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61.
Badruddin 《Journal of Astrophysics and Astronomy》2006,27(2-3):209-217
We discuss the effects of certain dynamic features of space environment in the heliosphere, the geo-magnetosphere, and the
earth’s atmosphere. In particular, transient perturbations in solar wind plasma, interplanetary magnetic field, and energetic
charged particle (cosmic ray) fluxes near 1 AU in the heliosphere have been discussed. Transient variations in magnetic activity
in geo-magnetosphere and solar modulation effects in the heliosphere have also been studied. Emphasis is on certain features
of transient perturbations related to space weather effects. Relationships between geomagnetic storms and transient modulations
in cosmic ray intensity (Forbush decreases), especially those caused by shock-associated interplanetary disturbances, have
been studied in detail. We have analysed the cosmic ray, geomagnetic and interplanetary plasma/field data to understand the
physical mechanisms of two phenomena namely, Forbush decrease and geomagnetic storms, and to search for precursors to Forbush
decrease (and geomagnetic storms) that can be used as a signature to forecast space weather. It is shown that the use of cosmic
ray records has practical application for space weather predictions. Enhanced diurnal anisotropy and intensity deficit of
cosmic rays have been identified as precursors to Forbush decreases in cosmic ray intensity. It is found that precursor to
smaller (less than 5%) amplitude Forbush decrease due to weaker interplanetary shock is enhanced diurnal anisotropy. However,
larger amplitude (greater than 5%) Forbush decrease due to stronger interplanetary shock shows loss cone type intensity deficit
as precursor in ground based intensity record. These precursors can be used as inputs for space weather forecast. 相似文献
62.
More than half of the C-type asteroids, which are the dominant type of asteroid in the outer half of the main belt, show evidence of hydration in their reflectance spectra. In order to understand the collisional evolution of asteroids, the production of interplanetary dust, and to model the infrared signature of small particles in the Solar System it is important to characterize the dust production from primary impact disruption events, and compare the disruption of hydrous and anhydrous targets. We performed impact disruption experiments of three “greenstone” targets, a hydrothermally metamorphosed basalt, and compared the results of these disruptions to our previous disruption experiments on porous, anhydrous basalt targets and to literature data on the disruption of non-porous, anhydrous basalt targets. The greenstone targets were selected because their major hydrous alteration phase is serpentine, the same hydrous alteration phase found in hydrous CM meteorites, like Murchison. The porous, anhydrous basalt targets were selected because their structure, consisting of millimeter-size olivine phenocrysts in a more porous, anhydrous matrix is similar to the structure of anhydrous chondritic meteorites, which consist of millimeter-size olivine chondrules embedded in a more porous, anhydrous matrix. The disruption measurements indicate the threshold collisional specific energy, Q
D*, is 570 J/kg for the greenstone, which is lower than the literature values for non-porous basalt targets, and significantly lower than the value of 2500 J/kg that we have measured for porous anhydrous basalt targets. We determined the mass-frequency distribution of the debris from the disruption of the greenstone targets, which ranged in mass from 80 to 280 g, over a nine order-of-magnitude mass range, from ~10−9 g to the mass of the largest fragment. The cumulative mass-frequency distribution from the greenstone targets is fit by two power–law segments, one for masses >10−2 g, which is significantly steeper than the corresponding segment from the disruption of similar-sized anhydrous basalt, and one in the range from 10−9 to 10−2 g, which is significantly flatter than the corresponding segment from the disruption of similar size anhydrous basalt. These hydrous greenstone targets overproduce small fragments (10−4 to 100 g) compared to anhydrous basalt targets, but underproduce dust-size grains (10−9 to 10−4 g) compared to anhydrous basalt targets. 相似文献
63.
Ballistic capture of spacecraft and celestial bodies by planets of the solar system is studied considering the elliptic restricted
three body model. A preferential region, due to the eccentricity of the planet and the Sun-gravity-gradient effect is found
for the capture phenomenon. An analytical formula is derived which determines the limiting value of the satellite capture
eccentricity ec as a function of the pericenter distance xp and planet’s true anomaly. The analytic values ec are tested by a numerical propagator, which makes use of planetary ephemeris, and only a small difference with respect to
numerical integration is found. It turns out that lower values of ec occur when the planet anomaly is close to zero; that is, capture is easier when the planet is at its perihelion. This fact
is confirmed by the capture of celestial bodies. It is shown that Jupiter comets are generally captured when Jupiter is in
its perihelion region. Ballistic capture is also important in interplanetary missions. The propellant saved using the minimum
ballistic capture eccentricity is evaluated for different missions and compared with respect to the case in which the insertion
orbit is a parabola: a significant saving can be accomplished. 相似文献
64.
65.
H. McNamara J. Jones B. Kauffman R. Suggs W. Cooke S. Smith 《Earth, Moon, and Planets》2004,95(1-4):123-139
In an attempt to overcome some of the deficiencies of existing meteoroid models, NASA’s Space Environments and Effects (SEE)
Program sponsored a 3 year research effort at the University of Western Ontario. The resulting understanding of the sporadic
meteoroid environment – particularly the nature and distribution of the sporadic sources – were then incorporated into a new
Meteoroid Engineering Model (MEM) by members of the Space Environments Team at NASA’s Marshall Space Flight Center. This paper
discusses some of the revolutionary aspects of MEM which include (a) identification of the sporadic radiants with real sources
of meteoroids, such as comets, (b) a physics-based approach which yields accurate fluxes and directionality for interplanetary
spacecraft anywhere from 0.2 to 2.0 astronomical units (AU), and (c) velocity distributions obtained from theory and validated
against observation. Use of the model, which gives penetrating fluxes and average impact speeds on the surfaces of a cube-like
structure, is also described along with its current limitations and plans for future improvements. 相似文献
66.
A mechanism for interstellar panspermia 总被引:2,自引:0,他引:2
W. M. Napier † 《Monthly notices of the Royal Astronomical Society》2004,348(1):46-51
67.
J. P. Younger I. M. Reid R. A. Vincent D. A. Holdsworth D. J. Murphy 《Monthly notices of the Royal Astronomical Society》2009,398(1):350-356
The 33.2 MHz interferometric meteor radars located at Davis Station, Antarctica and Darwin, Australia typically detect around 15 000 specular underdense meteor echoes every day. While the angle of arrival of the scattered radio wave can be inferred using phase differences between receive antennae, the direction of individual meteors is not known beyond a plane of ambiguity perpendicular to the angle of arrival. Using the great circle mapping technique with a Jones & Jones type weighting function, 37 meteor shower systems were detected in data collected at both locations over 2006–2007, including nine undocumented showers. The orbital elements of the parent debris streams were then calculated for the 31 showers where sufficiently precise measurements were available. 相似文献
68.
2001年3月31日观测到的大的多重磁云(Multi MC)事件造成了第23周太阳峰年(2000~2001)最大的地磁暴(Dst=-387nT). 通过分析ACE飞船的观测数据, 描述了这个多重磁云在1AU处的磁场和等离子体特征. 并且根据SOHO和GOES卫星的观测资料, 认证了它的太阳源. 在这次事件中, 由于多重磁云内部异常增强的南向磁场, 使之地磁效应变得更强, 它大大的延长了地磁暴的持续时间. 观测结果与理论分析表明, 多重磁云中子磁云的相互挤压使磁云内的磁场强度及其南向分量增强数倍, 从而加强了地磁效应. 因此, 研究认为多重磁云中子磁云之间的相互压缩是造成特大地磁暴的一种机制. 此外, 研究发现形成多重磁云的日冕物质抛射(CMEs)并不一定要来自同一太阳活动区. 相似文献
69.
深海沉积物中碲异常的成因 总被引:2,自引:0,他引:2
碲在地球上是一种稀有分散元素,碲在地壳中的丰度很低,仅为1.0ng/g;但碲在富钴结壳、海底多金属结核、党海沉积物和陨石中的丰度异常高。为了查明深海沉积物中碲异常的原因,笔者对比分析了深海沉积物中磁性部分与全岩的碲含量和氦同位素组成。结果显示磁性部分的He含量和3He/4He比值及Te含量较全岩的值明显偏高;3He含量与Te含量同步变化;在3He-Te关系图上,二者具明显的正相关关系。本文首次提出深海沉积物中的碲异常与氦同位素异常一样可能是由宇宙尘注入引起的;海底多金属结核和富钴结壳中的碲异常可能也与宇尘有关。 相似文献
70.
Marcos Rincon Voelzke 《Earth, Moon, and Planets》2005,97(3-4):399-409
This contribution makes a review and compares the current competitive theories, based on triggering mechanisms, in order to explain the episodic phenomena of disconnection events (DEs), i.e., when the plasma tail appears disconnected from the cometary head, namely: the ion production effects, the pressure effects and the magnetic reconnection effects are analysed. 相似文献