Physical Characteristics of Kazan Minor Showers as Determined by Correlations with the Arecibo UHF Radar

In the northern hemisphere, the month of February is characterized by a lack of major meteor shower activity yet a number of weak minor showers are present as seen by the Kazan radar. Using the Feller transformation to obtain the distribution of true meteor velocities from the distribution of radial velocities enables the angle of incidence to be obtained for the single beam AO (Arecibo Observatory) data. Thus the loci of AO radiants become beam-centered circles on the sky and one can, with simple search routines, find where these circles intersect on radiants determined by other means. Including geocentric velocity as an additional search criterion, we have examined a set of February radiants obtained at Kazan for coincidence in position and velocity. Although some may be chance associations, only those events with probabilities of association > 0.5 have been kept. Roughly 90 of the Kazan showers have been verified in this way with mass, radius and density histograms derived from the AO results. By comparing these histograms with those of the “background” in which the minor showers are found, a qualitative scale of dynamical minor shower age can be formulated. Most of the showers are found outside the usual “apex” sporadic source areas where it is easiest to detect discrete showers with less confusion from the background.     

Quantitative Comparison of a New Ab Initio Micrometeor Ablation Model with an Observationally Verifiable Standard Model

The Arecibo UHF radar is able to detect the head-echos of micron-sized meteoroids up to velocities of 75 km/s over a height range of 80–140 km. Because of their small size there are many uncertainties involved in calculating their above atmosphere properties as needed for orbit determination. An ab initio model of meteor ablation has been devised that should work over the mass range 10⁻¹⁶ kg to 10⁻⁷ kg, but the faint end of this range cannot be observed by any other method and so direct verification is not possible. On the other hand, the EISCAT UHF radar system detects micrometeors in the high mass part of this range and its observations can be fit to a “standard” ablation model and calibrated to optical observations (Szasz et al. 2007). In this paper, we present a preliminary comparison of the two models, one observationally confirmable. Among the features of the ab initio model that are different from the “standard” model are: (1) uses the experimentally based low pressure vaporization theory of O’Hanlon (A users’s guide to vacuum technology, 2003) for ablation, (2) uses velocity dependent functions fit from experimental data on heat transfer, luminosity and ionization efficiencies measured by Friichtenicht and Becker (NASA Special Publication 319: 53, 1973) for micron sized particles, (3) assumes a density and temperature dependence of the micrometeoroids and ablation product specific heats, (4) assumes a density and size dependent value for the thermal emissivity and (5) uses a unified synthesis of experimental data for the most important meteoroid elements and their oxides through least square fits (as functions of temperature, density, and/or melting point) of the tables of thermodynamic parameters given in Weast (CRC Handbook of Physics and Chemistry, 1984), Gray (American Institute of Physics Handbook, 1972), and Cox (Allen’s Astrophysical Quantities 2000). This utilization of mostly experimentally determined data is the main reason for calling this an ab initio model and is made necessary by the fact that individual average meteoroid mass densities are now derivable from Arecibo observations.     

Meteors in the IAU Meteor Data Center on Hyperbolic Orbits

The hyperbolic meteor orbits among the 4,581 photographic and 62,906 radar meteors of the IAU MDC have been analysed using statistical methods. It was shown that the vast majority of hyperbolic orbits has been caused by the dispersion of determined velocities. The large proportion of hyperbolic orbits among the known meteor showers strongly suggests the hyperbolicity of the meteors is not real. The number of apparent hyperbolic orbits increases inversely proportional to the difference between the mean heliocentric velocity of meteor shower and the parabolic velocity limit. The number of hyperbolic meteors in the investigated catalogues does not, in any case, represent the number of interstellar meteors in observational data. The apparent hyperbolicity of these orbits is caused by a high spread in velocity determination, shifting a part of the data through the parabolic limit.     

Meteor Orbit Determinations with Multistatic Receivers Using the MU Radar

The MU radar of RISH (Research Institute for Sustainable Humanosphere, Kyoto University), which is a MST radar (46.5 MHz, 1 MW peak power), has been successfully applied to meteor studies by using its very high versatility. The system has recently renewed with 25 channel digital receivers which significantly improved the sensitivity and precision of interferometer used in meteor observation. The transmission is now synchronized to GPS signals, and two external receiving sites with a ranging capability has additionally been operated in order to determine the trajectories and speeds of meteoroids.     

Power Fluctuations in Meteor Head Echoes Observed with the EISCAT VHF Radar

We present observations and preliminary results from a meteor experiment carried out with the 224 MHz EISCAT VHF radar in Tromsø, Norway, which was run for 6 h on November 26, 2003. The data set contains echoes with peculiar pulsations in received power in the frequency range 20–200 Hz, limited by instrumental parameters. The process causing the echo power pulsations has not yet been identified. Plasma effects are the most likely cause, a possible mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.     

Orbit characteristics of the tristatic EISCAT UHF meteors

The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s⁻¹ and a retrograde population peaking at 59 km s⁻¹. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.     

用陕西天文台流星雷达监测人为空间碎片可能性的分析

本文分析了利用中国科学院陕西天文台的流星雷达进行人为空间碎片监测的可能性．详细计算了到达接收机的回波的信噪比Ｓ／Ｎ依赖于目标散射横截面。和高度距离Ｒ的关系。理论计算表明，利用陕西天文台的流星雷达完全有可能监测在２００ｋｍ至１２００ｋｍ的高度范围内，半径大于０．５ｍ的人为空间碎片．     

Plasma and Electromagnetic Simulations of Meteor Head Echo Radar Reflections

Recently, meteor head echo detections from high powered large aperture radars (HPLA) have brought new measurements to bear on the study of sporadic interplanetary meteors. These same observations have demonstrated an ability to observe smaller meteoroids without some of the geometrical restrictions of specular radar techniques. Yet incorporating data from various radar reflection types and from different radars into a single consistent model has proven challenging. We believe this arises due to poorly understood radio scattering characteristics of the meteor plasma, especially in light of recent work showing that plasma turbulence and instability greatly influences meteor trail properties at every stage of evolution. In order to overcome some of the unknown relationships between meteoroid characteristics (such as mass and velocity) and the resulting head echo radar cross-sections (RCS), we present our results on meteor plasma simulations of head echo plasmas using particle in cell (PIC) ions, which show that electric fields strongly influence early stage meteor plasma evolution, by accelerating ions away from the meteoroid body at speeds as large as several kilometers per second. We also present the results of finite difference time domain electromagnetic simulations (FDTD), which can calculate the radar cross-section of the simulated meteor plasma electron distributions. These simulations have shown that the radar cross-section depends in a complex manner on a number of parameters. In this paper we demonstrate that for a given head echo plasma the RCS as a function of radar frequency peaks at sqrt (2*peak plasma frequency) and then decays linearly on a dB scale with increasing radar frequency. We also demonstrate that for a fixed radar frequency, the RCS increases linearly on a dB scale with increasing head echo plasma frequency. These simulations and resulting characterization of the head echo radar cross-section will both help relate HPLA radar observations to meteoroid properties and aid in determining a particular radar facility’s ability to observe various meteoroid populations.     

流星雷达测距精度的改进

针对目前流星雷达测距误差大的问题，本文提出了提高流星雷达测距精度的新方法，即提高采样速率，用相关分析确定回波脉冲参考点的方法。该方法使流星雷达的测距精度提高一个数量级，测距误差降到±１４ｍ，使流星雷达不仅可以用来观测研究流星，还可用于监测飞机、火箭的飞行等，扩大流星雷达的应用。     

Radio and Meteor Science Outcomes From Comparisons of Meteor Radar Observations at AMISR Poker Flat, Sondrestrom, and Arecibo

Radio science and meteor physics issues regarding meteor “head-echo” observations with high power, large aperture (HPLA) radars, include the frequency and latitude dependency of the observed meteor altitude, speed, and deceleration distributions. We address these issues via the first ever use and analysis of meteor observations from the Poker Flat AMISR (PFISR: 449.3 MHz), Sondrestrom (SRF: 1,290 MHz), and Arecibo (AO: 430 MHz) radars. The PFISR and SRF radars are located near the Arctic Circle while AO is in the tropics. The meteors observed at each radar were detected and analyzed using the same automated FFT periodic micrometeor searching algorithm. Meteor parameters (event altitude, velocity, and deceleration distributions) from all three facilities are compared revealing a clearly defined altitude “ceiling effect” in the 1,290 MHz results relative to the 430/449.3 MHz results. This effect is even more striking in that the Arecibo and PFISR distributions are similar even though the two radars are over 2,000 times different in sensitivity and at very different latitudes, thus providing the first statistical evidence that HPLA meteor radar observations are dominated by the incident wavelength, regardless of the other radar parameters. We also offer insights into the meteoroid fragmentation and “terminal” process.     

流星目视观测中人眼的感知函数

迄今为止,在流星的多种观测手段中,目视观测仍居主导地位.特别是在强流星雨来临的时候,遍布世界的爱好者绝大多数均使用目视观测.因此,人眼对流星的感知函数在研究中起了重要作用.上世纪九十年代,已经基本上完成了对人眼感知函数原始数据的测定.然而,对感知函数的数学表达的解析形式一直在探讨之中.作者近年来的研究,不仅给出了一种对人眼感知函数的一元表达,而且给出了二元表达形式,无论在理论还是实测方面,对流星的目视观测研究都有实际的用途.     

Probabilities of Perception and Population Index in Visual Observations of Meteor Showers

Earth, Moon, and Planets -     

Global Detection of Infrasonic Signals from Three Large Bolides

We present the infrasonic observations of three large bolides that were observed at numerous International Monitoring System (IMS) infrasound arrays on a global scale. First, a simple procedure for the global association of infrasound detections from large infrasound events is outlined. Infrasound signals are associated with large events based on arrival time, backazimuth and uniqueness at a given IMS array. Next, we apply the algorithm to three bolides and investigate some of the factors affecting the detectability of infrasound from large events. Our findings suggest that site-noise effects significantly degrade the capability of the IMS infrasound network, suggesting that more effort is required to reduce ambient site noise. These results have implications for the use of infrasound measurements (in particular those from IMS stations) as a tool for evaluating the global flux of near-Earth objects.     

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.     

Determination of the Velocity of Meteors Based on Sinodial Modulation and Frequency Analysis

In meteor photography the velocity of meteors is generally obtained from a chopper which blocks periodically the incident light beam in front of the camera lens. In this paper I examine modulation of the meteor trail instead with a sinodial function and use frequency analysis to compute accurately the mean atmospheric velocity.     

CTIO1m反射望远镜的色折射改正和星等差

利用ＣＴＩＯ（ＣｅｒｒｏＴｏｌｏｌｏＩｎｔｅｒ－ＡｍｅｒｉｃａｎＯｂｓｅｒｖａｔｏｒｙ）１ｍ反射望远镜观测球状星团ωＣｅｎ区域的Ｖ和Ｂ波段的照相底片，分析研究了色折射改正；描述了测定色折射改工项系数的方法，得到该望远镜在Ｖ和Ｂ波段照相观测的色折射改正项的系数分别为１．３５２１″和４．５４０３″．随后分析讨论了与星等有关的系统误差，结果表明，对大部分照相压片，星等差是很明显的．在利用底片常数归算时，星等的线性和二次项必须包括在内．     

Detection of large grains in the coma of Comet C/2001 A2 (LINEAR) from Arecibo radar observations

Arecibo S-band () radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least λ/2π or 2 cm in radius. The grains are moving at tens of m s⁻¹ with respect to the nucleus. The nondetection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ∼a⁻⁴) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture.     

The Meteors, Meteoroids and Interplanetary Dust Program of the International Heliophysical Year 2007/9

Under the title ‘Meteors, Meteoroids and Interplanetary Dust’, meteor research is included in the program of the International Heliophysical Year 2007/9.We list issues for coordinated meteor research within the framework of this global international program.