11,646 research outputs found
Coping with dating errors in causality estimation
We consider the problem of estimating causal influences between observed processes from time series possibly corrupted by errors in the time variable (dating errors) which are typical in palaeoclimatology, planetary science and astrophysics. "Causality ratio" based on the Wiener-Granger causality is proposed and studied for a paradigmatic class of model systems to reveal conditions under which it correctly indicates directionality of unidirectional coupling. It is argued that in the case of a priori known directionality, the causality ratio allows a characterization of dating errors and observational noise. Finally, we apply the developed approach to palaeoclimatic data and quantify the influence of solar activity on tropical Atlantic climate dynamics over the last two millennia. A stronger solar influence in the first millennium A.D. is inferred. The results also suggest a dating error of about 20 years in the solar proxy time series over the same period
Bidirectional cooperative motion of myosin-II motors on actin tracks with randomly alternating polarities
The cooperative action of many molecular motors is essential for dynamic
processes such as cell motility and mitosis. This action can be studied by
using motility assays in which the motion of cytoskeletal filaments over a
surface coated with motor proteins is tracked. In previous studies of
actin-myosin II systems, fast directional motion was observed, reflecting the
tendency of myosin II motors to propagate unidirectionally along actin
filaments. Here, we present a motility assay with actin bundles consisting of
short filamentous segments with randomly alternating polarities. These actin
tracks exhibit bidirectional motion with macroscopically large time intervals
(of the order of several seconds) between direction reversals. Analysis of this
bidirectional motion reveals that the characteristic reversal time,
, does not depend on the size of the moving bundle or on the number
of motors, . This observation contradicts previous theoretical calculations
based on a two-state ratchet model [Badoual et al., Proc. Natl. Acad. Sci. USA,
vol. 99, p. 6696 (2002)], predicting an exponential increase of
with . We present a modified version of this model that takes into account
the elastic energy due to the stretching of the actin track by the myosin II
motors. The new model yields a very good quantitative agreement with the
experimental results.Comment: A slightly revised version. Figures 2 and 7 were modified. Accepted
for publication in "Soft Matter
Finding the direction of disturbance propagation in a chemical process using transfer entropy
Published versio
All-optical spatio-temporal control of electron emission from SiO2 nanospheres with femtosecond two-color laser fields
Field localization by nanostructures illuminated with laser pulses of well-defined waveform enables spatio-temporal tailoring of the near-fields for sub-cycle control of electron dynamics at the nanoscale. Here, we apply intense linearly-polarized two-color laser pulses for all-optical control of the highest energy electron emission from SiO2 nanoparticles. For the size regime where light propagation effects become important, we demonstrate the possibility to control the preferential emission angle of a considerable fraction of the fastest electrons by varying the relative phase of the two-color field. Trajectory based semi-classical simulations show that for the investigated nanoparticle size range the directional steering can be attributed to the two-color effect on the electron trajectories, while the accompanied modification of the spatial distribution of the ionization rate on the nanoparticle surface has only a minor effect
Directional emission from asymmetric resonant cavities
Asymmetric resonant cavities (ARCs) with highly non-circular but convex
cross-sections are predicted theoretically to have high-Q whispering gallery
modes with very anisotropic emission. We develop a ray dynamics model for the
emission pattern and present numerical and experimental confirmation of the
theory.Comment: 7 pages LaTeX, 3 postscript figure
Transport in a Levy ratchet: Group velocity and distribution spread
We consider the motion of an overdamped particle in a periodic potential
lacking spatial symmetry under the influence of symmetric L\'evy noise, being a
minimal setup for a ``L\'evy ratchet.'' Due to the non-thermal character of the
L\'evy noise, the particle exhibits a motion with a preferred direction even in
the absence of whatever additional time-dependent forces. The examination of
the L\'evy ratchet has to be based on the characteristics of directionality
which are different from typically used measures like mean current and the
dispersion of particles' positions, since these get inappropriate when the
moments of the noise diverge. To overcome this problem, we discuss robust
measures of directionality of transport like the position of the median of the
particles displacements' distribution characterizing the group velocity, and
the interquantile distance giving the measure of the distributions' width.
Moreover, we analyze the behavior of splitting probabilities for leaving an
interval of a given length unveiling qualitative differences between the noises
with L\'evy indices below and above unity. Finally, we inspect the problem of
the first escape from an interval of given length revealing independence of
exit times on the structure of the potential.Comment: 9 pages, 12 figure
Cooperative molecular motors moving back and forth
We use a two-state ratchet model to study the cooperative bidirectional
motion of molecular motors on cytoskeletal tracks with randomly alternating
polarities. Our model is based on a previously proposed model [Badoual et al.,
{\em Proc. Natl. Acad. Sci. USA} {\bf 99}, 6696 (2002)] for collective motor
dynamics and, in addition, takes into account the cooperativity effect arising
from the elastic tension that develops in the cytoskeletal track due to the
joint action of the walking motors. We show, both computationally and
analytically, that this additional cooperativity effect leads to a dramatic
reduction in the characteristic reversal time of the bidirectional motion,
especially in systems with a large number of motors. We also find that
bidirectional motion takes place only on (almost) a-polar tracks, while on even
slightly polar tracks the motion is unidirectional. We argue that the origin of
these observations is the sensitive dependence of the cooperative dynamics on
the difference between the number of motors typically working in and against
the instantaneous direction of motion.Comment: Accepted for publication in Phys. Rev.
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