12,324 research outputs found
Spectral Diagnostics of Active Prominences
Active prominences exhibit plasma motions, resulting in difficulties with the
interpretation of spectroscopic observations. These solar features being
strongly influenced by the radiation coming from the solar disk, Doppler
dimming or brightening effects may arise, depending on which lines are observed
and on the velocity of the plasma. Interlocking between the different atomic
energy levels and non local thermodynamic equilibrium lead to non-trivial
spectral line profiles, and this calls for complex numerical modelling of the
radiative transfer in order to understand the observations. We present such a
tool, which solves the radiative transfer and statistical equilibrium for H, He
I, He II, and Ca II, in moving prominences where radial plasma motions are
taking place. It is found that for isothermal, isobaric prominence models, the
He II resonance lines are very sensitive to the Doppler effect and show a
strong Doppler dimming. The Ca II lines are not very sensitive to the Doppler
effect for the prominence models considered here. We illustrate how the code
makes it possible to retrieve the plasma thermodynamic parameters by comparing
computed and observed line profiles of hydrogen and helium resonance lines in a
quiescent prominence.Comment: 6 pages, 5 figures. In press,"Physics of Chromospheric Plasmas"
(Coimbra), ASP 368, 337 (2007). Revised version matches published version
The Helium spectrum in erupting solar prominences
Even quiescent solar prominences may become active and sometimes erupt. These
events are occasionally linked to coronal mass ejections. However we know very
little about the plasma properties during the activation and eruption
processes. We present new computations of the helium line profiles emitted by
an eruptive prominence. The prominence is modelled as a plane-parallel slab
standing vertically above the solar surface and moving upward as a solid body.
The helium spectrum is computed with a non local thermodynamic equilibrium
radiative transfer code. The effect of Doppler dimming / brightening is
investigated in the resonance lines of He I and He II formed in the EUV, as
well as on the He I 10830 A and 5876 A lines. We focus on the line profile
properties and the resulting integrated intensities. It is shown that the
helium lines are very sensitive to Doppler dimming effects. We also study the
effect of frequency redistribution in the formation mechanisms of the resonance
lines and find that it is necessary to use partial redistribution in frequency
for the resonance lines.Comment: 5 pages, 4 figures. Proceedings of IAU GA 2006, JD03: Solar Active
Regions and 3D Magnetic Structure. See also a more detailed paper at
astro-ph/060822
The problem with the SURF scheme
There is a serious problem with one of the assumptions made in the security
proof of the SURF scheme. This problem turns out to be easy in the regime of
parameters needed for the SURF scheme to work.
We give afterwards the old version of the paper for the reader's convenience.Comment: Warning : we found a serious problem in the security proof of the
SURF scheme. We explain this problem here and give the old version of the
paper afterward
Towards Sustainable Mobility Indicators: Application to the Lyons Conurbation
This paper applies the theme of sustainable development to the case of urban transport and daily mobility of the inhabitants of a city. A set of indicators which simultaneously takes the three dimensions of sustainabilityââenvironmental, economic, and socialââinto account is suggested. We present here the results of exploratory research funded by Renault Automobile Manufacturers, carried out to verify the feasibility and the usefulness of elaborating such sustainable mobility indicators. Values of the economics, environmental and social indicators are presented for the Lyons case. These estimations are mainly based on the household travel survey held in this city in 1994â1995. In the end, this set of indicators should allow the comparison of different urban transport strategies within an urban area, but also between different urban contexts, and through time. The conditions of generalization of these measurements of indicators are then discussed.Trip distance ; Daily mobility ; Sustainability indicators ; Household travel survey ; Methodology ; Pollutant emissions ; Expenditures ; Global costs
Recurrent Attempts to Renovate Management ControlRevisited through a Simons' Perspective
On propose de caractériser un systÚme de pilotage et de contrÎle à partir de trois dimensions: l'implication des managers, la spécificité des outils, la relation avec le systÚme d'incitation. Cette caractérisation constitue une extension directe de la typologie introduite par Simons (1995) pour distinguer entre contrÎle interactif et contrÎle diagnostic. Les tentatives récurrentes pour rénover le contrÎle de gestion suite aux critiques sévÚres formulées par Kaplan et Johnson (1987) sont alors re-examinées. Les approches du type contrÎle stratégique ou le management par la valeur apparaissent respectivement comme représentatives soit du contrÎle interactif soit du contrÎle diagnostic. Le cadre proposé permet de bien rendre compte des forces et faiblesses de ce type d'approches toujours trÚs en vogue auprÚs des entreprises.
Reversibility of laser filamentation
We investigate the reversibility of laser filamentation, a self-sustained,
non-linear propagation regime including dissipation and time-retarded effects.
We show that even losses related to ionization marginally affect the
possibility of reverse propagating ultrashort pulses back to the initial
conditions, although they make it prone to finite-distance blow-up susceptible
to prevent backward propagation.Comment: 12 pages, 3 figure
Multi-mode and long-lived quantum correlations between photons and spins in a crystal
The realization of quantum networks and quantum repeaters remains an
outstanding challenge in quantum communication. These rely on entanglement of
remote matter systems, which in turn requires creation of quantum correlations
between a single photon and a matter system. A practical way to establish such
correlations is via spontaneous Raman scattering in atomic ensembles, known as
the DLCZ scheme. However, time multiplexing is inherently difficult using this
method, which leads to low communication rates even in theory. Moreover, it is
desirable to find solid-state ensembles where such matter-photon correlations
could be generated. Here we demonstrate quantum correlations between a single
photon and a spin excitation in up to 12 temporal modes, in a Eu
doped YSiO crystal, using a novel DLCZ approach that is inherently
multimode. After a storage time of 1 ms, the spin excitation is converted into
a second photon. The quantum correlation of the generated photon pair is
verified by violating a Cauchy - Schwarz inequality. Our results show that
solid-state rare-earth crystals could be used to generate remote multi-mode
entanglement, an important resource for future quantum networks
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