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.     

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.     

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.     

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.     

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.     

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.     

Very precise orbits of 1998 Leonid meteors

Abstract— Seventy-five orbits of Leonid meteors obtained during the 1998 outburst are presented. Thirty-eight are precise enough to recognize significant dispersion in orbital elements. Results from the nights of 1998 November 16/17 and 17/18 differ, in agreement with the dominant presence of different dust components. The shower rate profile of 1998 November 16/17 was dominated by a broad component, rich in bright meteors. The radiant distribution is compact. The semimajor axis is confined to values close to that of the parent comet, whereas the distribution of inclination has a central condensation in a narrow range. On the other hand, 1998 November 17/18 was dominated by dust responsible for a more narrow secondary peak in the flux curve. The declination of the radiant and the inclination of the orbit are more widely dispersed. The argument of perihelion, inclination, and the perihelion distance are displaced. These data substantiate the hypothesis that trapping in orbital resonances is important for the dynamical evolution of the broad component.     

Preparing for the 1998/99 Leonid Storms

In order to further observing programs aimed at the possible meteor storms of November 1998 and 1999, we describe here how the Leonid shower is expected to manifest itself on the sky. We discuss: 1) the expected wavelength dependence of meteor (train) emission, 2) the meteor brightness distribution and influx, 3) the stream cross section, radiant and altitude of the meteors, 4) the apparent fluxes at various positions in the sky as a function of radiant elevation as well as 5) the trail length and radial velocity, and 6) the diameter and brightness of persistent trains as a function of radiant elevation. These topics were chosen to help researchers plan an observing strategy for imaging, spectroscopy, and LIDAR observations. Some applications are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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…     

Investigation on spectral character of ELF electromagnetic radiations during Leonid 2009 meteor shower

The problem of solitary electron acoustic (EA) wave propagation in a plasma with nonthermal hot electrons featuring the Tsallis distribution is addressed. A physically meaningful nonextensive nonthermal velocity distribution is outlined. It is shown that the effect of the nonthermal electron nonextensivity on EA waves can be quite important. Interestingly, we found that the phase speed of the linear EA mode increases as the entropic index q decreases. This enhancement is weak for q>1, and significant for q<1. For a given nonthermal state, the minimum value of the allowable Mach numbers is lowered as the nonextensive nature of the electrons becomes important. This critical limit is shifted towards higher values as the nonthermal character of the plasma is increased. Moreover, our plasma model supports rarefactive EA solitary waves the main quantities of which depend sensitively on q. This dependency (for q>1) becomes less noticeable as the nonthermal parameter decreases. Nevertheless, decreasing α yields for q<0 a different result, a trend which may be attributed to the functional form of the nonthermal nonextensive distribution. Our study (which is not aimed at putting the ad hoc Cairns distribution onto a more rigorous foundation) suggests that a background electron nonextensivity may influence the EA solitons.     

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.     

Visual Observations Of The Leonid Meteors 1998 In Yunnan

The 1998 Leonid meteor shower was observed at the Gaomeigu station of Yunnan Observatory during five successive days in November. The visual records indicate that the number of meteors increased suddenly, from a ZHR of about 140 to over 400, in the early morning of November 17th, Beijing time. But it decreased slowly in the following two days. During the maximum there was a high proportion – about 10 percent – of very bright fireballs with enduring trains. The brightest one was about -10 magnitude with a smoke train fading about three minutes after. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrasonic detection of a Leonid bolide: 1998 November 17

Abstract— During the early morning hours of the night of the peak of the annual Leonid meteor shower on 1998 November 17, a bright fireball (approximately ?12 to ?14 visual magnitude at 100 km in the zenith) was observed over northern New Mexico with visual sightings as far away from Los Alamos as Albuquerque (～150 km to the south of Los Alamos), including direct persistent trail observations at the U. S. A. F. Starfire Optical Range (SOR), which is also near Albuqerque. This event did not produce any sonic boom reports, presumably because of its high altitude. It was also detected locally by an infrared radiometer at Sandia National Laboratory and by an intensified charge-coupled device (CCD) camera located in Placitas, New Mexico. Subsequent investigations of the data from the six infrasound arrays used by Los Alamos National Laboratory (LANL) and operated for the Department of Energy as a part of the Comprehensive Test Ban Treaty (CTBT) Research and Development program for the International Monitoring System (IMS) showed the presence of an infrasonic signal from the proper direction at the correct time for this bolide from two of our six arrays (both located in Los Alamos). The infrasound recordings (i.e., the wave amplitude and period data) indicated that an explosion occurred in the atmosphere at a source height of ～93.5 km (with respect to sea level) or ～90 km with respect to the altitude of Los Alamos, having its origins slightly to the north and west of Los Alamos. Purely geometric solutions from the ground observers reports combined with direct measurements from the CCD camera at Placitas produced a source height of 91 ± 7 km. The signal characteristics analyzed from 0.5 to 3.0 Hz include a total duration of about 3–4 s for a source directed from Los Alamos toward 353.6 ± 0.4° measured from true north at a maximum elevation arrival angle of ～72.7°. The latter was deduced on the basis of the observed signal trace velocities (for the part of the recording with the highest cross-correlation) and ranged from a constant value of about 920–1150 m/s (depending on the window length used in the analysis) for a ray trajectory along a direct refractive path between the source and the Los Alamos arrays. The dominant signal frequency at maximum amplitude at Los Alamos was ～0.71 Hz. These highly correlated signals had a peak to peak, maximum amplitude of ～2.1 microbars (0.21 Pa). Using several methods that incorporate various observed signal characteristics, total distance traveled, etc., our analysis indicates that the bolide probably had a source energy of ～1.14 t (TNT equivalent) or 4.77 × 10⁹ J. This is ～14.1× smaller than the source energy estimate made using the infrasonic, empirical source energy relationship for low-altitude stationary point sources developed in the 1960s by the Air Force Technical Applications Center (AFTAC), Patrick Air Force Base, Florida. This relation was originally developed, however, for much larger source energies and at much longer ranges.     

Spaceborne ultraviolet 251–384 nm spectroscopy of a meteor during the 1997 Leonid shower

Abstract— We used the ultraviolet to visible spectrometers onboard the midcourse space experiment to obtain the first ultraviolet spectral measurements of a bright meteor during the 1997 Leonid shower. The meteor was most likely a Leonid with a brightness of about‐2 magnitude at 100 km altitude. In the region between 251 and 310 nm, the two strongest emission lines are from neutral and ionized magnesium. Ionized Ca lines, indicative of a hot T ? 10 000 K plasma, are not detected. The Mg and Mg+ line intensity ratio alone does not yield the ionization temperature, which can be determined only by assuming the electron density. A typical air plasma temperature of T = 4400 K would imply a very high electron density: n_e = 2.2 times 10¹⁸ m‐³, but at chondritic abundances of Fe/Mg and Si/Mg ? 1. For a more reasonable local‐thermodynamic‐equilibrium (LTE) air plasma electron density, the Mg and Mg+ line ratio implies a less than chondritic Fe/Mg = 0.06 abundance ratio and a cool non‐LTE T = 2830 K ionization temperature for the ablation vapor plasma. The present observations do not permit a choice between these alternatives. The new data provide also the first spectral confirmation of the presence of molecular OH and NO emission in meteor spectra.     

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.     

The 1998 Leonid multi-instrument aircraft campaign—an early review

Abstract— The 1998 return of the Leonid shower was the target of the Leonid multi-instrument aircraft campaign (Leonid MAC), an unusual two-aircraft astronomical research mission executed near Okinawa, Japan. The prospect of a meteor storm brought 28 researchers of 7 nationalities together in a concerted effort to observe the shower by imaging, spectroscopic, and ranging techniques. This paper is a review of the major science issues that are behind the deployment of each of the present array of instruments and describes the interconnection of the various experiments. This was NASA's first astrobiology mission. The mission also aimed to study contemporary issues in planetary astronomy, in atmospheric sciences, and concerning the satellite impact hazard. First results of the participating observers are discussed and put in context, in preparation for the deployment of a planned second mission in November of 1999.     

The 1998 outburst and history of the June Boötid meteor shower

The June Boötid meteor shower (sometimes referred to as the Draconids) surprised a number of regular and casual observers by an outburst with maximum zenithal hourly rates (ZHRs) near 100 on 1998 June 27 after a quiescent period of several decades. A total of 1217 June Boötid meteors were recorded during regular visual meteor observations throughout this outburst. An average population index of r =2.2±0.10 was derived from 1054 shower magnitude estimates. The broad activity profile with ZHR>40 lasting more than 12 h and the large spread of apparent radiants in 1998 resemble the 1916 and 1927 outbursts. The peak time is found to be at about λ _⊙=95°.7 (2000.0); peak ZHRs are of the order of 200, whereas reliable averages reach only 81±7. The period of high ZHRs covered by a single observer implies a full width at half-maximum of 3–4 h. The resulting maximum flux of particles causing meteors brighter than +6.5 mag is between 0.04 and 0.06 km⁻² h⁻¹. The average radiant from photographic, radar and visual records is α =224°.12, δ =+47°.77. The observed activity outbursts in 1916, 1927 and 1998 are not related to the orbital period or the perihelion passages of the parent comet 7P/Pons–Winnecke. These are probably a consequence of the effects of the 2:1 resonance with Jupiter.