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41.
David D. Meisel Johan Kero Csilla Szasz Vladimir Sidorov Stan Briczinski 《Earth, Moon, and Planets》2008,102(1-4):315-322
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. 相似文献
42.
Joseph A. Nuth III John L. Lowrance George R. Carruthers 《Earth, Moon, and Planets》2008,102(1-4):495-504
The prime measurement objective of the Near Earth Object Chemical Analysis Mission (NEOCAM) is to obtain the ultraviolet spectra
of meteors entering the terrestrial atmosphere from ∼125 to 300 nm in meteor showers. All of the spectra will be collected
using a slitless ultraviolet spectrometer in Earth orbit. Analysis of these spectra will reveal the degree of chemical diversity
in the meteors, as observed in a single meteor shower. Such meteors are traceable to a specific parent body and we know exactly
when the meteoroids in a particular shower were released from that parent body (Asher, in: Arlt (ed.) Proc. International
Meteor Conference, 2000; Lyytinen and van Flandern, Earth Moon Planets 82–83:149–166, 2000). By observing multiple apparitions
of meteor showers we can therefore obtain quasi-stratigraphic information on an individual comet or asteroid. We might also
be able to measure systematic effects of chemical weathering in meteoroids from specific parent bodies by looking for correlations
in the depletions of the more volatile elements as a function of space exposure (Borovička et al., Icarus 174:15–30, 2005).
By observing the relation between meteor entry characteristics (such as the rate of deceleration or breakup) and chemistry
we can determine if our meteorite collection is deficient in the most volatile-rich samples. Finally, we can obtain a direct
measurement of metal deposition into the terrestrial stratosphere that may act to catalyze atmospheric chemical reactions. 相似文献
43.
I.P. Williams 《Planetary and Space Science》2009,57(10):1228-1235
Meteor showers have been observed for a considerable time, and the cause, meteoroids from a meteoroid stream ablating in the Earth's atmosphere, has also been understood for centuries. The connection between meteoroid streams and comets was also established 150 years ago. Since that time our ability both to understand the physics and to numerically model the situation has steadily increased. We will review the current state of knowledge. However, just as there are differences between the behaviour of long period comets, Halley family comets and Jupiter family comets, so also differences exist between the associated meteoroid streams. Streams associated with Jupiter family comets show much more variety in their behaviour, driven by the gravitational perturbations from Jupiter. The more interesting showers associated with Jupiter family comets will be discussed individually. 相似文献
44.
Tafoni are pits formed by non‐uniform weathering in otherwise uniform rock. Two equations have been proposed for the rate of development of tafoni, both based on 2000‐year‐old outcrops from the coast of Japan. We have taken tafoni measurements from the Meteor Crater, Arizona, and vicinity that extend the equations back at least 50 000 years. As reported in earlier studies, we found pit depth to be the best tafone parameter to measure. The size of the pit decreases significantly with increasing inclination of the rock surface; however, the size of the pit can vary greatly for other reasons. In some cases the measurements are statistically significantly different between two stations taken from contiguous areas of similar inclination and aspect in an apparently homogeneous bed. It is clear, however, that over tens of thousands of years tafoni enlarge significantly. Our data are generally log‐normal and all are markedly heteroscedastic. The 1991 equation proposed by Matsukura and Matsuoka does not fit our data. The 1996 equation proposed by Sunamura provides a better fit. We propose a sigmoidal equation D = b1 + e(b2+(b3/t)) where D is the depth, t is the age, and b1, b2 and b3 vary with lithology. This new equation fits our data far better than the earlier published equations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
45.
Meteoric smoke forms in the mesosphere from the recondensation of the metallic species and silica produced by meteoric ablation. A photochemical flow reactor was used to generate meteoric smoke mimics using appropriate photolytic precursors of Fe and Si atoms in an excess of oxidant. The following systems were studied: (i) Fe+O3/O2, (ii) Fe+O3/O2+H2O, (iii) Fe+Si/SiO+O3/O2 and (iv) Si/SiO+O3/O2. The resulting nano-particles were captured for imaging by transmission electron microscopy, combined with elemental analysis using X-ray (EDX) and electron energy loss (EELS) techniques. These systems generated particle compositions consistent with: (i) Fe2O3 (hematite), (ii) FeOOH (goethite), (iii) Fe2SiO4 (fayalite) and (iv) SiO2 (silica). Electron diffraction revealed that the Fe-containing particles were entirely amorphous, while the SiO2 particles displayed some degree of crystallinity. The Fe-containing particles formed fractal aggregates with chain-like morphologies, whereas the SiO2 particles were predominantly spherical and compact in appearance. The optical extinction spectra of the Fe-containing particles were measured from 300 nm<λ<650 nm. Excellent agreement was found with the extinction calculated from Mie theory using the refractive indices for the bulk compounds, and assuming that the fractal aggregates are composed of poly-disperse distributions of constituent particles with radii ranging from 5 to 100 nm. These sizes were confirmed from measurements of the particle size distributions and microscopic imaging. Finally, the particle growth kinetics of the Fe-containing systems exhibit unexpectedly rapid agglomerative coagulation. This was modelled by assuming an initial period of coalescent particle growth resulting from diffusional (Brownian) coagulation to form primary particles; further growth of these particles is then dominated by long-range magnetic dipole–dipole interactions, leading to the fractal aggregates observed. The atmospheric implications of this work are then discussed. 相似文献
46.
Physical, Chemical, and Mineralogical Properties of Comet 81P/Wild 2 Particles Collected by Stardust
George James Flynn 《Earth, Moon, and Planets》2008,102(1-4):447-459
NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils.
The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging
from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in
the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates
of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks
were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar
to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal
oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu,
Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with
the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed
(>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior
of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range
sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical
compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in
the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and
for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately
volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous,
porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar
Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested
by elemental composition measurements on cometary meteors. 相似文献
47.
Peter S. Gural 《Earth, Moon, and Planets》2008,102(1-4):183-189
Recent work on the gravitational focusing of meteoroid streams and their threat to satellites and astronauts in the near-Earth
environment has concentrated on Earth acting as the gravitational attractor, totally ignoring the Moon. Though the Moon is
twelve-thousandths the mass of the Earth, it too can focus meteors, albeit at a much greater distance downstream from its
orbital position in space. At the Earth–Moon distance during particular phases of the Moon, slower speed meteoroid streams
with very compact radiant diameters can show meteoroid flux enhancements in Earth’s immediate neighborhood. When the right
geometric alignment occurs, this arises as a narrowed beam of particles of approximately 1,000 km width. For a narrow radiant
of one-tenth degree diameter there is a 10-fold increase in the level of flux passing through the near-Earth environment.
Meteoroid streams with more typical radiant sizes of 1° show at most two times enhancement. For sporadic sources, the enhancement
is found to be insignificant due to the wide angular spread of the diffuse radiant and thus may be considered of little importance. 相似文献
48.
The September 2007 encounter of Earth with the 1-revolution dust trail of comet C/1911 N1 (Kiess) is the most highly anticipated
dust trail crossing of a known long period comet in the next 50 years. The encounter was modeled to predict the expected peak
time, duration, and peak rate of the resulting outburst of Aurigid shower meteors. The Aurigids will radiate with a speed
of 67 km/s from a radiant at R.A. = 92°, Decl. = +39° (J2000) in the constellation Auriga. The expected peak time is 11:36 ± 20 min
UT, 2007 September 1, and the shower is expected to peak at Zenith Hourly Rate = 200/h during a 10-min interval, being above
half this value during 25 min. The meteor outburst will be visible by the naked eye from locations in Mexico, the Western
provinces of Canada, and the Western United States, including Hawaii and Alaska. A concerted observing campaign is being organized.
Added in proof: first impression of the shower.
Prepared as a contribution to the conference proceedings of “Meteoroids 2007”, to be published in the journal “Earth, Moon,
and Planets”. 相似文献
49.
Jürgen Rendtel 《Earth, Moon, and Planets》2008,102(1-4):103-110
Visual Orionid meteor data dating back to 1944 were transformed into the standard format of the Visual Meteor Data Base (VMDB)
of the International Meteor Organization (IMO) for systematic analysis. The strong 2006 Orionid return with a very low population
index (r = 1.6) and a peak ZHR of 60 (about 2.5 of the average peak strength) resembled meteor showers connected with the returns
of resonant meteoroids. An investigation of data dating back to 1928 yielded similar rate enhancements in 1936, further supporting
the assumption that meteoroids trapped in the 1:6 resonance with Jupiter caused the unusual 2006 Orionid return. 相似文献
50.
Radio meteor observations by Ham-band beacon or FM radio broadcasts using “Ham-band Radio meteor Observation Fast Fourier
Transform” (HROFFT) an automatic operating software have been performed widely in recent days. Previously, counting of meteor
echoes on the spectrograms of radio meteor observation was performed manually by observers. In the present paper, we introduce
an automatic meteor echo counting software application. Although output images of the HROFFT contain both the features of
meteor echoes and those of various types of noises, a newly developed image processing technique has been applied, resulting
in software that enables a useful auto-counting tool. There exists a slight error in the processing on spectrograms when the
observation site is affected by many disturbing noises. Nevertheless, comparison between software and manual counting revealed
an agreement of almost 90%. Therefore, we can easily obtain a dataset of detection time, duration time, signal strength, and
Doppler shift of each meteor echo from the HROFFT spectrograms. Using this software, statistical analyses of meteor activities
is based on the results obtained at many Ham-band Radio meteor Observation (HRO) sites throughout the world, resulting in
a very useful “standard” for monitoring meteor stream activities in real time. 相似文献