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.     

Atmospheric behavior and extreme beginning heights of the thirteen brightest photographic Leonid meteors from the ground‐based expedition to China

Abstract— Precise atmospheric trajectories including dynamic and photometric data on thirteen of the brightest Leonid fireballs have been determined from the double‐station photographic observations of Leonid meteors during the ground‐based expedition to China in 1998 November. the expedition was organized as a collaboration between the dutch and chinese academy of sciences and was supported by the leonid multi‐instrument aircraft campaign (mac) program (jenniskens and butow, 1999). All data presented here were taken at Xinglong Observatory and at a remote station, Lin Ting Kou near Beijing, on the night of 1998 November 16/17. At the Xinglong station, photographic cameras were accompanied by an all‐sky television camera equipped with an image intensifier and 15 mm fish‐eye objective in order to obtain precise timings for all observed meteors up to magnitude +2. Whereas beginning heights of photographed meteors are all lower than 130 km, those observed by the all‐sky television system are at ～160 km, and for three brightest events, even > 180 km. Such high beginnings for meteors have never before been observed. We also obtained a precise dynamic single‐body solution for the Leonid meteor 98003, including the ablation coefficient, which is an important material and structural quantity (0.16 s² km^?2). From this and from known photometry, we derived a density of this meteoroid of 0.7 g/cm³. Also, all PE coefficients indicate that these Leonid meteors belonged to the fireball group IIIB, which is typical for the most fragile and weak interplanetary bodies. From a photometric study of the meteor lightcurves, we found two typical shapes of light curves for these Leonid meteors.     

First results from video spectroscopy of 1998 Leonid meteors

Abstract— We report spectroscopic observations of meteors made from the FISTA aircraft on 1998 November 17 as a part of the Leonid multi-instrument aircraft campaign. Low-resolution spectra of 119 meteors of apparent visual magnitudes from +3 to ?4, corresponding to meteoroid masses from 10^?6 to 10^?3 kg, were obtained. After analyzing a representative sample of the spectra and comparing them to the spectra of Perseid meteors from the Ondrejov archive, the following conclusions were reached: Leonid meteoroids are very loose and disintegrate easily in the atmosphere. This leads to much faster evaporation of volatile Na than of other elements, an effect which is not observed in the Perseid meteors. Relative bulk abundances of Mg, Fe, Ca, and Na in Leonid meteors are nearly CI-chondritic within the uncertainty of the method (factor of 3). Smaller meteoroids tend to be poorer in Na, which is true also for Perseid meteors. Most meteoric vapor emissions could be reasonably well explained with the temperature of 4500 K. High-temperature meteoric emissions (Ca⁺, Mg⁺) are present only in bright meteors. Leonid spectra are very rich in atmospheric emissions of O, N, and N₂, even at high altitudes and in faint meteors. These emissions are therefore not connected with the meteor shock wave. Thermal continuum is also present in the spectra. Organic material was not revealed.     

Airborne intensified charge-coupled device observations of the 1998 Leonid shower

Abstract— We have used dual coaxial microchannel plate image-intensified monochrome charge-coupled device (CCD) detectors run at standard NTSC frame rates (30 frames per second, fps) to study the Leonid meteor shower on 1998 November 17 from an airborne platform at an altitude of ～13 km. These observations were part of NASA's 1998 Leonid multi-instrument aircraft campaign (MAC). The observing systems had fields of view (width) of 16.3° and 9.5°, and limiting stellar sensitivities of +8.3^m and +8.9^m. During 12 h of recording, 230 meteors were detected, of which 65 were Leonid meteors. Light curves are presented for 53 of these meteors. The magnitudes at peak brightness of the meteors investigated were generally in the range from +4.0^m to +6.0^m. The mass distribution indices for the two samples are 1.67 and 1.44, with the former being based on the wider field of view dataset. The light curves were skewed with the brightest point towards the beginning of the meteor trail. The F parameter for points one magnitude below maximum luminosity had a mean value of 0.47 for the wider field system and 0.37 for the more sensitive narrower field system. We provide leading and trailing edge light curve slopes for each meteor as another indication of light curve shape. There were few obvious flares on the light curves, indicating that in-flight fragmentation into a large number of grains is not common. There is variability in light curve shape from meteor to meteor. The light curves are inconsistent with single, compact body meteor theory, and we interpret the data as indicative of a two-component dustball model with metal or silicate grains bonded by a lower boiling point, possibly organic, substance. The variation in light curve shape may be indicative of differences in mass distribution of the constituent grains. We provide trail length vs. magnitude data. There is only a slight hint of a bend at +5^m in the data, representing the difference between meteors that have broken into a cluster of grains prior to grain ablation, and those that continue to fragment during the grain ablation phase. Two specific meteors show interesting light curve features. One meteor is nebulous in appearance, with significant transverse width. The apparent light production region extends for 450 m from the center of the meteor path. Another meteor has several main fragments, and evidence of significant separated fragments. We offer several suggestions for improvements for the 1999 Leonid MAC light curve experiment.     

Contour Observations of the Leonid Shower in 1999

Contour visual observations of the Leonid meteor shower were made on December 18, 1999 by the method in which different groups of observers counted meteors in zenith, near the horizon, and observed through binoculars to study the luminosity function and the space density of the swarm. The luminosity function was obtained in the range of magnitudes from –8 ^m to +9 ^m . Over a wide range of magnitudes, the luminosity function is found to be nonlinear and is adequately approximated with a second-order curve. The logarithm of the meteor space density (m) reaches saturation at about 10 ^m , indicating that particles that give rise to meteors fainter than 10 ^m are absent in the swarm. The meteor stream reached its activity peak at a solar longitude of 235°, 287 (2 ^h 05 ^m UT). The peak visual zenithal hourly rate was about 7000 per one observer over an averaging interval of 1 min. The swarm space density increased by a factor of 6 within 0.5 hours and exceeded 600 particles per 10⁹ km³ for meteors brighter than +4 ^m . In the peak night, the luminosity-function exponent demonstrates no regular trend and reflects the intercepting of certain swarm clouds by the Earth.     

Meteoric activities during the 11th century

We have analysed the meteor records in the chronicles that describe the era of the Song dynasty ( ad 960–1279). The data are complementary to the record-vacant 10th century of the Koryo dynasty ( ad 918–1392). The annual activity of sporadic meteors analysed shows a generic sinusoidal behaviour as in modern observations. In addition, we have also found that there are two prominent meteor showers, one in August and the other in November, appearing on the fluctuating sporadic meteors. The date of occurrence of the August shower indicates it to be the Perseids. By comparing the date of occurrence of the November shower with those of the Leonid showers of the Koryo dynasty, recent visual observations and the world-wide historical meteor storms, we conclude that the November shower is the Leonids. The regression rate of the Leonids is obtained to be     days per century, which agrees with recent observations.     

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…     

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.     

Searching for Light Curve Evidence of Meteoroid Structure and Fragmentation

We used light curve analysis to search for evidence of the dustball meteoroid model. Leonid, Taurid, Alpha Monocerotid and sporadic meteors from November 2003 were observed and analyzed using uniform methodology. Meteors from these four sources were examined for evidence of fragmentation by examining light curve shape and searching for light curve irregularities. Differences in meteoroid structure should be reflected by differences in meteor light curves. The resulting meteor light curve F-parameter values showed no statistically significant differences between the meteors from the various cometary showers or the sporadic meteors. The F-parameter values also suggest that the meteoroids from these sources do not follow a single body ablation model, which suggests that all four sources produce meteoroids with a dustball structure.     

Very small bodies in the solar system: The Earth's atmosphere as detector

Particles of mass less than about 1 gm are a minor fraction of the total matter impinging on the Earth averaged over millennia time scales. However, these particles dominate during a single particular year and produce the most obvious evidence of incoming extra-terrestrial matter in the form of ablation trails in the atmosphere which are visible at night as meteors.Observations of meteors give astronomical information on the composition, structure, and cometary associations of the particles. The composition is deduced from optical spectra of meteors, whilst telescopic studies of the trails during formation give information on the physical structure of the particles. Any cometary associations are deduced from measurement of meteor orbits determined photographically, using television, or by radar.Meteors occur in the atmosphere at heights from about 70 to 120 km. Optical observations are restricted to night-time and usually under conditions of low moonlight. A typical television based detector can record +8^M meteors with a sporadic rate of 15–20 per hour and velocities accurate to about 3%. The luminosity of the trail is strongly dependent on the velocity of the meteoroid (to about the third power).Radar observations of meteors are unrestricted by weather or time of day, and can readily detect meteors at least two orders of magnitude smaller in mass than those detectable optically. Again the observations are heavily biased toward the higher velocities as the electron line density varies approximately asV ^3.5. However, the higher the velocity of the meteoroid the greater the height of the meteor trail, and the reduced probability of radar detection due to rapid diffusion of the trail. Thus radar observations tend to select meteors in the intermediate velocity range 30–40 km s^–1.     

The beginning heights and light curves of high‐altitude meteors

Abstract— In this paper, we provide an overview of meteors with high beginning height. During the recent Leonid meteor storms, as well as within the regular double station video observations of other meteor showers, we recorded 164 meteors with a beginning height above 130 km. We found that beginning heights between 130 and 150 km are quite usual, especially for the Leonid meteor shower. Conversely, meteors with beginning heights above 160 km are very rare even among Leonids. From the meteor light curves, we are able to distinguish two different processes that govern radiation of the meteors at different altitudes. Light curves vary greatly above 130 km and exhibit sudden changes in meteor brightness. Sputtering from the meteoroid surface is the dominating process during this phase of the meteor luminous trajectory. Around 130 km, the process switches to ablation and the light curves become similar to the light curves of standard meteors. The sputtering model was successfully applied to explain the difference in the beginning heights of high‐altitude Leonid and Perseid meteors. We show also that this process in connection with high altitude fragmentation could explain the anomalously high beginning heights of several relatively faint meteors.     

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.     

Simultaneous observations of meteors with the radar and TV systems

We have carried out a simultaneous observation of radar and optical meteors with the MU radar (Middle and Upper Atmosphere Radar), Shigaraki and TV camera systems. We usually obtained about 20 meteors per an hour with 85 mm lens, but very small part of them are simultaneously observed by the MU radar (< 5%), suggesting the significance of rectangular scatterring. We have analyzed about 20 simultaneous meteors with magnitudes from 0 to +5.5, most of which are overdense meteors. For Geminid meteors, a linear relation between the logarithm of the echo duration and the absolute magnitude of the TV meteor, was deduced.     

Meteoroid Bulk Density Determination Using Radar Head Echo Observations

We present the results of a study of meteoroid bulk densities determined from meteor head echoes observed by radar. Meteor observations were made using the Advanced Research Projects Agency Long-Range Tracking And Instrumentation Radar (ALTAIR). ALTAIR is particularly well suited to the detection of meteor head echoes, being capable of detecting upwards of 1000 meteor head echoes per hour. Data were collected for 19 beam pointings and are comprised of approximately 70 min. of VHF observations. During these observations the ALTAIR beam was directed largely at the north apex sporadic source. Densities are calculated using the classical physical theory of meteors. Meteoroid masses are determined by applying a full wave scattering theory to the observed radar cross-section. Observed meteoroids are predominantly in the 10⁻¹⁰ to 10⁻⁶ kg mass range. We find that the vast majority of meteoroid densities are consistent with low density, highly porous objects as would be expected from cometary sources. The median calculated bulk density was found to be 900 kg/m³. The orbital distribution of this population of meteoroids was found to be highly inclined.     

New type of radiation of bright Leonid meteors above 130 km

Abstract— In this paper, we study the extremely high beginning parts of atmospheric trajectories of seven Leonid meteors recorded by sensitive TV systems equipped with image intensifiers up to apparent magnitude +6.5. For all seven cases, we observed comet‐like diffuse structures with sizes on the order of kilometers that developed quickly during the meteoroids' descent through the atmosphere. For the brightest event with a maximum absolute magnitude of ?12.5, we observed an arc similar to a solar protuberance and producing a jet detectable several kilometers sideways from the brightest parts of the meteor head, and moving with a velocity over 100 km/s. These jets are common features for the seven studied meteors. Precise position in trajectory, velocity, and brightness at each point is available for all seven meteors, because of double‐station records on 85 km base‐line. When these meteoroids reached 130 km height, their diffuse structures of the radiation quickly transformed to the usual meteor appearance resembling moving droplets, and meteor trains started to develop. These meteor phenomena above 130 km were not recognized before our observations, and they cannot be explained by standard ablation theory.     

First definitive observations of meteor shower particles using a high-power large-aperture radar

We present the first clear observations of meteor shower activity from meteor-head echoes detected by a high-power large-aperture radar (HPLAR). Such observations have been performed at the Jicamarca VHF radar using its interferometric capabilities allowing the discrimination of meteor shower echoes from the much more frequent sporadic meteors. Until now, HPLARs were unable to distinguish meteor shower from the much more common sporadic meteor ones. In this work we have been able to detect and characterize the η-Aquariids (ETA) as well as the Perseids (PER) showers. The shower activity is more conspicuous for the ETA than for the PER shower due to the more favorable geometry. Namely, PER meteors come from low elevation angles, experiencing more filtering due to the combined Earth-atmosphere-radar instrument. In both cases, there is an excellent agreement between the measured mean velocity of the shower echoes and their expected velocity, within a fraction of 1 km s⁻¹. Besides the good agreement with expected visual results, HPLARs observe meteors with a variety of particles sizes and masses, not observed by any other technique. Taking into account the different viewing volumes, compare to optical observations Jicamarca observes more than 1000 times more ETA meteors. Our results indicate that Jicamarca and other HPLARs are able to detect the echoes from meteor showers, but without interferometric capabilities such populations are difficult to identify just from their velocity distributions, particularly if their velocity distributions are expected to be similar to the more dominant distributions of sporadic meteors.     

Search for New Parent Bodies of Meteoroid Streams Among Comets. I. Showers of Comets 126P/1996 P1 and 161P/2004 V2 with Radiants on Southern Sky

We deal with theoretical meteoroid streams the parent bodies of which are two Halley-type comets in orbits situated at a relatively large distance from the orbit of Earth: 126P/1996 P1 and 161P/2004 V2. For two perihelion passages of each comet in the far past, we model the theoretical stream and follow its dynamical evolution until the present. We predict the characteristics of potential meteor showers according to the dynamical properties of theoretical particles currently approaching the orbit of the Earth. Our dynamical study reveals that the comet 161P/2004 V2 could have an associated Earth-observable meteor shower, although no significant number of theoretical particles are identified with real, photographic, video, or radar meteors. However, the mean radiant of the shower is predicted on the southern sky (its declination is about −23°) where a relatively low number of real meteors has been detected and, therefore, recorded in the databases used. The shower of 161P has a compact radiant area and a relatively large geocentric velocity of ∼53 km s⁻¹. A significant fraction of particles assumed to be released from comet 126P also cross the Earth’s orbit and, eventually, could be observed as meteors. However, their radiant area is largely dispersed (declination of radiants spans from about +60° to the south pole) and, therefore, mixed with the sporadic meteor background. An identification with real meteors is practically impossible.     

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.     

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.     

Do comets have chondrules and CAIs? Evidence from the Leonid meteors

Abstract— Chondrules, silicate spheres typically 0.1 to 1 mm in diameter, are the most abundant constituents in the most common meteorites falling on Earth, the ordinary chondrites. In addition, many primitive meteorites have calcium‐aluminum‐rich inclusions (CAIs). The question of whether comets have chondrules or CAIs is relevant to understanding what the interior of a comet is like and what a cometary meteorite might be like. In addition, one prominent model for forming chondrules and CAIs, the X‐wind model, predicts their presence in comets, while most other models do not. At present, the best way to search for chondrules and CAIs in comets is through meteor showers derived from comets, in particular, the Leonid meteor shower. Evidence potentially could be found in the overall mass distribution of the shower, in chemical analyses of meteors, or in light curves. There is no evidence for a chondrule abundance in the Leonid meteors similar to that found in chondritic meteorites. There is intriguing evidence for chondrule‐ or CAI‐sized objects in a small fraction of the light curves, but further work is required to generate a definitive test.