We discuss the significance of our zodiacal light models, which are based on Mie theory and refractive indices measured in the range 0.15 μm ≤ λ ≤ 100 μm. The models include scattering and thermal emission. They are absolute, since particle number density at 1 AU, size distribution and radial density gradient in the Ecliptic are fixed by lunar microcrater counts and by Helios and Pioneer results. The only free parameter is the chemical composition of the dust.

Meteoroid ablation in an atmosphere produces a weak plasma column. The processes important in governing the state of the meteoric species subsequent to column formation are reviewed. The actions of transport and of chemical reactions in controlling the life of the various species in the column are described. It is found that the interaction of meteoric ions with atmospheric gases leads to the dissipation of ionization and is the source of anomalous radio-meteor echo characteristics: recognition of these effects is important in permitting meaningful interpretations of radio-meteor data. Several modes of interaction between meteoric species and an atmosphere can yield enduring emissions. The fate of the various species is discussed.

The majority of radio studies of meteors have been carried out at frequencies higher than 17MHz and most of the rate observation at frequencies above 30MHz. At these frequencies a severe height selection of meteors occurs. In Figure 1(a) are shown the normalized height distributions of sporadic meteors observed at Adelaide on frequencies of 27MHz and 2MHz (Brown, 1976). The sharp cutoff of the latter distribution below 87 km is instrumental. The difference in the height distributions is due to the effect of the finite diameter of a meteor trail on its radar detectability. If the trail diameter is ≪ λ signals from the near and far edges reinforce but as the trail expands due to diffusion and the diameter becomes ≃ λ/4, interference reduces the amplitude. A meteor trail, produced by a particle with a velocity of 30 km s−1, has an initial diameter of 0.4m at 80 km, 2.0m at 104 km and 4.0m at 116 km.

Fluctuations in meteor rates. A fundamental concept in meteor astronomy is the rate of occurrence of meteors (hourly rate). The diurnal and seasonal behaviour of meteor radar rates has been the subject of many studies in the past. In these studies the tacit assumption is made that the observed rate variations are of an astronomical nature. Radar rate changes of an instrumental nature, or rate variations induced by changes in the atmospheric conditions, are generally neglected.

A problem commonly encountered in meteor studies is the need for a realistic interpretation of statistical data which contains uncertainties in two variables. A simple solution is provided and for illustration applied to diffusion-height data.

Difficulties in the interpretation of the meteor phenomenon have been mainly caused by the impossibility of dividing in time and space the information obtained from the observations. To a certain extent the use of the method of instantaneous exposures can help to overcome these difficulties. The present paper deals with the study of a bright meteor N 770533, broken into many fragments and its spectrum, which was doubly photographed in 1977, July, 19h 30m 56s UT in Dushanbe (Figure 1). The photographs were taken by the method of instantaneous exposures, which has been described by Babadzhanov and Kramer (1968). To take the photographs of spectra, transparent replica diffraction gratings with light concentration in the first order have been used. Rotating shutters made it possible to obtain meteor images in integrated light with duration of 0.00056s every 0.02s, as well as to obtain spectra with durations of 0.00056s and 0.0033s every 0.02s. A panchromatic emulsion with spectral sensitivity 3800 to 7000 Å was used.

Single station studies cannot provide detailed information such as zenith angle, velocity, heights and magnitude for individual meteors and Ceplecha (1976) has stressed the need for two station television observations which could provide such information through triangulation analyses. This paper deals with some of the results of the first two station television intensifier study of faint meteors, and the implications of these results concerning the validity of the current theories of the structure and ablation of dustball meteors.

Early visual observations of meteor spectra were instigated by A.S. Herschel and, in the period 1866-1880, resulted in the recording of several hundred spectra and the correct identification of the lines of neutral sodium and magnesium. Some 60 meteor spectra were photographed from 1897 to 1940, which resulted in the identification of 9 neutral and 4 singly ionized atoms. Improved cameras and techniques after World War II, and in particular the use of closed-circuit television equipment recording on video tape, increased the data-bank of meteor spectra to several thousand and made possible the identification in these spectra of between 15 and 20 neutral atoms, 9 singly ionized atoms and 6 diatomic molecules. Reliable luminous efficiencies of the atoms and molecules radiating in the observed light of meteors are available in only a few cases. Where relative abundances of the elements have been calculated, these agree in general with those found by other techniques for interplanetary dust, and with abundances for the solar system as a whole.

During the 1977 and 1978 Perseid showers, seventeen meteors were simultaneously photographed with a spectrograph and with a camera equipped with a rotating shutter. Of the eight best examples, the half in whose spectra the forbidden line of neutral oxygen at λ5577 was relatively strongest appeared and disappeared at heights about 9 km. greater than did the half in whose spectra the line at λ5577 was relatively weak. Structural or compositional differences in the meteoroidal particles appear to be the most likely explanation for these height variations. A by-product of the investigation was an average value of −8.8 km/sec2 for the deceleration of the meteors over the observed trajectories.

We study the long-term evolution of the orbits of meteoric particles subjected to planetary perturbations and to Poynting-Robertson drag. Solar wind erosion of the particle is considered. We compute the long-period elliptic elements of the meteoric orbit when it intersects the orbit of the Earth. We compare experimental results with those of the dynamical study.

The North American Air Defense Command (NORAD) has the responsibility of detecting and tracking artificial satellites in Earth orbit. Objects detected with a velocity in excess of the escape velocity of Earth are normally disregarded by the ground radar since they are obviously not orbiting satellites. During a special test conducted by NORAD for NASA to lower the detection threshold for small satellites, part of the radar software was modified, resulting in acquisition of what appears to be important new meteoroid data. The purpose of this paper is to describe the special test and give a preliminary analysis of the meteoroid data.

Continuous models of the meteoric background complex are discussed and analysed on the basis of observational data on radiants and hourly rates of sporadic meteors. A time-dependent, bi-elliptical model of the true radiant distribution fits the observational results.

The evolution of meteorid orbits over long time periods has been studied using the Gauss-Halphen-Gorjatschew method taking into account perturbations from the outer four planets (Galibina 1970). As initial elements, 15 different meteor orbits were taken from photographic observations (McCrosky 1968). Calculations were carried out from the modern epoch back in time for intervals of 10000 to 100000 years. Initial elements and the time intervals are given in Table 1.

Data on 240,000 meteor echoes recorded in 1958-1967 with the Springhill Meteor Radar, have been used for determining the structural features of the Eta Aquarid stream. A zone with a distinct enhancement in meteor rates was located at solar longitudes of 43° to 47°. The core of the stream, corresponding to the four peak days, extends from the orbit of the comet to a distance of 0.08 A.U. Other observations, from both hemispheres, support these results.

At the present time the orbit of the Quadrantid meteor stream not only intersects the orbit of Earth but also passes very close to the orbit of the planet Jupiter. This causes considerable perturbations. In a series of three papers (1,2,3) the authors replaced the myriad of meteoroids in the stream by ten test particles set at equal intervals of eccentric anomaly around the orbit. The equations of motion of these particles in the solar system were solved using a standard fourth order Runge–Kutta technique with self–adjusting step lengths. The orbits of the test particles were output at ten year intervals going back from the present to the year 300 B.C. and forward into the future to the year A.D. 3780.

The Geminids and Quadrantids belong to the most active annual meteor showers. The distinct difference between the orbit sizes of large and small meteoroids in these streams has been shown. From this, the estimated age of the streams greatly exceeds the time since their discovery in the XIX century (Geminids in 1862, Quadrantids in 1830) (Lovell 1954, Hindley 1972).

The Taurids, Geminids and Quadrantids represent meteor streams existing at different stages of stream evolution. This can be inferred from the duration of their activity, more than a month for Taurids, a few days for Geminids and only a few hours for Quadrantids. The structure of the streams can be deduced from a study of the variations in incident flux of shower meteors and their mass exponents as a function of solar longitude. This paper is an attempt to deduce such structure from recent observations in India combined with data available for these showers from the early years of this century (Lovell 1954, Olivier 1960, Rao et al. 1975,1976, Webster et al. 1966).

The Southern Delta Aquarid meteor shower which is believed to be of relatively recent origin because of its compact photographic radiant, is also possibly associated with the Arietid meteor shower (Kresak and Porubcan 1970). Williams, Murray and Hughes (1979) have studied the orbit of the Quantrantid stream and it is interesting to note that the orbital parameters of that shower approximately 1500 years ago were similar to the orbit of the present day Delta Aquarids.

Recent theoretical concepts of large meteoroid entry into the atmosphere are compared and an evident lack of a theory applicable to all observed fireballs is found. A new empirical criterion separating different fireball groups according to structure and composition of their bodies and containing only values directly obtained by photographic observations is proposed. The complex semi-empirical approach is found to be the best approximation to reality we can achieve for all fireballs at the moment. The interpretation of the fireball differences assuming variations in structure and composition of their bodies is the most natural explanation of the observational facts.

During the last decade much work has proceeded on the problem of understanding the complex physiochemical processes associated with the entry of large meteoroids into the atmosphere. In this paper the respective areas of ablation, luminosity and infrasonic wave generation are surveyed from the viewpoint of the physics of fluids. Related work on meteorite cratering processes is not considered in this brief survey. Companion papers by Baggaley and by Ceplecha consider additional areas of current interest in meteor and fireball research.