the transmitting station at Ongar, for arranging the special transmissions. We also wish to thank our assistant, Mr. A. B. RE:EDER, who gave invaluable help with the construction of the receivers, and the taking of the observations. 280
A search for radio echoes of long delay REFERENCES
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