Distribution Of Meteoroids Research Articles

Comparison of the structures of meteor streams of cometary and possible asteroidal origin

The structures of the meteor streams of cometary origin—Draconids, Ursids, Perseids, and Lyrids—and the streams presumably connected with asteroids—Taurids and α-Capricornids—are compared. The comparative analysis was performed by the mass distribution of meteoroids in the stream and the activity profile for the meteors with the maximum recorded stellar magnitude +3 m and brighter. Visual observations of 1987–2008 from the database of the International Meteor Organization (IMO) and earlier sources were considered. It has been shown that the structures of the meteor streams of cometary and, presumably, asteroidal origin differ somewhat by the activity profile and the mass distribution of meteoroids in the cross-section of a stream along the Earth’s orbit.

Mass distribution of lyrid meteoroids

Long-term visual observations of the Lyrid meteoroid shower have been analyzed to determine the mass distribution of Lyrid meteoroids. The value of the parameter S has been confirmed to be less than 1.8, which is normally assumed for meteoroid streams. The inclination of the descending and ascending branches of the S curve, depending on the longitude of the Sun, does not seem to exceed 3°. Observations carried out from 1987 until 2007 reveal that the minimum value of S corresponding to the longitude of the Sun 32.19 ± 0.04° is equal to 1.54 ± 0.02 (2000.0). The analysis of the S parameter derived from visual observations did not discover any particularities in the mass distribution of the meteoroids in the stream connected with the assumed 12-year enhancement period in the activity of the Lyrids.

Meteor Outburst Profiles and Cometary Ejection Models

The spatial structure of meteor streams, and the activity profiles of their corresponding meteor showers, depend firstly on the distribution of meteoroid orbits soon after ejection from the parent comet nucleus, and secondly on the subsequent dynamical evolution. The latter increases in importance as more time elapses. For younger structures within streams, notably the dust trails that cause sharp meteor outbursts, it is the cometary ejection model (meteoroid production rate as a function of time through the several months of the comet’s perihelion return, and velocity distribution of the meteoroids released) that primarily determines the shape and width of the trail structure. This paper describes how a trail cross section can be calculated once an ejection model has been assumed. Such calculations, if made for a range of ejection model parameters and compared with observed parameters of storms and outbursts, can be used to constrain quantitatively the process of meteoroid ejection from the nucleus, including the mass distribution of ejected meteoroids.

Characterization of the Meteoroid Spatial Flux Density during the 1999 Leonid Storm

The November 18, 1999 Leonid storm was rich in meteors and well observed by airborne intensified video cameras aimed low in the sky which enabled enhanced meteor counts over ground-based observations. The two- and three-dimensional distribution of meteoroids was investigated for signs of clustering that could provide evidence of meteoroid fragmentation shortly after lift-off from the parent comet 55P/Tempel-Tuttle, or much later due to space weathering. Analysis of the video tapes yields a refined estimation of the mass ratio during the peak of s = 1.65 and spatial flux density of 0.5 particles/km2 greater than those causing visual magnitude +6.5 during the 5 min centered around the peak of the storm. Furthermore, the projection of the individual trails into three-dimensional Heliocentric coordinates, shows non-homogeneity of the stream on spatial scales from hundreds to thousands of kilometers.

Commission 22: Meteors, Meteorites & Interplanetary Dust

There have been three international meetings where the subject area of the meeting was to significant extent within the area of interest of commission 22. These were: The Meteoroids 2004 Conference was held at the University of Western Ontario in London, Canada from August 15 to 21, 2004. This conference was the fifth in a series of meteoroid meetings which have been held approximately every three years since 1992, the previous one being in Kiruna, Sweden in 2001. Ingrid Mann chaired a scientific organizing committee which set the program for the conference. The meeting brought together scientists from more than twenty countries, to deliver 84 oral and 38 poster presentations. The papers represented the research contributions of more than 150 different scientists. The conference provided a comprehensive overview of leading edge research on topics ranging from the dynamics, sources and distribution of meteoroids, their chemistry and their physical processes in the interplanetary medium and the Earthõs atmosphere, and space and laboratory studies of meteorites, micrometeorites and interplanetary dust were also well represented. It was clear from the conference that the coordinated international campaigns for the Leonid showers provided a rich observational dataset and lead to the development of new observational and analysis techniques. Another trend obvious at the conference was the increasing use of sophisticated large aperture radars for meteor studies. High performance computing facilitates both dynamical model calculations and sophisticated ablation models. Significant progress was reported on ablation models for meteoroids ranging from dust to those producing bright fireballs. Study of solid particles entering the solar system from interstellar space and improved dust measuring capabilities on interplanetary spacecraft are an important research area which links astrophysical dust with solar system dust. The majority of papers presented at the conference (a total of 69 papers) are being published as a special issue of the journal Earth, Moon, and Planets (Vol. 95, Nos. 1–4) and also in the form of an associated book published by Springer: Modern Meteor Science: An Interdisciplinary View which was edited by R.Hawkes, I. Mann and P. Brown (ISBN 1-4020-4374-0). The book will be accompanied by a CD-ROM which includes a selection of conference photographs and the complete abstracts of all papers from the conference. As is reflected in the title of the spin-off book, this field is becoming increasingly interdisciplinary in nature, with researchers from astronomy, astrophysics, space science, space engineering, cosmochemistry, atmospheric science and geophysics, as well as others, now contributing to research in the field.

SINGLE AND MULTI-STATION RADAR OBSERVATIONS OF THE GEMINID/SEXTANTID METEOR STREAM SYSTEM

The Canadian Meteor Orbit Radar (CMOR) is used to look at the distribution of meteoroids which encounter the Earth. As a single-station operation, it is capable of determining radiant distributions on a statistical basis and the position and speed of individual meteors. The addition of two outlying receiving stations allows the determination of the orientation in space of the meteor leading to an estimate of the orbital parameters of the individual meteor and an independent additional estimate of its speed. Comparison is made of the effectiveness of the two modes of operation using observations on the Geminid and Sextantid meteor streams.

Velocity distribution of meteoroids in the vicinity of planets and satellites

Collisions in the Solar System play an important role in its history. Impact processes depend essentially on the velocity distribution of meteoroids colliding with a chosen planet. According to Carleman’s theorem it is sufficient to find the set of Mk = mathematical expectation of v k , v being the collisional velocity. We suppose that M k for meteoroids of asteroidal nature differs slightly from that for asteroids themselves. So among all numbered minor planets we select those which may potentially collide with the chosen major planet. Then we calculatevat intersection points and count the average over all such points and all selected asteroids. The gravitation of a body-target may be taken into account or not. Numerical results are collected in four Tables.

Velocity distribution of meteoroids colliding with planets and satellites. II. Numerical results

Abstract In the first part of the paper we proposed algorithm for describing velocity distribution of meteoroids colliding with planets and satellites. In the present part we show numerical characteristics of the distribution function. Namely, for each of terrestrial planets and their satellites we consider a swarm of encountering particles of asteroidal origin. They form a field of relative collisional velocities v. We consider momenta μ k (mathematical expectation of vk ), k = -1, 1, 2, 3, 4. The data are calculated under two different assumptions: taking into account gravitation of target body or without it. The main results are presented in a series of tables each containing five numbers and several useful functions of them.

Velocity distribution of meteoroids colliding with planets and satellites. I. theory

Abstract Algorithms for describing the velocity distribution of meteoroids colliding with planets and satellites are proposed providing a dense family of meteoroid orbits is known. The gravitation of the target body can be taken into account. The algorithms constructed here are semi-analytical, accomplishing operations with each known orbit via closed formulae mostly, whereas the average over all orbits of the family is to be calculated numerically. The characteristics of the distribution function will be published later. They may be obtained according to the above algorithms using Steel's idea to consider the family of asteroid orbits as representative for that of meteoroid orbits.

Face-dependent impact probabilities, velocities and angles upon LDEF by space debris and natural meteoroids

For specified geocentric orbits the impact probabilities, velocities and angles upon the different faces of the Long Duration Exposure Facility (LDEF) are calculated, and it is found that quite different distributions of micro-cratering are to be expected. In particular the flux to the East (leading) face should exceed that to the West (trailing) face by a very large ratio. The North and South faces receive exposures slightly in excess of the East face for lower-velocity impacts from low-inclination orbits, but much lower exposures than the East face for high-velocity impacts from high-inclination orbits. The Space face (pointing directly away from the Earth) and the Earth face (pointing directly towards the Earth) will have been subject to very few impacts from geocentric orbits. Therefore, whilst three sides (the East, North and South) will have been hit many times by artificial space debris, the other three (the West, Space and Earth) will have been impacted almost solely by natural meteoroids from heliocentric orbits, and may be used to determine the flux of such particles in the vicinity of the Earth. The ratios of impacts upon the East, West and Space faces will be useful indicators of the velocity/orbit distribution of meteoroids.

Mass distribution of massive meteoroids determined by occurrence of brilliant fireballs

DETERMINATION of the mass distribution function of bodies is of prime importance for an understanding of the evolutionary processes that determine the population of the Solar System. We report here a method for measuring mass distribution of meteoroids, by determining the occurrence frequency of very brilliant fireballs. In a certain mass regime the number distribution can be described by a power law dn = (const.)m−s dm where dn is the number of particles having a mass between m and m+dm and s is the mass distribution index. s is a slowly varying function of mass, and values of s have been well established for meteoroid particles 10−12< m ≲ 101 g by various techniques, and reasonable estimates are available for large bodies m ≳ 105 g (meteorites and asteroids). There is, however, a lack of information in the mass range 101 ≲ m ≲ 105 g corresponding1 to meteoroids which yield meteors of visual magnitude − 2 ≳ Mv □ − 12 ranging from bright visual objects to very brilliant fireballs (the term fireball generally refers to any meteor brighter than Venus, Mv ≃ −4). The problem is one of sampling: while meteor (radar, visual, optical) counts over periods of hours at a single station are suitable for estimates of s for m ≲ 1 g (Mv ≳ 0), such a procedure yields a prohibitively low count for larger objects. This difficulty would be overcome by observing over a large area of the Earth's surface for many years. We determine here the occurrence frequency of very brilliant fireballs, as a method of s measurement. A fireball of magnitude equal to that of the full Moon is an outstanding spectacle likely to be well reported in an area with a high population density. With good communications and active research groups analysing reports and recording data, a reliable estimate of the influx of such large objects could be secured. In the British Isles, extensive records of fireball events covering more than 40 yr, are available2,3. Most observers' estimates were based on the great illumination produced on the landscape or inside buildings, and the personnel analysing the reports were experienced workers in the field. A detailed examination of the reports reveals a consistent rate of about two lunar fireballs (apparent visual magnitude equal to or brighter than that of the full Moon) seen in the British Isles per year. Specifically, in the period 1900–36 the average occurrence frequency was 2.4 yr−1. This frequency can be compared with that of visual meteors to obtain an estimate of s.

On the origin and distribution of meteoroids

The influence of collisional and radiative processes on the population of sporadic and shower meteoroids is examined. It is found that the observed distribution of sporadic meteoroids in the photographic and radio ranges is unstable: collisional processes would rapidly change the distribution of these particles if they are not replenished by a source. It is shown that, in order to maintain the distribution of these sporadic meteoroids in a steady-state condition, a source is needed that replaces the sporadic meteoroids destroyed by collisions. The mass distribution of the required source is calculated and found to agree with the distribution of bright radio-shower meteors, provided that the population index α of sporadic meteors (in the photographic and radio ranges) is taken to be α = 13/6. The evolution of the population in a meteor shower under the influence of collisions with sporadic meteoroids is also examined. Since small particles have shorter lifetimes (with respect to collisions) than do larger particles, the population of faint shower meteors is more strongly affected. It is found that the mass distribution of faint radio meteors in the major showers considered here agrees with the results of the analysis, provided that α = 13/6, for sporadic meteors. Taking α = 13/6 therefore provides self-consistency for analysis and observation, thereby providing new evidence favoring the cometary origin of meteors. The rate at which the population of sporadic meteoroids is losing mass is calculated and found to compare favorably with F. L. Whipple's 1967 estimate as well as his estimate of the rate at which comets are injecting meteoritic mass into the solar system. The influence of the Poynting-Robertson effect on the meteoroid mass distribution is found to be minor; collisional processes dominate the evolution of meteoroids in the faint and brighter radio range. The influence of radiation pressure may be profound. By using previous work by Harwit and geometrical optics, it is shown that the smallest masses present in the major meteoroid showers are in the range of 10−6 to 10−10 gram. This is precisely the mass range where a leveling off of the distribution of sporadic meteoroids is indicated by satellite penetration measurements; these measurements are therefore consistent with the cometary origin of sporadic meteoroids. These theoretical results may not apply to particles with masses of 10−12 gram or smaller (i.e., submicron particles, for which details of physical optics become important).

Mass distribution of meteoroids as determined by radar observations of underdense meteor trails

The amplitudes of meteor echoes recorded on high-power radar equipment at Springhill Meteor Observatory have been measured for 57 samples of about 500 meteors each, obtained at various times of day on at least 1 day per month. The number of echoes as a function of echo amplitude is represented by a power law and the exponent in this relation calculated for each sample. Within the limits of error, no strong diurnal or seasonal variations in this exponent have been found. Similarly, there are no significant changes in the exponent when the data are divided into groups according to the height at which the echoes occurred and also according to the distance from the radar to meteor trail.Calibration of the equipment and the methods employed in obtaining and reducing the data are treated in detail. Theory relating the number distribution of echo amplitudes to the distribution of meteoroid masses is reviewed and assessed critically.A best value for the average mass index, s, of the sporadic meteor population in the range from 7th to 10th magnitude is s = 2.3 ± 0.1. The dispersion of individual sample values of s is about ± 0.3.

METEOR RADAR STATISTICS. II

Mean daily values of meteor flux for three categories of meteor echoes are tabulated for the period 1958 to 1962. The variation of echo rate with echo duration is studied. The difficulty of using this relation to determine the mass distribution of meteoroids in interplanetary space is noted.