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We have examined single dust particle dynamics in a plasma sheath near the surface of solid bodies in space, considering conditions which resemble those of planetary system bodies, when photoelectric effect can be neglected. The forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. As the dust particles will charge negatively in the sheath, these forces will act in opposite directions and may balance.The charge delay of a moving dust particle is responsible for many of the interesting dynamical properties, and we show that for a stationary plasma, dust motion is unstable to about one Debye length out from the surface of the solid body. This part of the sheath will therefore be devoid of dust particles as they will either fall down, escape completely from the solid body or collect and make damped oscillations at stable positions in the outer part of the sheath. With increasing plasma bulk speed towards the surface, the inner unstable part of the sheath will decrease in thickness.The sources for the dust in the sheath are assumed to be mainly ejecta from meteorites and micrometeorites, but may also, for the smallest solid bodies, be from electrostatic levitation of very small dust particles. We have for different sizes of solid bodies calculated the sizes of ejecta that can be floated in the sheath. For the solar wind plasma, the suspended dust particles range from less than 1 m for the Moon to about 80 m for an asteroid with radius 1 km. These particles create a dust atmosphere.The results in this paper hold when the dust particle density is so low that the charges on the dust particles do not contribute significantly to the total space charge; a higher density will lead to a modification of the sheath.Our calculations show that ejecta below a certain size will be accelerated in the sheath and totally escape from the body even if they have near zero initial vertical velocity, while ejecta above this size will need a much larger velocity to escape. This is especially significant for the small solid bodies (radius of order km and less) which will therefore act as important sources of micronsized dust. This could be of significance for the dust production and the size distribution of dust in planetary ring systems. 相似文献
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Peter F. Sale Tundi Agardy Cameron H. Ainsworth Blake E. Feist Johann D. Bell Patrick Christie Ove Hoegh-Guldberg Peter J. Mumby David A. Feary Megan I. Saunders Tim M. Daw Simon J. Foale Phillip S. Levin Kenyon C. Lindeman Kai Lorenzen Robert S. Pomeroy Edward H. Allison R.H. Bradbury Jennifer Corrin Alasdair J. Edwards David O. Obura Yvonne J. Sadovy de Mitcheson Melita A. Samoilys Charles R.C. Sheppard 《Marine pollution bulletin》2014
Over 1.3 billion people live on tropical coasts, primarily in developing countries. Many depend on adjacent coastal seas for food, and livelihoods. We show how trends in demography and in several local and global anthropogenic stressors are progressively degrading capacity of coastal waters to sustain these people. Far more effective approaches to environmental management are needed if the loss in provision of ecosystem goods and services is to be stemmed. We propose expanded use of marine spatial planning as a framework for more effective, pragmatic management based on ocean zones to accommodate conflicting uses. This would force the holistic, regional-scale reconciliation of food security, livelihoods, and conservation that is needed. Transforming how countries manage coastal resources will require major change in policy and politics, implemented with sufficient flexibility to accommodate societal variations. Achieving this change is a major challenge – one that affects the lives of one fifth of humanity. 相似文献
45.
The role of hydromechanical coupling in fractured rock engineering 总被引:19,自引:2,他引:17
This paper provides a review of hydromechanical (HM) couplings in fractured rock, with special emphasis on HM interactions
as a result of, or directly connected with human activities. In the early 1960s, the coupling between hydraulic and mechanical
processes in fractured rock started to receive wide attention. A series of events including dam failures, landslides, and
injection-induced earthquakes were believed to result from HM interaction. Moreover, the advent of the computer technology
in the 1970s made possible the integration of nonlinear processes such as stress–permeability coupling and rock mass failure
into coupled HM analysis. Coupled HM analysis is currently being applied to many geological engineering practices. One key
parameter in such analyses is a good estimate of the relationship between stress and permeability. Based on available laboratory
and field data, it was found that the permeability of fractured rock masses tends to be most sensitive to stress changes at
shallow depth (low stress) and in areas of low in-situ permeability. In highly permeable, fractured rock sections, fluid flow
may take place in clusters of connected fractures which are locked open as a result of previous shear dislocation or partial
cementation of hard mineral filling. Such locked-open fractures tend to be relatively insensitive to stress and may therefore
be conductive at great depths. Because of the great variability of HM properties in fractured rock, and the difficulties in
using laboratory data for deriving in-situ material properties, the HM properties of fractured rock masses are best characterized
in situ.
Electronic Publication 相似文献
46.