Knowledge of altitude and ionization curves of faint meteors is of great importance for the calculation of meteor detectability during radar observations (Greenhow and Hall, 1960; Lebedinec, 1963). Greenhow (1963), using the simplest physical theory of meteors (Herlofson, 1948) for altitude calculations, arrived at a conclusion that radar observations of very faint meteors were impossible for practically any radar power. The simplest physical theory of meteors is used for calculations of radar-meteor detectability by almost all the authors. Lebedinec (1963) has shown that this theory is inapplicable in the case of faint meteors since one should take into account energy losses caused by thermal radiation from the meteoroid surface.

Ablation profiles for compact meteoroids have been obtained which take account of the effects of thermal radiation, conduction, meteoroid heat capacity and deceleration, while retaining the simplifying assumption that the onset of massive ablation occurs at a well-defined temperature.

The dependence of the air density, ρa, at the onset of ablation on the initial meteoroid radius, R0, is in agreement with the results of Ceplecha and Padevět (1961). For a meteoroid of given composition, velocity and zenith angle there are four critical values for the radius, R1, R2, R3 and R4. R1 is the micrometeorite limit such that if R<R1, the ablation temperature is not reached. For R1<R<R2 the onset of ablation is delayed due to thermal radiation and occurs at air density ρa1, which is substantially independent of radius. If R2<R<R3 the delay in onset of ablation is due to the finite heat capacity of the meteoroid; it occurs at an air density ρa2, which is proportional to R0, i.e. to m01/3, where m0 is the initial mass. If R > R3 the meteoroid develops a marked thermal gradient and, if it is brittle, will fragment before ablation commences. The smallest fragments will ablate at the air density ρa1, which is shown to be close to the experimental result of Jacchia et al. (1967) for photographic meteors. If such a particle survives fragmentation, the molten droplet will fragment due to distortion if R > R4 (Lebedinec and Portnjagin, 1966); it is shown that R4 is not substantially different in magnitude from R3. Meteoroids in the size range R2<R<R3 produce underdense meteor trains and experimental measurements of echo decay as a function of echo amplitude are in agreement with the expectation that ρa2 ∝ m0n where n ~ 1/3 (Poole and Kaiser, 1967). This would support the view that the bulk of radio meteors are produced by compact particles and that the fragmentation observed with photographic meteors may be a result of thermal shock.

The ablation profiles have been computed for initial meteoroid velocities between 20 and 70 km sec−1 and radii between R1 and R3; the deceleration and residual size (at the end of ablation) have also been calculated. Even quite close to the micrometeorite limit, the residual particle is small compared with the initial size. Hence, meteoroids with R > R1 will make negligible contribution to the micrometeorites reaching the Earth's surface, except, perhaps, where a significant proportion of the mass of larger meteoroids fragments into particles with radius less than R1. By neglecting deceleration, it is possible to arrive at an analytic expression for the ablation profile; this is found to be in good agreement with the computed profiles except for particles near to the micrometeorite limit.

The separation of very small particles from meteor bodies as a part of the process of their ablation in the Earth's atmosphere is assumed by many investigators. But when the particle is small relative to the meteor body the path of the particle is a priori thought to be small relative to the length of the meteor path, i.e. the particle is expected to evaporate in close proximity to the point of separation. This assumption is not acceptable after McCrosky's work (1958) that has shown for one particular case the small particles (m = 10−5−10−6 g) go a long path before their mass is exhausted. The assumption made until now has caused incorrect conclusions, e.g. the luminosity at any point of the meteor path is proportional to the mass ablated from the meteor body at that point (Kramer, 1965).

The relation of physical characteristics of meteors to their orbital elements is investigated using Harvard Super-Schmidt data. A set of characteristic indices is defined, allowing for the effects of geocentric velocity, angle of incidence, magnitude and mass, wherever a correction appears appropriate according to the correlations found by Jacchia et al. (1967). The medians for representative meteor samples are plotted in the semi-major axis/eccentricity diagram and the distribution of each parameter is derived. Although the differences are moderate compared to the measuring errors, six regions of different nature can be distinguished.

The existence of two families of asteroidal meteors is indicated, one of them brought to crossing with the Earth's orbit by drag effects and the other by collision effects in the main asteroid belt. These meteors are characterized by low and uniform beginning heights, high fragmentation, low ablation, low deceleration, and bright wakes. A direct counterpart to this is represented by meteors moving in short-period orbits of higher eccentricity and shorter perihelion distance, which bear resemblance to the long-period and retrograde cometary meteors. Meteors with perihelion distances of less than 0·15 AU tend to resemble the bona fide asteroidal meteors by a progressive increase of fragmentation and decrease of reduced beginning heights and decelerations as the perihelion approaches the Sun. This is attributed to the selective destruction effects of solar radiation.

With the exception of the Draconids, the mean characteristics of meteor showers agree well with those of sporadic meteors moving in similar orbits. It is suggested that the Draconid stream includes a broad variety of meteoric material and that the two peculiar Super-Schmidt meteors on record represent only the less resistive, short-lived component which has already been eliminated from the other showers.

Possible origins for hyperbolic meteor orbits are noted, and photographic evidence on the nature of meteoroid structure is discussed.

Regular and numerous measurements of individual radiants and velocities of meteors were carried out in Jodrell Bank (Davies and Gill, 1960), Kharkov (Kaščeev et al., 1960; Kaščeev and Lebedinec, 1961), Adelaide (Nilsson, 1964) and as part of the Harvard Radio Meteor Project in U.S.A. (Hawkins, 1962). Observed distributions of radar meteor orbits obtained from a great number of systematic observations have been published up to now only at Jodrell Bank (Davies and Gill, 1960) and Kharkov (Kaščeev et al., 1960; Lebedinec and Kaščeev, 1966). 2474 orbits for meteors about + 5m to + 7m have been obtained in Jodrell Bank, 12500 orbits for meteors brighter than about + 7m have been obtained in Kharkov.

New photographic observations of meteors in the brightness range −5 > Mp > −18 (?) and with a median value of M = −8·5 have yielded orbits with a precision of better than 1° in the angular elements and of about 0·05 in e and 1/a. A comparison of 100 of these fireball orbits with Super-Schmidt orbits shows:

1. (1) A class of orbits, comprising about 25% of the faint meteors, of long period, is almost completely absent among the fireballs.

2. (2) Small perihelion distances (q < 0·2 AU) are not found. A maximum at q ~ 0·7 AU, which also appears in the Super-Schmidt orbits of short period, corresponds to the perihelion distances of all but two of the Apollo objects.

3. (3) The distribution of aphelia shows a marked decrease at Jupiter when compared to a similar distribution of small, faint meteor orbits. With the existing material, we cannot determine whether this represents an important evolutionary distinction between meteoroids of different mass or is simply the result of an inappropriate choice of the division between ‘large’ and ‘small’ orbits in the faint meteor data.

Provisional values of the bulk density of 28 of the fireballs as determined from the photometric mass and the observed mass-area ratio indicate that these meteoroids do not differ significantly from those observed with the Super-Schmidt cameras. Evidence of gross fragmentation of these large objects at high elevation and evidence of small or negligible terminal masses suggest that the material is friable. There is no certain evidence that any appreciable fraction of fireballs is produced by high-density, cohesive material.

If the above analysis is correct, we should expect an occasional (1 year−1 106 km−2?) meteorite fall of discoverable size originating from a low-density meteoroid.

1. Radio meteor rates for a calculated mass range 10−6 gm to 10−4 gm have been recorded semi-continuously at Havana, Ill., since July 1965. Automatic equipment samples the rate at four different levels of sensitivity each half hour. A flux vs. mass power law has been derived from these data for each of a number of different weeks of observation. Between September 1965 and December 1966 the mean cumulative influx I of meteors/m2/sec/2π ster can best be described by the equation: where m is the lower mass limit in grams.

2. A micrometeoroid detection system, which hopefully was to have measured particle velocities and directions of arrival for masses greater than 10−12 gm, has been flown on the OGO-II satellite. A comprehensive in-flight calibration system has confirmed the correct operation of the experiment for more than one year in space. 700 hours of data have been analyzed and no micrometeoroid events have been observed. This excludes several hundred spurious events from the microphone sensors. The effective detection area of the instrument is 0·8 cm2 ster. Thus, to a probability of 0·86, the average flux of particles of mass greater than 10−12 gm must be less than 6 × 10−2 particles/m2/sec/2π ster.

A research is described which had the purpose of deriving information about the physics and astronomy of sporadic meteors from the range distribution of radar echoes from meteor trails experimentally observed under given conditions.

The method is based on the comparison of the experimental distribution with a number of different theoretical distributions, corresponding to different sets of assumptions about the characteristics of sporadic meteors and the law of trail formation.

The quantities which can be derived by this method are: (1) the distribution of heliocentric radiants, (2) the type of meteors (stone, iron, dustballs), (3) the distribution of heliocentric velocities.

In the present paper are described the results of the first part of the theoretical computations. Evidence is obtained that, in order to fit the experimental curve, both the uniform distribution of heliocentric radiants over the celestial sphere and in the plane of the ecliptic must be taken into account.

The seasonal changes in the sporadic radiant distribution are investigated on the basis of the complete data for different classes of echo duration from the Ottawa 32·7 MHz radar taken over the years 1958–62, using a more accurate response function for the Ottawa radar equipment recently determined by McIntosh (1966). A model of the radiant distribution with four point sources (i.e. apex, helion, antihelion, and toroidal) and a uniform background is assumed. The strengths of the sources have been calculated by a least-squares fit to the observed mean hourly rates in 5-day intervals. The seasonal variations have different trends for meteors with different echo duration, showing stronger shower properties for long-duration echoes (4–8 sec). Meteors with short duration echoes (≤ 1 sec) show the same properties in radiant distribution, as was found in a previous paper based on preliminary Ottawa radar records. The toroidal source is without significant activity from August till November and shows a maximum in January. The diurnal variation calculated from this model is compared with the observed hourly rates. Some recent calculations have been made by Veverka and McIntosh (unpublished) using a continuous source model, elliptical in ecliptic longitude and exponential in latitude. The agreement of their results with observed diurnal rates is not as good. This suggests that the seasonal changes in the sporadic radiant distribution have a significant influence on the diurnal and annual variations of the hourly meteor rates.

Some results concerning the sporadic radiant distribution – or rather the sporadic radiant efficiency – obtained from meteor forward-scatter data are presented.

The works of E. Biot, published by the Institut National de France in 1848, made it possible to study material recorded in volumes 191 and 192 of the well-known 13th-century Encyclopaedia of Ma Touan-lin as well as records from other sources. They contain observational data of 24 centuries (especially from the 11th century) on more than 1500 fireballs, with descriptions of their positions with respect to the stars as well as descriptions of their physical, kinematic and other properties. The observation dates of the lunar calendar have been converted by Biot into dates of the Julian Calendar.

We have been able to process data on 1220 fireballs. As a result of this radiants were obtained for 153 meteor showers, seven of which belong to great showers. Out of the remaining 146 radiants of the minor showers, 80 radiants are more certain than the remainder.

The radiants were deduced from observations on dates recorded in short intervals from several years to several decades. First the dates of visibility were obtained along with the activity and radiants of great showers which are still active. In the Leonid shower, with retrograde motion, a shift of visibility dates to a much later period has been noted corresponding to a forward motion of the orbit's node, whereas a retrograde motion of the node is observed in the Quadrantids (i < 90°). In the Lyrids and Perseids, whose orbits are nearly perpendicular to the ecliptic plane, the nodes experienced no perturbations, and the visibility epochs for the showers remained the same during a period of 1000 years and longer. The motion of apsides resulted in a shift of the radiant; the increase of the ecliptical latitude indicated secular augmentation of the orbit's inclination (Geminids, η-Aquarids, Orionids, Leonids). The radiant of the Perseids was located in Cassiopeia, where the radiant of the present-day Cassiopeids is to be found. It appears that the Perseid stream began to cross the orbit of the Earth in 830 A.D.

In the δ-Aquarids the North branch was active, while there is no evidence that the South branch had existed earlier than 900 years ago. The Virginids, Librids, Scorpionids, Sagittarids and Aurigids were quite appreciable and their studies furnish much interesting data. Particularly active were the Taurids; their North and South branches were observed over 1000 years back. The South Taurids were about half as active as the North Taurids (at present this relation is reversed). Very active were the Cygnids (July–August), which presented at that time a compact shower, now disrupted into a series of minor showers with radiants spread over a large area of the celestial sphere. Of definitive interest is the radiant of the great meteor shower observed in 1037 (August 21 by the Julian Calendar, September 9 by the Gregorian Calendar, 1950–0), α = 324°, δ = + 1°(1950–0).

Some of the showers active in these early centuries are now unknown; on the other hand, some showers which are well known now were not observed in the Middle Ages. In the past millennium only those streams have survived whose orbits were so situated with respect to the orbits of the outer planets, that they were not subjected to any considerable perturbations produced by these planets.

The first part of the paper reviews the present state of knowledge of the characteristics of cosmic dust in interplanetary space. Since this is derived from a variety of observational techniques, some attempt is made critically to assess the difficulties in interpretation, particularly those due to differences in observational selection. Attention is drawn to the doubts that recently have arisen concerning the existence of a terrestrial dust cloud. The second part describes some radio investigations of the structure of meteor streams and of the sporadic background. Systematic variations in magnitude distribution with solar longitude which are observed in both the Geminids and Perseids cannot be simply interpreted as due to selective perturbation of the smaller meteoroids. Experimental data are described which point to the existence of considerable radiant structure in the sporadic background.

There are many problems in deducing the meteor mass distribution from radar echo observations. Some of these are reviewed in this paper, particular emphasis being placed on the effects of electron attachment and of equipment parameters on the statistics of long-duration echoes.

Theoretical hourly rates of radar echoes from meteor showers observed with narrow- and wide-aperture systems are calculated in terms of equipment parameters and the distribution of the number of meteors as a function of mass. From a comparison of the echo rates in selected range intervals, it is shown that it is possible to determine the value of the exponent in a simple inverse power flux law.

The effect on the echo rate of a change in the value of the exponent in the flux law with magnitude is described and the probability of detecting showers at different stations is discussed.

It is suggested that simple radar echo counts in several range intervals can be used to determine the mass distribution of meteor showers over a range of two or three magnitudes above the limiting magnitude of the system. The analysis is applied to some observations made at Christchurch and Ottawa.

The amplitude of meteor echoes is recorded on a logarithmic scale by a high-power radar equipment (λ = 9·2 m, PT=3 MW, G = 5·6) at Springhill Meteor Observatory near Ottawa. The smallest amplitude measured corresponds to a pulse power of 10−12 W, which represents a minimum electron line density of about 7 × 1011 el/m or a radio magnitude of + 10.

Distribution curves of number of echoes as a function of echo power have been obtained from some 50 samples of 500 meteors each, at various times of day on about 1 day per month. The slopes showed little variation throughout the year. The statistical error in the slope value for any one sample was small, ~ 2–3%. However, determination of the mass index s from these slopes involves several problems. On the basis of simplest theory we have obtained for the sporadic background, with no definite seasonal or diurnal variation.

During shower periods, lower values of s were obtained. For the 1966 Leonids, s for the shower was determined by estimating the percentage of shower meteors in the total sample. A value s = 1·7 ± 0·1 was obtained as the mean of 6 samples. It is not known to what extent the height-ceiling effect influences the observation of this shower.

At the Engelgardt Observatory of the V.I. Uljanov-Lenin Kazan State University systematic azimuthal radio echo observations are carried out at a wavelength of 8 m. Simple radio echo equipment is used with a five-element Yagi antenna. The observational program provides for a 30° turn of the antenna in azimuth every 5 min.

18 years have elapsed since the first data on meteoritic dust were obtained by means of the instruments installed on a rocket under the guidance of Professor Bohn. Since then these investigations, by means of different instrumentation, have been carried on in the vicinity of the Earth and in interplanetary space. By now rocket and satellite data have been obtained on the spatial density of meteoritic particles with masses of 10−7–10−16 grams and on their energy; a concept of the mass spectrum of these particles has been obtained as well as the first data on their velocities. Over recent years very difficult experiments have been initiated for obtaining samples of meteoritic particles directly from high altitudes with a view to determining their physical properties, chemical composition, and the destroying effect.

Our present knowledge of meteor streams is based on three types of meteor showers:

1. (1) About one or two dozen major showers that recur annually in optical and radar records of meteor activity.

2. (2) An indefinite number of minor showers, hardly distinguishable from the sporadic background, which can be traced only by comparing individual meteor orbits.

3. (3) Rare appearances of temporary showers, sometimes producing extraordinary meteor displays. These include prominent concentrations of meteors within some of the annual streams, meeting the Earth under specific conditions.

Southworth and Hawkins (1963) described a criterion for distinguishing stream meteors from sporadics. A function D of the orbital elements was proposed as a measure of the difference between two orbits. A sporadic meteor was then defined as one whose difference D from all other observed meteors exceeded an empirically determined minimum. Any two meteors that differed by less than the minimum were defined to be members of the same stream.

D may be taken to be a distance in a five-dimensional space, whose coordinates are orbital elements. It is now proposed that a stream would be better defined as a statistically significant concentration of meteors in this orbit-space. The procedure has been programmed for a computer, and applied to the Harvard-Smithsonian radar observations.