25,000 research outputs found
Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets
Strong shear flow regions found in astrophysical jets are shown to be
important dissipation regions, where the shear flow kinetic energy is converted
into electric and magnetic field energy via shear instabilities. The emergence
of these self-consistent fields make shear flows significant sites for
radiation emission and particle acceleration. We focus on electron-scale
instabilities, namely the collisionless, unmagnetized Kelvin-Helmholtz
instability (KHI) and a large-scale dc magnetic field generation mechanism on
the electron scales. We show that these processes are important candidates to
generate magnetic fields in the presence of strong velocity shears, which may
naturally originate in energetic matter outburst of active galactic nuclei and
gamma-ray bursters. We show that the KHI is robust to density jumps between
shearing flows, thus operating in various scenarios with different density
contrasts. Multidimensional particle-in-cell (PIC) simulations of the KHI,
performed with OSIRIS, reveal the emergence of a strong and large-scale dc
magnetic field component, which is not captured by the standard linear fluid
theory. This dc component arises from kinetic effects associated with the
thermal expansion of electrons of one flow into the other across the shear
layer, whilst ions remain unperturbed due to their inertia. The electron
expansion forms dc current sheets, which induce a dc magnetic field. Our
results indicate that most of the electromagnetic energy developed in the KHI
is stored in the dc component, reaching values of equipartition on the order of
in the electron time-scale, and persists longer than the proton
time-scale. Particle scattering/acceleration in the self generated fields of
these shear flow instabilities is also analyzed
Transverse electron-scale instability in relativistic shear flows
Electron-scale surface waves are shown to be unstable in the transverse plane
of a shear flow in an initially unmagnetized plasma, unlike in the
(magneto)hydrodynamics case. It is found that these unstable modes have a
higher growth rate than the closely related electron-scale Kelvin-Helmholtz
instability in relativistic shears. Multidimensional particle-in-cell
simulations verify the analytic results and further reveal the emergence of
mushroom-like electron density structures in the nonlinear phase of the
instability, similar to those observed in the Rayleigh Taylor instability
despite the great disparity in scales and different underlying physics.
Macroscopic () fields are shown to be generated by these
microscopic shear instabilities, which are relevant for particle acceleration,
radiation emission and to seed MHD processes at long time-scales
A practical perspective on ulvan extracted from green algae
Researchers have many times turned their attention
to nature and biological processes to develop novel technologies
and materials. In a medical perspective, nature-based
products are believed to be a strategic alternative approach
to the use of fully synthetic materials, particularly in the design
of medical devices. In the past decades, marine organisms
have become the focus of considerable attention as potential
sources of valuable materials. The sustainable exploitation
and valorisation of natural marine resources constitutes a
highly attractive and strategic platform for the development
of novel biomaterials, with both economic and environmental
benefits. In this context, algae are known to synthesise large
quantities of polysaccharides and are well established sources
of these particularly interesting molecules, many of which are
known for their applicability in the design of biomaterials.
Agar, carrageenan and alginates are some of the most known
examples, and their uses can range from food to biomedical applications. However, few of the world’s available seaweed
species are used commercially. Among the three main divisions
of macroalgae (Chlorophyta, Phaeophyta and Rhodophyta),
the green algae remain largely unexploited in this
biomedical arena. While the demand for novel materials and
technologies increases, so does the research of unexploited marine green algae including its unique polysaccharide ulvan.Anabela Alves is grateful for financial support from the Fundacao para a Ciencia e Tecnologia through the SFRH/BD/39359/2007 grant. This work was partially supported by the project IBEROMARE approved by the Operational Programme for Cross-border Cooperation: Spain-Portugal, 2007-2013 (POCTEP), with funding contribution through the European Regional Development Fund (ERDF co-funding) and POCTEP
Avaliação da gestão das UC do Sistema Estadual de Áreas Protegidas de Minas Gerais.
As Unidades de Conservação (UC) são espaços territoriais especialmente protegidos para a conservação dos recursos naturais. O estado de Minas Gerais abriga diversas UC federais, estaduais e municipais. Objetiva-se neste trabalho levantar, caracterizar e analisar como vem sendo implantado o Sistema Estadual de Unidades de Conservação em Minas Gerais. Especificamente, os objetivos foram: levantar as UC estaduais e sua distribuição por bioma e regionais do IEF, sua infra-estrutura básica, regularização fundiária, recursos humanos, existência de planos de manejo, conselhos consultivos, relação com a comunidade do entorno, existência de fluxo turístico, atividade de pesquisa e caracterizar os parceiros e voluntários que atuam na gestão das mesmas. A metodologia utilizada foi a de entrevistas semi-estruturadas e questionários sobre as UC aplicados a todos os gerentes, avaliando a situação espacial, fundiária, humana, estrutural entre outras. Os principais resultados encontrados foram: existem 74 UC estaduais que ocupam áreas significativas e representativas de todos os biomas presentes no estado; inexistem planos de manejo na maioria das UC; há carência de recursos humanos na gestão das UC, principalmente de gerentes; há sobreposição de funções, quantidade insuficiente de funcionários com capacitação para funções específicas; as UC com maior representatividade de área são as “Áreas de Proteção Ambiental” (APA) que enfrentam maiores carências em recursos humanos, materiais e financeiros, além da dificuldade de gerir o território em conjunto com os proprietários; as UC de proteção integral são as que possuem maiores problemas de regularização fundiári
Perfil e percepção dos chefes de unidades de conservação do sistema estadual de áreas protegidas em Minas Gerais.
As Unidades de Conservação (UC) são espaços territoriais especialmente protegidos para a proteção dos recursos naturais. Minas Gerais abriga diversas UC criadas pelos vários níveis de governo. Destes, o estadual vem se destacando, tanto pela criação como pelas medidas para implantação das UC. Os gerentes destas UC são importantes atores deste processo e estabelecem a interface entre governo e a realidade de campo. Objetivou-se com este trabalho definir o perfil profissional dos chefes de UC estaduais mineiras, seu nível de capacitação formal, sua distribuição pelo Sistema Estadual de Áreas Protegidas (SEAP), o tempo na função e no cargo e a satisfação e perspectivas de trabalho dos mesmos. A metodologia utilizada foi a de entrevistas semi-estruturadas e questionários aplicados a todos os gerentes. As principais conclusões foram: o quadro de gerentes cresceu ao longo do tempo, mas continua deficitário; a idade média dos gerentes diminuiu e a proporção de mulheres aumentou; há grande rotatividade na gerência das UC, impossibilitando a continuidade de muitos trabalhos; a formação acadêmica dos gerentes é considerada adequada; há apoio para o treinamento de funcionários, principalmente relacionados a cursos de caráter operacional, contudo, quase não ocorrem capacitações gerenciais; a remuneração é baixa e não estimula a maioria dos profissionais a fazer carreira na função; as condições desfavoráveis de atuação, por falta de recursos, desmotivam os gerentes; os gerentes lotados em sua região de origem apresentaram maior grau de satisfação, tendendo a se envolver mais em trabalhos de longo prazo e a se relacionar melhor com as comunidades; há acúmulo e desvio de função, comprometendo o desempenho gerencial
Exact solution for the energy density inside a one-dimensional non-static cavity with an arbitrary initial field state
We study the exact solution for the energy density of a real massless scalar
field in a two-dimensional spacetime, inside a non-static cavity with an
arbitrary initial field state, taking into account the Neumann and Dirichlet
boundary conditions. This work generalizes the exact solution proposed by Cole
and Schieve in the context of the Dirichlet boundary condition and vacuum as
the initial state. We investigate diagonal states, examining the vacuum and
thermal field as particular cases. We also study non-diagonal initial field
states, taking as examples the coherent and Schrodinger cat states.Comment: 10 pages, 8 figure
Slow down of a globally neutral relativistic beam shearing the vacuum
The microphysics of relativistic collisionless sheared flows is investigated
in a configuration consisting of a globally neutral, relativistic beam
streaming through a hollow plasma/dielectric channel. We show through
multidimensional PIC simulations that this scenario excites the Mushroom
instability (MI), a transverse shear instability on the electron-scale, when
there is no overlap (no contact) between the beam and the walls of the
hollow plasma channel. The onset of the MI leads to the conversion of the
beam's kinetic energy into magnetic (and electric) field energy, effectively
slowing down a globally neutral body in the absence of contact. The
collisionless shear physics explored in this configuration may operate in
astrophysical environments, particularly in highly relativistic and supersonic
settings where macroscopic shear processes are stable
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