SPECTROSCOPY OF A GEMINID FIREBALL: ITS SIMILARITY TO COMETARY METEOROIDS AND THE NATURE OF ITS PARENT BODY

A detailed analysis of a photographic spectrum of a Geminid fireball obtained in December 14, 1961 at the Ondrejov Observatory is presented. We have computed a synthetic spectrum for the fireball and compared it with the observed spectrum assuming chemical equilibrium in the meteor head. In this way we have determined relative chemical abundances in meteor vapors. Comparing the relative chemical abundances of this Geminid meteoroid with those obtained from meteoroids associated with comets 55P/Tempel-Tuttle and 109P/Swift-Tuttle we found no significant chemical differences in main rock-forming elements. Despite of this similarity, the deepest penetration of the Geminid meteoroids and their ability to reach high rotation rates in space without fragmentation suggest that thermal processing is affecting their physical properties. We suggest that as consequence of space weathering a high-strength envelope is produced around these particles. In this picture, heating processes of the mineral phases could result in the peculiar properties observed during atmospheric entry of the Geminid meteoroids, especially their strength, which is evidenced by its resistance to ablation. Finally, although one meteoroid cannot be obviously considered as representative of the composition of its parent body, we conclude that 3200 Phaethon is able to produce millimetre-size debris nearly chondritic in composition, but the measured slight overabundance of Na would support a cometary origin for this body.     

Cosmic Roulette: Comets In The Main Belt Asteroid Region

We have produced top ten ranked lists of impact velocity, mainbelt asteroid region dwell times and impact probabilities for a selection of short period comets. The comet with the combined highest ranking with respect to impact probability and impact velocity is Comet C/1766 G1 Helfenzrieder. Since it is not clear that this comet still exists, the highest ranked, presently active, comet with respect to the likelihood of suffering impacts from meter-sized objects while in the main belt asteroid region is Comet 28P/Neujmin 1. We find no evidence to support the existence of a distinctive sub-set of the short period comets liable to show repeated outburst or splitting behavioursdue to small body, meter-sized, asteroid impacts. This revised version was published online in July 2006 with corrections to the Cover Date.     

彗星等离子体某些特征的讨论

本文讨论了彗星中尘埃粒子的充电机制 ,带电特性和平衡电势的变化规律 ,分析了彗星尘埃的破碎特性和临界半径 ,得出了很有意义的结果     

埃尔塔宁小行星质量的下限估计

埃尔塔宁小行星是一颗２１５ 万年前在南太平洋撞击地球的小行星,它也是目前唯一的深海区与地球碰撞的小行星。根据埃尔塔宁号和Ｐｏｌａｒｓｔｅｒｎ 号海洋调查船在陨击海区勘查的小行星残骸分布密度,我们估计埃尔塔宁小行星质量为２ ．３ ×１０１３ｇ ,对应于它的直径为０ ．２３ 公里。这比Ｋｙｔｅ 等人（１９８８） 估计的质量小了１０ 倍,然而它更接近埃尔塔宁小行星撞击事件的实际情况,可以作为埃尔塔宁小行星质量下限和半径下限更合理的估计值。     

太阳黑子半影的形成和返回磁通量的来源

本文从无半影的黑子成长为带半影的黑子伴随着磁场强度增强和延伸运动这个观测事实出发，通过MHD数值模拟，证明了：（1）对流层内黑子中涡旋流动的自然形成；（2）仅在黑子表面附近磁矢才急速向外移动，最终形成我们观测到的半影磁场位形；（3）光球之上由于β迅速减少，这么小的延伸速度（0.2公里／秒）仅在β～1的光球区200公里厚的层里使磁矢有效地旋转，在约几小时至一天量级时间内将近似垂直的本影磁场向水平方向旋转，形成Osherovich所期的ReturnFlux磁位形，将注入日冕空间的本影主磁流同在色球和光球内就返回的半影磁流自然地划分开来．     

1997年3月4日海尔-波普彗星的小规模分裂

１　引言彗核分裂是彗星的最重要特征之一．至１９８２年止文献记录共观测到２２个彗核分裂,３３个次核［１］．１９８６年哈雷彗星回归时观测到彗核的分裂［２］．１９９６年紫金山天文台２００ｍｍ赤道望远镜和青岛观象台同时观测到百武彗星的分裂［３］．海尔波普彗星从１９９５年８月到１０月曾爆发５次［４］．１９９６年３月至１２月不断有喷流射出,彗核活动日趋剧烈［５］．１９９７年３月４日我们观测到彗核的小规模分裂及喷流特征,３月３日我们观测到海尔波普彗星近核照片上有喷流,这可能是３月４日彗核小块分裂的前奏．３…     

关于太阳白光耀斑连续发射起源的讨论

白光耀斑研究近年来获得较大进展，对其连续发射起源机制目前流行三种解释．一是耀斑形成过程中所产生的色球凝聚[1-3]，二是非热粒子注入大气致氢原子非热激发和电离[4-6]，三是色球强加热所引起的对大气深层的辐射反加热[7]．这三类解释具有一个共性，即除了自身直接对连续谱