Shower Meteoroids: Constraints From Interplanetary Dust Particles And Leonid Meteors

The cometary Leonid meteoroids represent a size range in between largest carbon-richIDPs and the smallest CI meteorites. Their dustball structure and chemistry offer anopportunity to constrain hierarchical dust accretion inferred from petrologic studies ofaggregate and cluster IDPs. The Leonid shower meteoroids of known ``comet ejection'ages provide an opportunity to study space weathering of cometary dust over periodsof up to several hundred years. The meteors and aggregate and cluster IDPs displaycontinuous thermal modification of organics and volatile element (Na, K-bearing phases), that occur as discrete minerals and amorphous solids each different response during kinetically controlled ablation. Leonid meteoroids are not excessively Na-rich. The occurrences of Leonid meteors can now be accurate predicted and combined withknowledge better models for the settling rates, collections of surviving dust becomea comet nucleus-sampling mission. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the 2001 Leonid meteor shower using VHF meteor radar

Every year the Earth crosses or passes near one of the dust trails left by Comet 55P/Tempel-Tuttle in its pass through the Solar System every 33.2 years. This produces a meteor shower Commonly called the Leonid. The 2001 Leonid meteor shower is one of the strongest in recent years. We present observations made by the 50 MHz all-sky meteor radar located at the Platteville Atmospheric Observatory in Colorado (40° N, 105° W). The spatial and temporal distributions of the meteor activity detected by the radar during the 2001 Leonid shower differs from the observed sporadic activity detected by VHF radars. Estimation of the radiant flux of the meteor shower of the shower by a well-known methodology is presented, and the intensity of the phenomena is discussed.     

The Third Peak of the 1998 Leonid Meteor Shower

1 INTRODUCTIONThe Leonid meteor shower is a well-known periodic meteor shower. Its history is tied upwith the development of the theory of meteor stream astronomy itself. It was the very st.rongshowers of 1799 and 1833 that played a sghficant pat in the recoghtion of the ealstence ofmeteoroid streams. These evellts started the obse~ions of Leoaid meteor shower and broughtabout the birth of meteoritiCS. It is known that the Leould parent comet, 55P/Tempel-TUttle,has an orbital period a…     

Comet Tempel-Tuttle and the Leonid meteors

The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval.     

Meteor stream activity. III. Measurement of the first in a new series of Leonid outburst

Abstract— In 1994 November, a shower of bright Leonid meteors signaled what is likely the first meteor outburst of Leonids associated with the upcoming return of comet P/Tempel-Tuttle to perihelion. Measurements of meteor activity and the meteor brightness distribution are presented. By comparing the present observation with those of past Leonid returns, a forecast is made of the time, the duration, the intensity, and the mean meteor brightness of Leonid outbursts that may occur if previously observed patterns are repeated in the forthcoming years.     

The Leonid outburst of 1996

There is strong anticipation that the Leonid meteor shower could produce storm-level activity in 1998 and/or 1999. The well-documented Leonid outburst in 1996 and the more poorly observed one in 1994 have been taken by many observers to imply that a storm is imminent, This article explores the possible relationship between the 1996 outburst in activity and possible Leonid storms. The curve of activity is found to be much closer to that of normal activity, although with greater hourly rates, than it is to the very brief, steeply rising activity curve of a storm. It is probable that the 1996 outburst is thus completely unrelated to any future storm which may appear.     

近20年来狮子座流星雨预报进展

对近20年来狮子座流星雨的预报工作，进行了系统的阐述和分析。1998年Tempel-Tuttle彗星的回归，再度带来了狮子座流星雨的观测热，也大大促进了对狮子座流星雨预报工作的研究与验证。有的研究在时间预报准确度方面已显示出其模型的优越性，有的在流星雨的强度方面显示出一定的准确度。指出了两大类不同的方法实际上是在三维空间强调了不同的方面。将不同方法的优势结合起来，可能会使流星雨的预报更加成熟。     

Flux of very faint Leonid meteors observed with a 3 meter liquid mirror telescope intensified charge‐coupled device system

Abstract— We have used a 3.0 m diameter liquid mirror telescope (LMT) coupled to a microchannel plate image‐intensified charge‐coupled device (CCD) detector to study the 1999 Leonid meteor shower. This is the largest aperture optical instrument ever utilized for meteor detection. While the observing system is sensitive down to stars of +18 astronomical magnitude under optimum conditions, when corrections for meteor motion are applied the majority of the meteors collected fall in the absolute magnitude range from +5 to +10, corresponding to photometric masses from about 10^?7 to 10^?9 kg. This is largely due to the fact that the field of view of the LMT was only 0.28°, so that only a small portion of the luminous meteor trail was recorded. While the flux of these small (1.4 times 10^?9 kg) Leonid meteors is low (on the order of one Leonid meteor per hour per square kilometer perpendicular to the Leonid), we do have clear evidence that the Leonid stream contains particles in the mass range studied here. The data showed a possibly significant peak in Leonid flux (9.3 ± 3.5) for the 1 h period from 11:00 to 12:00 u.t. 1999 November 17 (solar longitude 234.653 to 234.695, epoch 2000.0), although the main trend of these results is a broad low‐level Leonid activity. There is evidence that small meteoroids are more widely distributed in the Leonid stream, as would be expected from cometary ejection stream models. As would be expected from an extrapolation of mass distribution indices for brighter meteors, the vast majority of meteors at this size are sporadic. The LMT is a powerful detector of sporadic meteors, with an average non‐Leonid detection rate of more than 140 meteor events per hour.     

Detailed visual observations and modelling of the 1998 Leonid shower

We present a detailed activity profile for the 1998 Leonid shower from visual observations. The shower displayed at least two distinct components – a broad component peaking between 2344 and 2350, and two narrower filaments near 23521 and 23533 probably of younger origin based on modelling results. This dual-peaked structure in the flux profile has peak fluxes to a limiting magnitude of +6.5 of 0.03 Leonid km⁻² h⁻¹. The distribution of particles also changes dramatically across the stream in 1998, with large meteoroids dominating the early peak and smaller meteoroids relatively more abundant near the time of the nodal passage of the comet. Detailed comparison of the observed activity with models in 1998 shows that the early component comes from material ejected between 500 and 1000 yr ago. Our modelling results suggest that the later dual peaks are caused by high- β meteoroids with large ejection velocities released during the 1932 and 1965 passages of Comet 55P/Tempel–Tuttle.     

Image‐intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure

Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10^?6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ～1.0 × 10‐⁷ kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.     

Johnson Space Center's Leonids optical observations

Abstract— The 1998 Leonid meteor shower was videotaped by NASA Johnson Space Center (JSC) personnel, Jim Pawlowski and Jerry Winkler, at Houston, Texas, and Anna Scott at Cloudcroft, New Mexico. The videotapes were screened and the Leonid meteors in the videotapes were analyzed. The outcome of this effort was tables of counts per hour over the viewing period and a comparison to the Leonids meteors mass distribution model (Matney, 1998, unpubl. data) used for risk assessment calculations associated with space shuttle missions. The comparison exhibited a difference between the observed data and the model. Perhaps this difference can be resolved when observations from other parts of the Earth are assimilated with JSC observations.     

Observations of interplanetary meteoroids with TIRA

The Tracking and Imaging RAdar (TIRA) at the Research Establishment for Applied Science (FGAN) was used in the L-band (1.33 GHz) to observe the Leonid shower in 1999. The radar beam was pointed directly into the radiant in the constellation Leo to receive “head echoes” from meteoroids when they ablate in the atmosphere at altitudes around 100 km. Two hundred and eighty-seven meteors were observed during 21 h in the early hours of November 17 and 18, 1999. The individual velocities, radiants and rough heliocentric orbits are calculated. Criteria are derived from optically observed Leonids which are then applied to decide whether an echo was created by a Leonid or a background meteoroid. However, in most cases the accuracy in the observational data is not good enough to allow for a clear distinction. Only for 100 meteors the velocity errors were less than 10 km/s. Out of those, 71 could be excluded on a 3σ level to be a Leonid (95 are excluded on a 1σ level). This confirms the theory that the Leonids have dominantly sizes of optical meteoroids with no significant extension in the lower mass range. Therefore, the risk of meteoroid impacts on spacecraft does not increase considerably during a Leonid storm. Background measurements 9 days after the Leonids maximum were taken in 2001 which corroborated the overall results obtained in 1999.     

Video Observations of the 2006 Leonid Outburst

We carried out double station observations of the Leonid meteor shower outburst, which occurred in the morning hours of November 19, 2006. Using image-intensified cameras we recorded approximately 100 Leonid meteors. As predicted, the outburst was rich especially in fainter meteors. The activity profile shows that the peak of the outburst occurred at 4:40 ± 0:05 UT. The maximum reached flux was 0.03 meteoroids km⁻² hod⁻¹ for meteors brighter than +6.5 magnitude.     

THE LEONIDS BY RADAR—1957 TO 19681

Radar observations of the Leonid meteor shower, made near Ottawa during the years from 1957 to 1968 inclusive, are analyzed and reduced to give comparative flux rates. A strength classification has been made in terms of the ratio of shower rates to background rates. The relative strengths found by radar, showing marked variability from year to year, are confirmed by analysis of available visual observations. There is also great variation in the distribution of particle sizes. The high rates of the 1966 return were accompanied by a relatively high percentage of small particles. In 1965 there was a much higher proportion of large particles, and the high rates of 1961 showed a mass or size distribution intermediate between that of 1966 and of 1965.     

Gigamasers: the key to the dust-obscured star formation history of the Universe?

A numerical model of the Leonid stream is developed, based on an earlier model which has been applied to the Perseid stream. The results for this model are applied to the 2001 Leonid return. By examining the full three-dimensional dispersion of individual 'streamlets' released from the Leonid parent comet, 55P/Tempel–Tuttle, we have derived an estimate for the temporal change in spatial density of each trail. Using this result along with an estimate for the location of the centres for individual streamlets and fits to previous Leonid storm profiles, we estimate that the activity from the shower will be broad and relatively strong (zenithal hourly rates perhaps in excess of 1000). In particular, streamlets from the 1766 and 1799 ejections contribute to activity peaking near 10 and 12 ut on 2001 November 18, respectively. Additional older material from 1633, 1666 and 1699, as well as more recent ejections from 1866 and 1833, contributes to a much broader secondary maximum near 17.5 ut on November 18. Comparison with other published models of predicted Leonid activity in 2001 shows general agreement in terms of timing, but the models differ significantly in terms of the relative magnitude of the activity (which other models suggest will be larger). Significant anisotropy in the impact hazard exists for satellites in the geostationary belt, with those over western longitudes most likely to be affected. Integrated fluences for the 2001 Leonid return suggest a hazard of order one magnitude greater than occurred for the 1999 Leonid storm.     

The Composition of the Y2K Meteor

During the Leonid meteor shower of November 1999 a very bright meteor train, subsequently called the Y2K meteor, was observed. Analysis of the trajectory of the meteor suggests that it was composed of two distinct materials. The bulk of the meteor was composed of a comet-like material, while a much smaller fraction was of a denser carbonaceous material. A simple model is used to analytically determine the mass of the meteor fragments. This revised version was published online in July 2006 with corrections to the Cover Date.     

A micrometeor component of the 1998 Leonid shower

Most astronomers expected a significant meteor shower associated with the Leonid meteoroid stream to appear in 1998 and 1999. An enhanced shower was widely observed in both years, and details can be found in many published articles. In 1998, one remarkable feature was the appearance of a strong component, rich in bright meteors, which appeared about 16 h before the expected maximum of the main shower, but another observed feature was an abnormal peak in the ionosphere characteristic value f _b E _s which was detected about 18 h after the main shower. A very high value of f _b E _s persisted for over an hour. The likely explanation is that the ionosphere was bombarded by an additional swarm of meteoroids, much smaller than those that produce a visible trail or an ionization trail that can be picked up by radio detectors. The different dynamical behaviours between small and large meteoroids are investigated and, in consequence, an explanation for the observed phenomena is offered and 1933 is suggested as being the likely ejection time.     

The relationship between the semi-major axis, mass index and age of the Leonid meteor shower

Owing to sublimation of ice, comet nuclei eject dust particles when they are near to the sun. Those particles assume velocities and then vary their orbits to ones similar to that of the comet. The most notable difference between the orbit of the parent comet and those of the particles is their semi-major axes. This difference (Δ a ) has been widely used in modern meteor shower predictions. Observational evidence of the distribution showed that it is a function of Δ a , and the age of the dust trail. However, the relation is not well known. In this paper, a simplified relation between Δ a , the mass index ( s ) and the age of the dust trail is presented, taking the instance of a recent Leonid meteor shower.     

Lightcurves of 1999 Leonid Impact Flashes on the Moon

Optical flashes observed on the night side of the Moon during the 1999 Leonid meteor shower have attracted the interest of astronomers. These flashes are attributed to high-velocity impacts of Leonid meteoroids on the lunar surface. Here, we report five lunar flashes detected over a 5.8-h observation period centered at 11:25 UT on Nov. 18, 1999, in Japan. The flashes are characterized by an abrupt brightening. Three flashes exhibited afterglows that remained visible for at least 50 ms, which is longer than the duration predicted for radiation from an impact-generated plasma cloud. We show that thermal radiation from hot droplets ejected from the lunar surface during high-velocity impacts could be the cause of the afterglows.     

Leonid fluxes: 1994–1998 activity patterns

Abstract— The Leonid meteor shower was observed worldwide in 1998 November in an intensive campaign without precedent. During this international effort ～35 500 meteors were reported by members and collaborators of the International Meteor Organization (IMO) using a standard methodology. Despite the absence of a meteor storm in 1998, the rich observational data allow us to obtain a detailed unprecedented knowledge of the stream structure between 1994 and 1998.