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An active region on the surface of a cometarynucleus is considered as a conic hole in the surface dust mantle with icy bottom
and dusty side-walls. This conic structureshould concentrate solar energy onto the bottom andtherefore enhance sublimation.
Preliminary results of thecalculation of this effect are given. The temperature distributionat the bottom of the crater is
calculated for different sets ofits geometrical parameters. Effects of intensified sublimation depending on the geometrical
parameters areconsidered for the specific case when a single active region islocated exactly at the pole of the nucleus and
the pole is directed tothe Sun. 相似文献
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A.M.T. Pollock 《Astronomische Nachrichten》2012,333(4):351-354
Charge exchange occurs between charged ions with enough energy to overcome Coulomb repulsion, a condition satisfied for collisions at velocities like those of the winds driven from hot stars by radiation pressure. X‐ray line ratios in some hot stars are inconsistent with those expected from thermal plasmas excited by electron impact. Ion‐ion interactions including charge exchange might be responsible instead if high‐velocity collisions between ions are enabled by the presence of a magnetic field in the wind, suggesting a possible alternative mechanism to the widely accepted instability‐driven shock model. The nature of a plasma in charge‐exchange equilibrium is yet to be determined (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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During the eighties, microgravity research focussed predominantly on the investigation of fundamental phenomena, often with
limited industrial support. Although this approach led to some rather impressive breakthroughs in terms of new theoretical
insights and microgravity experimentation, the need for increased co-ordination and interest from industry became increasingly
apparent. In this decade, a user-driven research strategy has been instigated by ESA to promote microgravity research. The objective is to coordinate ESA, national
activities and industry into an overall European strategy, which will allow valuable application-oriented microgravity research
to be performed aboard the International Space Station (ISS). On this basis, it is expected that scientific progress will
evolve even more rapidly due to the easier planning, regular access and longer experiment-durations associated with the ISS.
This paper highlights the wealth of microgravity research being co-ordinated by ESA in the field of physical sciences. A number of key areas of research under microgravity conditions are currently being explored such as alloy solidification,
crystal growth,measurement of thermophysical properties, combustion mechanisms, fluid flow, cold atom physics and complex
plasmas, to name but a few. The following sections will provide background information relating to the various ESA research
programmes, as well as emphasising their microgravity relevance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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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. 相似文献
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G. Z. Machabeli Qinghuan Luo D. B. Melrose S. Vladimirov 《Monthly notices of the Royal Astronomical Society》2000,312(1):51-56
Gamma-ray emission in pulsar magnetospheres is attributed to synchrotron radiation, which tends to decrease the pitch angle of the particle, being balanced by plasma processes tending to increase the pitch angle. The plasma processes are non-resonant instabilities that drive non-resonant quasilinear diffusion (NQD), thereby pumping energy from waves and the parallel motion of the particle into the perpendicular motion of the particle. It is shown that NQD can maintain the pitch angles for particles near the light-cylinder such that they radiate synchrotron radiation at MeV energies. Compared to conventional emission mechanisms (such as polar cap or outer gap models), the resulting spectrum has a relatively low upper cut-off from about a few to 100 MeV. Possible observational consequences of this mechanism are discussed. 相似文献
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激光驱动亥姆霍兹电容线圈靶的磁重联实验已经提出并进行了多年.当实验中的金属板被强激光照射时产生自由电子,这些自由电子的运动在连接两金属板的两个平行线圈中产生电流,由两个平行线圈内部电流产生的磁场之间随即发生重联.该实验不同于其他直接由Biermann电池效应所产生高β(等离子体热压与磁压的比值)环境下的磁重联实验.对该类实验进行了3维磁流体动力学数值模拟,首次展示了亥姆霍兹电容器线圈靶如何驱动磁重联的过程.数值模拟结果清楚地表明,磁重联的出流等离子体在线圈周围发生与实验结果相一致的堆积现象.线圈电流产生的磁场可高达100 T,使得磁重联区域周围的等离子体β值达到10^-2.与实验室结果进行比较,数值模拟重复了实验展示的大多数特征,可有助于深入认识和理解实验结果背后的物理学原理. 相似文献
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