636 research outputs found
CT guided mini-laparotomy for adhesional small bowel obstruction – A novel surgical approach
On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium
We consider the dissipation by Fermi acceleration of magnetosonic turbulence
in the Reynolds Layer of the interstellar medium. The scale in the cascade at
which electron acceleration via stochastic Fermi acceleration (STFA) becomes
comparable to further cascade of the turbulence defines the inner scale. For
any magnetic turbulent spectra equal to or shallower than Goldreich-Sridhar
this turns out to be cm, which is much larger than the shortest
length scales observed in radio scintillation measurements. While STFA for such
spectra then contradict models of scintillation which appeal directly to an
extended, continuous turbulent cascade, such a separation of scales is
consistent with the recent work of \citet{Boldyrev2} and \citet{Boldyrev3}
suggesting that interstellar scintillation may result from the passage of radio
waves through the galactic distribution of thin ionized boundary surfaces of
HII regions, rather than density variations from cascading turbulence. The
presence of STFA dissipation also provides a mechanism for the non-ionizing
heat source observed in the Reynolds Layer of the interstellar medium
\citep{Reynolds}. STFA accommodates the proper heating power, and the input
energy is rapidly thermalized within the low density Reynolds layer plasma.Comment: 12 Pages, no figures. Accepted for publication in MNRA
Galactic Magnetic Turbulence from Radio data
Fluctuations in the Galactic synchrotron emission can be traced by the
angular power spectrum of radio maps at low multipoles. At frequencies below
few GHz, large-scale anisotropies are mainly induced by magnetic field
turbulence. By performing an analysis of five radio maps, we extract
constraints on turbulence spectral index and halo scale. Results favour a power
spectrum significantly flatter than for 3D Kolmogorov-like turbulence, and a
thin halo. This can be interpreted as an indication supporting non-conventional
models of propagation of cosmic-ray particles in the Galaxy, or as a suggestion
of a spectral-index break in the observed magnetic turbulence power spectrum.Comment: 15 pages, 3 figures. v2: discussions and references improved, to
appear in Astropart.Phys.
Canals beyond Mars: Beam depolarization in radio continuum maps of the warm ISM
Multi-frequency radio polarimetric observations of the diffuse Galactic
synchrotron background enable us to study the structure of the diffuse ionized
gas via rotation measure maps. However, depolarization will introduce artifacts
in the resulting rotation measure, most notably in the form of narrow,
elongated ``depolarization canals''. We use numerical models of a non-emitting
Faraday rotating medium to study the RM distribution needed to create
depolarization canals by depolarization due to a finite beam width, and to
estimate the influence of this depolarization mechanism on the determination of
RM. We argue that the depolarization canals indeed can be caused by beam
depolarization, which in turn is a natural consequence when observing a
turbulent medium with limited resolution. Furthermore, we estimate that beam
depolarization can induce an additional error of about 20% in RM
determinations, and considerably less in regions that are not affected by
depolarization canals.Comment: 9 pages, 9 figures, accepted by A&
Turbulent Density Spectrum in Solar Wind Plasma
The density fluctuation spectrum in the solar wind reveals a Kolmogorov-like
scaling with a spectral slope of -5/3 in wavenumber space. The energy transfer
process in the magnetized solar wind, characterized typically by MHD
turbulence, over extended length-scales remains an unresolved paradox of modern
turbulence theories, raising the question of how a compressible magnetofluid
exhibits a turbulent spectrum that is characteristic of an incompressible
hydrodynamic fluid. To address these questions, we have undertaken
three-dimensional time dependent numerical simulations of a compressible
magnetohydrodynamic fluid describing super-Alfv\'enic, supersonic and strongly
magnetized plasma fluid. It is shown that a Kolmogorov-like density spectrum
can develop by plasma motions that are dominated by Alfv\'enic cascades whereas
compressive modes are dissipated.Comment: Paper is to appear in Monthly Notices of the Royal Astronomical
Society Main Journa
Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the Gerasimov-Drell-Hearn Sum Rule
Photoproduction of pairs at small angles is investigated as a tool
to determine the functions and entering the real-photon forward
Compton scattering amplitude. The method is based on an interference of the
Bethe-Heitler and the virtual Compton scattering mechanisms, generating an
azimuthal asymmetry in the versus yield. The general case of a
circularly polarized beam and a longitudinally polarized target allows one to
determine both the real and imaginary parts of as well as . The
imaginary part of requires target polarization only. We calculate cross
sections and asymmetries of the reaction , estimate
corrections and backgrounds, and propose suitable kinematical regions to
perform the experiment. Our investigation shows that photoproduction of
-pairs off the proton and light nuclei may serve as a rather sensitive
test of the validity of the Gerasimov-Drell-Hearn sum rule.Comment: 22 pages; revtex; 5 postscript figures included in submission;
submitted to Phys. Rev.
Module-based multiscale simulation of angiogenesis in skeletal muscle
<p>Abstract</p> <p>Background</p> <p>Mathematical modeling of angiogenesis has been gaining momentum as a means to shed new light on the biological complexity underlying blood vessel growth. A variety of computational models have been developed, each focusing on different aspects of the angiogenesis process and occurring at different biological scales, ranging from the molecular to the tissue levels. Integration of models at different scales is a challenging and currently unsolved problem.</p> <p>Results</p> <p>We present an object-oriented module-based computational integration strategy to build a multiscale model of angiogenesis that links currently available models. As an example case, we use this approach to integrate modules representing microvascular blood flow, oxygen transport, vascular endothelial growth factor transport and endothelial cell behavior (sensing, migration and proliferation). Modeling methodologies in these modules include algebraic equations, partial differential equations and agent-based models with complex logical rules. We apply this integrated model to simulate exercise-induced angiogenesis in skeletal muscle. The simulation results compare capillary growth patterns between different exercise conditions for a single bout of exercise. Results demonstrate how the computational infrastructure can effectively integrate multiple modules by coordinating their connectivity and data exchange. Model parameterization offers simulation flexibility and a platform for performing sensitivity analysis.</p> <p>Conclusions</p> <p>This systems biology strategy can be applied to larger scale integration of computational models of angiogenesis in skeletal muscle, or other complex processes in other tissues under physiological and pathological conditions.</p
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
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