10,683 research outputs found
Dendritic Actin Filament Nucleation Causes Traveling Waves and Patches
The polymerization of actin via branching at a cell membrane containing
nucleation-promoting factors is simulated using a stochastic-growth
methodology. The polymerized-actin distribution displays three types of
behavior: a) traveling waves, b) moving patches, and c) random fluctuations.
Increasing actin concentration causes a transition from patches to waves. The
waves and patches move by a treadmilling mechanism which does not require
myosin II. The effects of downregulation of key proteins on actin wave behavior
are evaluated.Comment: 10 pages, 4 figure
Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results
An approximate method for solving the rate equations for the hydrogen
populations was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with
six energy levels and fixed radiative rates. It has been tested extensively in
one-dimensional simulations. The extended method has been used to create a
three-dimensional model that extends from the upper convection zone to the
chromosphere. The ionisation degree of hydrogen in our time-dependent
simulation is comparable to the corresponding equilibrium value up to 500 km
above optical depth unity. Above this height, the non-equilibrium ionisation
degree is fairly constant over time and space, and tends to be at a value set
by hot propagating shock waves. The hydrogen level populations and electron
density are much more constant than the corresponding values for statistical
equilibrium, too. In contrast, the equilibrium ionisation degree varies by more
than 20 orders of magnitude between hot, shocked regions and cool, non-shocked
regions. The simulation shows for the first time in 3D that the chromospheric
hydrogen ionisation degree and electron density cannot be calculated in
equilibrium. Our simulation can provide realistic values of those quantities
for detailed radiative transfer computations.Comment: 8 pages, 7 figure
The Functional Form of Angular Forces around Transition Metal Ions in Biomolecules
A method for generating angular forces around -bonded transition
metal ions is generalized to treat -bonded configurations. The theoretical
approach is based on an analysis of a ligand-field Hamiltonian based on the
moments of the electron state distribution. The functional forms that are
obtained involve a modification of the usual expression of the binding energy
as a sum of ligand-ligand interactions, which however requires very little
increased in CPU time. The angular interactions have simple forms involving sin
and cos functions, whose relative weights depend on whether the ligands are
- or -bonded. They describe the ligand-field stabilization energy
to an accuracy of about 10%. The resulting force field is used to model the
structure of small clusters, including fragments of the copper blue protein
structure. Large deviations from the typical square copper coordination are
found when -bonded ligands are present.Comment: Latex source, 9 postscript figure
Convergence of density-matrix expansions for nuclear interactions
We extend density-matrix expansions in nuclei to higher orders in derivatives
of densities and test their convergence properties. The expansions allow for
converting the interaction energies characteristic to finite- and short-range
nuclear effective forces into quasi-local density functionals. We also propose
a new type of expansion that has excellent convergence properties when
benchmarked against the binding energies obtained for the Gogny interaction.Comment: 4 pages, 3 figure
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Morphology and Dynamics of the Low Solar Chromosphere
The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.Comment: accepted by ApJ, 16 pages, 8 figure
Twisted flux tube emergence from the convection zone to the corona
3D numerical simulations of a horizontal magnetic flux tube emergence with
different twist are carried out in a computational domain spanning the upper
layers of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along the magnetic field lines. The emergence
of the magnetic flux tube input at the bottom boundary into a weakly magnetized
atmosphere is presented. The photospheric and chromospheric response is
described with magnetograms, synthetic images and velocity field distributions.
The emergence of a magnetic flux tube into such an atmosphere results in varied
atmospheric responses. In the photosphere the granular size increases when the
flux tube approaches from below. In the convective overshoot region some 200km
above the photosphere adiabatic expansion produces cooling, darker regions with
the structure of granulation cells. We also find collapsed granulation in the
boundaries of the rising flux tube. Once the flux tube has crossed the
photosphere, bright points related with concentrated magnetic field, vorticity,
high vertical velocities and heating by compressed material are found at
heights up to 500km above the photosphere. At greater heights in the magnetized
chromosphere, the rising flux tube produces a cool, magnetized bubble that
tends to expel the usual chromospheric oscillations. In addition the rising
flux tube dramatically increases the chromospheric scale height, pushing the
transition region and corona aside such that the chromosphere extends up to 6Mm
above the photosphere. The emergence of magnetic flux tubes through the
photosphere to the lower corona is a relatively slow process, taking of order 1
hour.Comment: 53 pages,79 figures, Submitted to Ap
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