50,358 research outputs found
Solar Orbiter: Exploring the Sun-heliosphere connection
The heliosphere represents a uniquely accessible domain of space, where
fundamental physical processes common to solar, astrophysical and laboratory
plasmas can be studied under conditions impossible to reproduce on Earth and
unfeasible to observe from astronomical distances. Solar Orbiter, the first
mission of ESA's Cosmic Vision 2015-2025 programme, will address the central
question of heliophysics: How does the Sun create and control the heliosphere?
In this paper, we present the scientific goals of the mission and provide an
overview of the mission implementation.Comment: 52 pages, 21 figures, 125 references; accepted for publication in
Solar Physic
A Wildfire Prediction Based on Fuzzy Inference System for Wireless Sensor Networks
The study of forest fires has been traditionally considered as an important
application due to the inherent danger that this entails. This phenomenon
takes place in hostile regions of difficult access and large areas. Introduction of
new technologies such as Wireless Sensor Networks (WSNs) has allowed us to
monitor such areas. In this paper, an intelligent system for fire prediction based
on wireless sensor networks is presented. This system obtains the probability of
fire and fire behavior in a particular area. This information allows firefighters to
obtain escape paths and determine strategies to fight the fire. A firefighter can
access this information with a portable device on every node of the network. The
system has been evaluated by simulation analysis and its implementation is being
done in a real environment.Junta de Andalucía P07-TIC-02476Junta de Andalucía TIC-570
The Difficulty of Getting High Escape Fractions of Ionizing Photons from High-redshift Galaxies: a View from the FIRE Cosmological Simulations
We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological
zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE)
project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame
ultraviolet magnitude Muv = -9 to -19. These simulations include explicit
models of the multi-phase ISM, star formation, and stellar feedback, which
produce reasonable galaxy properties at z = 0-6. We post-process the snapshots
with a radiative transfer code to evaluate the escape fraction (fesc) of
hydrogen ionizing photons. We find that the instantaneous fesc has large time
variability (0.01%-20%), while the time-averaged fesc over long time-scales
generally remains ~5%, considerably lower than the estimate in many
reionization models. We find no strong dependence of fesc on galaxy mass or
redshift. In our simulations, the intrinsic ionizing photon budgets are
dominated by stellar populations younger than 3 Myr, which tend to be buried in
dense birth clouds. The escaping photons mostly come from populations between
3-10 Myr, whose birth clouds have been largely cleared by stellar feedback.
However, these populations only contribute a small fraction of intrinsic
ionizing photon budgets according to standard stellar population models. We
show that fesc can be boosted to high values, if stellar populations older than
3 Myr produce more ionizing photons than standard stellar population models (as
motivated by, e.g., models including binaries). By contrast, runaway stars with
velocities suggested by observations can enhance fesc by only a small fraction.
We show that "sub-grid" star formation models, which do not explicitly resolve
star formation in dense clouds with n >> 1 cm^-3, will dramatically
over-predict fesc.Comment: 17 pages, 16 figures, MNRAS in pres
The gradient of diffuse gamma-ray emission in the Galaxy
We show that the well-known discrepancy between the radial dependence of the
Galactic cosmic ray (CR) nucleon distribution, as inferred most recently from
EGRET observations of diffuse gamma-rays above 100 MeV, and of the most likely
CR source distribution (supernova remnants, pulsars) can be explained purely by
PROPAGATION effects. Contrary to previous claims, we demonstrate that this is
possible, if the dynamical coupling between the escaping CRs and thermal plasma
is taken into account, and thus a self-consistent GALACTIC WIND calculation is
carried out. Given a dependence of the CR source distribution on Galactocentric
radius, r, our numerical wind solutions show that the CR outflow velocity,
V(r,z) depends both on r, and on vertical distance, z, at reference level z_C.
The latter is defined as the transition boundary from diffusion to advection
dominated CR transport and is therefore also a function of r. In fact, the CR
escape time averaged over particle energies decreases with increasing CR source
strength. Such an increase is counteracted by a reduced average CR residence
time in the gas disk. Therfore pronounced peaks in the radial source
distribution result in mild radial gamma-ray gradients at GeV energies, as it
has been observed. This effect is enhanced by anisotropic diffusion, assuming
different radial and vertical diffusion coefficients. We have calculated 2D
analytic solutions of the stationary diffusion-advection equation, including
anisotropic diffusion, for a given CR source distribution and a realistic
outflow velocity field V(r,z), inferred from self-consistent numerical Galactic
Wind simulations. At TeV energies the gamma-rays from the sources are expected
to dominate the observed "diffuse" flux from the disk. Its observation should
allow an empirical test of the theory presented.Comment: 23 pages, 12 figures; accepted for publication in Astronomy and
Astrophysics Main Journa
Selection of dune shapes and velocities. Part 1: Dynamics of sand, wind and barchans
Almost fifty years of investigations of barchan dunes morphology and dynamics
is reviewed, with emphasis on the physical understanding of these objects. The
characteristics measured on the field (shape, size, velocity) and the physical
problems they rise are presented. Then, we review the dynamical mechanisms
explaining the formation and the propagation of dunes. In particular a complete
and original approach of the sand transport over a flat sand bed is proposed
and discussed. We conclude on open problems by outlining future research
directions.Comment: submitted to Eur. Phys. J. B, 20 pages, 20 figure
Enhanced propagation of motile bacteria on surfaces due to forward scattering
How motile bacteria move near a surface is a problem of fundamental
biophysical interest and is key to the emergence of several phenomena of
biological, ecological and medical relevance, including biofilm formation.
Solid boundaries can strongly influence a cell's propulsion mechanism, thus
leading many flagellated bacteria to describe long circular trajectories stably
entrapped by the surface. Experimental studies on near-surface bacterial
motility have, however, neglected the fact that real environments have typical
microstructures varying on the scale of the cells' motion. Here, we show that
micro-obstacles influence the propagation of peritrichously flagellated
bacteria on a flat surface in a non-monotonic way. Instead of hindering it, an
optimal, relatively low obstacle density can significantly enhance cells'
propagation on surfaces due to individual forward-scattering events. This
finding provides insight on the emerging dynamics of chiral active matter in
complex environments and inspires possible routes to control microbial ecology
in natural habitats
- …