327 research outputs found
Self-organized Pattern Formation in Motor-Microtubule Mixtures
We propose and study a hydrodynamic model for pattern formation in mixtures
of molecular motors and microtubules. The steady state patterns we obtain in
different regimes of parameter space include arrangements of vortices and
asters separately as well as aster-vortex mixtures and fully disordered states.
Such stable steady states are observed in experiments in vitro. The sequence of
patterns obtained in the experiments can be associated with smooth trajectories
in a non-equilibrium phase diagram for our model.Comment: 11 pages Latex file, 2 figures include
Spaces of finite element differential forms
We discuss the construction of finite element spaces of differential forms
which satisfy the crucial assumptions of the finite element exterior calculus,
namely that they can be assembled into subcomplexes of the de Rham complex
which admit commuting projections. We present two families of spaces in the
case of simplicial meshes, and two other families in the case of cubical
meshes. We make use of the exterior calculus and the Koszul complex to define
and understand the spaces. These tools allow us to treat a wide variety of
situations, which are often treated separately, in a unified fashion.Comment: To appear in: Analysis and Numerics of Partial Differential
Equations, U. Gianazza, F. Brezzi, P. Colli Franzone, and G. Gilardi, eds.,
Springer 2013. v2: a few minor typos corrected. v3: a few more typo
correction
Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data
We analyze ozone observations recorded over Equatorial Africa between April 1997 and March 2003 by the MOZAIC programme, providing the first ozone climatology deriving from continental in-situ data over this region. Three-dimensional streamlines strongly suggests connections between the characteristics of the ozone monthly mean vertical profiles, the most persistent circulation patterns in the troposphere over Equatorial Africa (on a monthly basis) such as the Harmattan, the African Easterly Jet, the Trades and the regions of ozone precursors emissions by biomass burning. During the biomass burning season in each hemisphere, the lower troposphere exhibits layers of enhanced ozone (i.e. 70 ppbv over the coast of Gulf of Guinea in December-February and 85 ppbv over Congo in June-August). The characteristics of the ozone monthly mean vertical profiles are clearly connected to the regional flow regime determined by seasonal dynamic forcing. The mean ozone profile over the coast of Gulf of Guinea in the burning season is characterized by systematically high ozone below 650hPa ; these are due to the transport by the Harmattan and the AEJ of the pollutants originating from upwind fires. The confinement of high ozone to the lower troposphere is due to the high stability of the Harmattan and the blocking Saharan anticyclone which prevents efficient vertical mixing. In contrast, ozone enhancements observed over Central Africa during the local dry season (June-August) are not only found in the lower troposphere but throughout the troposphere. Moreover, this study highlights a connection between the regions of the coast of Gulf of Guinea and regions of Congo to the south that appears on a semi annual basis. Vertical profiles in wet-season regions exhibit ozone enhancements in the lower troposphere due to biomass burning products transport from fires situated in the opposite dry-season hemisphere
Measurement of air and nitrogen fluorescence light yields induced by electron beam for UHECR experiments
Most of the Ultra High Energy Cosmic Ray (UHECR) experiments and projects
(HiRes, AUGER, TA, EUSO, TUS,...) use air fluorescence to detect and measure
extensive air showers (EAS). The precise knowledge of the Fluorescence Light
Yield (FLY) is of paramount importance for the reconstruction of UHECR. The
MACFLY - Measurement of Air Cherenkov and Fluorescence Light Yield - experiment
has been designed to perform such FLY measurements. In this paper we will
present the results of FLY in the 290-440 nm wavelength range for dry air and
pure nitrogen, both excited by electrons with energy of 1.5 MeV, 20 GeV and 50
GeV. The experiment uses a 90Sr radioactive source for low energy measurement
and a CERN SPS electron beam for high energy. We find that the FLY is
proportional to the deposited energy (E_d) in the gas and we show that the air
fluorescence properties remain constant independently of the electron energy.
At the reference point: atmospheric dry air at 1013 hPa and 23C, the ratio
FLY/E_d=17.6 photon/MeV with a systematic error of 13.2%.Comment: 19 pages, 8 figures. Accepted for publication in Astroparticle
Physic
Wildfire smoke in the Siberian Arctic in summer: source characterization and plume evolution from airborne measurements
We present airborne measurements of carbon dioxide (CO<sub>2</sub>), carbon monoxide (CO), ozone (O<sub>3</sub>), equivalent black carbon (EBC) and ultra fine particles over North-Eastern Siberia in July 2008 performed during the YAK-AEROSIB/POLARCAT experiment. During a "golden day" (11 July 2008) a number of biomass burning plumes were encountered with CO mixing ratio enhancements of up to 500 ppb relative to a background of 90 ppb. Number concentrations of aerosols in the size range 3.5–200 nm peaked at 4000 cm<sup>&minus;3</sup> and the EBC content reached 1.4 &mu;g m<sup>&minus;3</sup>. These high concentrations were caused by forest fires in the vicinity of the landing airport in Yakutsk where measurements in fresh smoke could be made during the descent. We estimate a combustion efficiency of 90 &plusmn; 3% based on CO and CO<sub>2</sub> measurements and a CO emission factor of 65.5 &plusmn; 10.8 g CO per kilogram of dry matter burned. This suggests a potential increase in the average northern hemispheric CO mixing ratio of 3.0–7.2 ppb per million hectares of Siberian forest burned. For BC, we estimate an emission factor of 0.52 &plusmn; 0.07 g BC kg<sup>&minus;1</sup>, comparable to values reported in the literature. The emission ratio of ultra-fine particles (3.5–200 nm) was 26 cm<sup>&minus;3</sup> (ppb CO)<sup>&minus;1</sup>, consistent with other airborne studies. <br><br> The transport of identified biomass burning plumes was investigated using the FLEXPART Lagrangian model. Based on sampling of wildfire plumes from the same source but with different atmospheric ages derived from FLEXPART, we estimate that the e-folding lifetimes of EBC and ultra fine particles (between 3.5 and 200 nm in size) against removal and growth processes are 5.1 and 5.5 days respectively, supporting lifetime estimates used in various modelling studies
PT-symmetric models in curved manifolds
We consider the Laplace-Beltrami operator in tubular neighbourhoods of curves
on two-dimensional Riemannian manifolds, subject to non-Hermitian parity and
time preserving boundary conditions. We are interested in the interplay between
the geometry and spectrum. After introducing a suitable Hilbert space framework
in the general situation, which enables us to realize the Laplace-Beltrami
operator as an m-sectorial operator, we focus on solvable models defined on
manifolds of constant curvature. In some situations, notably for non-Hermitian
Robin-type boundary conditions, we are able to prove either the reality of the
spectrum or the existence of complex conjugate pairs of eigenvalues, and
establish similarity of the non-Hermitian m-sectorial operators to normal or
self-adjoint operators. The study is illustrated by numerical computations.Comment: 37 pages, PDFLaTeX with 11 figure
Annihilation of low energy antiprotons in silicon
The goal of the AEIS experiment at the Antiproton
Decelerator (AD) at CERN, is to measure directly the Earth's gravitational
acceleration on antimatter. To achieve this goal, the AEIS
collaboration will produce a pulsed, cold (100 mK) antihydrogen beam with a
velocity of a few 100 m/s and measure the magnitude of the vertical deflection
of the beam from a straight path. The final position of the falling
antihydrogen will be detected by a position sensitive detector. This detector
will consist of an active silicon part, where the annihilations take place,
followed by an emulsion part. Together, they allow to achieve 1 precision on
the measurement of with about 600 reconstructed and time tagged
annihilations.
We present here, to the best of our knowledge, the first direct measurement
of antiproton annihilation in a segmented silicon sensor, the first step
towards designing a position sensitive silicon detector for the
AEIS experiment. We also present a first comparison with
Monte Carlo simulations (GEANT4) for antiproton energies below 5 MeVComment: 21 pages in total, 29 figures, 3 table
Prospects for measuring the gravitational free-fall of antihydrogen with emulsion detectors
The main goal of the AEgIS experiment at CERN is to test the weak equivalence
principle for antimatter. AEgIS will measure the free-fall of an antihydrogen
beam traversing a moir\'e deflectometer. The goal is to determine the
gravitational acceleration g for antihydrogen with an initial relative accuracy
of 1% by using an emulsion detector combined with a silicon micro-strip
detector to measure the time of flight. Nuclear emulsions can measure the
annihilation vertex of antihydrogen atoms with a precision of about 1 - 2
microns r.m.s. We present here results for emulsion detectors operated in
vacuum using low energy antiprotons from the CERN antiproton decelerator. We
compare with Monte Carlo simulations, and discuss the impact on the AEgIS
project.Comment: 20 pages, 16 figures, 3 table
Force and Motion Generation of Molecular Motors: A Generic Description
We review the properties of biological motor proteins which move along linear
filaments that are polar and periodic. The physics of the operation of such
motors can be described by simple stochastic models which are coupled to a
chemical reaction. We analyze the essential features of force and motion
generation and discuss the general properties of single motors in the framework
of two-state models. Systems which contain large numbers of motors such as
muscles and flagella motivate the study of many interacting motors within the
framework of simple models. In this case, collective effects can lead to new
types of behaviors such as dynamic instabilities of the steady states and
oscillatory motion.Comment: 29 pages, 9 figure
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