372 research outputs found
Activation of MHD reconnection on ideal timescales
Magnetic reconnection in laboratory, space and astrophysical plasmas is often
invoked to explain explosive energy release and particle acceleration. However,
the timescales involved in classical models within the macroscopic MHD regime
are far too slow to match the observations. Here we revisit the tearing
instability by performing visco-resistive two-dimensional numerical simulations
of the evolution of thin current sheets, for a variety of initial
configurations and of values of the Lunquist number , up to . Results
confirm that when the critical aspect ratio of is reached in the
reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic)
macroscopic timescales, as required to explain observations. Moreover, the same
scaling is seen to apply also to the local, secondary reconnection events
triggered during the nonlinear phase of the tearing instability, thus
accelerating the cascading process to increasingly smaller spatial and temporal
scales. The process appears to be robust, as the predicted scaling is measured
both in inviscid simulations and when using a Prandtl number in the
viscous regime.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Implications of Food Subsistence for Monetary Policy and Inflation
The chapter introduces subsistence requirements in food consumption into a simple New Keynesian model with flexible food and sticky non-food prices. It shows how the endogenous structural transformation that results from subsistence affects the dynamics of the economy, the design of monetary policy, and the properties of inflation at different levels of development. A calibrated version of the model encompasses both rich and poor countries and broadly replicates the properties of inflation across the development spectrum, including the dominant role played by changes in the relative price of food in poor countries. The authors derive a welfare-based loss function for the monetary authority and show that optimal policy calls for complete (in some cases near-complete) stabilization of sticky-price non-food inflation, despite the presence of a food-subsistence threshold. Subsistence amplifies the welfare losses of policy mistakes, however, raising the stakes for monetary policy at earlier stages of development.</p
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
Erratum to: A study of vorticity formation in high energy nuclear collisions
Due to an oversight of ours in proofreading and a communication problem with the publisher, the figures published in F. Becattini et al. Eur. Phys. J. C (2015) 75: 406 were not correct. This Erratum contains the correct figures as in arXiv 1501.04468v2, submitted on March 12 2015, and the post-publication version arXiv 1501.04468v3, submitted on August 17 2015
Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields
This paper presents an application of the recent relativistic HLLC
approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing
normal component of the magnetic field.
The numerical scheme is validated in two dimensions by investigating the
propagation of axisymmetric jets with toroidal magnetic fields.
The selected jet models show that the HLLC solver yields sharper resolution
of contact and shear waves and better convergence properties over the
traditional HLL approach.Comment: 12 pages, 5 figure
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
Building productive relationships with young people with SEBD in transition: the role of identity
This article reports a study of the experiences of school leavers with social, emotional and behavioural difficulties (SEBD), which identified supportive relationships as key elements in young people demonstrating resilience through this transitional period. Almost all the young people involved in the study had access to potential helpers, but few managed to establish productive relationships with them. Analysis of interviews, conducted over a 15 month period with a group of 15 school leavers, their parents and those who worked with them, suggested that barriers and facilitators to relationship development existed at two levels: institutional and individual. This article focuses on the individual level, in which identity processes appear to play a key role. These processes are used to explain why some school leavers built productive relationships and thrived, whilst many failed to do so, and struggled. These findings have implications for policy, practice and theory
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