2,367 research outputs found
Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks
Plasma processes close to SNR shocks result in the amplification of magnetic
fields and in the acceleration of electrons, injecting them into the diffusive
acceleration mechanism. The acceleration of electrons and the B field
amplification by the collision of two plasma clouds, each consisting of
electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel
guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of
25 keV are considered. A quasi-planar shock forms at the front of the dense
plasma cloud. It is mediated by a circularly left-hand polarized
electromagnetic wave with an electric field component along the guiding
magnetic field. Its propagation direction is close to that of the guiding field
and orthogonal to the collision boundary. It has a low frequency and a
wavelength that equals several times the ion inertial length, which would be
indicative of a dispersive Alfven wave close to the ion cyclotron resonance
frequency of the left-handed mode (ion whistler), provided that the frequency
is appropriate. However, it moves with the super-alfvenic plasma collision
speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such
as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the
magnetic amplitude of this wave to values well in excess of those of the
quasi-parallel guiding field and of the filamentation modes results in a
quasi-perpendicular shock. We present evidence for the instability of this mode
to a four wave interaction. The waves developing upstream of the dense cloud
give rise to electron acceleration ahead of the collision boundary. Energy
equipartition between the ions and the electrons is established at the shock
and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability
We examine the formation and evolution of the density enhancement (density
spike) that appears downstream of strong, cosmic-ray-modified shocks. This
feature results from temporary overcompression of the flow by the combined
cosmic-ray shock precursor/gas subshock. Formation of the density spike is
expected whenever shock modification by cosmic-ray pressure increases strongly.
That occurence may be anticipated for newly generated strong shocks or for
cosmic-ray-modified shocks encountering a region of higher external density,
for example. The predicted mass density within the spike increases with the
shock Mach number and with shocks more dominated by cosmic-ray pressure. We
find this spike to be linearly unstable under a modified Rayleigh-Taylor
instability criterion at the early stage of its formation. We confirm this
instability numerically using two independent codes based on the two-fluid
model for cosmic-ray transport. These two-dimensional simulations show that the
instability grows impulsively at early stages and then slows down as the
gradients of total pressure and gas density decrease. Observational discovery
of this unstable density spike behind shocks, possibly through radio emission
enhanced by the amplified magnetic fields would provide evidence for the
existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap
Hip precautions after hip operation (HippityHop): protocol for a before and after study evaluating hip precautions following total hip replacement
Introduction
Hip precautions are routinely used despite inconclusive evidence that they reduce dislocations, and concern that they impede activities of daily living. HippityHop compares a change in practice locally from implementing routine hip precautions to no routine precautions, in order to: 1. Compare patient outcomes in quality of life, functional performance, pain, sleep, mood and satisfaction. 2. Ascertain staff and patient perceptions of the two regimes. 3. Determine the cost of precautions.
Methods
Before and after study: phase one patients will receive hip precautions, while phase two patients will receive no routine precautions. We propose to collect data from 342 participants at baseline, and at one week, six weeks, and three months postoperatively. Interviews will be conducted with 20 staff and 20 patients, and data collected relating to costs.
Results
Statistical analysis will be conducted to compare the two groups to determine any differences in patient outcomes. Thematic analysis will be used to identify and report themes within the interview data.
Conclusion
If there are no additional advantages to hip precautions, patients could resume everyday activities more quickly, potentially improving their quality of life. Conversely, if withdrawing hip precautions is detrimental, evidence for precautions will be provided
Energetics of X-ray Cavities and Radio Lobes in Galaxy Clusters
We describe the formation and evolution of X-ray cavities in the hot gas of
galaxy clusters. The cavities are formed only with relativistic cosmic rays
that eventually diffuse into the surrounding gas. We explore the evolution of
cavities formed with a wide range of cosmic ray diffusion rates. In previous
numerical simulations cavities are formed by injecting ultra-hot but
non-relativistic gas which increases the global thermal energy, offsetting
radiative losses in the gas and helping to solve the cooling flow problem.
Contrary to these results, we find that X-ray cavities formed solely by cosmic
rays have a global cooling effect. As the cluster gas is displaced by cosmic
rays, a global expansion of the cluster gas occurs with associated cooling that
exceeds the heating by shock waves as the cavity forms. Most cosmic rays in our
cavity evolutions do not move beyond the cooling radius even after 1 Gyr. The
gas density is depressed by cosmic rays, becomes buoyant, and undergoes a
significant outward mass transfer within the cooling radius, carrying cosmic
rays and relatively low entropy gas to distant regions in the cluster where it
remains for times exceeding the local cooling time in the hot gas. This
post-cavity mass outflow due to cosmic ray buoyancy may contribute toward
solving the cooling flow problem. We describe the energetics, size, stability
and buoyant rise of X-ray cavities in detail, showing how each depends on the
rate of cosmic ray diffusion.Comment: 17 pages, 8 figures, accepted by Ap
The Effects of Isokinetic Contraction Velocity on the Concentric to Eccentric Strength Relationship of the Biceps Brachii
The purpose of this investigation was to determine the influence of contraction velocity on the eccentric (ECC) and concentric (CON) torque production of the biceps brachii. After performing warm-up procedures, each male subject (n = 11) completed 3 sets of 5 maximal bilateral CON and ECC isokinetic contractions of the biceps at three different speeds on a Biodex System 3 dynamometer. The men received a 3-minute rest between sets and the order of exercises was randomized. Peak torque (Nm) values were obtained for CON and ECC contractions at each speed. Peak torque scores (ECC vs. CON) were compared using a t-test at each speed. A repeated measures analysis of variance was used to determine differences between speeds. ECC peak torque scores were greater than CON peak torque scores at each given speed. No differences were found between the ECC peak torque scores (p = 0.62) at any of the speeds. Differences were found among the CON scores (p = 0.004). Post hoc analysis revealed differences. The data suggests that ECC contractions of the biceps brachii were somewhat resistant to a force decrement as the result of an increase in velocity, whereas CON muscular actions of the biceps brachii were unable to maintain force as velocity increased
The Littlewood-Gowers problem
We show that if A is a subset of Z/pZ (p a prime) of density bounded away
from 0 and 1 then the A(Z/pZ)-norm (that is the l^1-norm of the Fourier
transform) of the characterstic function of A is bounded below by an absolute
constant times (log p)^{1/2 - \epsilon} as p tends to infinity. This improves
on the exponent 1/3 in recent work of Green and Konyagin.Comment: 31 pp. Corrected typos. Updated references
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