351 research outputs found
Thermal instability in ionized plasma
We study magnetothermal instability in the ionized plasmas including the
effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single
fluid approximation does not allow transverse thermal condensations, however,
non-ideal effects highly diminish the stabilizing role of the magnetic field in
thermally unstable plasmas. Therefore, enhanced growth rate of thermal
condensation modes in the presence of the diffusion mechanisms speed up the
rate of structure formation.Comment: Accepted for publication in Astrophysics & Space Scienc
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
Emergence and Control of Rottboelia spp. in Different Months in Sugarcane Cultivation1
Impacto de sulfentrazona, isoxaflutol e oxyfluorfem sobre a microbiota de dois solos florestais
Determination of Fundamental Supersymmetry Parameters from Chargino Production at Lepii
If accessible at LEP II, chargino production is likely to be one of the few
available supersymmetric signals for many years. We consider the prospects for
the determination of fundamental supersymmetry parameters in such a scenario.
The study is complicated by the dependence of observables on a large number of
these parameters. We propose a straightforward procedure for disentangling
these dependences and demonstrate its effectiveness by presenting a number of
case studies at representative points in parameter space. Working in the
context of the minimal supersymmetric standard model, we find that chargino
production by itself is a fairly sensitive probe of the supersymmetry-breaking
sector. For significant regions of parameter space, it is possible to test the
gaugino mass unification hypothesis and to measure the gaugino contents of the
charginos and neutralinos, thereby testing the predictions of grand unification
and the viability of the lightest supersymmetric particle as a dark matter
candidate. For much of the parameter space, it is also possible to set limits
on the mass of the electron sneutrino, which provide a valuable guide for
future particle searches.Comment: 52pp, Revtex, 30 figures available upon request, SLAC-PUB-6497,
RU-94-67 (text and figures available in ps form by anonymous ftp from
preprint.slac.stanford.edu, directory pub/preprints/hep-ph/9408
Cristallochemical characterization of synthetic Zn-substituted maghemites (g-Fe2-xZn xO3)
How distributed processing produces false negatives in voxel-based lesion-deficit analyses
In this study, we hypothesized that if the same deficit can be caused by damage to one or another part of a
distributed neural system, then voxel-based analyses might miss critical lesion sites because preservation of each
site will not be consistently associated with preserved function. The first part of our investigation used voxelbased
multiple regression analyses of data from 359 right-handed stroke survivors to identify brain regions
where lesion load is associated with picture naming abilities after factoring out variance related to object recognition,
semantics and speech articulation so as to focus on deficits arising at the word retrieval level. A highly
significant lesion-deficit relationship was identified in left temporal and frontal/premotor regions. Post-hoc
analyses showed that damage to either of these sites caused the deficit of interest in less than half the affected
patients (76/162 = 47%). After excluding all patients with damage to one or both of the identified regions, our
second analysis revealed a new region, in the anterior part of the left putamen, which had not been previously
detected because many patients had the deficit of interest after temporal or frontal damage that preserved the
left putamen. The results illustrate how (i) false negative results arise when the same deficit can be caused by
different lesion sites; (ii) some of the missed effects can be unveiled by adopting an iterative approach that
systematically excludes patients with lesions to the areas identified in previous analyses, (iii) statistically significant
voxel-based lesion-deficit mappings can be driven by a subset of patients; (iv) focal lesions to the
identified regions are needed to determine whether the deficit of interest is the consequence of focal damage or
much more extensive damage that includes the identified region; and, finally, (v) univariate voxel-based lesiondeficit
mappings cannot, in isolation, be used to predict outcome in other patients
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