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Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images
of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring
outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one
source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion.
The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized
to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows
even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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萝卜纹是田黄的重要鉴定特征。本文通过岩石薄片、X射线衍射分析和电子探针分析等手段,确定萝卜纹的组成矿物为伊利石;萝卜纹的成因为田黄的原岩受到构造应力作用后产生了显微裂隙,次生的伊利石填充了显微裂隙;显微裂隙中的伊利石与岩石中的主要矿物迪开石光学性质差别较大而呈丝缕状(萝卜纹)。 相似文献
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We reanalyse positions and motion of the striae in the dust tails of the bright comets Mrkos 1957 V and West 1976 VI. Two theories are compared to the data: the high speed particle ejection theory of NOTNI (1964) and the two-step kinematical sequence proposed by SEKANINA and FARRELL (1980) in their particle fragmentation theory. The final decision is in favour of the two step sequence, though the real situation may be a mixtum compositum of both. The parent clouds (first step) are found to have low lateral velocity dispersion and higher than expected acceleration. The question how these clouds originate remains unsolved. It is suggested that they comprise particles which left the coma into a tailward region of high magnetic field and plasma density. Another possibility, additionally accounting for the sometimes apparent duplicity, is an origin out of an optically thick coma at angles orthogonal to the radius vector. 相似文献
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Sekanina and Farrell's model for the striated dust tails of comets describes the formation of striae as a two-step fragmentation
process that is characterized by an ejection time te of parent particles, by their radiation pressure acceleration βp, and by their fragmentation time tf. Of these three, tf is the weakest parameter in that a range oftf offers a set of nearly equivalent solutions. In this context, we comment on Nishioka et al.'s finite-lifetime model, which
is a modification of the fragmentation model. We propose a truncated Gaussian function as a first-approximation distribution
law for particle fragmentation times. We apply this generalized model to a stria in comet Hale-Bopp detected on March 5–15,
1997 and analyzed by Pittichová et al. in a recent paper. We find that in order to fit the stria's estimated width of ∼150
000 km, the fragmentation times cannot be distributed over a period of more than approximately 2 to 3 days.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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浙江“萧山萝卜干”肉质鲜美、产量高、抗逆性强,易于种植和加工。这些优良品质除了与品种有关外,还与其产地的土壤地质和地球化学背景密切相关。与浙江平原土壤元素平均值对比,产地的MgO、B、P、CaO、Mn含量较高,N、K、Mo、Fe、Zn含量较低;从该区土壤养分和微量元素有效态分析结果来看,碱解氮、Fe、Mn、Cu含量丰富,速效磷、有机质和Zn含量一般。由于产地土壤成熟度不高,加上该区复种指数高等问题,造成土壤N、K流失和B、Mo缺乏,影响和制约了萝卜产量和质量。建议及时追加微肥和有机肥,做到科学养地。 相似文献
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