1,590 research outputs found
The Career of Moses Shapira, Bookseller and Antiquarian
publishedVersio
The sodium-potassium pump controls the intrinsic firing of the cerebellar Purkinje neuron
In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na+/K+ pump activity sets the Purkinje cell's operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na+/K+ pump. The model can replicate these recordings. We propose that Na+/K+ pump activity controls the intrinsic firing mode of cerbellar Purkinje cells
Final spins from the merger of precessing binary black holes
The inspiral of binary black holes is governed by gravitational radiation
reaction at binary separations r < 1000 M, yet it is too computationally
expensive to begin numerical-relativity simulations with initial separations r
> 10 M. Fortunately, binary evolution between these separations is well
described by post-Newtonian equations of motion. We examine how this
post-Newtonian evolution affects the distribution of spin orientations at
separations r ~ 10 M where numerical-relativity simulations typically begin.
Although isotropic spin distributions at r ~ 1000 M remain isotropic at r ~ 10
M, distributions that are initially partially aligned with the orbital angular
momentum can be significantly distorted during the post-Newtonian inspiral.
Spin precession tends to align (anti-align) the binary black hole spins with
each other if the spin of the more massive black hole is initially partially
aligned (anti-aligned) with the orbital angular momentum, thus increasing
(decreasing) the average final spin. Spin precession is stronger for
comparable-mass binaries, and could produce significant spin alignment before
merger for both supermassive and stellar-mass black hole binaries. We also
point out that precession induces an intrinsic accuracy limitation (< 0.03 in
the dimensionless spin magnitude, < 20 degrees in the direction) in predicting
the final spin resulting from the merger of widely separated binaries.Comment: 20 pages, 16 figures, new PN terms, submitted to PR
Controlled nanochannel lattice formation utilizing prepatterned substrates
Solid substrates can be endued with self-organized regular stripe patterns of
nanoscopic lengthscale by Langmuir-Blodgett transfer of organic monolayers.
Here we consider the effect of periodically prepatterned substrates on this
process of pattern formation. It leads to a time periodic forcing of the
oscillatory behavior at the meniscus. Utilizing higher order synchronization
with this forcing, complex periodic patterns of predefined wavelength can be
created. The dependence of the synchronization on the amplitude and the
wavelength of the wetting contrast is investigated in one and two spatial
dimensions and the resulting patterns are discussed. Furthermore, the effect of
prepatterned substrates on the pattern selection process is investigated
Polar confinement of the Sun's interior magnetic field by laminar magnetostrophic flow
The global-scale interior magnetic field needed to account for the Sun's
observed differential rotation can be effective only if confined below the
convection zone in all latitudes, including the polar caps. Axisymmetric
nonlinear MHD solutions are obtained showing that such confinement can be
brought about by a very weak downwelling flow U~10^{-5}cm/s over each pole.
Such downwelling is consistent with the helioseismic evidence. All three
components of the magnetic field decay exponentially with altitude across a
thin "magnetic confinement layer" located at the bottom of the tachocline. With
realistic parameter values, the thickness of the confinement layer ~10^{-3} of
the Sun's radius. Alongside baroclinic effects and stable thermal
stratification, the solutions take into account the stable compositional
stratification of the helium settling layer, if present as in today's Sun, and
the small diffusivity of helium through hydrogen, chi. The small value of chi
relative to magnetic diffusivity produces a double boundary-layer structure in
which a "helium sublayer" of smaller vertical scale is sandwiched between the
top of the helium settling layer and the rest of the confinement layer.
Solutions are obtained using both semi-analytical and purely numerical,
finite-difference techniques. The confinement-layer flows are magnetostrophic
to excellent approximation. More precisely, the principal force balances are
between Lorentz, Coriolis, pressure-gradient and buoyancy forces, with relative
accelerations and viscous forces negligible. This is despite the kinematic
viscosity being somewhat greater than chi. We discuss how the confinement
layers at each pole might fit into a global dynamical picture of the solar
tachocline. That picture, in turn, suggests a new insight into the early Sun
and into the longstanding enigma of solar lithium depletion.Comment: Accepted by JFM. 36 pages, 10 figure
A Model for the Propagation of Sound in Granular Materials
This paper presents a simple ball-and-spring model for the propagation of
small amplitude vibrations in a granular material. In this model, the
positional disorder in the sample is ignored and the particles are placed on
the vertices of a square lattice. The inter-particle forces are modeled as
linear springs, with the only disorder in the system coming from a random
distribution of spring constants. Despite its apparent simplicity, this model
is able to reproduce the complex frequency response seen in measurements of
sound propagation in a granular system. In order to understand this behavior,
the role of the resonance modes of the system is investigated. Finally, this
simple model is generalized to include relaxation behavior in the force network
-- a behavior which is also seen in real granular materials. This model gives
quantitative agreement with experimental observations of relaxation.Comment: 21 pages, requires Harvard macros (9/91), 12 postscript figures not
included, HLRZ preprint 6/93, (replacement has proper references included
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
Dynamics of a thin liquid film with surface rigidity and spontaneous curvature
The effect of rigid surfaces on the dynamics of thin liquid films which are
amenable to the lubrication approximation is considered. It is shown that the
Helfrich energy of the layer gives rise to additional terms in the
time-evolution equations of the liquid film. The dynamics is found to depend on
the absolute value of the spontaneous curvature, irrespective of its sign. Due
to the additional terms, a novel finite wavelength instability of flat rigid
interfaces can be observed. Furthermore, the dependence of the shape of a
droplet on the bending rigidity as well as on the spontaneous curvature is
discussed.Comment: 4 pages, 5 figure
Summary of a Crew-Centered Flight Deck Design Philosophy for High-Speed Civil Transport (HSCT) Aircraft
Past flight deck design practices used within the U.S. commercial transport aircraft industry have been highly successful in producing safe and efficient aircraft. However, recent advances in automation have changed the way pilots operate aircraft, and these changes make it necessary to reconsider overall flight deck design. Automated systems have become more complex and numerous, and often their inner functioning is partially or fully opaque to the flight crew. Recent accidents and incidents involving autoflight system mode awareness Dornheim, 1995) are an example. This increase in complexity raises pilot concerns about the trustworthiness of automation, and makes it difficult for the crew to be aware of all the intricacies of operation that may impact safe flight. While pilots remain ultimately responsible for mission success, performance of flight deck tasks has been more widely distributed across human and automated resources. Advances in sensor and data integration technologies now make far more information available than may be prudent to present to the flight crew
Two-photon ionization of Helium studied with the multiconfigurational time-dependent Hartree-Fock method
The multiconfigurational time-dependent Hartree-Fock method (MCTDHF) is
applied for simulations of the two-photon ionization of Helium. We present
results for the single- and double ionization from the groundstate for photon
energies in the non-sequential regime, and compare them to direct solutions of
the Schr\"odinger equation using the time-dependent (full) Configuration
Interaction method (TDCI). We find that the single-ionization is accurately
reproduced by MCTDHF, whereas the double ionization results correctly capture
the main trends of TDCI
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