29,588 research outputs found
Heliospheric plasma sheets
[1] As a high-beta feature on scales of hours or less, the heliospheric plasma sheet (HPS) encasing the heliospheric current sheet shows a high degree of variability. A study of 52 sector boundaries identified in electron pitch angle spectrograms in Wind data from 1995 reveals that only half concur with both high-beta plasma and current sheets, as required for an HPS. The remaining half lack either a plasma sheet or current sheet or both. A complementary study of 37 high-beta events reveals that only 5 contain sector boundaries while nearly all (34) contain local magnetic field reversals, however brief. We conclude that high-beta plasma sheets surround current sheets but that most of these current sheets are associated with fields turned back on themselves. The findings are consistent with the hypothesis that high-beta plasma sheets, both at and away from sector boundaries, are the heliospheric counterparts of the small coronal transients observed at the tips of helmet streamers, in which case the proposed mechanism for their release, interchange reconnection, could be responsible for the field inversions
Oscillations of General Relativistic Multi-fluid/Multi-layer Compact Stars
We develop the formalism for determining the quasinormal modes of general
relativistic multi-fluid compact stars in such a way that the impact of
superfluid gap data can be assessed. Our results represent the first attempt to
study true multi-layer dynamics, an important step towards considering
realistic superfluid/superconducting compact stars. We combine a relativistic
model for entrainment with model equations of state that explicity incorporate
the symmetry energy. Our analysis emphasises the many different parameters that
are required for this kind of modelling, and the fact that standard tabulated
equations of state are grossly incomplete in this respect. To make progress,
future equations of state need to provide the energy density as a function of
the various nucleon number densities, the temperature (i.e. entropy), and the
entrainment among the various components
Comment on "Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2 yH2O"
The effect of surface degradation of the thermolectric cobaltite on Raman
spectra is discussed and compared to experimental results from Co3O4 single
crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer
of Co3O4 easily forms that leads to the observation of an intense phonon around
700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved
crystals from optical floating zone ovens do not show such effects and have a
high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96,
167204 (2006)]. We discuss the relation of structural dimensionality,
electronic correlations and the high frequency phonon cut-off of the
thermolectric cobaltite.Comment: 2 pages, 1 figure, to be published in Phys. Rev. B (2007
New Approximability Results for the Robust k-Median Problem
We consider a robust variant of the classical -median problem, introduced
by Anthony et al. \cite{AnthonyGGN10}. In the \emph{Robust -Median problem},
we are given an -vertex metric space and client sets . The objective is to open a set of
facilities such that the worst case connection cost over all client sets is
minimized; in other words, minimize . Anthony
et al.\ showed an approximation algorithm for any metric and
APX-hardness even in the case of uniform metric. In this paper, we show that
their algorithm is nearly tight by providing
approximation hardness, unless . This hardness result holds even for uniform and line
metrics. To our knowledge, this is one of the rare cases in which a problem on
a line metric is hard to approximate to within logarithmic factor. We
complement the hardness result by an experimental evaluation of different
heuristics that shows that very simple heuristics achieve good approximations
for realistic classes of instances.Comment: 19 page
Volatility of Linear and Nonlinear Time Series
Previous studies indicate that nonlinear properties of Gaussian time series
with long-range correlations, , can be detected and quantified by studying
the correlations in the magnitude series , i.e., the ``volatility''.
However, the origin for this empirical observation still remains unclear, and
the exact relation between the correlations in and the correlations in
is still unknown. Here we find analytical relations between the scaling
exponent of linear series and its magnitude series . Moreover, we
find that nonlinear time series exhibit stronger (or the same) correlations in
the magnitude time series compared to linear time series with the same
two-point correlations. Based on these results we propose a simple model that
generates multifractal time series by explicitly inserting long range
correlations in the magnitude series; the nonlinear multifractal time series is
generated by multiplying a long-range correlated time series (that represents
the magnitude series) with uncorrelated time series [that represents the sign
series ]. Our results of magnitude series correlations may help to
identify linear and nonlinear processes in experimental records.Comment: 7 pages, 5 figure
Hepatic fibrogenesis requires sympathetic neurotransmitters
Background and aims: Hepatic stellate cells (HSC) are activated by liver injury to become proliferative fibrogenic myofibroblasts. This process may be regulated by the sympathetic nervous system (SNS) but the mechanisms involved are unclear. Methods: We studied cultured HSC and intact mice with liver injury to test the hypothesis that HSC respond to and produce SNS neurotransmitters to promote fibrogenesis. Results: HSC expressed adrenoceptors, catecholamine biosynthetic enzymes, released norepinephrine (NE), and were growth inhibited by α- and β-adrenoceptor antagonists. HSC from dopamine β-hydroxylase deficient (Dbh(−/−)) mice, which cannot make NE, grew poorly in culture and were rescued by NE. Inhibitor studies demonstrated that this effect was mediated via G protein coupled adrenoceptors, mitogen activated kinases, and phosphatidylinositol 3-kinase. Injury related fibrogenic responses were inhibited in Dbh(−/−) mice, as evidenced by reduced hepatic accumulation of α-smooth muscle actin(+ve) HSC and decreased induction of transforming growth factor β1 (TGF-β1) and collagen. Treatment with isoprenaline rescued HSC activation. HSC were also reduced in leptin deficient ob/ob mice which have reduced NE levels and are resistant to hepatic fibrosis. Treating ob/ob mice with NE induced HSC proliferation, upregulated hepatic TGF-β1 and collagen, and increased liver fibrosis. Conclusions: HSC are hepatic neuroglia that produce and respond to SNS neurotransmitters to promote hepatic fibrosis
Comments on AdS2 solutions of D=11 Supergravity
We study the supersymmetric solutions of 11-dimensional supergravity with a
factor of made of M2-branes. Such solutions can provide gravity duals
of superconformal quantum mechanics, or through double Wick rotation, the
generic bubbling geometry of M-theory which are 1/16-BPS. We show that, when
the internal manifold is compact, it should take the form of a warped
U(1)-fibration over an 8-dimensional Kahler space.Comment: 11 pages, no figure, JHEP3.cl
Bifurcation analysis of the behavior of partially wetting liquids on a rotating cylinder
We discuss the behavior of partially wetting liquids on a rotating cylinder
using a model that takes into account the effects of gravity, viscosity,
rotation, surface tension and wettability. Such a system can be considered as a
prototype for many other systems where the interplay of spatial heterogeneity
and a lateral driving force in the proximity of a first- or second-order phase
transition results in intricate behavior. So does a partially wetting drop on a
rotating cylinder undergo a depinning transition as the rotation speed is
increased, whereas for ideally wetting liquids the behavior \bfuwe{only changes
quantitatively. We analyze the bifurcations that occur when the rotation speed
is increased for several values of the equilibrium contact angle of the
partially wetting liquids. This allows us to discuss how the entire bifurcation
structure and the flow behavior it encodes changes with changing wettability.
We employ various numerical continuation techniques that allow us to track
stable/unstable steady and time-periodic film and drop thickness profiles. We
support our findings by time-dependent numerical simulations and asymptotic
analyses of steady and time-periodic profiles for large rotation numbers
Resonance structure in the Li^- photodetachment cross section
We report on the first observation of resonance structure in the total cross
section for the photodetachment of Li^-. The structure arises from the
autodetaching decay of doubly excited ^1P states of Li^- that are bound with
respect to the 3p state of the Li atom. Calculations have been performed for
both Li^- and H^- to assist in the identification of these resonances. The
lowest lying resonance is a symmetrically excited intrashell resonance. Higher
lying asymmetrically excited intershell states are observed which converge on
the Li(3p) limit.Comment: 4 pages, 2 figure, 19 references, RevTeX, figures in ep
Interference of multi-mode photon echoes generated in spatially separated solid-state atomic ensembles
High-visibility interference of photon echoes generated in spatially
separated solid-state atomic ensembles is demonstrated. The solid state
ensembles were LiNbO waveguides doped with Erbium ions absorbing at 1.53
m. Bright coherent states of light in several temporal modes (up to 3) are
stored and retrieved from the optical memories using two-pulse photon echoes.
The stored and retrieved optical pulses, when combined at a beam splitter, show
almost perfect interference, which demonstrates both phase preserving storage
and indistinguishability of photon echoes from separate optical memories. By
measuring interference fringes for different storage times, we also show
explicitly that the visibility is not limited by atomic decoherence. These
results are relevant for novel quantum repeaters architectures with photon echo
based multimode quantum memories
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