Copies of author's previously published articles are inserted between p. 272-283 and are not numbered as part of main text. / Bibliography: p. 283-293. / xxii, 293 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis is concerned with the observations of meteors with a narrow beam, high gain, VHF radar system, operating at a frequency of 54.1 MHz. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1996

Small near-Earth asteroids (NEAs) (< 20 m) are interesting, because they are progenitors for meteorites in our terrestrial collection. The physical characteristics of these small NEAs are crucial to our understanding of the effectiveness of our atmosphere in filtering low-strength impactors. In the past, the characterization of small NEAs has been a challenge, because of the difficulty in detecting them prior to close Earth flyby. In this study, we physically characterized the 2 m diameter NEA 2015 TC25 using ground-based optical, near-infrared and radar assets during a close flyby of the Earth (distance 128,000 km) in 2015 October 12. Our observations suggest that its surface composition is similar to aubrites, a rare class of high-albedo differentiated meteorites. Aubrites make up only 0.14% of all known meteorites in our terrestrial meteorite collection. 2015 TC25 is also a very fast rotator with a period of 133 +/- 6 s. We combined the spectral and dynamical properties of 2015 TC25 and found the best candidate source body in the inner main belt to be the 70 km diameter E-type asteroid (44) Nysa. We attribute the difference in spectral slope between the two objects to the lack of regolith on the surface of 2015 TC25. Using the albedo of E-type asteroids (50%-60%) we refine the diameter of 2015 TC25 to 2 m, making it one of the smallest NEAs ever to be characterized.

Parallel double-station video observations paired with spectroscopic video observations are a good way to study millimetre-sized meteoroids. Almost two decades of video observations of meteors at the Ondřejov observatory give us broad database to study large quantities of meteoroids and their properties. In this work we combined spectral video observations and results of the modelling of the fragmentation of meteoroids. Along with complex infor- mation about meteoroid's trajectories and orbits, this can give us better understanding about origin, internal structure etc. of these millimetre-sized interplanetary bodies. Meteoroids that contained small grains tend to release the sodium early. Since there is a smaller amount of sodium for Na depleted meteoroids, the sodium was released earlier than it was released for meteoroids with same grain sizes and without the sodium depletion. Overall, meteoroids with sodium depletion showed different composition: they were composed of stronger material without very small grains and they did not showed very bright wakes. Two iron meteoroids on Halley type orbits were observed. They are probably remnants of complicated early years of our solar system. The dis- tribution of grain sizes of Jupiter-family members was in a good agreements with results from the COSIMA...

Thesis. 1976. B.S.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Includes bibliographical references. / by Joseph B. Hamilton. / B.S.

This thesis is concerned with the observation and study of meteors with a narrow beam VHF radar operated by the University of Adelaide at the Buckland Park research station, in particular the study of the structure and characteristics of meteor showers and the geocentric speeds of meteors. There have been several observations of meteors with the radar previously (Steel & Elford 1991, Cervera 1996, Taylor et al. 1996), but this is the first with an automated data analysis directed to a systematic study of the properties of meteor showers. The Buckland Park VHF radar offers significant advantages over the wide beam radars traditionally used for meteor observation. The narrow beam, while reducing the collecting area of the radar, allows observations of much lower electron line densities than a wide beam radar of similar power. It also allows the determination of meteor shower radiants by the use of the radar response function. Pulse repetition frequencies of up to 2000 Hz allow excellent time resolution, and the ability to record in-phase and quadrature data allows the phase information to be used. This phase information is important as it allows the use of the phase information to accurately determine radial wind drifts, and the atmospheric speeds of meteoroids. During 1998, 1999 and 2000, observations were made of a number of meteor showers and the sporadic background. These showed that the η-Aquarid meteor shower was active in these years, and the Orionid and the Leonid showers were detected in 1999. Analysis of the η-Aquarid activity revealed multiple peaks which show that the shower is produced by at least four distinct "filaments", subsets of the meteoroid stream which produces the shower. Not only does the stream have spatial structure, containing groups of particles in different orbits, but also the presence or absence of the peaks in a random fashion from day to day shows that the filaments are themselves made up of clumps of particles. The radar response function is developed and used to determine radiants for the four η-Aquarid filaments. Evidence is given of a significant number of meteors detected at heights which are above the "radar ceiling", a height at which the theoretical initial radius attenuation factor is near zero for radars operating at the frequency of the Buckland Park VHF Radar, and underdense echoes should be impossible to detect. Investigation showed that over 60 % of meteor trails at heights above the ceiling (105 km) were underdense. Not only does the expected meteor height distribution extend up to 130 km, but also another distribution, peaking at 145 km is uncovered. Diffusion coefficients estimated from the decay of echoes are compared to theoretical calculations. There is a general agreement, but a number of meteor trails show slower diffusion than expected. This is attributed to the effect of the Earth's magnetic field. Three methods are used to determine meteoroid speeds using the phase data, each applying to a different type of meteor echo, and in combination, speeds could be determined for over 90 % of all meteor echoes. The first, the pre-t0 method can be applied to transverse meteor echoes with great success, although it may underestimate the speed of weak echoes with speeds under 15 km s-¹. Using the Cauchy approximations to the Fresnel integrals allows speed determination from head echoes which were aliased near the t0-point. Meteor trails which form at a small angle to the boresight of the radar beam are called "down-the-beam" echoes, and a new method is developed to determine the meteoroid speed and deceleration from these. The speed measurements of meteors detected during the η-Aquarids show a strong peak in the distribution at 66 km s-¹, as well as a smaller peak at 50 km s-¹, which may due to a minor shower. The sporadic background shows a broad peak at 25 km s-¹, with a smaller peak at 58 km s-¹. Distributions of the speed of meteors in the sporadic background show good agreement with previous observations (McCrosky & Posen 1961, Nilsson 1962, Elford et al. 1995, Cervera 1996), with the exception of meteoroid speeds smaller than 15 km s-¹, which can be underestimated by the pre-t0 technique. / Thesis (Ph.D.)--Physics and Mathematical Physics, 2002.

In this thesis, we study two different topics: collisional probability between two bodies and dynamics of the sporadic meteoroids in the Solar System. Determination of the collision probabilities in the Solar System is one of the important problems in mod- ern celestial mechanics. Here, we generalize classical theories of the collisions between two bodies by Öpik, Wetherill or Greenberg by including the Kozai-Lidov oscillations, a mechanism that significantly change orbital eccentricity and inclination in the Solar System. Sporadic meteors have been studied for many decades providing a wealthy re- source of data. Here, we build dynamical steady-state models for all known populations observed in the sporadic meteoroid complex based on the latest and most precise data provided by Canadian Meteor Orbit Radar (CMOR). Our models using the latest theo- ries for cometary populations in the Solar System accurately describe observed sporadic background population. Our results are in agreement with observations provided by space probes IRAS and LDEF.

Many topics in planetary studies demand an estimate of the collision probability of two objects moving on nearly Keplerian orbits. In the classic works of Opik and Wetherill, the collision probability was derived by linearizing the motion near the collision points, and there is now a vast amount of literature using their method. We present here a simpler and more physically motivated derivation for non-tangential collisions in Keplerian orbits, as well as for tangential collisions that were not previously considered. Our formulas have the added advantage of being manifestly symmetric in the parameters of the two colliding bodies. In common with the Opik-Wetherill treatments, we linearize the motion of the bodies in the vicinity of the point of orbit intersection (or near the points of minimum distance between the two orbits) and assume a uniform distribution of impact parameter within the collision radius. We point out that the linear approximation leads to singular results for the case of tangential encounters. We regularize this singularity by use of a parabolic approximation of the motion in the vicinity of a tangential encounter.

Pourquoi est-il si difficile de trouver les corps parents des pluies de météores ? Malgré le développement constant des réseaux de détection dédiés à leur observation, environ 80% des corps parents des pluies de météores établies attendent toujours d'être identifiés. Ce faible taux de réussite est principalement expliqué par le manque de précision des mesures d'orbites de météoroïdes. Les erreurs d'estimation de la vitesse et de la décélération des météores sont particulièrement responsables de la divergence actuelle entre les orbites théoriques et mesurées et entravent la recherche de l'origine orbitale des météoroïdes. Dans ce travail, nous avons démontré que les observations visuelles de météores ne permettent pas d'estimer ces paramètres, et qu'elles sont donc à exclure pour toute détermination d'orbite. À partir d'observations et de milliers de simulations, nous avons également comparé de multiples techniques de détermination de la vitesse de météores capturés par des réseaux de détection photographiques et vidéo. Il est alors apparu que notre nouvelle implémentation de la méthode d'ajustement à paramètres multiples est la technique la plus précise et plus fiable pour évaluer la trajectoire et la vitesse de ces objets, et permet d'atteindre une précision de l'ordre de 1% sur l'estimation de la vitesse pré-atmosphérique des météoroïdes détectés par le réseau CABERNET. Finalement, nous avons mis en place un protocole pour évaluer la durée pendant laquelle la position d'un météoroïde et celle de son corps parents sont suffisamment bien déterminées pour établir une relation entre eux. Cette date limite de traçabilité dépend considérablement de la précision des techniques de réduction des observations de météores et peut dans le cas de mauvaises mesures ne pas excéder quelques dizaines d'années. À l'heure actuelle, le projet CABERNET démarre une phase d'exploitation prolifique qui permettra de contribuer de façon efficace et maîtrisée à l'identification de nouveaux corps parents de pluies de météores. / Why are parent bodies of well-known meteors showers so hard to find ? Despite the development of cameras networks dedicated to their observation, still 80% of the parent bodies of established meteor showers still need to be identified. The main reason for this lack of efficiency is an important uncertainty on the orbit determination of meteoroids. The inaccurate estimates of the velocity and deceleration measurements of meteors are highly responsible for the actual discrepancy between the theoretical and measured orbits and prevent the determination of the orbital origin of meteoroids. In this work, we have shown that visual observations of fireballs cannot lead to a correct estimate of these parameters, and should not be taken into account for any orbit computation. From observations and thousands of simulations, we have compared several methods widely used to compute the velocity of meteors recorded by photographic and video networks. It appears that our implementation of the multi-parameter fitting method is the most reliable and precise technique to compute the trajectory and the velocity of these objects, and leads to an accuracy of about 1% on the pre-atmospheric velocity of the meteoroids recorded by CABERNET. Finally, we have set up a protocol to estimate the duration the position of a meteoroid and its parent body are known with enough precision to be correlated. This traceability duration limit (TDL) highly depends on the accuracy of the data reduction techniques, and may not exceed a few decades in the case of poor quality measurements. Nowadays, the CABERNET project is entering a prolific exploitation phase and will efficiently and reliably contribute to the identification of new parent bodies of meteor showers.

No description available.

Submitted by Jean Medeiros (jeanletras@uepb.edu.br) on 2016-03-03T17:22:59Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) PDF - Sebastião Nascimento de Souza.pdf: 2316347 bytes, checksum: 7e719d9c14cbff4227a3c49a84dc164f (MD5) / Made available in DSpace on 2016-03-03T17:22:59Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) PDF - Sebastião Nascimento de Souza.pdf: 2316347 bytes, checksum: 7e719d9c14cbff4227a3c49a84dc164f (MD5) Previous issue date: 2015-05-29 / In this paper, data from the heights of maximum occurrence of meteor echoes were used to investigate a possible relationship between variations in the height of meteors and solar flux at low latitudes during the solar cycle 23. The heights of meteors were obtained from measurements by SKiYMET meteor radar system at Cachoeira Paulista (22.7 ° S, 45.0 ° W). The data used include measurements taken from March 1999 to July 2006 and from September 2007 to October 2008. The echo of the reach and the zenith angle with respect to each radar echo are used for the heights of occurrence of meteors. The maximum height of daily occurrence was obtained from the Gaussian distribution through vertical adjustment. The series of height data of peak occurrence of meteors was subjected to multiple linear regression analysis to investigate the trend and possible variations induced by solar activity. The results showed a downward trend of approximately 68 m / year at the time of maximum occurrence of meteor and 447 m / 100sfu using monthly data after removal of the solar effect, resulting in a decrease in the peak height of approximately 248 m for all observation period after the time trend removal. Considering only the period of decline of the solar cycle (2002-2008) the decline was approximately 1054 m using monthly data in the analysis. The decrease of the meteors occurrence height can be attributed, in part, the effects of solar activity. Knowing that the atmosphere expands with the growth of solar activity the neutral density shows a downward trend over time. / Neste trabalho, dados das alturas de máxima ocorrência de ecos meteóricos foram usados para investigar uma possível relação entre as variações na altura dos meteoros e fluxo solar em baixas latitudes, durante o ciclo solar 23. As alturas dos meteoros foram obtidos a partir das medidas por radar meteórico SKiYMET em Cachoeira Paulista (22,7°S, 45,0°O). Os dados utilizados incluem as medidas realizadas de março de 1999 a julho de 2006 e de setembro de 2007 a outubro de 2008. O alcance do eco e o ângulo zenital referente a cada eco do radar são usados para obter as alturas de ocorrência dos meteoros. A altura de máxima ocorrência diária foi obtida a partir da distribuição vertical através de ajuste Gaussiano. A série de dados da altura do pico de ocorrência de meteoros foi submetida a análise de regressão linear múltipla para investigar a tendência e possíveis variações induzidas pela atividade solar. Os resultados indicam uma tendência de queda de aproximadamente 68 m/ano na altura de ocorrência máxima dos meteoros e de 447 m/100sfu utilizando os dados mensais após remoção do efeito solar, acarretando numa diminuição da altura do pico de aproximadamente 248 m para todo o período observado após remoção da tendência do tempo. Considerando apenas o período de declínio do ciclo solar (de 2002 a 2008) a queda foi de aproximadamente 1054 m utilizando nas análises os dados mensais. O decréscimo da altura de ocorrência dos meteoros pode ser atribuído, em parte, a efeitos da atividade solar. Sabendo que a atmosfera se expande com o crescimento da atividade solar a densidade neutra apresenta uma tendência decrescente ao longo do tempo.