2,194 research outputs found
A more accurate numerical scheme for diffusive shock acceleration
We present a more accurate numerical scheme for the calculation of diffusive
shock acceleration of cosmic rays using Stochastic Differential Equations. The
accuracy of this scheme is demonstrated using a simple analytical flow profile
that contains a shock of finite width and a varying diffusivity of the cosmic
rays, where the diffusivity decreases across the shock. We compare the results
for the slope of the momentum distribution with those obtained from a
perturbation analysis valid for finite but small shock width. These
calculations show that this scheme, although computationally more expensive,
provides a significantly better performance than the Cauchy-Euler type schemes
that were proposed earlier in the case where steep gradients in the cosmic ray
diffusivity occur. For constant diffusivity the proposed scheme gives similar
results as the Cauchy-Euler scheme.Comment: 9 pages, 2 figures. Published in MNRA
Branching on multi-aggregated variables
open5siopenGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, DomenicoGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, Domenic
Evolution of Magnetic Fields in Supernova Remnants
Supernova remnants (SNR) are now widely believed to be a source of cosmic
rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate
CRs to sufficiently high energies need to be much higher than can result from
compression of the circumstellar medium (CSM) by a factor 4, as is the case in
strong shocks. Non-thermal synchrotron maps of these regions indicate that
indeed the magnetic field is much stronger, and for young SNRs has a dominant
radial component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for young
remnants, is not yet fully understood. We use an adaptive mesh refinement MHD
code, AMRVAC, to simulate the evolution of supernova remnants and to see if we
can reproduce a mainly radial magnetic field in early stages of evolution. We
follow the evolution of the SNR with three different configurations of the
initial magnetic field in the CSM: an initially mainly toroidal field, a
turbulent magnetic field, and a field parallel to the symmetry axis. Although
for the latter two topologies a significant radial field component arises at
the contact discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears not
sufficient to explain observations. Possibly a higher compression ratio and
additional turbulence due to dominant presence of CRs can help us to better
reproduce the observations in future studies.Comment: 5 pages, 3 figures. To appear in conference proceedings of "Magnetic
Fields in the Universe II" (2008), RevMexA
A review of consumer awareness, understanding and use of food-based dietary guidelines
Copyright @ 2011 Cambridge University PressFood-based dietary guidelines (FBDG) have primarily been designed for the consumer to encourage healthy, habitual food choices, decrease chronic disease risk and improve public health. However, minimal research has been conducted to evaluate whether FBDG are utilised by the public. The present review used a framework of three concepts, awareness, understanding and use, to summarise consumer evidence related to national FBDG and food guides. Searches of nine electronic databases, reference lists and Internet grey literature elicited 939 articles. Predetermined exclusion criteria selected twenty-eight studies for review. These consisted of qualitative, quantitative and mixed study designs, non-clinical participants, related to official FBDG for the general public, and involved measures of consumer awareness, understanding or use of FBDG. The three concepts of awareness, understanding and use were often discussed interchangeably. Nevertheless, a greater amount of evidence for consumer awareness and understanding was reported than consumer use of FBDG. The twenty-eight studies varied in terms of aim, design and method. Study quality also varied with raw qualitative data, and quantitative method details were often omitted. Thus, the reliability and validity of these review findings may be limited. Further research is required to evaluate the efficacy of FBDG as a public health promotion tool. If the purpose of FBDG is to evoke consumer behaviour change, then the framework of consumer awareness, understanding and use of FBDG may be useful to categorise consumer behaviour studies and complement the dietary survey and health outcome data in the process of FBDG evaluation and revision.This study is funded by the European Commission Sixth Framework Programme (contract no. 036196)
Jets as diagnostics of the circumstellar medium and the explosion energetics of supernovae: the case of Cas A
We present hydrodynamical models for the Cassiopeia A (Cas A) supernova
remnant and its observed jet / counter-jet system. We include the evolution of
the progenitor's circumstellar medium, which is shaped by a slow red supergiant
wind that is followed by a fast Wolf-Rayet (WR) wind.
The main parameters of the simulations are the duration of the WR phase and
the jet energy. We find that the jet is destroyed if the WR phase is
sufficiently long and a massive circumstellar shell has formed. We therefore
conclude that the WR phase must have been short (a few thousand yr), if present
at all. Since the actual jet length of Cas A is not known we derive a lower
limit for the jet energy, which is ~10^{48} erg. We discuss the implications
for the progenitor of Cas A and the nature of its explosion.Comment: 9 pages, 5 figures, ApJ accepted. Version with high resolution
figures available at http://www.phys.uu.nl/~schure/CasA_jet.pd
Comment on the first-order Fermi acceleration at ultra-relativistic shocks
The first-order Fermi acceleration process at an ultra-relativistic shock
wave is expected to create a particle spectrum with the unique asymptotic
spectral index sigma_{gamma >> 1} approximately 2.2. Below, we discuss this
result and differences in its various derivations, which -- explicitly or
implicitly -- always require highly turbulent conditions downstream of the
shock. In the presence of medium amplitude turbulence the generated particle
spectrum can be much steeper than the above asymptotic one. We also note
problems with application of the pitch angle diffusion model for particle
transport near the ultra-relativistic shocks.Comment: Substantially modified and shorted version, accepted to A&
Stochastic Acceleration of Low Energy Electrons in Plasmas with Finite Temperature
This paper extends our earlier work on the acceleration of low-energy
electrons by plasma turbulence to include the effects of finite temperature of
the plasma. We consider the resonant interaction of thermal electrons with the
whole transverse branch of plasma waves propagating along the magnetic field.
We show that our earlier published results for acceleration of low-energy
electrons can be applied to the case of finite temperature if a sufficient
level of turbulence is present. From comparison of the acceleration rate of the
thermal particles with the decay rate of the waves with which they interact, we
determine the required energy density of the waves as a fraction of the
magnetic energy density, so that a substantial fraction of the background
plasma electrons can be accelerated. The dependence of this value on the plasma
parameter alpha = omega_pe / Omega_e (the ratio of electron plasma frequency to
electron gyrofrequency), plasma temperature, and turbulence spectral parameters
is quantified. We show that the result is most sensitive to the plasma
parameter alpha. We estimate the required level of total turbulence by
calculating the level of turbulence required for the initial acceleration of
thermal electrons and that required for further acceleration to higher
energies
Induced Scattering and Two-Photon Absorption of Alfven Waves with Arbitrary Propagation Angles
The equation for temporary evolution of spectral energy of collisionless
Alfven waves is derived in framework of weak turbulence theory. The main
nonlinear processes for such conditions are induced scattering and two quantum
absorption of Alfven waves by thermal ions. The equation for velocity
distribution of thermal particles is derived that describes diffusion in
momentum space due to this nonlinear processes. Comparison is done with the
results of another authors. Results obtained are qualitatively differ from the
ones obtained for the case of Alfven waves propagation along mean magnetic
field.Comment: 8 page
Stochastic Acceleration of Low Energy Electrons in Cold Plasmas
We investigate the possibility of stochastic acceleration of background
low-energy electrons by turbulent plasma waves. We consider the resonant
interaction of the charged particles with all branches of the transverse plasma
waves propagating parallel to a uniform magnetic field. Numerical results and
asymptotic analytic solutions valid at non-relativistic and ultra-relativistic
energies are obtained for the acceleration and scattering times of electrons.
These times have a strong dependence on plasma parameter alpha = Omega_pe /
Omega_e (the ratio of electron plasma frequency to electron gyrofrequency) and
on the spectral index of plasma turbulence. It is shown that particles with
energies above certain critical value may interact with higher frequency
electromagnetic plasma waves and this interaction is allowed only in plasmas
with alpha < 1. We show that for non-relativistic and semi-relativistic
electrons in low-alpha plasmas the ratio of the acceleration time to the
scattering time can be less than unity for a wide range of energies. From this
we conclude that the transport equation derived for cosmic rays which requires
this ratio to be much larger than one is not applicable at these energies. An
approximate "critical" value of particle energy above which the dynamics of
charged particles may be described by this transport equation is determined as
a function of plasma parameters. We propose new transport equation for the
opposite limit (energies less than this critical value) when the acceleration
rate is much faster than the pitch angle scattering rate. This equation is
needed to describe the electron dynamics in plasmas with alpha <= 0.1.Comment: 22 pages, 13 figures, Latex, submitted to Astrophysical Journa
Genetic and environmental influences on MRI scan quantity and quality.
The current study provides an overview of quantity and quality of MRI data in a large developmental twin
sample (N = 512, aged 7–9), and investigated to what extent scan quantity and quality were influenced by
genetic and environmental factors. This was examined in a fixed scan protocol consisting of two functional MRI
tasks, high resolution structural anatomy (3DT1) and connectivity (DTI) scans, and a resting state scan. Overall,
scan quantity was high (88% of participants completed all runs), while scan quality decreased with increasing
session length. Scanner related distress was negatively associated with scan quantity (i.e., completed runs), but
not with scan quality (i.e., included runs). In line with previous studies, behavioral genetic analyses showed that
genetics explained part of the variation in head motion, with heritability estimates of 29% for framewise displacement and 65% for absolute displacement. Additionally, our results revealed that subtle head motion (after
exclusion of excessive head motion) showed lower heritability estimates (0–14%), indicating that findings of
motion-corrected and quality-controlled MRI data may be less confounded by genetic factors. These findings
provide insights in factors contributing to scan quality in children, an issue that is highly relevant for the field of
developmental neuroscience
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