148 research outputs found
Magnetic collimation of meridional-self-similar general relativistic MHD flows
We present a model for the spine of relativistic MHD outflows in the Kerr
geometry. Meridional self-similarity is invoked to derive semi-analytical
solutions close to the polar axis. The study of the energy conservation along a
particular field line gives a simple criterion for the collimation of jets.
Such parameter have already been derived in the classical case by Sauty et al.
1999 and also extended to the Schwarzschild metric by Meliani et al. 2006. We
generalize the same study to the Kerr metric. We show that the rotation of the
black hole increases the magnetic self-confinement.Comment: 16 pages, 6 figures, accepted for publication in Physical Review
Solar wind density turbulence and solar flare electron transport from the Sun to the Earth
Solar flare accelerated electron beams propagating away from the Sun can
interact with the turbulent interplanetary media, producing plasma waves and
type III radio emission. These electron beams are detected near the Earth with
a double power-law energy spectrum. We simulate electron beam propagation from
the Sun to the Earth in the weak turbulent regime taking into account the
self-consistent generation of plasma waves and subsequent wave interaction with
density fluctuations from low frequency MHD turbulence. The rate at which
plasma waves are induced by an unstable electron beam is reduced by background
density fluctuations, most acutely when fluctuations have large amplitudes or
small wavelengths. This suppression of plasma waves alters the wave
distribution which changes the electron beam transport. Assuming a 5/3
Kolmogorov-type power density spectrum of fluctuations often observed near the
Earth, we investigate the corresponding energy spectrum of the electron beam
after it has propagated 1 AU. We find a direct correlation between the spectrum
of the double power-law below the break energy and the turbulent intensity of
the background plasma. For an initial spectral index of 3.5, we find a range of
spectra below the break energy between 1.6-2.1, with higher levels of
turbulence corresponding to higher spectral indices.Comment: 9 pages, 9 figures, to be published in Ap
Severe Clostridial Pyomyoma following an Abortion Does Not Always Require Surgical Intervention
Background. Clostridial infection following pregnancy may be fatal, and surgery is considered as the treatment of choice. We suggest a conservative management in selected cases when preservation of fertility is of major importance. Case. A 41-year-old primigravida presented with abdominal pain and fever, one day following dilatation and curettage at 20 weeks of gestation. Her abdomen was diffusely tender, with a uterus enlarged to 20 weeks' gestation. Laboratory studies were consistent with sepsis and hemolysis. CT demonstrated a gas-containing mass compressing the uterine cavity, and presence of air in pelvic veins. Blood cultures were positive for Clostridium perfringens. The patient was treated conservatively, with IV antibiotics and fluid resuscitation, and recovered. Conclusion. In selected cases of infected myoma complicated by clostridial sepsis, refraining from surgical intervention is a possible therapeutic approach
Constraining Low-Frequency Alfvenic Turbulence in the Solar Wind Using Density Fluctuation Measurements
One proposed mechanism for heating the solar wind, from close to the sun to
beyond 10 AU, invokes low-frequency, oblique, Alfven-wave turbulence. Because
small-scale oblique Alfven waves (kinetic Alfven waves) are compressive, the
measured density fluctuations in the solar wind place an upper limit on the
amplitude of kinetic Alfven waves and hence an upper limit on the rate at which
the solar wind can be heated by low-frequency, Alfvenic turbulence. We evaluate
this upper limit for both coronal holes at 5 solar radii and in the near-Earth
solar wind. At both radii, the upper limit we find is consistent with models in
which the solar wind is heated by low-frequency Alfvenic turbulence. At 1 AU,
the upper limit on the turbulent heating rate derived from the measured density
fluctuations is within a factor of 2 of the measured solar wind heating rate.
Thus if low-frequency Alfvenic turbulence contributes to heating the near-Earth
solar wind, kinetic Alfven waves must be one of the dominant sources of solar
wind density fluctuations at frequencies of order 1 Hz. We also present a
simple argument for why density fluctuation measurements do appear to rule out
models in which the solar wind is heated by non-turbulent high-frequency waves
``sweeping'' through the ion-cyclotron resonance, but are compatible with
heating by low-frequency Alfvenic turbulence.Comment: 8 pages, 3 figures, submitted to Ap
The Heating of Test Particles in Numerical Simulations of Alfvenic Turbulence
We study the heating of charged test particles in three-dimensional numerical
simulations of weakly compressible magnetohydrodynamic (MHD) turbulence
(``Alfvenic turbulence''); these results are relevant to particle heating and
acceleration in the solar wind, solar flares, accretion disks onto black holes,
and other astrophysics and heliospheric environments. The physics of particle
heating depends on whether the gyrofrequency of a particle is comparable to the
frequency of a turbulent fluctuation that is resolved on the computational
domain. Particles with these frequencies nearly equal undergo strong
perpendicular heating (relative to the local magnetic field) and pitch angle
scattering. By contrast, particles with large gyrofrequency undergo strong
parallel heating. Simulations with a finite resistivity produce additional
parallel heating due to parallel electric fields in small-scale current sheets.
Many of our results are consistent with linear theory predictions for the
particle heating produced by the Alfven and slow magnetosonic waves that make
up Alfvenic turbulence. However, in contrast to linear theory predictions,
energy exchange is not dominated by discrete resonances between particles and
waves; instead, the resonances are substantially ``broadened.'' We discuss the
implications of our results for solar and astrophysics problems, in particular
the thermodynamics of the near-Earth solar wind. We conclude that Alfvenic
turbulence produces significant parallel heating via the interaction between
particles and magnetic field compressions (``slow waves''). However, on scales
above the proton Larmor radius, Alfvenic turbulence does not produce
significant perpendicular heating of protons or minor ions.Comment: Submitted to Ap
Solar Wind Turbulence and the Role of Ion Instabilities
International audienc
An analysis of the reliability and design optimization of aluminium ribbon bonds in power electronics modules using computer simulation method
Ribbon bonding technique has recently been used as an alternative to wire bonding in order to improve the reliability, performance and reduce cost of power modules. In this work, the reliability of aluminium and copper ribbon bonds for an Insulated Gate Bipolar Transistors (IGBT) power module under power cycling is compared with that of wire bonds under power and thermal cycling loading conditions. The results show that a single ribbon with a cross section of 2000 μm × 200 μm can be used to replace three wire bonds of 400 μm in diameter to achieve similar module temperature distribution under the same power loading and ribbon bonds have longer lifetime than wire bonds under cyclic power and thermal cycling conditions. In order to find the optimal ribbon bond design for both power cycling and thermal cycling conditions, multi-objective optimization method has been used and the Pareto optimal solutions have been obtained for trade off analysis
A study of hyperons produced by negative kaons having a momentum of 3.5 Gevc in a hydrogen bubble chamber
Imperial Users onl
- …