排序方式: 共有68条查询结果,搜索用时 62 毫秒
51.
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
52.
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
53.
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
54.
55.
We have carried out double-station TV meteor observations between 1990 and 1994. The orbits of 326 meteors have been determined from doubly observed meteors, and radiant distributions are studied. The mean magnitude of the observed meteors was as faint as +4.7, since I.I. (Image Intensifier) and Video cameras were used. Radiants were widely distributed over the celestial sphere. The velocity distribution showed some similarity with the distribution predicted by the theoretical radiant distribution from comets rather than that from asteroids. In all 13 showers including both major and minor meteor showers were detected from radiant distributions of the observed meteors; from the orbital elements and meteor velocities as well as from the radiant directions. 相似文献
56.
A procedure of selection of meteoroids from major streams is suggested and applied to the IAU Lund photographic database modified by a check for internal consistency among orbital elements (3411 orbits). Limits for choice of stream members were defined by break points on the plots of the cumulative numberN
C vs. the Southworth-HawkinsD discriminant. For the break points were considered the points from which the dependenceN
C vs.D changes to a quasi-linear one, and with the increasingD, N
C changes only moderately. Except for the Taurids which desire a separate analysis, theN
C vs.D diagrams are presented for the following major meteoroid streams: Quadrantids, Lyrids, Aquarids, Capricornids, N and S Aquarids, Perseids, Orionids, Leonids and Geminids. The mean orbits, velocities and radiants of the streams are derived and compared with the osculating orbits of their parent bodies. The limitingD
B was found to be a function of the number of the stream membersN
CB. Omitting the exceptionally concentrated Geminids, the relation is in the formD
B = 0.058 *ln(N
CB) – 0.04. 相似文献
57.
We are able to determine neutral air temperatures by examining the fading times of meteor trail echoes in the upper mesosphere/lower thermosphere. It has been suggested that solar proton events may stimulate both dynamic and aeronomic changes in the middle atmosphere and we have endeavoured to investigate this. Despite a variety of approaches to determine the background temperature above which we might expect to see enhancements under conditions of strong proton precipitation, we are unable to detect any significant changes. We have repeated the search during selected seasons and also with various proton flux thresholds, similarly to no avail. We conclude, therefore, that at 90 km altitude, 78°N and 16°E, at least, we are unable to detect enhanced neutral temperatures due to solar proton events. At best, any enhancements, predicted to be of the order of a few K only, are likely to be completely masked by the day-to-day variability of the temperature field. 相似文献
58.
利用昆明电波观测站(25.6°N,103.8°E)两台不同工作频率的全天空流星雷达在2011年特殊联合观测试验期间的数据,基于Hocking的方法利用不同的温度梯度,在确定了昆明地区中层顶位于流星峰值高度之上的情况下,反演了昆明地区上空88 km和85 km高度的大气温度,并与Aura卫星观测的温度进行比较.对比研究发现,两台流星雷达可以分别正确获得88 km和85 km高度的大气温度,但其中由全球温度梯度模式反演得到的大气温度与卫星观测温度相关性不是很好,而利用卫星观测的温度梯度,两台雷达反演出的大气温度与卫星观测温度存在很好的相关性.结果表明了准确的温度梯度在流星雷达观测大气温度过程中是至关重要的. 相似文献
59.
Meteoroids that orbit the Sun encounter the Earth with speeds between 11 and 74 km/sec. However, the distribution of the velocities of meteoroids between these limits is not well known. The uncertainty is caused by the difficulty in measuring the true flux of meteors at the extrema of the velocity distribution. Whilst the most comprehensive measurements of meteor flux are those obtained using radio techniques, meteors with speeds > 50 km/sec occur at heights where the effects of initial radius of the trail and diffusion significantly reduce the radio reflection from the trails; on the other hand the high dependence of the collisional ionization probability on velocity (to the power 3.5) significantly inhibits the detection of meteors with speeds < 20 km/sec. Recent developments in meteor radar systems are now making it possible to measure the velocity of meteors at the extrema of the distribution. For meteoroids ablating at heights between 100 and 120 km the speed of entry can be measured at 2 and 6 MHz using a radar with a 1 km diameter array located near Adelaide; these observations will commence early in 1995. In the meantime a 54 MHz MST radar is being operated at a pulse repetition frequency of 1024 Hz to search for the presence of interstellar (speed > 74 km/sec) meteors. Both these radars exploit the phase information available prior to the closest-approach (to) point. 相似文献
60.
Presented are results of Perseid 1993 meteor shower from radar observation at Ondejov observatory. Investigation of the shower activity profiles in four echo duration intervals proved the position of dominant peak at solar longitude L = 138.°8±0.°05 (epoch 1950.0) followed by series of secondary maxima positions of which depend on examined echo duration class. Extremely low value of the mass distribution indexs = 1.27 ± 0.01 near the maximum activity peak associated with high proportion of fragmenting particles leads to the suggestion that meteor particles concentrated in this filament are younger than those which form the other parts of the stream. 相似文献