1998～2000年狮子座流星雨的预报和观测

“1998年狮子座流星雨可能是暴雨，最佳观测地在东亚”的预报，客观上加速了中国对流星雨的现代研究。几年来，对狮子座流星雨的观测和研究，是在全国狮子座流星雨联测小组的组织和领导下发展起来的。这包括使用目视方法、照相方法和无线电方法的观测，以及组织广大天文爱好者在全国范围内的观测。我们的确也观测到了很多重要的现象，如流星雨的纤维结构、尺度分布和质量密度分布的变化等等。我们发展了数据处理方法，将处理其它天文观测数据的“时间窗”方法移植到了流星雨的数据处理中。这一方法如今在流星雨研究中已经在世界上普遍使用了。我们还提出了流星雨的质量流参量ZHMR，它对于今后流星雨的观测比较和预报可能更加有用。在雷达观测方面，我们观测到了1998年所期待的流星雨强度主峰。在此主峰过了大约18h之后，又观测到了地球电离层的反常爆发。从1998年连续两天狮子座流星雨不同的表现，以及从全国不同地区的观测得到的流星质量分布系数的差异，我们提出了大、小2种不同粒子的混合流模型，很好地解释了观测到的现象。从天体力学的基本公式出发，我们发现流星群轨道升交点和其母彗星轨道升交点之差，不仅与当时的抛射速度，而且与彗星抛出这些尘埃粒子时在轨道上的位置有关。对于1998年的观测，最可能的是这些粒子是彗星在1933年回归的时候、在距离太阳1．44A．U．距离的时候抛射出来的。按照传统的统计图分析方法，我们还对1999—2001年的狮子座流星雨强度进行了预报。事实证明，我们对1999和2000年的预报的准确性还是很高的。此外，我们还在狮子座流星雨的照相观测、轨道计算以及其它方面做了一些工作，本文仪仅是对我们所做的一些主要工作进行了一个小结。     

狮子座流星暴雨真的来了

根据国际流星组织和狮子座流星雨研究中心的初步统计显示,11月18日在中东和欧洲地区都观测到狮子座流星雨的大规模爆发,极大就发生在预测地球通过坦普尔塔特尔彗星轨道面时的格林尼治时间18日凌晨2点钟。狮子座流星暴雨真的来了。欧洲和中东的观测　天文学家和天文爱好者?..     

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。     

流星的摄影观测技巧

一、流星摄影观测的意义１９９８年１１月狮子座流星雨已经过去,但是随着坦普尔－塔特尔彗星的回归,１９９９年的１１月我们还可能看到狮子座流星雨的再现,这无疑又给天文爱好者们一线希望。许多人都想用相机记录下那一令人兴奋的场面,留作永久的纪念。然而,流星的摄...     

举办2001年狮子座流星雨摄影赛通知

鉴于2001年11月19日前后,在中国东部、中部,有可能观测到每小时标准天顶计数峰值达到一万颗以上流星的狮子座流星暴雨。为促进这一可能的、罕见的流星雨观测和保留珍贵的流星雨资料,我们决定举办《2001狮子座流星雨摄影赛》。我们在全国范围内(包括港、澳、台)征集狮子座流星雨的观测照片。来稿要求为: 1.提供拍摄有狮子座流星的5对以上     

流星雨爆发的反响

2001年11月19日凌晨,曾经让爱好者们悲观失望的狮子座流星雨,出色地表现了一回。其实,几年来有关狮子座流星雨的预报和观测过程就是一个生动的科学探索过程。天文学家在不懈地探索总结流星雨的预报理论,今年又坚定地对狮子座流星雨的爆发作出判断,我们《天文爱好者》也及时作了报导;而广大爱好者中,所有坚持者都饱尝了成功的欢乐。 喜悦的心情总是希望能和大家分享。最近,观测这次流星雨的总结报告、摄影作品、心得体会等等,像雪片一样纷纷飞到我们编辑部。但是由于版面限制,我们只能选择其中的一小部分发表在这里(而且还不得不作了大量的删节)。我们还用“流星雨爆发的反响”为题,写了一篇摘编报导。挂一漏万,不妥之处,只能请求您的谅解了。至于流星雨观测数据,敬请及时上报有关组织。     

Tempel—Tuttle彗星与近年的狮子座流星雨

对狮子座流雨的历史进行了回顾和讨论，并利用“彗星－地球轨道分离（CEOS）及地球滞后彗星时间（TE－C）”统计图进行分析，发现几乎所有的狮子座流星都位于一个倾斜的方框内，而这倾斜方框械右边界的斜率大约为15m/s，方框的宽度大约为4yr，它表明，33年一度的狮子座流星雨一般不会有超过4年的爆发期，更细致的分析表明，最强的流星暴位于一弯曲的细窄条带，在慧星一次回归期，亮流星的比例将年衰减，这些事实，可以用运动，碎裂，扩散和尘埃彗尾模型进行解释，由15m/s速度得到的流星体尺度大小也与事实相容，并且，这表明与地球相遇的流星体粒子是以有限的速度偏离彗星时间（TE－C）就越长，由此倾斜方框的存在，可以对未来狮子座流星雨进行了预报，表明在1998－2000年期间将有较强的狮子座流星雨，中心在1999年，至于2000年以后，要在100多年以后才会有较强的流星暴，而狮子座流星雨的辉煌期可以说已经过去。     

彗星的喷发速度

彗星的喷发速度在流星群的演化过程中起着重要作用。本文简述了 (1 )流星群轨道升交点的变化和周期的变化都与喷发速度密切相关 ,(2 )从流星雨的观测结果可推断出喷发速度 ,(3)喷发速度的理论模型和结果。     

1998年德令哈狮子座流星雨目视观测及初步分析

根据中国－荷兰狮子座流星雨合观测1998年11月13日至18是日在紫金山天文台德令哈观测站观测所得的数据，以及同期中国其他地方的观测数据，分析了1998年狮子座流星雨的分布特征，提出此次地球所遭遇的流星群带呈3层结构。     

北京市天文爱好者拍摄的狮子座流星雨

为配合2001年11月的狮子座流星雨观测活动,北京市青少年天文爱好者协会于2001年底在全市举办了相关的天文摄影比赛活动。摄影赛作品征集题材除流星雨项目外,还包括月球、太阳以及行星、星云等,考虑到本次征集到的狮子座流星雨作品较多,故征集的其他作品待下一次活动中评审。此项活动得到了全市     

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.     

The origin of the June Bootid outburst in 1998 and determination of cometary ejection velocities

Numerical integrations are used to show that the main contribution to the outburst observed in the June Bootid meteor shower in 1998 was a subset of meteoroids released from the parent comet, 7P/Pons–Winnecke, at its 1825 return. A substantial part of the June Bootid stream is in 2:1 resonance with Jupiter. This inhibits chaotic motion, allowing structures in the stream to remain compact enough over centuries that meteor outbursts can still be produced. Circumstances of ejection in 1825 are calculated that exactly result in orbits capable of producing meteors at the observed time in 1998. Required ejection velocities are  10–20 m s^-1  .     

Spectroscopic Observations of the 2011 Draconids Meteor Shower

Some past October Draconid shower meteoroids fell apart in a spray of fragments at the end of their trajectory before slowing down, from which it was concluded that these were among the most fragile meteoroids known. In those instances, the dust could not be reliably traced to a particular return of the parent comet 21P/Giaconini-Zinner. On October 8th, 2011, Earth was predicted to transverse the 1900 A.D. dust ejecta of the comet. In 1900, the comet’s perihelion distance first moved significantly inwards to the Sun and ejection conditions could have been unusual. An airborne observing campaign was organized, with several teams contributing imaging and spectrographic cameras to study the manner in which these meteoroids released the volatile element sodium during the ablation process in the Earth’s atmosphere. IMCCE, ESA, and the SETI Institute contributed spectrographic cameras based on low-light WATEC 902H2 Ultimate, low-light LCC1, and GenII XX1332 image intensified cameras. An outburst was observed, much as predicted. Despite a lack of bright meteors, a total of 15 Draconid spectra were recorded. All show evidence of an early release of sodium. The loss of sodium was observed to coincide with the formation of a distinct wake of fragments. The observations show that 21P/Giacobini-Zinner ejected fragile meteoroids during the return in 1900. Those grains may have lost some sodium even before impacting Earth.     

Geminid Meteor shower activity 2003–2005 as observed by Gadanki radar

The distribution of meteor signals reflected from a backscatter radar is considered according to their duration. This duration time (T) is used to classify the meteor echoes and to calculate the mass index (S) of different meteoroids of shower plus sporadic background. Observational data on particle size distribution of the Geminid meteor shower are very scarce, particularly at low latitudes. In this paper the observational data from Gadanki radar (13.46°N, 79.18°E) have been used to determine the particle size distribution and the number density of meteoroids inside the stream of the Geminid meteor shower. The mean variation of meteor number density across the stream has been determined for three echo duration classes, T<0.4, T=0.4–1 and T>1 s. We are more interested in the appearance of echoes of various durations and therefore meteors of various masses in order to understand more on the filamentary structure of the stream. It is observed that the faint particle flux peaks earlier than the larger particles. We found a decreasing trend in the mass index values from the day of peak activity to the next observation days. The mass index profile was found to be U-shaped with a minimum value near the time of peak activity. The observed minimum s values are 1.64±0.05 and 1.65±0.04 in the years 2003 and 2005, respectively. The activity of the shower indicates the mass segregation of meteoroids inside the stream. Our results are best comparable with the “scissors” structure model of the meteoroid stream formation of Ryabova [2007. Mathematical modeling of the Geminid meteoroid stream. Mon. Not. R. Astron. Soc. 375, 1371–1380] by considering the asteroid 3200 Phaethon as an extinct comet.     

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…     

A meteoroid stream survey using the Canadian Meteor Orbit Radar: I. Methodology and radiant catalogue

Using a meteor orbit radar, a total of more than 2.5 million meteoroids with masses ∼10⁻⁷ kg have had orbits measured in the interval 2002-2006. From these data, a total of 45 meteoroid streams have been identified using a wavelet transform approach to isolate enhancements in radiant density in geocentric coordinates. Of the recorded streams, 12 are previously unreported or unrecognized. The survey finds >90% of all meteoroids at this size range are part of the sporadic meteoroid background. A large fraction of the radar detected streams have q<0.15 AU suggestive of a strong contribution from sungrazing comets to the meteoroid stream population currently intersecting the Earth. We find a remarkably long period of activity for the Taurid shower (almost half the year as a clearly definable radiant) and several streams notable for a high proportion of small meteoroids only, among these a strong new shower in January at the time of the Quadrantids (January Leonids). A new shower (Epsilon Perseids) has also been identified with orbital elements almost identical to Comet 96P/Machholz.     

A parameterization for the radio emission of air showers as predicted by CoREAS simulations and applied to LOFAR measurements

Measuring radio emission from air showers provides excellent opportunities to directly measure all air shower properties, including the shower development. To exploit this in large-scale experiments, a simple and analytic parameterization of the distribution of the radio signal at ground level is needed. Data taken with the Low-Frequency Array (LOFAR) show a complex two-dimensional pattern of pulse powers, which is sensitive to the shower geometry. Earlier parameterizations of the lateral signal distribution have proven insufficient to describe these data. In this article, we present a parameterization derived from air-shower simulations. We are able to fit the two-dimensional distribution with a double Gaussian, requiring five fit parameters. All parameters show strong correlations with air shower properties, such as the energy of the shower, the arrival direction, and the shower maximum. We successfully apply the parameterization to data taken with LOFAR and discuss implications for air shower experiments.     

SPECTROSCOPY OF A GEMINID FIREBALL: ITS SIMILARITY TO COMETARY METEOROIDS AND THE NATURE OF ITS PARENT BODY

A detailed analysis of a photographic spectrum of a Geminid fireball obtained in December 14, 1961 at the Ondrejov Observatory is presented. We have computed a synthetic spectrum for the fireball and compared it with the observed spectrum assuming chemical equilibrium in the meteor head. In this way we have determined relative chemical abundances in meteor vapors. Comparing the relative chemical abundances of this Geminid meteoroid with those obtained from meteoroids associated with comets 55P/Tempel-Tuttle and 109P/Swift-Tuttle we found no significant chemical differences in main rock-forming elements. Despite of this similarity, the deepest penetration of the Geminid meteoroids and their ability to reach high rotation rates in space without fragmentation suggest that thermal processing is affecting their physical properties. We suggest that as consequence of space weathering a high-strength envelope is produced around these particles. In this picture, heating processes of the mineral phases could result in the peculiar properties observed during atmospheric entry of the Geminid meteoroids, especially their strength, which is evidenced by its resistance to ablation. Finally, although one meteoroid cannot be obviously considered as representative of the composition of its parent body, we conclude that 3200 Phaethon is able to produce millimetre-size debris nearly chondritic in composition, but the measured slight overabundance of Na would support a cometary origin for this body.     

The 'silent world' of Comet 15P/Finlay

Comet 15P/Finlay is unusual in that, contrary to ab initio expectations, it demonstrates no apparent linkage to any known meteor shower. Using data contained within the Electronic Atlas of Dynamical Evolutions of Short-Period Comets, we evaluate theoretical shower radiants for Comet 15P/Finlay, but find no evidence to link it to any meteoric anomalies in recorded antiquity. This result, however, must be tempered by the fact that any Comet 15P/Finlay-derived meteoroids will have a low, 16 km s⁻¹, encounter velocity with Earth's atmosphere. Typically, therefore, one would expect mostly faint meteors to be produced during an encounter with a Comet 15P/Finlay-derived meteoroid stream. We have conducted a D -criterion survey of meteoroid orbits derived from three southern hemisphere meteor radar surveys conducted during the 1960s, and again we find no evidence for any Comet 15P/Finlay-related activity. Numerical calculations following the orbital evolution of hypothetical meteoroids ejected from the comet, at each perihelion epoch since 1886, indicate that Jovian perturbations effectively 'drive' the meteoroids to orbits with nodal points beyond the Earth's orbit. The numerical calculations indicate that, even if Comet 15P/Finlay had been a copious emitter of meteoroids during the past 100 years, virtually none of them would have evolved into orbits capable of being sampled by the Earth. There are good observational data, however, to suggest that Comet 15P/Finlay is becoming a transitional comet–asteroid object, and that it has probably not been an efficient producer of meteoroids during the past several hundreds of years.