4,508 research outputs found
On the radial distribution of Galactic cosmic rays
The spectrum and morphology of the diffuse Galactic gamma-ray emission
carries valuable information on cosmic ray (CR) propagation. Recent results
obtained by analyzing Fermi-LAT data accumulated over seven years of
observation show a substantial variation of the CR spectrum as a function of
the distance from the Galactic Center. The spatial distribution of the CR
density in the outer Galaxy appears to be weakly dependent upon the
galactocentric distance, as found in previous studies as well, while the
density in the central region of the Galaxy was found to exceed the value
measured in the outer Galaxy. At the same time, Fermi-LAT data suggest a
gradual spectral softening while moving outward from the center of the Galaxy
to its outskirts. These findings represent a challenge for standard
calculations of CR propagation based on assuming a uniform diffusion
coefficient within the Galactic volume. Here we present a model of non-linear
CR propagation in which transport is due to particle scattering and advection
off self-generated turbulence. We find that for a realistic distribution of CR
sources following the spatial distribution of supernova remnants and the space
dependence of the magnetic field on galactocentric distance, both the spatial
profile of CR density and the spectral softening can easily be accounted for.Comment: 6 pages, 3 figures. Accepted for publivation to MNRAS letter
Cosmic ray driven Galactic winds
The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic
ray pressure that acts as a force on the background plasma, in the direction
opposite to the gravitational pull. If this force is large enough to win
against gravity, a wind can be launched that removes gas from the Galaxy,
thereby regulating several physical processes, including star formation. The
dynamics of these cosmic ray driven winds is intrinsically non-linear in that
the spectrum of cosmic rays determines the characteristics of the wind
(velocity, pressure, magnetic field) and in turn the wind dynamics affects the
cosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution
function causes excitation of Alfven waves, that in turn determine the
scattering properties of cosmic rays, namely their diffusive transport. These
effects all feed into each other so that what we see at the Earth is the result
of these non-linear effects. Here we investigate the launch and evolution of
such winds, and we determine the implications for the spectrum of cosmic rays
by solving together the hydrodynamical equations for the wind and the transport
equation for cosmic rays under the action of self-generated diffusion and
advection with the wind and the self-excited Alfven waves.Comment: 14 pages, 15 figures. Accepted for publication to MNRAS main journa
Instabilities of noncommutative two dimensional BF model
The noncommutative extension of two dimensional BF model is considered. It is
shown that the realization of the noncommutative map via the Groenewold-Moyal
star product leads to instabilities of the action, hence to a non
renormalizable theory.Comment: 9 page
General Solution Of Linear Vector Supersymmetry
We give the general solution of the Ward identity for the linear vector
supersymmetry which characterizes all topological models. Such solution, whose
expression is quite compact and simple, greatly simplifies the study of
theories displaying a supersymmetric algebraic structure, reducing to a few
lines the proof of their possible finiteness. In particular, the cohomology
technology usually involved for the quantum extension of these theories, is
completely bypassed. The case of Chern-Simons theory is taken as an example.Comment: 18 pages, LaTeX, no figure
High Energy Phenomena in Clusters of Galaxies
Several phenomena in high energy astrophysics have been recently related to
clusters of galaxies and to cosmic ray interactions occurring inside these
structures. In many of these phenomena the observable effects depend on the
energy density of cosmic rays confined in the Intra Cluster (IC) medium, which
is a poorly known quantity. We propose here that useful indications about this
quantity can be obtained from present and future observations of galaxy
clusters in the radio and hard X-ray frequency ranges.Comment: 5 pages, 3 Figures, Latex (using espcrc2,epsfig), to appear in the
Proceedings of the TAUP97, Eds. A. DiCredico et al., in press. Send comments
to S.Colafrancesco: [email protected]
A dip in the UHECR spectrum and the transition from galactic to extragalactic cosmic rays
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE)
protons caused by electron-positron pair production on the cosmic microwave
background (CMB) radiation. For a power-law generation spectrum , the
calculated position and shape of the dip is confirmed with high accuracy by the
spectra observed by the Akeno-AGASA, HiRes, Yakutsk and Fly's Eye detectors.
When the particle energies, measured in these detectors, are calibrated by the
dip, their fluxes agree with a remarkable accuracy. The predicted shape of the
dip is quite robust. The dip is only modified strongly when the fraction of
nuclei heavier than protons is high at injection, which imposes some
restrictions on the mechanisms of acceleration operating in UHECR sources. The
existence of the dip, confirmed by observations, implies that the transition
from galactic to extragalactic cosmic rays occurs at E \lsim 1\times 10^{18}
eV. We show that at energies lower than a characteristic value eV, the spectrum of extragalactic cosmic rays
flattens in all cases of interest, and it provides a natural transition to a
steeper galactic cosmic ray spectrum. This transition occurs at some energy
below , corresponding to the position of the so-called second knee.
We discuss extensively the constraints on this model imposed by current
knowledge of acceleration processes and sources of UHECR and compare it with
the traditional model of transition at the ankle.Comment: Version Accepted for Publication in Astroparticle Physics (minor
changes
the role of plant sociology in the study and management of european forest ecosystems
Abstract: Forest composition is a faithful indicator of the stressors and disturbances that influence forest ecosystems, and it should be accounted for in Sustainable Forest Management policies. Indeed, the classification of forest ecosystems in forest types is considered as a key tool to improve the assessment and monitoring of forest biological diversity, and for the definition of management guidelines. Accordingly, the Ministerial Conference on the Protection of Forests in Europe has recognized the need of developing a pan-European forest classification in forest types, and has identified indicators of Sustainable Forest Management that should be applied by forest types. The classification of vegetation has always been among the main aims of the plant sociology. The quantitative and qualitative analysis of plant species composition, performed through the plant sociological approach, condenses compositional and structural information within a hierarchical system, and expresses all historical, sociological and habitat factors that influence the actual and potential vegetation. In a modern perspective the integration of plant sociology and ecological analysis represents a key to a hierarchical land classification and to the understanding of vegetation dynamics; furthermore the long history of plant sociology determined the availability of large datasets of vegetation data throughout Europe. Starting from these considerations, in this paper we briefly describe how plant sociology could represent a tool for the assessment of the indicators of SFM that should be applied by forest types, giving insights on how this discipline could contribute to the assessment of each of these indicators
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