256 research outputs found
RĂŽle hydrogĂ©ologique des linĂ©aments structuraux en milieu cristallin et cristallophyllien : cas du bassin versant du Sassandra, Sud-Ouest de la CĂŽte dâIvoire
La zone dâĂ©tude est situĂ©e en milieu tropical humide au sud-ouest de la CĂŽte dâIvoire dans les formations plutoniques et mĂ©tamorphiques prĂ©cambriennes. Lâobjectif est dâĂ©tudier lâeffet des linĂ©aments structuraux dans lâhydrodynamisme souterrain dans un tel environnement en se basant sur des donnĂ©es de tĂ©lĂ©dĂ©tection, de forages, des teneurs SO42- et Cl- des eaux souterraines. Le traitement des images satellitaires a donnĂ© la carte des linĂ©aments qui a servi au choix des sites dâĂ©chantillonnage dâeau souterraine et Ă lâindividualisation des « blocs ». Les rapports de SO42- et Cl- a permis dâidentifier les linĂ©aments participant Ă la circulation de lâeau souterraine Ă lâĂ©chelle rĂ©gionale. Lâanalyse des donnĂ©es gĂ©omĂ©triques des blocs et des dĂ©bits des forages a permis de mettre en Ă©vidence le rĂŽle hydrogĂ©ologique prĂ©pondĂ©rant des petits linĂ©aments par rapport aux linĂ©aments majeurs. Confirmant ainsi lâimportance de lâhorizon fissurĂ© et la topographie dans lâĂ©coulement souterrain en milieu de socle cristallin et cristallophyllien.Mots-clĂ©s: aquifĂšres de socle, bloc, dĂ©bit, horizon fissurĂ©, hydrodynamisme.Hydrogeological role of structural lineaments in plutonic and metamorphic environment: Case of Sassandra Watershed (South-Western CĂŽte d'Ivoire)The survey area is located south-west of Ivory Coast in Precambrian metamorphic and plutonic zone. The aim is to study the contribution of structural lineaments in the underground hydrodynamics in such environment. To highlight the network of kilometer-scale fractures in hard-rock, remote sensing techniques, drilling and concentrations SOSO42- and Cl- in groundwater were used. The map of lineaments obtained after processing of satellite images has been discretized into blocks aquifers which are delimited by opened or clogged fractures that can have a conducting or watertight function. Analysis of the geometric block parameters and the drilling instantaneous discharge helped highlight the leading role hydrogeological small fractures (fissured layer) compared to major lineaments in groundwater flow of in hard-rock. Thus confirming the importance of fissured topography and groundwater flow in crystallophyllian crystalline basement.Keywords: hard-rock aquifer, block, hydrodynamism, fissured zone, and yield
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo
The possibility of the magnetic flux expulsion from the Galaxy in the
superbubble (SB) explosions, important for the Alpha-Omega dynamo, is
considered. Special emphasis is put on the investigation of the downsliding of
the matter from the top of the shell formed by the SB explosion which is able
to influence the kinematics of the shell. It is shown that either Galactic
gravity or the development of the Rayleigh-Taylor instabilities in the shell
limit the SB expansion, thus, making impossible magnetic flux expulsion. The
effect of the cosmic rays in the shell on the sliding is considered and it is
shown that it is negligible compared to Galactic gravity. Thus, the question of
possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure
Development of a new method to estimate the incident solar flux on central receivers from deteriorated heliostats
This work proposes a new empirical direct methodology to estimate both the solar flux distribution and intensity on the surface of central receivers. In solar power tower plants with deteriorated heliostats, the numerical simulations to estimate the incident solar flux are not precise. Hence the thermal behaviour of the receivers cannot be determined. In those cases, direct measurement or semi-empirical methodologies are required to characterize the radiant power on the receiver. The new methodology proposed, named "Superposition method", consists in the hourly characterization of the reflected solar beam of each individual heliostat by means of a pyrheliometer, a passive screen, a flux sensor, a camera and digital image analysis. According to the aiming strategy used during receiver operation, each individual solar flux distribution and intensity can be gathered to obtain the total incident radiant power on the solar receiver. This non-real-time method has the advantage of reproducing any solar flux distribution on the receiver at present and past time
In Situ Origin of Large Scale Galactic Magnetic Fields Without Kinetic Helicity?
The origin and sustenance of large scale galactic magnetic fields has been a
long standing and controversial astrophysical problem. Here an alternative to
the ``standard'' \a-\Omega mean field dynamo and primordial theories is
pursued. The steady supply of supernovae induced turbulence exponentiates the
total field energy, providing a significant seed mean field that can be
linearly stretched by shear. The observed micro-Gauss fields would be produced
primarily within one vertical diffusion time since it is only during this time
that linear stretching can compete with diffusion. This approach does not
invoke exponential mean field dynamo growth from the helicity \a-effect but
does employ turbulent diffusion, which limits the number of large scale
reversals. The approach could be of interest if the helicity effect is
suppressed independently of the turbulent diffusion. This is an important but
presently unresolved issue.Comment: 15 pages TeX, accepted, ApJ
The spatial energy spectrum of magnetic fields in our Galaxy
Interstellar magnetic fields exist over a broad range of spatial scales,
extending from the large Galactic scales ( kpc) down to the very small
dissipative scales ( pc). In this paper, we use a set of 490 pulsars
distributed over roughly one third of the Galactic disk out to a radius kpc (assuming kpc) and combine their observed
rotation and dispersion measures with their estimated distances to derive the
spatial energy spectrum of the Galactic interstellar magnetic field over the
scale range kpc. We obtain a nearly flat spectrum, with a 1D
power-law index for and an
rms field strength of approximately G over the relevant scales. Our
study complements the derivation of the magnetic energy spectrum over the scale
range pc by \citet{ms96b}, showing that the magnetic spectrum
becomes flatter at larger scales. This observational result is discussed in the
framework of current theoretical and numerical models.Comment: 7 pages, 6 figures, ApJ accepte
Nonlinear dynamics and intermittency in a long-term copepod time series
We consider the nonlinear dynamics of a long-term copepod (small crustaceans) time series sampled weekly in the Mediterranean sea from 1967 to 1992. Such population dynamics display a high variability that we consider here in an interdisciplinary study, using tools borrowed from the field of statistical physics. We analyse the extreme events of male and female abundances, and of the total population, and show that they both have heavy tailed probability density functions (pdf). We provide hyperbolic fits of the form p(x) ⌠1/xÎŒ+1, and estimate the value of ÎŒ using Hillâs estimator. We then study the ratio of male to female abundances, compared to the female abundances. Using conditional probability density functions and conditional averages, we show that this ratio is independent of the female density, when the latter is larger than a given threshold. This property is very useful for modelization. We also consider the product of male to female abundances, which can be ecologically related to the encounters. We show that this product is extremely intermittent, and link its pdf to the female pdf
Evidence for dark matter in the inner Milky Way
The ubiquitous presence of dark matter in the universe is today a central
tenet in modern cosmology and astrophysics. Ranging from the smallest galaxies
to the observable universe, the evidence for dark matter is compelling in
dwarfs, spiral galaxies, galaxy clusters as well as at cosmological scales.
However, it has been historically difficult to pin down the dark matter
contribution to the total mass density in the Milky Way, particularly in the
innermost regions of the Galaxy and in the solar neighbourhood. Here we present
an up-to-date compilation of Milky Way rotation curve measurements, and compare
it with state-of-the-art baryonic mass distribution models. We show that
current data strongly disfavour baryons as the sole contribution to the
galactic mass budget, even inside the solar circle. Our findings demonstrate
the existence of dark matter in the inner Galaxy while making no assumptions on
its distribution. We anticipate that this result will compel new
model-independent constraints on the dark matter local density and profile,
thus reducing uncertainties on direct and indirect dark matter searches, and
will shed new light on the structure and evolution of the Galaxy.Comment: First submitted version of letter published in Nature Physics on
Febuary 9, 2015:
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3237.htm
The supernova rate-velocity dispersion relation in the interstellar medium
We investigate the relationship between the velocity dispersion of the gas
and the SN rate and feedback efficiency in the ISM. We explore the constancy of
the velocity dispersion profiles in the outer parts of galactic disks at~6-8 km
s^-1, and the transition to the starburst regime. Our results show that a) SN
driving leads to constant velocity dispersions of sig~6 km s^-1 for the total
gas and sigHI~3 km s^-1 for the HI gas, independent of the SN rate, for values
of the rate between 0.01-0.5 the Galactic rate R_{G},b) the position of the
transition to the starburst regime at SFR/Area~5*10^-3-10^-2 M_sol yr^-1 kpc^-2
observed in the simulations, is in good agreement with the transition to the
starburst regime in the observations, c) for the high SN rates, no HI gas is
present in the simulations box, however, for the total gas velocity dispersion,
there is good agreement between the models and the observations,d) at the
intermediate SN rates R/R_{G}~0.5-1, taking into account the thermal broadening
of the HI line helps reach a good agreement in that regime between the models
and the observations,e) for R/R_{G}<0.5, sig and sigHI fall below the observed
values by a factor of~2. However, a set of simulation with different values of
epsilon indicates that for larger values of the supernova feedback
efficiencies, velocity dispersions of the HI gas of the order of 5-6 km s^{-1}
can be obtained, in closer agreement with the observations. The fact that for
R/R_{G}<0.5, the HI gas velocity dispersions are a factor ~2 smaller than the
observed values could result from the fact that we might have underestimated
the SN feedback efficiency. It might also be an indication that other physical
processes couple to the stellar feedback in order to produce the observed level
of turbulence in galactic disks.Comment: 44 pages, 22 figures. Accepted to Ap
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