General properties and analytical approximations of photorefractive solitons

We investigate general properties of spatial 1-dimensional bright photorefractive solitons and suggest various analytical approximations for the soliton profile and the half width, both depending on an intensity parameter r

Vertical pairing of identical particles suspended in the plasma sheath

It is shown experimentally that vertical pairing of two identical microspheres suspended in the sheath of a radio-frequency (rf) discharge at low gas pressures (a few Pa), appears at a well defined instability threshold of the rf power. The transition is reversible, but with significant hysteresis on the second stage. A simple model, which uses measured microsphere resonance frequencies and takes into account besides Coulomb interaction between negatively charged microspheres also their interaction with positive ion wake charges, seems to explain the instability threshold quite well.Comment: 4 pages, 6 figures. to appear in Phys. Rev. Lett. 86, May 14th (2001

Biological Soil Crusts as Modern Analogues for the Archean Continental Biosphere: Insights from Carbon and Nitrogen Isotopes

Stable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ13C and δ15N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogues for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses

Kinetic formulation and global existence for the Hall-Magneto-hydrodynamics system

This paper deals with the derivation and analysis of the the Hall Magneto-Hydrodynamic equations. We first provide a derivation of this system from a two-fluids Euler-Maxwell system for electrons and ions, through a set of scaling limits. We also propose a kinetic formulation for the Hall-MHD equations which contains as fluid closure different variants of the Hall-MHD model. Then, we prove the existence of global weak solutions for the incompressible viscous resistive Hall-MHD model. We use the particular structure of the Hall term which has zero contribution to the energy identity. Finally, we discuss particular solutions in the form of axisymmetric purely swirling magnetic fields and propose some regularization of the Hall equation

Improving food security, nutrition and incomes: the contribution of small stock

Despite years of work, the development indicators relating to sub-Saharan Africa (SSA) often disappoint. For example, the highest incidence of undernourishment in the world occurs in SSA where one out of every three people suffers from chronic hunger (WDR, 2007). Per capita food consumption in SSA is on the decline (Shapouri et al., 1999). Sub-Saharan Africa is the only region in the world where the number of rural poor is on the rise (WDR, 2007). The majority (86%) of SSA’s combined population of 200 million relies on the agriculture sector. Therefore, it is no surprise that we look to this sector to provide solutions to the issues of chronic poverty, food insecurity, and malnutrition. Improvements here have the possibility of impacting millions. This brief looks at the contribution that livestock, small stock in particular, can make towards achieving food and nutritional security and improving social wellbeing in southern Africa

Assisting smallholder farmers in mixed crop-livestock systems to understand the potential effects of technologies and climate change through participatory modeling

Smallholder farming systems in the semi-arid areas of Zimbabwe are characterized by low production. This low production is not solely due to lack of technologies but also due to a lack of integrating a diversity of viewpoints belonging to local, expert and specialized stakeholders during technology development. Participatory approaches combined with computer-based modeling are increasingly being recognized as valuable approaches to jointly develop sustainable agricultural pathways. The paper discusses the application of this integrated and iterative process in developing and evaluating the impact of interventions aimed at improving food and feed production. The paper concludes that the process allows farmers to determine the impact of their decisions, evaluate new options and define realistic production and management options tailored to their particular circumstances. While in-turn scientists and other stakeholders learn more about the farmers’ decision-making process, input and managerial potentials as well as knowledge gaps

Goat production and marketing: Baseline information for semi-arid Zimbabwe

The role of livestock in rural communities is changing rapidly. Goats are increasingly used to augment cash income and enhance food security, thus serving as an important component in household’s livelihood strategies, particularly in drought-prone areas. While much has been done to improve agricultural production in the small-scale sector, little is known and documented about the current state of goat production and marketing in the communal areas of Zimbabwe. This report aims to set a baseline of the current status of goat production and marketing in southwestern Zimbabwe. The report illustrates the main functions of goats. Basic statistics describe goat ownership patterns with regard to the socioeconomic profi les of goat keepers, and the responsibilities of different household members in day-to-day goat management and marketing. The current productivity of goat fl ocks and seasonal trends are illustrated, indicating major challenges that farmers face in goat production and marketing. The report then provides a detailed description of the current management practices and existing marketing systems, and illustrates differences between districts, proximity to markets, flock sizes, levels of education and gender. Goat mortality has been found to be the most important constraint. Farmers with few goats are unable to sustain their flocks, whereas those with larger flocks do not realize the potential benefits from goats due to high mortality rates. Poor access to animal health support, dry season feed shortages and inadequate housing are the most important immediate factors contributing to high mortalities and can generally be ascribed to a lack of information and poor service structures, both resulting from limited support given to the small stock sector by government and NGO support services. The study also shows that although many farmers attempt to sell goats, markets are underdeveloped, infrastructure is inadequate and market information is not readily available. This results in poor confidence in markets, high transaction costs and low prices for goats. It is hypothesized that improved market access will act as an incentive for farmers to invest more in goat production. Market development is thus singled out as the next important step in further developing the goat industry in Zimbabwe. Practical options to enhance the contribution of goats to food security and income growth are discussed, and priority interventions are recommended to service providers, development agents and policymakers in Zimbabwe

National and Regional Livestock Markets: Opportunities for Growth in SADC

The last forty years has seen a continuous rise in the demand for meat, milk, and other livestock products worldwide. Fueled by trends such as increased urbanization, growing populations, and income growth in the urban areas, it is clear that this demand will only continue to grow (Delgado et al., 1999). Although decision makers in agriculture have traditionally focused on crop production, the growing demand for livestock products makes a good case for improving the livestock sector. Gains made here will ultimately benefit the many small-scale farmers who collectively own more than 80% of the livestock in southern Africa (SADC RISDP, 2006)

CD4 memory T cells divide poorly in response to antigen because of their cytokine profile

Immunological memory is a hallmark of adaptive immunity, and understanding T cell memory will be central to the development of effective cell-mediated vaccines. The characteristics and functions of CD4 memory cells have not been well defined. Here we demonstrate that the increased size of the secondary response is solely a consequence of the increased antigen-specific precursor frequency within the memory pool. Memory cells proliferated less than primary responding cells, even within the same host. By analyzing the entry of primary and memory cells into the cell cycle, we found that the two populations proliferated similarly until day 5; after this time, fewer of the reactivated memory cells proliferated. At this time, fewer of the reactivated memory cells made IL-2 than primary responding cells, but more made IFNγ. Both these factors affected the low proliferation of the memory cells, because either exogenous IL-2 or inhibition of IFNγ increased the proliferation of the memory cells

Coupling of morphological instability and kinetic instability: Chemical waves in hydrogen oxidation on a bimetallic Ni/Rh(111) surface

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming O2+H2 reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in H2 produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the O2+H2 reaction involve a cyclic transformation between 3D-NiO and 2D-NiO