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Radio echo studies of meteors
Radio Echo Studies of Meteors J. G. DAVIES and A. (~. B. LOVELL University of Manchester, Jodrell Bank Experimental Station SUMMARY
A description is given of the radio-eetm techniques for the study of meteors. Particular attention is given to those which are relevant to the work on the astronomy of meteors, such as methods for the determination of meteor radiants and velocities. The discovery and elucidation of the complex series of day-time meteor streams is described as an illustration of the ability of these methods to work under conditions when photographic and visual observations are impossible. A short account is also given o f the work on the interstellar meteor problem which has resolved the uncertainty as to whether the sporadic meteors are members of the solar system or come from interstellar space. Brief reference is made to the use of the radio techniques in meteor physics.
1. HISTORICAL NOTE ALTHOUGH the appearance of a meteor, or shooting star, in the night s k y is a familiar sight, the serious s t u d y of meteoric p h e n o m e n a dates b a c k little more t h a n 100 years. D u r i n g the t r e m e n d o u s display of meteors which occurred in November, 1833, OLMSTED, TWINING, a n d others noticed t h a t the meteors appeared to be diverging from a point in the constellation of Leo. I t was soon realized t h a t the divergence was the effect of perspective a n d t h a t the meteors must actually be travelling in nearly parallel p a t h s t h r o u g h space outside the E a r t h ' s atmosphere. I n t e r e s t was further heightened b y the r e t u r n of the great N o v e m b e r display t h i r t y - t h r e e years later. At this time SCHIAPARELLI, H. A. NEWTON, and ADAMS elucidated the n a t u r e of the orbits of some of the p r o m i n e n t meteor streams a n d the close relation of the Lyrids with Comet 1861 I, of the Perseids with Comet 1862 I I I , a n d of the Leonids with Comet 1866 I, was established b e y o n d doubt. U n f o r t u n a t e l y the great Leonid shower failed to reappear as predicted in 1899 a n d this u n d o u b t e d l y had a most serious effect on the progress of the science. I n t e r e s t in meteor a s t r o n o m y waned, and a p a r t from the H a r v a r d expedition to Arizona in !932 and the work of HOFFMEISTER in G e r m a n y , the main contributions during the first f o r t y years of the twentieth c e n t u r y came from a m a t e u r observing organizations. Recently, however, the subject has been revitalized b y the application of two new techniques to the observation of meteors. The double camera, r o t a t i n g shutter, p h o t o g r a p h i c work of WHIPPLE at H a r v a r d is providing f u n d a m e n t a l d a t a of high a c c u r a c y from which the spatial orbits of individual meteors can be derived, and which is also an i m p o r t a n t tool for the s t u d y of the physical properties of the high atmosphere. I n the other technique the observation of radio waves reflected from the ionized trails left when the meteor evaporates provides the only k n o w n m e t h o d of s t u d y i n g meteors in daylight and u n d e r cloudy conditions. I t is the purpose of this contribution to review some of the achievements of this latter technique. 2. THE RADIO ECHO TECHNIQUE The c o n t e m p o r a r y radio echo m e t h o d s for the s t u d y of meteors have evolved from the r a d a r techniques developed during the Second World W a r for the location of 585
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Radio echo studies of meteors
aircraft and ships. The radio waves are generated in a t r a n s m i t t e r T (Fig. 1) connected to an aerial system A, which can either be of simple design 6o radiate uniformly over the sky, or a large complex structure designed to concentrate the radiation in a specific direction. In the case of large aerial systems it is c o m m o n practice to use an electronic switching device, S, so t h a t the same aerial can be used to receive the PULSE GENERATII'~G CIRCUIT
TRANSMITTER T
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Fig. l. Block diagram of radio echo a p p a r a t u s
radiation after scattering from the ionized meteor trail. The o u t p u t of the receiver X, is connected to an indicator or recording a p p a r a t u s I. In m a n y systems T transmits pulses of radio waves with a duration of a few microseconds at a recurrence rate of several h u n d r e d per second. The simplest form of display I is a cathode r a y t u b e indicator and the radio waves scattered from the
Fig. 2. A radio echo h,oIn a meteor trail observed on a range-time cathode ray tube display
meteor trail are t h e n observed as a transient echo as in Fig. 2. This t y p e of display gives immediately the range R of the meteor from the observing station, and the strength of the received signal as measured b y the amplitude of the echo. In recent years detailed investigations have been made of the relation between the various parameters of the radio a p p a r a t u s and the strength of the signal received from a meteor trail of given visual magnitude. The results m a y be summarized as follows.
,L G. DAVIESAND A. C. B. LOVELL
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I f the electron density in the meteor trail is less than the critical density for the wavelength used and if the diameter of the trail is less than the wavelength, then the incident wave penetrates to all the electrons, which scatter freely and in phase giving a received power ~, where e-
PG2N223 ( e 2 ~ 2 127r2Ra \mc~ ] .
.
.
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.
(1)
In this expression, which was first derived by LOVELL and CLEGG (1948), P is the peak transmitter power, G the power gain of the aerial system, referred to as a halfwave dipole (assumed the same for transmission and reception), 2 the radio wavelength, N the number of electrons per centimetre in the meteor trail, and (e2/mc 2) the classical electron radius. When the diameter of the trail is small compared with 4, and the volume density is somewhat greater than the critical density, the nature of the scattering depends on the orientation of the electric vector with respect to the column of ionization, and under certain conditions plasma resonance effects occur. The full analysis has been given by KAISER and CLOSS (1952). When the electron trail is so dense th at the radio wave does not penetrate, a different situation arises, and it has been shown by GREENHOW (1952a) and by KAISER and CLOSS (1952) t hat for N >> 1012 electrons per cm, (1) becomes = ~40~R ~ \ i n c h
. . . . (2)
In the first case HERLOFSON (1948) has shown t hat the echo duration to 1/e of the initial amplitude is independent of N and is given by 22 T -- 167r2D.
.
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.
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(3)
where D is the diffusion coefficient. In the latter case, however, the duration depends directly on N and is given by N22 T oc D-. . . . (4) MILLMAN and McKINLEY (MILLMAN, 1950) have made a detailed study of the relation between T, for echoes observed on an equipment with 2 ~ 9.1 m, and the visual magnitude of the corresponding meteor. From their results, and by using equation (4), GREENHOW and HAWKINS (1952) have found the electron line density produced by a meteor of given zenithal magnitude. The results indicate t h a t a meteor of zenithal magnitude ~ 6 produces approximately 1012 electrons per cm path, and a meteor of zero magnitude about l014 electrons per cm path. The value of D (4 x l04 cm 2 per sec.) was determined from echoes of short duration using equation (3). From these results and the above equations it is possible to determine the parameters of the apparatus necessary to detect meteors down to a given limiting magnitude. Here it is only necessary to mention t hat with contemporary apparatus working in the wavelength range from about 4 to l0 m, all meteors within the visual range of magnitudes can be readily detected with simple aerial systems, and t h a t echoes of comparatively long duration (determined by (4)) will result. With equipments of higher power, using larger aerial systems, the limit has been extended to about
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Radio echo studies of meteors
magnitude -- 9. In these cases equations (1) and (3) a p p l y and the echo duration is only a fraction of a second. 3. THE MEASUREMENT OF METEOR RADIANTS AND VELOCITIES Radio echo a p p a r a t u s employing simple aerials and indicating systems can be used to s t u d y the incidence of meteors i n d e p e n d e n t l y of daylight or cloud. However, significant contributions to meteor a s t r o n o m y d e m a n d the measurement of meteor radiant positions and velocities. F o r t u n a t e l y , appropriate techniques were soon developed and the more i m p o r t a n t will be described briefly here. The basis of the m e t h o d of radiant d e t e r m i n a t i o n depends on the fact t h a t in the m a j o r i t y of eases the radio reflection from the ionized column is specular (HEy and STEWART, 1947; LOVELL, BANWELL, and CLEGG, 1947). This means t h a t a radio echo can be obtained from a trail only if the perpendicular from the station to the ionized p a r t of the trail lies within the aerial beam. Thus if a narrow beam aerial is used it is possible to establish the plane in which the trails of the meteors lie. This p r o p e r t y was utilized b y CLEGO (1948) in a m e t h o d for determining the right ascension and declination of an active m e t e o r radiant. The final form of this a p p a r a t u s (ASPINALLet al., 1951) is shown in a preceding article (p. 545, Fig. 4). Two aerial arrays, each producing a narrow beam are erected on either side of the hut, one directed 25 ° N of W and the other 25 ° S of W. The o u t p u t from the t r a n s m i t t e r is fed equally into both aerials. An electronic device connects the receiver for alternate pulses to the two aerials in t u r n and simultaneously the o u t p u t of the receiver is switched to separate cathode r a y tube displays which are p h o t o g r a p h e d continuously on a moving film. B y this means continuous records are obtained of the range and time of occurrence of the echoes in each aerial. Considering for a m o m e n t the idealized case of infinitely narrow aerial beams and meteors radiating from a point radiant, it will be evident t h a t in view of the specular reflecting properties of the trails echoes will be observed only when the radiant is at right angles to the beam axis. F r o m the time at which the echoes appeared in each aerial b o t h the right ascension and declination of the meteor radiant could be computed. In practice the beam widths are finite and it is necessary to consider the g e o m e t r y of the intersection of the aerial beam with the zone in which the meteors ionize. I t can be shown t h a t as the E a r t h rotates, echoes from a given radiant will first appear at short range and t h a t the ranges will increase to a m a x i m u m as tim radiant moves into a position perpendicular to the beam axis. The echo rate will then fall suddenly. After an interval depending on the declination of the radiant the process will be repeated in the aerial directed to the north west. Examples of the results obtained with this system during the s u m m e r d a y t i m e streams are shown in Fig. 3. The a p p a r a t u s has now been in regular use for several years and has given valuable information a b o u t the radiant co-ordinates and a c t i v i t y of all the d a y and night time meteor streams active in the n o r t h e r n hemisphere. First success in determining velocities b y the radio echo technique was achieved b y HEY, STEWART, and PARSONS (1947) during the great Giacobinid shower of 10th October, 1946. F o r a small n u m b e r of v e r y densely ionizing meteors t h e y succeeded in photographing the echo r e t u r n e d from the ionization near the head of the meteor and from the change of range with time the velocity was determined. Such range-time methods are restricted to relatively large meteors which create sufficient ionization to r e t u r n an echo from the vicinity of the moving head of the
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meteor. They have been largely superseded in general use by amplitude-time methods in which the diffraction of radio waves from the trail is observed. The original method was devised by DAVIES and ELLYETT (1949) using a pulsed transmitter, and subsequently McKINLEy (1951) used an alternative method with the transmitter radiating continuous waves. The principle of the method is as follows. X AMPLITUDE
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:Fig. 4. D i f f r a c t i o n o f r a d i o w a v e s from. a m e t e o r t r a i l
The amplitude and phase of the wave scattered by an element ds at the point Y on a meteor trail X Y, Fig. 4, is given by
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.
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where /c is a constant for a given trail, equation (l), ~,~/27r is the radio frequency, 2 the wavelength, and R the range of Y from the station O on the ground. From this it can be shown t hat the amplitude received from the whole length of trail X Y is
where C and S are the Fresnel integrals of optical diffraction theory taken between the appropriate limits. The variations in the value of IAI as the point Y moves along the trail are shown in Fig. 4. The values of V, the variable of the Fresnel integrals at the maxima and minima can be determined. The length of the trail between these points is then ½(V2 -- V i ) ~ R 0 ~ and the velocity is calculated fl'om
where T is the time taken by the meteor in travelling between points 1 and 2. In the pulse technique of DAVIES and ELLYETT (1949) the transmitter radiated 600 pulses per second, each of 15 #sec. duration. The first pulse received back from a meteor trail initiated the time bases of two cathode ray tubes, each of which displayed the amplitude of the receiver output. One of these time bases lasted for one-tenth of a second, and thus the first 60 pulses received from the meteor trail were shown side by side. An example of this display is illustrated in Fig. 5. The value of T was then obtained by counting the pulses between successive maxima and minima. The second time base lasted for 7 msec., and was used to determine the range
Fig. 5. The diffraction p a t t e r n o f r a d i o waves scattered from a m e t e o r trail as p h o t o g r a p h e d on a cathode ray t u b e display. The velocity of the m e t e o r is calculated from the distance between the successive m a x i m a a n d minima. The range of the meteor is m e a s u r e d on a separate display O r d i n a t c : Amplitude. A b s c i s s a : Time, increasing from right to left. (each pulse corresponds to an interval of 1/600 sec.).
Fig. 6. The a r r a y of Yagi aerials working on a wavelength of 8.2 m used in some of the e x p e r i m e n t s on the velocity distribution of sporadic meteors
592
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R 0 of the echo. Wavelengths of 4 m and 8 In have been used with this technique, and aerials varying from single dipoles, radiating over a large part of the sky, to arrays of Yagis, as shown in Fig. 6, designed to give a narrow beam, for the observation of faint meteors. In each case the same aerial was used for transmission and reception. With this apparatus velocities have been obtained from meteors of magnitudes down to + 9. In the continuous wave technique used by McKINLEY (1951) the transmitter and receiver were separated by several kilometres in order to reduce the direct wave from transmitter to receiver to an amount of the same order as the wave reflected from the meteor trail. In this case the theory is complicated by the addition of the ground wave, but the practical result is that the oscillations in amplitude of the signal can be obtained both before and after the meteor reaches the perpendicular reflecting point. 4. SOME EXAMPLES OF RADIO ECHO RESULTS
The radiant and velocity measuring techniques described above have been in use at the Jodrell Bank Experimental Station of the University of Manchester since 1948, and have been applied both to shower and sporadic meteors. Some of the more notable results are described briefly below. (i) The Daytime Meteor Streams A remarkable series of meteor streams active during the daytime in the months of May, June, and J u l y were discovered in 1947, and have since been observed systematically (CLEGG et al., 1947; ASPINALL et al., 1949; ELLYETT, 1949; ASPINALL and HAWKINS, 1951 ; DAVIES a n d GREENHOW, 1951 ; ALMOND, 1951 ; HAWKINS a n d ALMOND, 1952; ALMONDet al., 1952). From a large active area in transit during the
three hours before noon, four major radiants appear; the first, near o Ceti, is active for several days around 15th May, then from 31st May to 16th June, two intense streams appear simultaneously, one in Aries, and the other near ~ Persei. At the end of June and the beginning of J u l y the fourth major radiant is active near fl-Tauri. Of these the Arietid radiant, active for three weeks, and reaching an equivalent visual rate of 70 to 80 meteors per hour at maximum constitutes the greatest annual display of meteors at the present time. In the course of its three weeks duration the radiant point moves some l 2° across tile sky, the right ascension increasing from 39 ° to 48 °, and the declination from ~- 20 ° to ~- 27 °. Several other radiants have also been found in the same general part of the sky, but these have been much less active, and have not reappeared consistently each year. The velocities of each of the four major daytime streams have been determined over several years, and by combining these with the radiant measurements, orbits have been computed. In each case the inclination of the orbits is small, so the projection of these on to the ecliptic (Fig. 7) gives a fair picture of the true orbit. The orbital periods range from 1.5 years for the o Cetids to 3.2 years for the fl Taurids, and are amongst the shortest known periods of meteor streams, being closer to those of certain minor planets rather than comets. Since the inclination of the orbits is small, the streams will pass close to the Earth's orbit again as t he y approach the Sun. In fact, it is now realized that night time streams with orbits closely similar to the daytime orbits have long been known to visual observers. Thus the Arietid stream, observed in June as it recedes from the Sun is seen again at the end of J ul y as it approaches the Sun, and is then known
J. G. DAVIES AND A. C. B. LOVELL
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urids
Arietids
Fig. 7. Orbits of some of the day-time m e t e o r s t r e a m s projected on to the plane of the ecliptic
as the 5 Aquarid stream. Similarly the ~ Perseids correspond to the Southern Arietid stream in October, and the daytime fi Taurids to the night time Taurids in November. The fi Taurid orbit is compared with the orbits of three individual night time Taurids determined by WHIeeLE'S photographic technique ( W H I P P L E , 1940; WRIGHT and WHIPPLE, 1950) in Fig. S. (ii) The Interstellar Meteor problem One of the major controversies in astronomy during the last fifty years has ariuen over the question of the origin of sporadic meteors. One group, led by 0eIK (1934a, b; 1940; 1941) and HOFFMEISTER (1948), believe that the sporadic meteors have an interstellar origin, and are merely visitors to the solar system. The other group, led by PRENTICE ([948) and PORTER (1943; 1944), believe that they move in elliptical orbits round the Sun. HOFFMEISTER'S analysis of the variation of meteor rates throughout the year and throughout the night was based on the assumption, since shown to be false by radio observations, that the meteor orbits were uniformly distributed in space ; and visual estimates of velocity, based as they are on durations usually less than a second, are notoriously inaccurate. Even the photographic results, although extremely accurate did not give a clear answer, since they were limited to 39
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the brightest meteors, and the method is more sensitive to slowly moving particles. Thus in 1948, when the radio echo technique was applied to the problem in England by ALMOND, DAVIES, and LOVELL, and in Canada by McKINLEY, there were wide divergencies of opinion as to the orbits of the sporadic meteors. In order to determine whether the orbit of a meteor is elliptical or hyperbolic, it is necessary to know its heliocentric velocity. Unfortunately, no radio technique qO
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Fig. 10. V e l o c i t y d i s t r i b u t i o n o f sporadic m e t e o r s as m e a s u r e d in t h e e x p e r i m e n t s o f ALMOND, DAVIES, a n d LOVELL. This distribution refers to the m e t e o r s w i t h radiants close to the a p e x of t h e E a r t h ' s w a y , m e a s u r e d during the a u t u n m of 1950. The s m o o t h c u r v e s h o w s t h e v e l o c i t y d i s t r i b u t i o n calculated on the a s s u m p t i o n t h a t t h e m e t e o r s are t r a v e l l i n g in parabolic orbits in r a n d o m directions
yet used will give this for individual meteors, since the radiant point can only be determined for meteor showers. Therefore statistical experiments have been performed on groups of meteors, and the resulting histograms compared with those expected from different values of the heliocentric velocity. Since heliocentric velocities in excess of 42 km per sec. would indicate hyperbolic orbits, and the Earth's orbital velocity is about 30 km per sec., the greatest geocentric velocity which can be observed for a meteor moving in an elliptical orbit is about 72 km per sec. Velocities in excess of this limit would indicate the existence of hyperbolic meteors. McKINLEY (1951), using his continuous wave technique and dipole aerials, measured velocities for periods of 48 hr or 72 hr each month for 15 months. In this time he observed more than 10,000 velocities, the distribution being given in Fig. 9. The effect of the major showers is clear in some of the histograms. Of these
59[;
Hadio echo studies of meteors
only t h i r t y - t w o meteors had velocities in the range 75-79 k m per sec., and there were none with velocities above 80 k m per see. The errors of measurement were such t h a t in no case could a n y meteor be definitely stated to have a hyperbolic velocity. Simultaneously A L M O N D , D A V I E S , and LOVELL ( 1 9 5 l ; 1952; 1953) started a series of experiments, which lasted for four years, using the pulse technique and a b e a m e d ~erial system directed to receive echoes from meteors with radiants close to the apex of the E a r t h ' s way. In this case the velocity distribution to be expected from meteors travelling in parabolic orbits should show a peak close to 72 k m per sec. In all, S60 such meteors were observed, the distribution for one such e x p e r i m e n t being given in Fig. I0. A second group of experiments, receiving meteors from the
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ALMOND, DAVIES, and LOVEIA. This distribution refers to tho meteors with r a d i a n t s close to th~ antape,~ of the E a r t h ' s way, m e a s u r e d during the spring of 1951. Tile s m o o t h curve shows the velocity distribution caleulate:t oll the a s s u m p t i o n t h a t the meteors are travelling in parabolic orbits in r a n d o m directions
region of the a n t a p e x gave the distribution of Fig. l l, and p r o v e d t h a t the high velocity cut off observed in the main experiments was not due to i n s t r u m e n t a l limitations. The peak of the observed distributions is in all cases some l0 k m per sec. lower t h a n t h a t derived from the assumption of parabolic velocity, and it is concluded t h a t the m a j o r i t y of the sporadic meteors move in orbits of short periods, similar to those found for the d a y t i m e showers. As in MCKINLEY'S experiment, no definite ease of a hyperbolic velocity was found, and the results are overwhelmingly in f a v o u r of the view t h a t sporadic meteors are localized in the solar system. (iii) Problems in Meteor Physics In addition to astronomic~fl problems, such as those discussed above, the radio echo technique has been applied to the s t u d y of meteor ionization and the physics of the upper atmosphere. The m e a s u r e m e n t of high altitude winds provides an illustration of the value of this technique. Meteors of zenithal magnitude brighter t h a n a b o u t + 6, produce echoes whose duration is proportional to the electron density in the tr~il, and which m a y last for m a n y seconds (equation (3)). The violent fluctuations in echo amplitude observed under these circumstances have been studied b y GREE~-HOW (1952b). An example, observed on both 4 m and S m is shown in Fig. 12. I t will be seen t h a t the period of fluctuation is closely proportional to the wavelength. I m m e d i a t e l y after formation the meteor trail is subjected to the motion of the
3. G. DAVIES AND A. C. B. LOVELL
597
winds in the 80 k m region where the trail is formed. These winds are b o t h high and t u r b u l e n t , and the trail rapidly gets distorted out of its initially straight condition. U n d e r these circumstances more t h a n one section of the trail m a y produce reflection, and these sections m a y move with different radial velocities relative to the observing station. The signals received from different parts of the trail will t h e n interfere, and produce the a m p l i t u d e fluctuations observed. F r o m the period of these fluctuations the degree of turbulence can be inferred, and velocity gradients of the order of 5 m per sec. per k m are usually observed for points separated b y 5-12 km. In addition, b y observing the slow drift in range of the long duration echoes, Greenhow was able to measure the radial c o m p o n e n t of the velocities of individual echoing
4.2rn ECHO AMPLITUDE
8.4 m ECHO AMPLITUDE
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Fig. 12. The thmtuations in amp|itude in a radio echo fl'om a meteor trail photographed simultaneously on wavelengths of 4.2 m and ~.4 m. The fluctuations are caused by winds in the high atmosphere centres. Changes in range of 50 m were observable and drifts of as m u c h as a kilometre in 20 sec. have been recorded. An average value of the winds observed b y this m e t h o d is 50 m per sec. 5. THE FUTURE OF METEOR ASTRONOMY I t must be a d m i t t e d t h a t in spite of the great advances made b y the radio and photographic techniques during the last few years the existence of meteors remains an enigma. The c o m e t a r y association of the Lyrids, Lconids, and Perseids was established eighty years ago and in the intervening period the list of associations has not increased greatly. I n fact, only the association of the night time Taurids and (tay time fl Taurids with ENCKE'S Comet, and of the Giacobinids with the GiacobiniZinner Comet, can be said to be firmly established. The relationship, if any, of the Aquarids and Orionids with HALLEY'S comet remains v e r y problematical. As regards the other major showers the recent measurements of their orbits has created an e x t r e m e l y puzzling situation. Whereas it was believed with some confidence t h a t c o m e t a r y associations could be found when the orbits were known, this now seems to be impossible. The Geminids, Quadrantids, and the s u m m e r d a y t i m e streams have orbits with unique characteristics, in some cases of shorter period even t h a n the asteroidal bodies. The p a r e n t a g e of these streams now presents a problem of great interest. Although the major streams provide the most notable events in meteor a s t r o n o m y , their contribution to the total n u m b e r of meteors entering the a t m o s p h e r e is smaller
598
Radio echo studies of meteors
t h a n t h a t of t h e s p o r a d i c m e t e o r s . I n v i e w of t h e r e c e n t m e a s u r e m e n t s r e f e r r e d to a b o v e , i t c a n s c a r c e l y be d o u b t e d t h a t t h e s e s p o r a d i c m e t e o r s are m o v i n g i n closed o r b i t s a r o u n d t h e S u n , a n d , i n fact, t h a t a large p e r c e n t a g e o f t h e o r b i t s m u s t be of s h o r t period. T h e s o l u t i o n of t h e p r o b l e m of t h e o r b i t s m e r e l y serves to raise a g a i n t h e p r o b l e m of t h e i r origin. Are t h e y d i s p e r s e d fl'om c o m p a c t s t r e a m s w h i c h were o n c e m o v i n g i n o r b i t s like t h e G e m i n i d s a n d t h e s u m m e r d a y t i m e s t r e a m s , or h a v e t h e y a n i n d e p e n d e n t o r i g i n f r o m t h e f o r m a t i o n o f t h e solar s y s t e m ? F r o m t h e s e r e m a r k s it will be a p p r e c i a t e d t h a t all t h e f u n d a m e n t a l q u e s t i o n s i n m e t e o r a s t r o n o m y r e m a i n u n s o l v e d . A l t h o u g h s o m e c o m e t s a n d m e t e o r s t r e a m s are a s s o c i a t e d , t h e n a t u r e of t h e a s s o c i a t i o n is u n k n o w n . D i d t h e c o m e t s c o m e before t h e m e t e o r s , do c o m e t s f o r m m e t e o r s , or m e t e o r s c o m e t s ; or are t h e c o m e t s a n d m e t e o r s m o v i n g i n t h e s a m e o r b i t s w i t h a s t a b l e r e l a t i o n s h i p ? Are t h e s h o r t p e r i o d s t r e a m s t h e r e m a i n s o f a u n i q u e f a m i l y of c o m e t s w h i c h h a s n o w d i s a p p e a r e d , or of disi n t e g r a t e d m i n o r p l a n e t s ? F i n a l l y , was t h e large s p o r a d i c m e t e o r c o m p o n e n t o r i g i n a l l y r e l a t e d t o t h i s t y p e of m e t e o r s t r e a m ? T h e r a d i o echo s t u d i e s of m e t e o r s are s t i m u l a t i n g a n e w i n t e r e s t i n t h e s e f u n d a m e n t a l p r o b l e m s of m e t e o r a s t r o n o m y a n d t h e n e w t e c h n i q u e s are o p e n i n g fresh fields of w o r k i n m e t e o r p h y s i c s a n d t h e s t u d i e s of t h e h i g h a t m o s p h e r e .
REFERENCES ALMOND, M.
. . . . . . . . . . . ALMOND, M. BULLOUGH, K. a n d HA'WKINS, C. S . . ALMOND, M., DAVIES, J. G. an(] LOVELL, A. C . B .
1951 1952 1951
1952 1953 ASPINALL, A., CLEGG, J. A. and LOVELL, A. C . B . ASPINALL, A., CLEGG, J. A. and HAWKINS,G. S . . ASPI~CALL,A. and HAWKINS,G. S. . . . . . CLEOG, J. A . . . . . . . . . . . . CLEGO,J. A., HUOHES,V. A. and LOVELL, A. C.B. DAVIES, J. G. and ELLYETT, C. D . . . . . . DAVIES, J. G. and GREENHOW,J. S. . . . . ELLYETT,
C. D . . . . . . . . . . . GREENHO~V, J. S . . . . . . . . .
.
.
1949
1951 1951 1948 ]947 1949 1951 1949 1952a
GREENHOW, J. S. and HAWKINS,G. S. . . . . HAWKINS, G. S. a n d ALMOND, M. . . . . . HEY, J. S. and STEWART,G. S. . . . . . . HEY, J. S., STEWART,G. S. and PARSONS, S.J. HERLOFSON, N . . . . . . . . . . .
1952b 1952 1952 1947 1947 1948
HOFFMEISTER, C .
1948
.
.
.
.
.
.
.
.
.
.
KAISER, T. R. and CLOSS, R. L . . . . . . .
1952
LOVELL,
1947
A. C. B., BANWELL, C. J. and CLEGC~, J.A. LOVELL, A. C. B. and CLEGG, J. A. . . . . .
McKINLEY, D. W. R . . . . . . . . . . MILLMAN, P. M . . . . . . . . . . . 0PIK, E. J . . . . . . . . . . . .
PORTER, J. G. . . . . . . . . . . . PRENTICE, J. P. M.
. . . . . . . . . . . . . . . . . . . WRIOItT, F. W. and WHIPPLE, F. L . . . . . WHIPPLE, F. L .
1948 1951 1950 ] 934a 1934b 1940 1941 1943 1944 1948 1940 1950
M.N., 111, 37. Jodrell Bank Annals, 1, 13. M.N., 111, 585. M.N., 112, 21. M.N., 113, 411. M.N., 109, 352. Phil. Mag., 42, 501. M.N., 111, 18. Phil. Mag., 39, 577. M.N., 107, 369. Phil. Mag., 40, 614. M.N., 111, 26. M.N., 109, 359. Proc. Phys. Soc., 65, B, 169. J. atmosph, terr. Phys., 2, 282. Nat~re (London), 170, 355. Jodrell Bank Annals, 1, 2. Proc. Phys. Soc., 59, 858. M.N., 107, 176. Phys. Soc. Rep. Prog. Phys., 11, 444. Die Meteorstr6me, Weimar. Phil. Mag., 48, 1. M.N., 107, 164. Proc. Phys. Soe., 60, 491. Astrophys. J., 118, 225. J. Roy. Ast. Soe. Can., 44, 209. Circ. Harv. Coll. Ob.~., I~o. 389. Circ. Harv. Coll. Obs., No. 391. Publ. Tart~¢. Obs., 8{), No. 5. Publ. Tart~z. Obs., 80, No. 6. M.N., 103, 134. M.N., 1{)4, 257. Phys. Soc. Rep. Prog. Phys., 11, 389. Proc. Amer. Phil. Soc., 83, 711. Teeh. Rep. Harv. CoU.Obs., No. 6.
A description is given of the radio-eetm techniques for the study of meteors. Particular attention is given to those which are relevant to the work on the astronomy of meteors, such as methods for the determination of meteor radiants and velocities. The discovery and elucidation of the complex series of day-time meteor streams is described as an illustration of the ability of these methods to work under conditions when photographic and visual observations are impossible. A short account is also given o f the work on the interstellar meteor problem which has resolved the uncertainty as to whether the sporadic meteors are members of the solar system or come from interstellar space. Brief reference is made to the use of the radio techniques in meteor physics.
1. HISTORICAL NOTE ALTHOUGH the appearance of a meteor, or shooting star, in the night s k y is a familiar sight, the serious s t u d y of meteoric p h e n o m e n a dates b a c k little more t h a n 100 years. D u r i n g the t r e m e n d o u s display of meteors which occurred in November, 1833, OLMSTED, TWINING, a n d others noticed t h a t the meteors appeared to be diverging from a point in the constellation of Leo. I t was soon realized t h a t the divergence was the effect of perspective a n d t h a t the meteors must actually be travelling in nearly parallel p a t h s t h r o u g h space outside the E a r t h ' s atmosphere. I n t e r e s t was further heightened b y the r e t u r n of the great N o v e m b e r display t h i r t y - t h r e e years later. At this time SCHIAPARELLI, H. A. NEWTON, and ADAMS elucidated the n a t u r e of the orbits of some of the p r o m i n e n t meteor streams a n d the close relation of the Lyrids with Comet 1861 I, of the Perseids with Comet 1862 I I I , a n d of the Leonids with Comet 1866 I, was established b e y o n d doubt. U n f o r t u n a t e l y the great Leonid shower failed to reappear as predicted in 1899 a n d this u n d o u b t e d l y had a most serious effect on the progress of the science. I n t e r e s t in meteor a s t r o n o m y waned, and a p a r t from the H a r v a r d expedition to Arizona in !932 and the work of HOFFMEISTER in G e r m a n y , the main contributions during the first f o r t y years of the twentieth c e n t u r y came from a m a t e u r observing organizations. Recently, however, the subject has been revitalized b y the application of two new techniques to the observation of meteors. The double camera, r o t a t i n g shutter, p h o t o g r a p h i c work of WHIPPLE at H a r v a r d is providing f u n d a m e n t a l d a t a of high a c c u r a c y from which the spatial orbits of individual meteors can be derived, and which is also an i m p o r t a n t tool for the s t u d y of the physical properties of the high atmosphere. I n the other technique the observation of radio waves reflected from the ionized trails left when the meteor evaporates provides the only k n o w n m e t h o d of s t u d y i n g meteors in daylight and u n d e r cloudy conditions. I t is the purpose of this contribution to review some of the achievements of this latter technique. 2. THE RADIO ECHO TECHNIQUE The c o n t e m p o r a r y radio echo m e t h o d s for the s t u d y of meteors have evolved from the r a d a r techniques developed during the Second World W a r for the location of 585
586
Radio echo studies of meteors
aircraft and ships. The radio waves are generated in a t r a n s m i t t e r T (Fig. 1) connected to an aerial system A, which can either be of simple design 6o radiate uniformly over the sky, or a large complex structure designed to concentrate the radiation in a specific direction. In the case of large aerial systems it is c o m m o n practice to use an electronic switching device, S, so t h a t the same aerial can be used to receive the PULSE GENERATII'~G CIRCUIT
TRANSMITTER T
RECORDER
TRANSNITRECEIVESWITCH
AERIAL h
RECEIVER X
FCATHODERAY ITUSE ~NDICATOR
Fig. l. Block diagram of radio echo a p p a r a t u s
radiation after scattering from the ionized meteor trail. The o u t p u t of the receiver X, is connected to an indicator or recording a p p a r a t u s I. In m a n y systems T transmits pulses of radio waves with a duration of a few microseconds at a recurrence rate of several h u n d r e d per second. The simplest form of display I is a cathode r a y t u b e indicator and the radio waves scattered from the
Fig. 2. A radio echo h,oIn a meteor trail observed on a range-time cathode ray tube display
meteor trail are t h e n observed as a transient echo as in Fig. 2. This t y p e of display gives immediately the range R of the meteor from the observing station, and the strength of the received signal as measured b y the amplitude of the echo. In recent years detailed investigations have been made of the relation between the various parameters of the radio a p p a r a t u s and the strength of the signal received from a meteor trail of given visual magnitude. The results m a y be summarized as follows.
,L G. DAVIESAND A. C. B. LOVELL
587
I f the electron density in the meteor trail is less than the critical density for the wavelength used and if the diameter of the trail is less than the wavelength, then the incident wave penetrates to all the electrons, which scatter freely and in phase giving a received power ~, where e-
PG2N223 ( e 2 ~ 2 127r2Ra \mc~ ] .
.
.
.
.
(1)
In this expression, which was first derived by LOVELL and CLEGG (1948), P is the peak transmitter power, G the power gain of the aerial system, referred to as a halfwave dipole (assumed the same for transmission and reception), 2 the radio wavelength, N the number of electrons per centimetre in the meteor trail, and (e2/mc 2) the classical electron radius. When the diameter of the trail is small compared with 4, and the volume density is somewhat greater than the critical density, the nature of the scattering depends on the orientation of the electric vector with respect to the column of ionization, and under certain conditions plasma resonance effects occur. The full analysis has been given by KAISER and CLOSS (1952). When the electron trail is so dense th at the radio wave does not penetrate, a different situation arises, and it has been shown by GREENHOW (1952a) and by KAISER and CLOSS (1952) t hat for N >> 1012 electrons per cm, (1) becomes = ~40~R ~ \ i n c h
. . . . (2)
In the first case HERLOFSON (1948) has shown t hat the echo duration to 1/e of the initial amplitude is independent of N and is given by 22 T -- 167r2D.
.
.
.
.
(3)
where D is the diffusion coefficient. In the latter case, however, the duration depends directly on N and is given by N22 T oc D-. . . . (4) MILLMAN and McKINLEY (MILLMAN, 1950) have made a detailed study of the relation between T, for echoes observed on an equipment with 2 ~ 9.1 m, and the visual magnitude of the corresponding meteor. From their results, and by using equation (4), GREENHOW and HAWKINS (1952) have found the electron line density produced by a meteor of given zenithal magnitude. The results indicate t h a t a meteor of zenithal magnitude ~ 6 produces approximately 1012 electrons per cm path, and a meteor of zero magnitude about l014 electrons per cm path. The value of D (4 x l04 cm 2 per sec.) was determined from echoes of short duration using equation (3). From these results and the above equations it is possible to determine the parameters of the apparatus necessary to detect meteors down to a given limiting magnitude. Here it is only necessary to mention t hat with contemporary apparatus working in the wavelength range from about 4 to l0 m, all meteors within the visual range of magnitudes can be readily detected with simple aerial systems, and t h a t echoes of comparatively long duration (determined by (4)) will result. With equipments of higher power, using larger aerial systems, the limit has been extended to about
588
Radio echo studies of meteors
magnitude -- 9. In these cases equations (1) and (3) a p p l y and the echo duration is only a fraction of a second. 3. THE MEASUREMENT OF METEOR RADIANTS AND VELOCITIES Radio echo a p p a r a t u s employing simple aerials and indicating systems can be used to s t u d y the incidence of meteors i n d e p e n d e n t l y of daylight or cloud. However, significant contributions to meteor a s t r o n o m y d e m a n d the measurement of meteor radiant positions and velocities. F o r t u n a t e l y , appropriate techniques were soon developed and the more i m p o r t a n t will be described briefly here. The basis of the m e t h o d of radiant d e t e r m i n a t i o n depends on the fact t h a t in the m a j o r i t y of eases the radio reflection from the ionized column is specular (HEy and STEWART, 1947; LOVELL, BANWELL, and CLEGG, 1947). This means t h a t a radio echo can be obtained from a trail only if the perpendicular from the station to the ionized p a r t of the trail lies within the aerial beam. Thus if a narrow beam aerial is used it is possible to establish the plane in which the trails of the meteors lie. This p r o p e r t y was utilized b y CLEGO (1948) in a m e t h o d for determining the right ascension and declination of an active m e t e o r radiant. The final form of this a p p a r a t u s (ASPINALLet al., 1951) is shown in a preceding article (p. 545, Fig. 4). Two aerial arrays, each producing a narrow beam are erected on either side of the hut, one directed 25 ° N of W and the other 25 ° S of W. The o u t p u t from the t r a n s m i t t e r is fed equally into both aerials. An electronic device connects the receiver for alternate pulses to the two aerials in t u r n and simultaneously the o u t p u t of the receiver is switched to separate cathode r a y tube displays which are p h o t o g r a p h e d continuously on a moving film. B y this means continuous records are obtained of the range and time of occurrence of the echoes in each aerial. Considering for a m o m e n t the idealized case of infinitely narrow aerial beams and meteors radiating from a point radiant, it will be evident t h a t in view of the specular reflecting properties of the trails echoes will be observed only when the radiant is at right angles to the beam axis. F r o m the time at which the echoes appeared in each aerial b o t h the right ascension and declination of the meteor radiant could be computed. In practice the beam widths are finite and it is necessary to consider the g e o m e t r y of the intersection of the aerial beam with the zone in which the meteors ionize. I t can be shown t h a t as the E a r t h rotates, echoes from a given radiant will first appear at short range and t h a t the ranges will increase to a m a x i m u m as tim radiant moves into a position perpendicular to the beam axis. The echo rate will then fall suddenly. After an interval depending on the declination of the radiant the process will be repeated in the aerial directed to the north west. Examples of the results obtained with this system during the s u m m e r d a y t i m e streams are shown in Fig. 3. The a p p a r a t u s has now been in regular use for several years and has given valuable information a b o u t the radiant co-ordinates and a c t i v i t y of all the d a y and night time meteor streams active in the n o r t h e r n hemisphere. First success in determining velocities b y the radio echo technique was achieved b y HEY, STEWART, and PARSONS (1947) during the great Giacobinid shower of 10th October, 1946. F o r a small n u m b e r of v e r y densely ionizing meteors t h e y succeeded in photographing the echo r e t u r n e d from the ionization near the head of the meteor and from the change of range with time the velocity was determined. Such range-time methods are restricted to relatively large meteors which create sufficient ionization to r e t u r n an echo from the vicinity of the moving head of the
J. G. DAVIES AND A . C. B . LOVELL
589
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R a d i o echo s t u d i e s of m e t e o r s
meteor. They have been largely superseded in general use by amplitude-time methods in which the diffraction of radio waves from the trail is observed. The original method was devised by DAVIES and ELLYETT (1949) using a pulsed transmitter, and subsequently McKINLEy (1951) used an alternative method with the transmitter radiating continuous waves. The principle of the method is as follows. X AMPLITUDE
0
:Fig. 4. D i f f r a c t i o n o f r a d i o w a v e s from. a m e t e o r t r a i l
The amplitude and phase of the wave scattered by an element ds at the point Y on a meteor trail X Y, Fig. 4, is given by
dA = k s i n
(V) (ot-- - -
ds .
.
.
.
.
(5)
where /c is a constant for a given trail, equation (l), ~,~/27r is the radio frequency, 2 the wavelength, and R the range of Y from the station O on the ground. From this it can be shown t hat the amplitude received from the whole length of trail X Y is
where C and S are the Fresnel integrals of optical diffraction theory taken between the appropriate limits. The variations in the value of IAI as the point Y moves along the trail are shown in Fig. 4. The values of V, the variable of the Fresnel integrals at the maxima and minima can be determined. The length of the trail between these points is then ½(V2 -- V i ) ~ R 0 ~ and the velocity is calculated fl'om
where T is the time taken by the meteor in travelling between points 1 and 2. In the pulse technique of DAVIES and ELLYETT (1949) the transmitter radiated 600 pulses per second, each of 15 #sec. duration. The first pulse received back from a meteor trail initiated the time bases of two cathode ray tubes, each of which displayed the amplitude of the receiver output. One of these time bases lasted for one-tenth of a second, and thus the first 60 pulses received from the meteor trail were shown side by side. An example of this display is illustrated in Fig. 5. The value of T was then obtained by counting the pulses between successive maxima and minima. The second time base lasted for 7 msec., and was used to determine the range
Fig. 5. The diffraction p a t t e r n o f r a d i o waves scattered from a m e t e o r trail as p h o t o g r a p h e d on a cathode ray t u b e display. The velocity of the m e t e o r is calculated from the distance between the successive m a x i m a a n d minima. The range of the meteor is m e a s u r e d on a separate display O r d i n a t c : Amplitude. A b s c i s s a : Time, increasing from right to left. (each pulse corresponds to an interval of 1/600 sec.).
Fig. 6. The a r r a y of Yagi aerials working on a wavelength of 8.2 m used in some of the e x p e r i m e n t s on the velocity distribution of sporadic meteors
592
Radio echo studies of meteors
R 0 of the echo. Wavelengths of 4 m and 8 In have been used with this technique, and aerials varying from single dipoles, radiating over a large part of the sky, to arrays of Yagis, as shown in Fig. 6, designed to give a narrow beam, for the observation of faint meteors. In each case the same aerial was used for transmission and reception. With this apparatus velocities have been obtained from meteors of magnitudes down to + 9. In the continuous wave technique used by McKINLEY (1951) the transmitter and receiver were separated by several kilometres in order to reduce the direct wave from transmitter to receiver to an amount of the same order as the wave reflected from the meteor trail. In this case the theory is complicated by the addition of the ground wave, but the practical result is that the oscillations in amplitude of the signal can be obtained both before and after the meteor reaches the perpendicular reflecting point. 4. SOME EXAMPLES OF RADIO ECHO RESULTS
The radiant and velocity measuring techniques described above have been in use at the Jodrell Bank Experimental Station of the University of Manchester since 1948, and have been applied both to shower and sporadic meteors. Some of the more notable results are described briefly below. (i) The Daytime Meteor Streams A remarkable series of meteor streams active during the daytime in the months of May, June, and J u l y were discovered in 1947, and have since been observed systematically (CLEGG et al., 1947; ASPINALL et al., 1949; ELLYETT, 1949; ASPINALL and HAWKINS, 1951 ; DAVIES a n d GREENHOW, 1951 ; ALMOND, 1951 ; HAWKINS a n d ALMOND, 1952; ALMONDet al., 1952). From a large active area in transit during the
three hours before noon, four major radiants appear; the first, near o Ceti, is active for several days around 15th May, then from 31st May to 16th June, two intense streams appear simultaneously, one in Aries, and the other near ~ Persei. At the end of June and the beginning of J u l y the fourth major radiant is active near fl-Tauri. Of these the Arietid radiant, active for three weeks, and reaching an equivalent visual rate of 70 to 80 meteors per hour at maximum constitutes the greatest annual display of meteors at the present time. In the course of its three weeks duration the radiant point moves some l 2° across tile sky, the right ascension increasing from 39 ° to 48 °, and the declination from ~- 20 ° to ~- 27 °. Several other radiants have also been found in the same general part of the sky, but these have been much less active, and have not reappeared consistently each year. The velocities of each of the four major daytime streams have been determined over several years, and by combining these with the radiant measurements, orbits have been computed. In each case the inclination of the orbits is small, so the projection of these on to the ecliptic (Fig. 7) gives a fair picture of the true orbit. The orbital periods range from 1.5 years for the o Cetids to 3.2 years for the fl Taurids, and are amongst the shortest known periods of meteor streams, being closer to those of certain minor planets rather than comets. Since the inclination of the orbits is small, the streams will pass close to the Earth's orbit again as t he y approach the Sun. In fact, it is now realized that night time streams with orbits closely similar to the daytime orbits have long been known to visual observers. Thus the Arietid stream, observed in June as it recedes from the Sun is seen again at the end of J ul y as it approaches the Sun, and is then known
J. G. DAVIES AND A. C. B. LOVELL
~ ~ .
593
urids
Arietids
Fig. 7. Orbits of some of the day-time m e t e o r s t r e a m s projected on to the plane of the ecliptic
as the 5 Aquarid stream. Similarly the ~ Perseids correspond to the Southern Arietid stream in October, and the daytime fi Taurids to the night time Taurids in November. The fi Taurid orbit is compared with the orbits of three individual night time Taurids determined by WHIeeLE'S photographic technique ( W H I P P L E , 1940; WRIGHT and WHIPPLE, 1950) in Fig. S. (ii) The Interstellar Meteor problem One of the major controversies in astronomy during the last fifty years has ariuen over the question of the origin of sporadic meteors. One group, led by 0eIK (1934a, b; 1940; 1941) and HOFFMEISTER (1948), believe that the sporadic meteors have an interstellar origin, and are merely visitors to the solar system. The other group, led by PRENTICE ([948) and PORTER (1943; 1944), believe that they move in elliptical orbits round the Sun. HOFFMEISTER'S analysis of the variation of meteor rates throughout the year and throughout the night was based on the assumption, since shown to be false by radio observations, that the meteor orbits were uniformly distributed in space ; and visual estimates of velocity, based as they are on durations usually less than a second, are notoriously inaccurate. Even the photographic results, although extremely accurate did not give a clear answer, since they were limited to 39
594
Radio echo studies of meteors
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F i g . 9. T h e v e l o c i t y d i s t r i b u t i o n o f s p o r a d i c m e t e o r s a s m e a s u r e d b y M c K I N L E Y u s i n g t h e r a d i o e c h o t e c h n i ( t u e f r o m D e c e m b e r , }948, t o M a r c h , 1950
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the brightest meteors, and the method is more sensitive to slowly moving particles. Thus in 1948, when the radio echo technique was applied to the problem in England by ALMOND, DAVIES, and LOVELL, and in Canada by McKINLEY, there were wide divergencies of opinion as to the orbits of the sporadic meteors. In order to determine whether the orbit of a meteor is elliptical or hyperbolic, it is necessary to know its heliocentric velocity. Unfortunately, no radio technique qO
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\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\
20-
I0
\\
X\\ \\N 0
I I0
b,\\
20
~0
40
SO
bO
70
80
I qO
Fig. 10. V e l o c i t y d i s t r i b u t i o n o f sporadic m e t e o r s as m e a s u r e d in t h e e x p e r i m e n t s o f ALMOND, DAVIES, a n d LOVELL. This distribution refers to the m e t e o r s w i t h radiants close to the a p e x of t h e E a r t h ' s w a y , m e a s u r e d during the a u t u n m of 1950. The s m o o t h c u r v e s h o w s t h e v e l o c i t y d i s t r i b u t i o n calculated on the a s s u m p t i o n t h a t t h e m e t e o r s are t r a v e l l i n g in parabolic orbits in r a n d o m directions
yet used will give this for individual meteors, since the radiant point can only be determined for meteor showers. Therefore statistical experiments have been performed on groups of meteors, and the resulting histograms compared with those expected from different values of the heliocentric velocity. Since heliocentric velocities in excess of 42 km per sec. would indicate hyperbolic orbits, and the Earth's orbital velocity is about 30 km per sec., the greatest geocentric velocity which can be observed for a meteor moving in an elliptical orbit is about 72 km per sec. Velocities in excess of this limit would indicate the existence of hyperbolic meteors. McKINLEY (1951), using his continuous wave technique and dipole aerials, measured velocities for periods of 48 hr or 72 hr each month for 15 months. In this time he observed more than 10,000 velocities, the distribution being given in Fig. 9. The effect of the major showers is clear in some of the histograms. Of these
59[;
Hadio echo studies of meteors
only t h i r t y - t w o meteors had velocities in the range 75-79 k m per sec., and there were none with velocities above 80 k m per see. The errors of measurement were such t h a t in no case could a n y meteor be definitely stated to have a hyperbolic velocity. Simultaneously A L M O N D , D A V I E S , and LOVELL ( 1 9 5 l ; 1952; 1953) started a series of experiments, which lasted for four years, using the pulse technique and a b e a m e d ~erial system directed to receive echoes from meteors with radiants close to the apex of the E a r t h ' s way. In this case the velocity distribution to be expected from meteors travelling in parabolic orbits should show a peak close to 72 k m per sec. In all, S60 such meteors were observed, the distribution for one such e x p e r i m e n t being given in Fig. I0. A second group of experiments, receiving meteors from the
15 14 12 N IO 8 b 4 2 0
Fig. 1 l.
I I0
I
i
I
I
1
20
30
40
SO V
bO
I 70
I 80
Velocity distribution of sporadic meteors as m e a s u r e d in tile e x p e r i m e n t s of
ALMOND, DAVIES, and LOVEIA. This distribution refers to tho meteors with r a d i a n t s close to th~ antape,~ of the E a r t h ' s way, m e a s u r e d during the spring of 1951. Tile s m o o t h curve shows the velocity distribution caleulate:t oll the a s s u m p t i o n t h a t the meteors are travelling in parabolic orbits in r a n d o m directions
region of the a n t a p e x gave the distribution of Fig. l l, and p r o v e d t h a t the high velocity cut off observed in the main experiments was not due to i n s t r u m e n t a l limitations. The peak of the observed distributions is in all cases some l0 k m per sec. lower t h a n t h a t derived from the assumption of parabolic velocity, and it is concluded t h a t the m a j o r i t y of the sporadic meteors move in orbits of short periods, similar to those found for the d a y t i m e showers. As in MCKINLEY'S experiment, no definite ease of a hyperbolic velocity was found, and the results are overwhelmingly in f a v o u r of the view t h a t sporadic meteors are localized in the solar system. (iii) Problems in Meteor Physics In addition to astronomic~fl problems, such as those discussed above, the radio echo technique has been applied to the s t u d y of meteor ionization and the physics of the upper atmosphere. The m e a s u r e m e n t of high altitude winds provides an illustration of the value of this technique. Meteors of zenithal magnitude brighter t h a n a b o u t + 6, produce echoes whose duration is proportional to the electron density in the tr~il, and which m a y last for m a n y seconds (equation (3)). The violent fluctuations in echo amplitude observed under these circumstances have been studied b y GREE~-HOW (1952b). An example, observed on both 4 m and S m is shown in Fig. 12. I t will be seen t h a t the period of fluctuation is closely proportional to the wavelength. I m m e d i a t e l y after formation the meteor trail is subjected to the motion of the
3. G. DAVIES AND A. C. B. LOVELL
597
winds in the 80 k m region where the trail is formed. These winds are b o t h high and t u r b u l e n t , and the trail rapidly gets distorted out of its initially straight condition. U n d e r these circumstances more t h a n one section of the trail m a y produce reflection, and these sections m a y move with different radial velocities relative to the observing station. The signals received from different parts of the trail will t h e n interfere, and produce the a m p l i t u d e fluctuations observed. F r o m the period of these fluctuations the degree of turbulence can be inferred, and velocity gradients of the order of 5 m per sec. per k m are usually observed for points separated b y 5-12 km. In addition, b y observing the slow drift in range of the long duration echoes, Greenhow was able to measure the radial c o m p o n e n t of the velocities of individual echoing
4.2rn ECHO AMPLITUDE
8.4 m ECHO AMPLITUDE
TIME
Fig. 12. The thmtuations in amp|itude in a radio echo fl'om a meteor trail photographed simultaneously on wavelengths of 4.2 m and ~.4 m. The fluctuations are caused by winds in the high atmosphere centres. Changes in range of 50 m were observable and drifts of as m u c h as a kilometre in 20 sec. have been recorded. An average value of the winds observed b y this m e t h o d is 50 m per sec. 5. THE FUTURE OF METEOR ASTRONOMY I t must be a d m i t t e d t h a t in spite of the great advances made b y the radio and photographic techniques during the last few years the existence of meteors remains an enigma. The c o m e t a r y association of the Lyrids, Lconids, and Perseids was established eighty years ago and in the intervening period the list of associations has not increased greatly. I n fact, only the association of the night time Taurids and (tay time fl Taurids with ENCKE'S Comet, and of the Giacobinids with the GiacobiniZinner Comet, can be said to be firmly established. The relationship, if any, of the Aquarids and Orionids with HALLEY'S comet remains v e r y problematical. As regards the other major showers the recent measurements of their orbits has created an e x t r e m e l y puzzling situation. Whereas it was believed with some confidence t h a t c o m e t a r y associations could be found when the orbits were known, this now seems to be impossible. The Geminids, Quadrantids, and the s u m m e r d a y t i m e streams have orbits with unique characteristics, in some cases of shorter period even t h a n the asteroidal bodies. The p a r e n t a g e of these streams now presents a problem of great interest. Although the major streams provide the most notable events in meteor a s t r o n o m y , their contribution to the total n u m b e r of meteors entering the a t m o s p h e r e is smaller
598
Radio echo studies of meteors
t h a n t h a t of t h e s p o r a d i c m e t e o r s . I n v i e w of t h e r e c e n t m e a s u r e m e n t s r e f e r r e d to a b o v e , i t c a n s c a r c e l y be d o u b t e d t h a t t h e s e s p o r a d i c m e t e o r s are m o v i n g i n closed o r b i t s a r o u n d t h e S u n , a n d , i n fact, t h a t a large p e r c e n t a g e o f t h e o r b i t s m u s t be of s h o r t period. T h e s o l u t i o n of t h e p r o b l e m of t h e o r b i t s m e r e l y serves to raise a g a i n t h e p r o b l e m of t h e i r origin. Are t h e y d i s p e r s e d fl'om c o m p a c t s t r e a m s w h i c h were o n c e m o v i n g i n o r b i t s like t h e G e m i n i d s a n d t h e s u m m e r d a y t i m e s t r e a m s , or h a v e t h e y a n i n d e p e n d e n t o r i g i n f r o m t h e f o r m a t i o n o f t h e solar s y s t e m ? F r o m t h e s e r e m a r k s it will be a p p r e c i a t e d t h a t all t h e f u n d a m e n t a l q u e s t i o n s i n m e t e o r a s t r o n o m y r e m a i n u n s o l v e d . A l t h o u g h s o m e c o m e t s a n d m e t e o r s t r e a m s are a s s o c i a t e d , t h e n a t u r e of t h e a s s o c i a t i o n is u n k n o w n . D i d t h e c o m e t s c o m e before t h e m e t e o r s , do c o m e t s f o r m m e t e o r s , or m e t e o r s c o m e t s ; or are t h e c o m e t s a n d m e t e o r s m o v i n g i n t h e s a m e o r b i t s w i t h a s t a b l e r e l a t i o n s h i p ? Are t h e s h o r t p e r i o d s t r e a m s t h e r e m a i n s o f a u n i q u e f a m i l y of c o m e t s w h i c h h a s n o w d i s a p p e a r e d , or of disi n t e g r a t e d m i n o r p l a n e t s ? F i n a l l y , was t h e large s p o r a d i c m e t e o r c o m p o n e n t o r i g i n a l l y r e l a t e d t o t h i s t y p e of m e t e o r s t r e a m ? T h e r a d i o echo s t u d i e s of m e t e o r s are s t i m u l a t i n g a n e w i n t e r e s t i n t h e s e f u n d a m e n t a l p r o b l e m s of m e t e o r a s t r o n o m y a n d t h e n e w t e c h n i q u e s are o p e n i n g fresh fields of w o r k i n m e t e o r p h y s i c s a n d t h e s t u d i e s of t h e h i g h a t m o s p h e r e .
REFERENCES ALMOND, M.
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1952 1953 ASPINALL, A., CLEGG, J. A. and LOVELL, A. C . B . ASPINALL, A., CLEGG, J. A. and HAWKINS,G. S . . ASPI~CALL,A. and HAWKINS,G. S. . . . . . CLEOG, J. A . . . . . . . . . . . . CLEGO,J. A., HUOHES,V. A. and LOVELL, A. C.B. DAVIES, J. G. and ELLYETT, C. D . . . . . . DAVIES, J. G. and GREENHOW,J. S. . . . . ELLYETT,
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1949
1951 1951 1948 ]947 1949 1951 1949 1952a
GREENHOW, J. S. and HAWKINS,G. S. . . . . HAWKINS, G. S. a n d ALMOND, M. . . . . . HEY, J. S. and STEWART,G. S. . . . . . . HEY, J. S., STEWART,G. S. and PARSONS, S.J. HERLOFSON, N . . . . . . . . . . .
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HOFFMEISTER, C .
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KAISER, T. R. and CLOSS, R. L . . . . . . .
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A. C. B., BANWELL, C. J. and CLEGC~, J.A. LOVELL, A. C. B. and CLEGG, J. A. . . . . .
McKINLEY, D. W. R . . . . . . . . . . MILLMAN, P. M . . . . . . . . . . . 0PIK, E. J . . . . . . . . . . . .
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1948 1951 1950 ] 934a 1934b 1940 1941 1943 1944 1948 1940 1950
M.N., 111, 37. Jodrell Bank Annals, 1, 13. M.N., 111, 585. M.N., 112, 21. M.N., 113, 411. M.N., 109, 352. Phil. Mag., 42, 501. M.N., 111, 18. Phil. Mag., 39, 577. M.N., 107, 369. Phil. Mag., 40, 614. M.N., 111, 26. M.N., 109, 359. Proc. Phys. Soc., 65, B, 169. J. atmosph, terr. Phys., 2, 282. Nat~re (London), 170, 355. Jodrell Bank Annals, 1, 2. Proc. Phys. Soc., 59, 858. M.N., 107, 176. Phys. Soc. Rep. Prog. Phys., 11, 444. Die Meteorstr6me, Weimar. Phil. Mag., 48, 1. M.N., 107, 164. Proc. Phys. Soe., 60, 491. Astrophys. J., 118, 225. J. Roy. Ast. Soe. Can., 44, 209. Circ. Harv. Coll. Ob.~., I~o. 389. Circ. Harv. Coll. Obs., No. 391. Publ. Tart~¢. Obs., 8{), No. 5. Publ. Tart~z. Obs., 80, No. 6. M.N., 103, 134. M.N., 1{)4, 257. Phys. Soc. Rep. Prog. Phys., 11, 389. Proc. Amer. Phil. Soc., 83, 711. Teeh. Rep. Harv. CoU.Obs., No. 6.