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Magnetotail energy dissipation during an auroral substorm.
Violent releases of space plasma energy from the Earth's magnetotail during substorms produce strong electric currents and bright aurora. But what modulates these currents and aurora and controls dissipation of the energy released in the ionosphere? Using data from the THEMIS fleet of satellites and ground-based imagers and magnetometers, we show that plasma energy dissipation is controlled by field-aligned currents (FACs) produced and modulated during magnetotail topology change and oscillatory braking of fast plasma jets at 10-14 Earth radii in the nightside magnetosphere. FACs appear in regions where plasma sheet pressure and flux tube volume gradients are non-collinear. Faster tailward expansion of magnetotail dipolarization and subsequent slower inner plasma sheet restretching during substorm expansion and recovery phases cause faster poleward then slower equatorward movement of the substorm aurora. Anharmonic radial plasma oscillations build up displaced current filaments and are responsible for discrete longitudinal auroral arcs that move equatorward at a velocity of about 1km/s. This observed auroral activity appears sufficient to dissipate the released energy
Step-wedge cluster-randomised community-based trials: An application to the study of the impact of community health insurance
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.BACKGROUND: We describe a step-wedge cluster-randomised community-based trial which has been conducted since 2003 to accompany the implementation of a community health insurance (CHI) scheme in West Africa. The trial aims at overcoming the paucity of evidence-based information on the impact of CHI. Impact is defined in terms of changes in health service utilisation and household protection against the cost of illness. Our exclusive focus on the description and discussion of the methods is justified by the fact that the study relies on a methodology previously applied in the field of disease control, but never in the field of health financing. METHODS: First, we clarify how clusters were defined both in respect of statistical considerations and of local geographical and socio-cultural concerns. Second, we illustrate how households within clusters were sampled. Third, we expound the data collection process and the survey instruments. Finally, we outline the statistical tools to be applied to estimate the impact of CHI. CONCLUSION: We discuss all design choices both in relation to methodological considerations and to specific ethical and organisational concerns faced in the field. On the basis of the appraisal of our experience, we postulate that conducting relatively sophisticated trials (such as our step-wedge cluster-randomised community-based trial) aimed at generating sound public health evidence, is both feasible and valuable also in low income settings. Our work shows that if accurately designed in conjunction with local health authorities, such trials have the potential to generate sound scientific evidence and do not hinder, but at times even facilitate, the implementation of complex health interventions such as CHI
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Dissection of hippocampal CRH-CRHR1 signalling in early life stress-induced learning and memory deficits
Yangian in the Twistor String
We study symmetries of the quantized open twistor string. In addition to
global PSL(4|4) symmetry, we find non-local conserved currents. The associated
non-local charges lead to Ward identities which show that these charges
annihilate the string gluon tree amplitudes, and have the same form as
symmetries of amplitudes in N=4 super conformal Yang Mills theory. We describe
how states of the open twistor string form a realization of the PSL(4|4)
Yangian superalgebra.Comment: 37 pages, 4 figure
Quantification of the performance of chaotic micromixers on the basis of finite time Lyapunov exponents
Chaotic micromixers such as the staggered herringbone mixer developed by
Stroock et al. allow efficient mixing of fluids even at low Reynolds number by
repeated stretching and folding of the fluid interfaces. The ability of the
fluid to mix well depends on the rate at which "chaotic advection" occurs in
the mixer. An optimization of mixer geometries is a non trivial task which is
often performed by time consuming and expensive trial and error experiments. In
this paper an algorithm is presented that applies the concept of finite-time
Lyapunov exponents to obtain a quantitative measure of the chaotic advection of
the flow and hence the performance of micromixers. By performing lattice
Boltzmann simulations of the flow inside a mixer geometry, introducing massless
and non-interacting tracer particles and following their trajectories the
finite time Lyapunov exponents can be calculated. The applicability of the
method is demonstrated by a comparison of the improved geometrical structure of
the staggered herringbone mixer with available literature data.Comment: 9 pages, 8 figure
Human cooperation in groups: variation begets variation
Many experiments on human cooperation have revealed that individuals differ systematically in their tendency to cooperate with others. It has also been shown that individuals condition their behaviour on the overall cooperation level of their peers. Yet, little is known about how individuals respond to heterogeneity in cooperativeness in their neighbourhood. Here, we present an experimental study investigating whether and how people respond to heterogeneous behaviour in a public goods game. We find that a large majority of subjects does respond to heterogeneity in their group, but they respond in quite different ways. Most subjects contribute less to the public good when the contributions of their peers are more heterogeneous, but a substantial fraction of individuals consistently contributes more in this case. In addition, we find that individuals that respond positively to heterogeneity have a higher general cooperation tendency. The finding that social responsiveness occurs in different forms and is correlated with cooperativeness may have important implications for the outcome of cooperative interactions
Using LES to Study Reacting Flows and Instabilities in Annular Combustion Chambers
Great prominence is put on the design of aeronautical gas turbines due to increasingly stringent regulations and the need to tackle rising fuel prices. This drive towards innovation has resulted sometimes in new concepts being prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is estimating the adequate grid, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on three grids, respectively made of 38, 93 and 336 million elements. Results are compared in terms of mean and fluctuating fields. LES captures self-established azimuthal modes. The presence and structure of the modes is discussed. This study therefore highlights the potential of LES for studying combustion instabilities in annular gas turbine combustors
Observation of second-harmonic generation induced by pure spin currents
Extensive efforts are currently being devoted to developing a new electronic
technology, called spintronics, where the spin of electrons is explored to
carry information. [1,2] Several techniques have been developed to generate
pure spin currents in many materials and structures. [3-10] However, there is
still no method available that can be used to directly detect pure spin
currents, which carry no net charge current and no net magnetization.
Currently, studies of pure spin currents rely on measuring the induced spin
accumulation with optical techniques [5, 11-13] or spin-valve configurations.
[14-17] However, the spin accumulation does not directly reflect the spatial
distribution or temporal dynamics of the pure spin current, and therefore
cannot monitor the pure spin current in a real-time and real-space fashion.
This imposes severe constraints on research in this field. Here we demonstrate
a second-order nonlinear optical effect of the pure spin current. We show that
such a nonlinear optical effect, which has never been explored before, can be
used for the non-invasive, non-destructive, and real-time imaging of pure spin
currents. Since this detection scheme does not rely on optical resonances, it
can be generally applied in a wide range of materials with different electronic
bandstructures. Furthermore, the control of nonlinear optical properties of
materials with pure spin currents may have potential applications in photonics
integrated with spintronics.Comment: 19 pages, 3 figures, supplementary discussion adde
Electric-field controlled spin reversal in a quantum dot with ferromagnetic contacts
Manipulation of the spin-states of a quantum dot by purely electrical means
is a highly desirable property of fundamental importance for the development of
spintronic devices such as spin-filters, spin-transistors and single-spin
memory as well as for solid-state qubits. An electrically gated quantum dot in
the Coulomb blockade regime can be tuned to hold a single unpaired spin-1/2,
which is routinely spin-polarized by an applied magnetic field. Using
ferromagnetic electrodes, however, the properties of the quantum dot become
directly spin-dependent and it has been demonstrated that the ferromagnetic
electrodes induce a local exchange-field which polarizes the localized spin in
the absence of any external fields. Here we report on the experimental
realization of this tunneling-induced spin-splitting in a carbon nanotube
quantum dot coupled to ferromagnetic nickel-electrodes. We study the
intermediate coupling regime in which single-electron states remain well
defined, but with sufficiently good tunnel-contacts to give rise to a sizable
exchange-field. Since charge transport in this regime is dominated by the
Kondo-effect, we can utilize this sharp many-body resonance to read off the
local spin-polarization from the measured bias-spectroscopy. We show that the
exchange-field can be compensated by an external magnetic field, thus restoring
a zero-bias Kondo-resonance, and we demonstrate that the exchange-field itself,
and hence the local spin-polarization, can be tuned and reversed merely by
tuning the gate-voltage. This demonstrates a very direct electrical control
over the spin-state of a quantum dot which, in contrast to an applied magnetic
field, allows for rapid spin-reversal with a very localized addressing.Comment: 19 pages, 11 figure
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