1,283 research outputs found
The MHD Kelvin-Helmholtz Instability II: The Roles of Weak and Oblique Fields in Planar Flows
We have carried out high resolution MHD simulations of the nonlinear
evolution of Kelvin-Helmholtz unstable flows in 2 1/2 dimensions. The modeled
flows and fields were initially uniform except for a thin shear layer with a
hyperbolic tangent velocity profile and a small, normal mode perturbation. The
calculations consider periodic sections of flows containing magnetic fields
parallel to the shear layer, but projecting over a full range of angles with
respect to the flow vectors. They are intended as preparation for fully 3D
calculations and to address two specific questions raised in earlier work: 1)
What role, if any, does the orientation of the field play in nonlinear
evolution of the MHD Kelvin-Helmholtz instability in 2 1/2 D. 2) Given that the
field is too weak to stabilize against a linear perturbation of the flow, how
does the nonlinear evolution of the instability depend on strength of the
field. The magnetic field component in the third direction contributes only
through minor pressure contributions, so the flows are essentially 2D. Even a
very weak field can significantly enhance the rate of energy dissipation. In
all of the cases we studied magnetic field amplification by stretching in the
vortex is limited by tearing mode, ``fast'' reconnection events that isolate
and then destroy magnetic flux islands within the vortex and relax the fields
outside the vortex. If the magnetic tension developed prior to reconnection is
comparable to Reynolds stresses in the flow, that flow is reorganized during
reconnection. Otherwise, the primary influence on the plasma is generation of
entropy. The effective expulsion of flux from the vortex is very similar to
that shown by Weiss for passive fields in idealized vortices with large
magnetic Reynolds numbers. We demonstrated that thisComment: 23 pages of ApJ Latex (aaspp4.sty) with 10 figures, high resolution
postscript images for figs 4-9 available through anonymous at
ftp://ftp.msi.umn.edu/pub/twj To appear in the June 10, 1997 Ap
Building global capacity for brain and nervous system disorders research.
The global burden of neurological, neuropsychiatric, substance-use and neurodevelopmental disorders in low- and middle-income countries is worsened, not only by the lack of targeted research funding, but also by the lack of relevant in-country research capacity. Such capacity, from the individual to the national level, is necessary to address the problems within a local context. As for many health issues in these countries, the ability to address this burden requires development of research infrastructure and a trained cadre of clinicians and scientists who can ask the right questions, and conduct, manage, apply and disseminate research for practice and policy. This Review describes some of the evolving issues, knowledge and programmes focused on building research capacity in low- and middle-income countries in general and for brain and nervous system disorders in particular
Lessons from development: A role for asymmetric stem cell division in cancer
AbstractAsymmetric stem cell division has emerged as a major regulatory mechanism for physiologic control of stem cell numbers. Reinvigoration of the cancer stem cell theory suggests that tumorigenesis may be regulated by maintaining the balance between asymmetric and symmetric cell division. Therefore, mutations affecting this balance could result in aberrant expansion of stem cells. Although a number of molecules have been implicated in regulation of asymmetric stem cell division, here, we highlight known tumor suppressors with established roles in this process. While a subset of these tumor suppressors were originally defined in developmental contexts, recent investigations reveal they are also lost or mutated in human cancers. Mutations in tumor suppressors involved in asymmetric stem cell division provide mechanisms by which cancer stem cells can hyperproliferate and offer an intriguing new focus for understanding cancer biology. Our discussion of this emerging research area derives insight from a frontier area of basic science and links these discoveries to human tumorigenesis. This highlights an important new focus for understanding the mechanism underlying expansion of cancer stem cells in driving tumorigenesis
High Order Methods for a Class of Volterra Integral Equations with Weakly Singular Kernels
Soil Moisture Active Passive (SMAP) Mission Level 4 Surface and Root Zone Soil Moisture (L4_SM) Product Specification Document
This is the Product Specification Document (PSD) for Level 4 Surface and Root Zone Soil Moisture (L4_SM) data for the Science Data System (SDS) of the Soil Moisture Active Passive (SMAP) project. The L4_SM data product provides estimates of land surface conditions based on the assimilation of SMAP observations into a customized version of the NASA Goddard Earth Observing System, Version 5 (GEOS-5) land data assimilation system (LDAS). This document applies to any standard L4_SM data product generated by the SMAP Project
Soil Moisture Active Passive (SMAP) Mission Level 4 Surface and Root Zone Soil Moisture (L4_SM) Product Specification Document
This is the Product Specification Document (PSD) for Level 4 Surface and Root Zone Soil Moisture (L4_SM) data for the Science Data System (SDS) of the Soil Moisture Active Passive (SMAP) project. The L4_SM data product provides estimates of land surface conditions based on the assimilation of SMAP observations into a customized version of the NASA Goddard Earth Observing System, Version 5 (GEOS-5) land data assimilation system (LDAS). This document applies to any standard L4_SM data product generated by the SMAP Project. The Soil Moisture Active Passive (SMAP) mission will enhance the accuracy and the resolution of space-based measurements of terrestrial soil moisture and freeze-thaw state. SMAP data products will have a noteworthy impact on multiple relevant and current Earth Science endeavors. These include: Understanding of the processes that link the terrestrial water, the energy and the carbon cycles, Estimations of global water and energy fluxes over the land surfaces, Quantification of the net carbon flux in boreal landscapes Forecast skill of both weather and climate, Predictions and monitoring of natural disasters including floods, landslides and droughts, and Predictions of agricultural productivity. To provide these data, the SMAP mission will deploy a satellite observatory in a near polar, sun synchronous orbit. The observatory will house an L-band radiometer that operates at 1.40 GHz and an L-band radar that operates at 1.26 GHz. The instruments will share a rotating reflector antenna with a 6 meter aperture that scans over a 1000 km swath
High Temperature Limit of the Confining Phase
The deconfining transition in non-Abelian gauge theory is known to occur by a
condensation of Wilson lines. By expanding around an appropriate Wilson line
background, it is possible at large to analytically continue the confining
phase to arbitrarily high temperatures, reaching a weak coupling confinement
regime. This is used to study the high temperature partition function of an
electric flux tube. It is found that the partition function corresponds
to that of a string theory with a number of world-sheet fields that diverges at
short distance.Comment: 13 page
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On the Analysis of Genome-Wide Association Studies in Family-Based Designs: A Universal, Robust Analysis Approach and an Application to Four Genome-Wide Association Studies
For genome-wide association studies in family-based designs, we propose a new, universally applicable approach. The new test statistic exploits all available information about the association, while, by virtue of its design, it maintains the same robustness against population admixture as traditional family-based approaches that are based exclusively on the within-family information. The approach is suitable for the analysis of almost any trait type, e.g. binary, continuous, time-to-onset, multivariate, etc., and combinations of those. We use simulation studies to verify all theoretically derived properties of the approach, estimate its power, and compare it with other standard approaches. We illustrate the practical implications of the new analysis method by an application to a lung-function phenotype, forced expiratory volume in one second (FEV1) in 4 genome-wide association studies
Soil Moisture Active Passive (SMAP) Mission Level 4 Carbon (L4_C) Product Specification Document
This is the Product Specification Document (PSD) for Level 4 Surface and Root Zone Soil Moisture (L4_SM) data for the Science Data System (SDS) of the Soil Moisture Active Passive (SMAP) project. The L4_SM data product provides estimates of land surface conditions based on the assimilation of SMAP observations into a customized version of the NASA Goddard Earth Observing System, Version 5 (GEOS-5) land data assimilation system (LDAS). This document applies to any standard L4_SM data product generated by the SMAP Project
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