23,458 research outputs found
Surface Critical Behavior in Systems with Absorbing States
We present a general scaling theory for the surface critical behavior of
non-equilibrium systems with phase transitions into absorbing states. The
theory allows for two independent surface exponents which satisfy generalized
hyperscaling relations. As an application we study a generalized version of
directed percolation with two absorbing states. We find two distinct surface
universality classes associated with inactive and reflective walls. Our results
indicate that the exponents associated with these two surface universality
classes are closely connected.Comment: latex, 4 pages, to appear in PR
Persistence in the Voter model: continuum reaction-diffusion approach
We investigate the persistence probability in the Voter model for dimensions
d\geq 2. This is achieved by mapping the Voter model onto a continuum
reaction-diffusion system. Using path integral methods, we compute the
persistence probability r(q,t), where q is the number of ``opinions'' in the
original Voter model. We find r(q,t)\sim exp[-f_2(q)(ln t)^2] in d=2;
r(q,t)\sim exp[-f_d(q)t^{(d-2)/2}] for 2<d<4; r(q,t)\sim exp[-f_4(q)t/ln t] in
d=4; and r(q,t)\sim exp[-f_d(q)t] for d>4. The results of our analysis are
checked by Monte Carlo simulations.Comment: 10 pages, 3 figures, Latex, submitted to J. Phys. A (letters
Lunar radar measurements of the earth's magnetospheric wake Scientific report no. 11
Lunar radar echo measurments to determine size and electron density of earth magnetospheric wak
When it Pays to Rush: Interpreting Morphogen Gradients Prior to Steady-State
During development, morphogen gradients precisely determine the position of
gene expression boundaries despite the inevitable presence of fluctuations.
Recent experiments suggest that some morphogen gradients may be interpreted
prior to reaching steady-state. Theoretical work has predicted that such
systems will be more robust to embryo-to-embryo fluctuations. By analysing two
experimentally motivated models of morphogen gradient formation, we investigate
the positional precision of gene expression boundaries determined by
pre-steady-state morphogen gradients in the presence of embryo-to-embryo
fluctuations, internal biochemical noise and variations in the timing of
morphogen measurement. Morphogens that are direct transcription factors are
found to be particularly sensitive to internal noise when interpreted prior to
steady-state, disadvantaging early measurement, even in the presence of large
embryo-to-embryo fluctuations. Morphogens interpreted by cell-surface receptors
can be measured prior to steady-state without significant decrease in
positional precision provided fluctuations in the timing of measurement are
small. Applying our results to experiment, we predict that Bicoid, a
transcription factor morphogen in Drosophila, is unlikely to be interpreted
prior to reaching steady-state. We also predict that Activin in Xenopus and
Nodal in zebrafish, morphogens interpreted by cell-surface receptors, can be
decoded in pre-steady-state.Comment: 18 pages, 3 figure
Comprehensive Observations of a Solar Minimum CME with STEREO
We perform the first kinematic analysis of a CME observed by both imaging and
in situ instruments on board STEREO, namely the SECCHI, PLASTIC, and IMPACT
experiments. Launched on 2008 February 4, the CME is tracked continuously from
initiation to 1 AU using the SECCHI imagers on both STEREO spacecraft, and is
then detected by the PLASTIC and IMPACT particle and field detectors on board
STEREO-B. The CME is also detected in situ by ACE and SOHO/CELIAS at Earth's L1
Lagrangian point. The CME hits STEREO-B, ACE, and SOHO on 2008 February 7, but
misses STEREO-A entirely. This event provides a good example of just how
different the same event can look when viewed from different perspectives. We
also demonstrate many ways in which the comprehensive and continuous coverage
of this CME by STEREO improves confidence in our assessment of its kinematic
behavior, with potential ramifications for space weather forecasting. The
observations provide several lines of evidence in favor of the observable part
of the CME being narrow in angular extent, a determination crucial for deciding
how best to convert observed CME elongation angles from Sun-center to actual
Sun-center distances.Comment: 27 pages, 10 figures, AASTEX v5.2, accepted by Ap
First Stereoscopic Coronal Loop Reconstructions from Stereo Secchi Images
We present the first reconstruction of the three-dimensional shape of
magnetic loops in an active region from two different vantage points based on
simultaneously recorded images. The images were taken by the two EUVI
telescopes of the SECCHI instrument onboard the recently launched STEREO
spacecraft when the heliocentric separation of the two space probes was 12
degrees. We demostrate that these data allow to obtain a reliable
three-dimensional reconstruction of sufficiently bright loops. The result is
compared with field lines derived from a coronal magnetic field model
extrapolated from a photospheric magnetogram recorded nearly simultaneously by
SOHO/MDI. We attribute discrepancies between reconstructed loops and
extrapolated field lines to the inadequacy of the linear force-free field model
used for the extrapolation.Comment: 6 pages, 5 figure
Coupling of Two Motor Proteins: a New Motor Can Move Faster
We study the effect of a coupling between two motor domains in
highly-processive motor protein complexes. A simple stochastic discrete model,
in which the two parts of the protein molecule interact through some energy
potential, is presented. The exact analytical solutions for the dynamic
properties of the combined motor species, such as the velocity and dispersion,
are derived in terms of the properties of free individual motor domains and the
interaction potential. It is shown that the coupling between the motor domains
can create a more efficient motor protein that can move faster than individual
particles. The results are applied to analyze the motion of helicase RecBCD
molecules
The Relationship of Coronal Mass Ejections to Streamers
We have examined images from the Large Angle Spectroscopic Coronagraph
(LASCO) to study the relationship of Coronal Mass Ejections (CMEs) to coronal
streamers. We wish to test the suggestion (Low 1996) that CMEs arise from flux
ropes embedded in a streamer erupting, thus disrupting the streamer. The data
span a period of two years near sunspot minimum through a period of increased
activity as sunspot numbers increased. We have used LASCO data from the C2
coronagraph which records Thomson scattered white light from coronal electrons
at heights between 1.5 and 6R_sun. Maps of the coronal streamers have been
constructed from LASCO C2 observations at a height of 2.5R_sun at the east and
west limbs. We have superposed the corresponding positions of CMEs observed
with the C2 coronagraph onto the synoptic maps. We identified the different
kinds of signatures CMEs leave on the streamer structure at this height
(2.5R_sun). We find four types of CMEs with respect to their effect on
streamers:
1. CMEs that disrupt the streamer 2. CMEs that have no effect on the
streamer, even though they are related to it. 3. CMEs that create streamer-like
structures 4. CMEs that are latitudinally displaced from the streamer.
This is the most extensive observational study of the relation between CMEs
and streamers to date. Previous studies using SMM data have made the general
statement that CMEs are mostly associated with streamers, and that they
frequently disrupt it. However, we find that approximately 35% of the observed
CMEs bear no relation to the pre-existing streamer, while 46% have no effect on
the observed streamer, even though they appear to be related to it. Our
conclusions thus differ considerably from those of previous studies.Comment: Accepted, Journal of Geophysical Research. 8 figs, better versions at
http://www.science.gmu.edu/~prasads/streamer.htm
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