41,935 research outputs found
The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23
For the first time, we present and discuss EUV radiances of the solar
transition region (TR) and corona obtained during a solar cycle. The
measurements were obtained with the SOHO/coronal diagnostic spectrometer (CDS)
during the period from 1996 to 2010. We find that limb-brightening
significantly affects any characterisation of the solar radiances. We present
the limb-brightening function for the main lines and find that it does not
change measurably during the cycle. We confirm earlier findings that the
radiance histogram of the cooler lines have a well defined, log-normal
quiet-Sun component, although our results differ from previous ones. The width
of the lowest-radiance log-normal distribution is constant along the cycle.
Both the analysis of the centre-to-limb variation and of the radiance
statistical distribution point to a constant QS emission along solar cycle 23.
Lines formed above 1 MK are dramatically affected by the presence of active
regions, and indeed, no "quiet Sun" region can be defined during periods of
maximum activity. Much of the irradiance variability in lines formed below 1.5
MK is due to a change in the emitting area. For hotter lines, the emitting area
saturates to almost 100% of full solar disk at the maximum of activity, while
simultaneously the emission due to active regions increases by more than an
order of magnitude. We show that structures around active regions, sometimes
referred to as dark halos or dark canopies, are common and discuss their
similarities and differences with coronal holes. In particular, we show how
they are well visible in TR lines, contrary to coronal holes.Comment: 15 pages, 11 figures, 2 tables; in press in: Astronomy & Astrophysic
On azimuthal spin correlations in Higgs plus jet events at LHC
We consider the recent proposal that the distribution of the difference
between azimuthal angles of the two accompanying jets in gluon-fusion induced
Higgs-plus-two-jet events at LHC reflects the CP of the Higgs boson produced.
We point out that the hierarchy between the Higgs boson mass and the jet
transverse energy makes this observable vulnerable to logarithmically enhanced
higher-order perturbative corrections. We present an evolution equation that
describes the scale variation of the azimuthal angular correlation for the two
jets. The emission of extra partons leads to a significant suppression of the
correlation. Using the HERWIG Monte Carlo event generator, we carry out a
parton-shower analysis to confirm the findings.Comment: Published version. 11 pages, 4 figures, uses JHEP3.cl
Lorentzian compact manifolds: isometries and geodesics
In this work we investigate families of compact Lorentzian manifolds in
dimension four. We show that every lightlike geodesic on such spaces is
periodic, while there are closed and non-closed spacelike and timelike
geodesics. Their isometry groups are computed. We also show that there is a non
trivial action by isometries of \Heis_3(\RR) on the nilmanifold S^1\times
(\Gamma_k \bsh \Heis_3(\RR)) for a lattice of \Heis_3(\RR).Comment: 17 page
Stability of spinor Fermi gases in tight waveguides
The two and three-body correlation functions of the ground state of an
optically trapped ultracold spin-1/2 Fermi gas (SFG) in a tight waveguide (1D
regime) are calculated in the plane of even and odd-wave coupling constants,
assuming a 1D attractive zero-range odd-wave interaction induced by a 3D p-wave
Feshbach resonance, as well as the usual repulsive zero-range even-wave
interaction stemming from 3D s-wave scattering. The calculations are based on
the exact mapping from the SFG to a ``Lieb-Liniger-Heisenberg'' model with
delta-function repulsions depending on isotropic Heisenberg spin-spin
interactions, and indicate that the SFG should be stable against three-body
recombination in a large region of the coupling constant plane encompassing
parts of both the ferromagnetic and antiferromagnetic phases. However, the
limiting case of the fermionic Tonks-Girardeau gas (FTG), a spin-aligned 1D
Fermi gas with infinitely attractive p-wave interactions, is unstable in this
sense. Effects due to the dipolar interaction and a Zeeman term due to a
resonance-generating magnetic field do not lead to shrinkage of the region of
stability of the SFG.Comment: 5 pages, 6 figure
Pre-test analysis of protected loss of primary pump transients in CIRCE-HERO facility
In the frame of LEADER project (Lead-cooled European Advanced Demonstration Reactor), a new configuration of the steam generator for ALFRED (Advanced Lead Fast Reactor European Demonstrator) was proposed. The new concept is a super-heated steam generator, double wall bayonet tube type with leakage monitoring [1]. In order to support the new steam generator concept, in the framework of Horizon 2020 SESAME project (thermal hydraulics Simulations and Experiments for the Safety Assessment of MEtal cooled reactors), the ENEA CIRCE pool facility will be refurbished to host the HERO (Heavy liquid mEtal pRessurized water cOoled tubes) test section to investigate a bundle of seven full scale bayonet tubes in ALFRED-like thermal hydraulics conditions. The aim of this work is to verify thermofluid dynamic performance of HERO during the transition from nominal to natural circulation condition. The simulations have been performed with RELAP5-3D© by using the validated geometrical model of the previous CIRCE-ICE test section [2], in which the preceding heat exchanger has been replaced by the new bayonet bundle model. Several calculations have been carried out to identify thermal hydraulics performance in different steady state conditions. The previous calculations represent the starting points of transient tests aimed at investigating the operation in natural circulation. The transient tests consist of the protected loss of primary pump, obtained by reducing feed-water mass flow to simulate the activation of DHR (Decay
Heat Removal) system, and of the loss of DHR function in hot conditions, where feed-water mass flow rate is absent. According to simulations, in nominal conditions, HERO bayonet bundle offers excellent thermal hydraulic behavior and, moreover, it allows the operation in natural circulation
Post-test simulation of a PLOFA transient test in the CIRCE-HERO facility
CIRCE is a lead–bismuth eutectic alloy (LBE) pool facility aimed to simulate the primary system of a heavy liquid metal (HLM) cooled pool-type fast reactor. The experimental facility was implemented with a new test section, called HERO (Heavy liquid mEtal pRessurized water cOoled tubes), which consists of a steam generator composed of seven double-wall bayonet tubes (DWBT) with an active length of six meters. The experimental campaign aims to investigate HERO behavior, which is representative of the tubes that will compose ALFRED SG. In the framework of the Horizon 2020 SESAME project, a transient test was selected for the realization of a validation benchmark. The test consists of a protected loss of flow accident (PLOFA) simulating the shutdown of primary pumps, the reactor scram and the activation of the DHR system. A RELAP5-3D© nodalization scheme was developed in the pre-test phase at DIAEE of “Sapienza” University of Rome, providing useful information to the experimentalists. The model consisted to a mono-dimensional scheme of the primary flow path and the SG secondary side, and a multi-dimensional component simulating the large LBE pool. The analysis of experimental data, provided by ENEA, has suggested to improve the thermal–hydraulic model with a more detailed nodalization scheme of the secondary loop, looking to reproduce the asymmetries observed on the DWBTs operation. The paper summarizes the post-test activity performed in the frame of the H2020 SESAME project as a contribution of the benchmark activity, highlighting a global agreement between simulations and experiment for all the primary circuit physical quantities monitored. Then, the attention is focused on the secondary system operation, where uncertainties related to the boundary conditions affect the computational results
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