Techniques For Measuring Radar Meteor Speeds

We compare the results from the application of four different methods to determine the speed of meteoroids from single station radar data. The methods used are the pre-t ₀ amplitude, post-t ₀ amplitude, pre-t ₀ phase and the Fresnel transform (FT) methods. Speeds from the first three methods are compared to the FT method since, requiring the use of the entire records of both the amplitude and phase data, this method is the most accurate of the four.     

A Fine Structure of the Perseid Meteoroid Stream

A fine structure of the Perseid stream in the range of photographic magnitudes is studied using the method of indices. A new completed 2003 version of the IAU Meteor Data Center Catalogue of 4581 photographic orbits is used. The method of indices is used to acquire a basic data set for the Perseids. Subsequently, the method is applied on the chosen Perseids to study their structure. Sixty four percent of chosen Perseids taken into account are attached to one of the 17 determined filaments of orbits. The filaments are not distributed in the space accidentally, but they form a higher structure consisting of at least four well-defined and distinguished “branches”.     

Interstellar dust in the solar system

Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic exploration.     

Slow Diffusion and Effective Stability of Dust Particles Orbiting Asteroids

This paper presents quantitative estimates of orbital lifetimes for small dust particles in the vicinity of a spherical asteroid orbiting the Sun on a circular orbit. In particular, the modeling accounts for gravitational interactions with the asteroid and the Sun and radiation pressure from the Sun. The analysis focuses on slow drift of particles in a state-space neighborhood of weakly unstable periodic orbits corresponding to the most tightly bound orbits about the asteroid. Through a Hamiltonian normal-form computation, Nekhoroshev-type estimates are derived that yield regions in state space of effective stability, that is, regions in which the grains would remain over significant periods of time in the absence of other depletion and replenishment mechanisms.     

THE CORE OF THE QUADRANTID METEOROID STREAM IS TWO HUNDRED YEARS OLD

The Quadrantids are one of the most active annual meteor showers and have a number of unusual features. One is a sharp brief maximum, 12–14 h in length. A second is the Quadrantids, relatively recent appearance in our skies, the first observation having likely been made in 1835. Until recently no likely parent with a similar orbit had been observed and previous investigators concluded that the stream was quite old, with the stream’s recent appearance and sharp peak attributed to a recent fortuitous convergence of meteoroid orbits. The recent discovery of the near-Earth asteroid 2003 EH1 on an orbit very similar to that of the Quadrantids has almost certainly uncovered the parent body of this stream. From the simulations of the orbit of this body and of meteoroids released at intervals from it in the past, we find that both the sharp peak and recent appearance of the Quadrantids can most easily be explained assuming meteoroids were ejected in substantial numbers near 1800 AD.     

The Dynamics of Low-Perihelion Meteoroid Streams

The Canadian Meteor Orbit Radar (CMOR) has collected information on a number of weak meteor showers that have not been well characterized in the literature. A subsample of these showers (1) do not show a strong orbital resemblance to any known comets or asteroids, (2) have highly inclined orbits, (3) are at low perihelion distances ( AU) and (4) are at small semimajor axes (<2 AU). Though one might conclude that the absence of a parent object could be the result of its disruption, it is unclear how this relatively inaccessible (dynamically speaking) region of phase space might have been populated by parents in the first place. It will be shown that the Kozai secular resonance and/or Poynting–Robertson drag can modify meteor stream orbits rapidly (on time scales comparable to a precession cycle) and may be responsible for placing some of these streams into their current locations. These same effects are also argued to act on these streams so as to contribute to the high-ecliptic latitude north and south toroidal sporadic meteor sources. There remain some differences between the simple model results presented here and observations, but there may be no need to invoke a substantial population of high-inclination parents for the observed high-inclination meteoroid streams with small perihelion distances.     

A weakly non-gaussian model of wave height distribution for random wave train

The wave height distribution with Edgeworth’s form of a cumulative expansion of probability density function (PDF) of surface elevation are investigated. The results show that a non-Gaussian model of wave height distribution reasonably agrees with experimental data. It is discussed that the fourth order moment (kurtosis) of water surface elevation corresponds to the first order nonlinear correction of wave heights and is related with wave grouping.     

A determination of the strengths of the sporadic radio-meteor sources

We have calculated the response functions for the Springhill (Ottawa) and Christchurch (New Zealand) radars. Using the sizes and locations of each of the six sporadic radio-meteor sources found in our previous study (Jones and Brown, 1993), we have determined the response of each radar to each of the sources. Then, using least-squares fit to data collected over several years, we have determined the strength of each source. Excellent agreement with the non-radar data is obtained for the total flux after application of a correction for the initial train radius. The results from the two radars show good consistency for the sources which are visible to both of them. For individual years the fits are good to between 5 and 10% but there does appear to be some variability from year to year which may be due to atmospheric effects. Presented at theInternational Conference on Meteoroids 28–31 August 1994 in Bratislava.Submited to Earth, Moon and Planets     

1961-2005年中国霾日气候特征及变化分析

2015年1月15—16日, 宜昌及周边地区经历了连续性严重污染天气过程。文中利用污染物浓度数据和严重污染时段内气象资料及后向轨迹模型, 分别分析了严重污染天气发生发展过程中的气象条件和污染物源地。分析表明, 细颗粒物浓度(PM2.5, 大气中直径小于等于2.5 μm的颗粒物)具有明显的日变化特征, 峰值一般出现在午夜, 次峰值出现在中午12时前后, 两个谷值分别出现在日出前和下午16—17时。通过与历史气候数据比较发现, 2015年1月14—15日污染物浓度快速上升的时段内, 宜昌站的气象条件表现为明显的相对湿度正距平(10%)以及较大的最大风速负距平(－1m·s^-1); 宜昌城区位于地形辐合线影响区域, 有利于细颗粒物的聚集。宜昌站的探空数据表明, 在1月14—15日期间, 垂直方向上, 1 km以下的大气中存在明显的逆温层, 并且相对湿度保持较高的值(80%~90%)。风廓线雷达监测结果表明, 1月14—15日, 边界层顶较低, 边界层以内为小风或静风流场, 且以下沉气流为主。以上气象条件有利于气溶胶的吸湿增长和浓度的聚集。对1月1—20日后向轨迹分析表明, 在宜昌地区, 与4—5日相比, 在15—16日持续性严重污染过程中, 偏南轨迹造成的污染物在本地滞留性、往返性运动更明显, 而且在到达本地之前6 h, 均从城区以东进入, 配合向东开口的喇叭口地形, 更易造成持续性严重污染。

     

滇南中小尺度灾害天气的多普勒统计特征及识别研究

利用普洱CIND3830-CC新一代天气雷达资料、地面观测资料、探空资料,对2004—2009年滇南普洱、西双版纳典型的中小尺度强对流天气的多普勒雷达回波特征进行统计分析,总结冰雹、大风、短时强降水的识别方法和预报指标。结果表明：冰雹云初始回波中心强度在40 dBz左右,高度在5 km左右,接近0℃层高度。冰雹云径向速度≥10m·s^-1,辐合特征明显,97%的移速≥30 km·h^-1,中心强度为55～69 dBz。97%的冰雹云的45 dBz回波顶高≥7.5 km,92%的冰雹云的45 dBz回波顶高超过-20℃层高度;大风回波可分为4种类型。96%的大风回波径向速度≥10 m·s^-1,50%的辐合特征明显,85%的移速≥30 km·h^-1,大风回波中心强度为30～55 dBz;强降水回波的辐合特征明显,79%的回波径向速度〈10 m·s^-1,85%的移速〈30 km·h^-1,回波强度集中在40～45 dBz,高度集中在6.5 km以下,强中心高度低于4.5 km,85%的强降水回波移速〈30 km·h^-1。这些特征可为短时临近预报提供参考。