96 research outputs found
Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation
Current coronal mass ejection (CME) models set their lower boundary to be in
the lower corona. They do not calculate accurately the transfer of free
magnetic energy from the convection zone to the magnetically dominated corona
because they model the effects of flux emergence using kinematic boundary
conditions or simply assume the appearance of flux at these heights. We test
the importance of including dynamical flux emergence in CME modeling by
simulating, in 2.5D, the emergence of sub-surface flux tubes into different
coronal magnetic field configurations. We investigate how much free magnetic
energy, in the form of shear magnetic field, is transported from the convection
zone to the corona, and whether dynamical flux emergence can drive CMEs. We
find that multiple coronal flux ropes can be formed during flux emergence, and
although they carry some shear field into the corona, the majority of shear
field is confined to the lower atmosphere. Less than 10% of the magnetic energy
in the corona is in the shear field, and this, combined with the fact that the
coronal flux ropes bring up significant dense material, means that they do not
erupt. Our results have significant implications for all CME models which rely
on the transfer of free magnetic energy from the lower atmosphere into the
corona but which do not explicitly model this transfer. Such studies of flux
emergence and CMEs are timely, as we have new capabilities to observe this with
Hinode and SDO, and therefore to test the models against observations
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Overview of mathematical approaches used to model bacterial chemotaxis II: bacterial populations
We review the application of mathematical modeling to understanding the behavior of populations of chemotactic bacteria. The application of continuum mathematical models, in particular generalized KellerâSegel models, is discussed along with attempts to incorporate the microscale (individual) behavior on the macroscale, modeling the interaction between different species of bacteria, the interaction of bacteria with their environment, and methods used to obtain experimentally verified parameter values. We allude briefly to the role of modeling pattern formation in understanding collective behavior within bacterial populations. Various aspects of each model are discussed and areas for possible future research are postulated
DOT tomography of the solar atmosphere. IV. Magnetic patches in internetwork areas
We use G-band and Ca II H image sequences from the Dutch Open Telescope (DOT)
to study magnetic elements that appear as bright points in internetwork parts
of the quiet solar photosphere and chromosphere. We find that many of these
bright points appear recurrently with varying intensity and horizontal motion
within longer-lived magnetic patches. We develop an algorithm for detection of
the patches and find that all patches identified last much longer than the
granulation. The patches outline cell patterns on mesogranular scales,
indicating that magnetic flux tubes are advected by granular flows to
mesogranular boundaries. Statistical analysis of the emergence and
disappearance of the patches points to an average patch lifetime as long as
530+-50 min (about nine hours), which suggests that the magnetic elements
constituting strong internetwork fields are not generated by a local turbulent
dynamo.Comment: 8 pages, 6 figure
Protons in near earth orbit
The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured
by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at
an altitude of 380 km. Above the geomagnetic cutoff the observed spectrum is
parameterized by a power law. Below the geomagnetic cutoff a substantial second
spectrum was observed concentrated at equatorial latitudes with a flux ~ 70
m^-2 sec^-1 sr^-1. Most of these second spectrum protons follow a complicated
trajectory and originate from a restricted geographic region.Comment: 19 pages, Latex, 7 .eps figure
Search for antihelium in cosmic rays
The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle
Discovery during flight STS-91 in a 51.7 degree orbit at altitudes between 320
and 390 km. A total of 2.86 * 10^6 helium nuclei were observed in the rigidity
range 1 to 140 GV. No antihelium nuclei were detected at any rigidity. An upper
limit on the flux ratio of antihelium to helium of < 1.1 * 10^-6 is obtained.Comment: 18 pages, Latex, 9 .eps figure
A Study of Cosmic Ray Secondaries Induced by the Mir Space Station Using AMS-01
The Alpha Magnetic Spectrometer (AMS-02) is a high energy particle physics
experiment that will study cosmic rays in the to range and will be installed on the International Space Station
(ISS) for at least 3 years. A first version of AMS-02, AMS-01, flew aboard the
space shuttle \emph{Discovery} from June 2 to June 12, 1998, and collected
cosmic ray triggers. Part of the \emph{Mir} space station was within the
AMS-01 field of view during the four day \emph{Mir} docking phase of this
flight. We have reconstructed an image of this part of the \emph{Mir} space
station using secondary and emissions from primary cosmic rays
interacting with \emph{Mir}. This is the first time this reconstruction was
performed in AMS-01, and it is important for understanding potential
backgrounds during the 3 year AMS-02 mission.Comment: To be submitted to NIM B Added material requested by referee. Minor
stylistic and grammer change
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Mathematical analysis of the Escherichia coli chemotaxis signalling pathway
We undertake a detailed mathematical analysis of a recent nonlinear ordinary differential equation (ODE) model describing the chemotactic signalling cascade within an {\it Escherichia coli} cell. The model includes a detailed description of the cell signalling cascade and an average approximation of the receptor activity. A steady-state stability analysis reveals the system exhibits one positive real steady-state which is shown to be asymptotically stable. Given the occurrence of a negative feedback between phosphorylated CheB (CheB-P) and the receptor state, we ask under what conditions, the system may exhibit oscillatory type behaviour. A detailed analysis of parameter space reveals that whilst variation in kinetic rate parameters within known biological limits is unlikely to lead to such behaviour, changes in the total concentration of the signalling proteins does. We postulate that experimentally observed overshoot behaviour can actually be described by damped oscillatory dynamics and consider the relationship between overshoot amplitude, total cell protein concentration and the magnitude of the external ligand stimulus. Model reductions of the full ODE model allow us to understand the link between phosphorylation events and the negative feedback between CheB-P and receptor methylation, as well as elucidate why some mathematical models exhibit overshoot and others do not. Our manuscript closes by discussing intercell variability of total protein concentration as means of ensuring the overall survival of a population as cells are subjected to different environments
Ponte et stratégie de la reproduction chez les femelles du tacaud Trisopterus luscus (Gadidés) du plateau continental de la Galice, nord-ouest de l'Espagne
[EN] Pouting, Trisopterus luscus is harvested commercially on the Galician shelf by the Spanish inshore artisanal fleet. In spite of a substantial decrease in pouting catches, fishery regulations are limited to size length restrictions. This study provides biological data including length-at-maturity based on histological methods, seasonal maturation, spawning and fecundity. A collection 443 females, from 17 to 42 cm in total length, were sampled from landings (December 2003 to December 2004). Pouting length-at-maturity was estimated as 19.2 cm on average. Pouting females in spawning condition were observed throughout the year and the number of developing oocytes ranged from 20 000 to 1 327 000. Peak spawning was observed between February and April, which correlated well with trends in gonadosomatic index, and inverse to condition factor and hepatosomatic index. Histological examination of the gonads revealed that pouting ovarian development organization is asynchronous, and fecundity is probably determinate.[FR] Le tacaud, Trisopterus luscus est pĂȘchĂ© par la flotte artisanale espagnole. En dĂ©pit d'une diminution substantielle des captures de tacaud, la pĂȘche est rĂ©glementĂ©e uniquement Ă partir d'une taille minimum commerciale. Cette Ă©tude fournit des donnĂ©es biologiques comprenant la taille Ă maturitĂ© sexuelle basĂ©e sur l'histologie, la maturation saisonniĂšre, la ponte et la fĂ©conditĂ©. Un Ă©chantillon de 443 femelles, de 17 Ă 42 cm longueur totale, est examinĂ© Ă partir des dĂ©barquements durant une annĂ©e (de dĂ©cembre 2003 Ă dĂ©cembre 2004). La taille moyenne Ă maturitĂ© sexuelle est estimĂ©e Ă 19,2 cm. Les femelles de tacaud en condition de ponte sont observĂ©e tout au long de l'annĂ©e et le nombre d'ovocytes s'Ă©tend de 20 000 to 1 327 000 par individu. Les pics de ponte sont observĂ©s entre fĂ©vrier et avril, ce qui est bien corrĂ©lĂ© avec l'Ă©volution du rapport gonado-somatique et inverse de celle du facteur de condition et du rapport hĂ©pato-somatique. L'Ă©tude histologique rĂ©vĂšle que le dĂ©veloppement ovarien chez le tacaud est asynchrone, et que la fĂ©conditĂ© est probablement dĂ©terminĂ©e.Peer reviewe
Evolution of response dynamics underlying bacterial chemotaxis
© 2011 Soyer and Goldstein; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Background: The ability to predict the function and structure of complex molecular mechanisms underlying cellular behaviour is one of the main aims of systems biology. To achieve it, we need to understand the evolutionary routes leading to a specific response dynamics that can underlie a given function and how biophysical and environmental factors affect which route is taken. Here, we apply such an evolutionary approach to the bacterial chemotaxis pathway, which is documented to display considerable complexity and diversity.Results: We construct evolutionarily accessible response dynamics starting from a linear response to absolute levels of attractant, to those observed in current-day Escherichia coli. We explicitly consider bacterial movement as a two-state process composed of non-instantaneous tumbling and swimming modes. We find that a linear response to attractant results in significant chemotaxis when sensitivity to attractant is low and when time spent tumbling is large. More importantly, such linear response is optimal in a regime where signalling has low sensitivity. As sensitivity increases, an adaptive response as seen in Escherichia coli becomes optimal and leads to 'perfect' chemotaxis with a low tumbling time. We find that as tumbling time decreases and sensitivity increases, there exist a parameter regime where the chemotaxis performance of the linear and adaptive responses overlap, suggesting that evolution of chemotaxis responses might provide an example for the principle of functional change in structural continuity.Conclusions: Our findings explain several results from diverse bacteria and lead to testable predictions regarding chemotaxis responses evolved in bacteria living under different biophysical constraints and with specific motility machinery. Further, they shed light on the potential evolutionary paths for the evolution of complex behaviours from simpler ones in incremental fashion
Cosmic-ray positron fraction measurement from 1 to 30 GeV with AMS-01
A measurement of the cosmic ray positron fraction e+/(e+ + e-) in the energy range of 1-30 GeV is presented. The measurement is based on data taken by the AMS-01 experiment during its 10 day Space Shuttle flight in June 1998. A proton background suppression on the order of 10^6 is reached by identifying converted bremsstrahlung photons emitted from positrons
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