6,697 research outputs found
Coupling the solar surface and the corona: coronal rotation, Alfv\'en wave-driven polar plumes
The dynamical response of the solar corona to surface and sub-surface
perturbations depends on the chromospheric stratification, and specifically on
how efficiently these layers reflect or transmit incoming Alfv\'en waves. While
it would be desirable to include the chromospheric layers in the numerical
simulations used to study such phenomena, that is most often not feasible. We
defined and tested a simple approximation allowing the study of coronal
phenomena while taking into account a parametrised chromospheric reflectivity.
We addressed the problems of the transmission of the surface rotation to the
corona and that of the generation of polar plumes by Alfv\'en waves (Pinto et
al., 2010, 2011). We found that a high (yet partial) effective chromospheric
reflectivity is required to properly describe the angular momentum balance in
the corona and the way the surface differential rotation is transmitted
upwards. Alfv\'en wave-driven polar plumes maintain their properties for a wide
range of values for the reflectivity, but they become bursty (and eventually
disrupt) when the limit of total reflection is attained.Comment: Solar Wind 13: Proceedings of the Thirteenth International Solar Wind
Conferenc
Quantum size effects in the low temperature layer-by-layer growth of Pb on Ge(001)
The electronic properties of thin metallic films deviate from the
corresponding bulk ones when the film thickness is comparable with the
wavelength of the electrons at the Fermi level due to quantum size effects
(QSE). QSE are expected to affect the film morphology and structure leading to
the low temperature (LT) ``electronic growth'' of metals on semiconductors. In
particular, layer-by-layer growth of Pb(111) films has been reported for
deposition on Ge(001) below 130 K. An extremely flat morphology is preserved
throughout deposition from four up to a dozen of monolayers. These flat films
are shown to be metastable and to reorganize into large clusters uncovering the
first Pb layer, pseudomorphic to the substrate, already at room temperature.
Indications of QSE induced structural variations of the growing films have been
reported for Pb growth on Ge(001), where the apparent height of the Pb(111)
monatomic step was shown to change in an oscillatory fashion by He atom
scattering (HAS) during layer-by-layer growth. The extent of the structural QSE
has been obtained by a comparison of the HAS data with X-ray diffraction (XRD)
and reflectivity experiments. Whereas step height variations as large as 20 %
have been measured by HAS reflectivity, the displacement of the atomic planes
from their bulk position, as measured by XRD, has been found to mainly affect
the topmost Pb layer, but with a lower extent, i.e. the QSE observed by HAS are
mainly due to a perpendicular displacement of the topmost layer charge density.
The effect of the variable surface relaxation on the surface vibration has been
studied by inelastic HAS to measure the acoustic dispersion of the low energy
phonons.Comment: 28 pages (laTex,elsart) and 13 figures (eps); updated reference
Study of the isotropic contribution to the analysis of photoelectron diffraction experiments at the ALOISA beamline
The angular distribution of the intensity in photoemission experiments is
affected by electron diffraction patterns and by a smoothly varying ISO
contribution originated by both intrumental details and physical properties of
the samples. The origin of the various contributions to the ISO component has
been identified since many years. Nonetheless in this work we present original
developement of the ED analysis, which arises from the evolution of
instrumental performance, in terms of analyzers positioning and angular
resolution, as well as collimation and size of X-ray beams in third generation
synchrotron sources. The analytical treatement of the instrumental factors is
presented in detail for the end station of the ALOISA beamline (Trieste
Synchrotron), where a wide variety of scattering geometries is available for ED
experiments. We present here the basic formulae and their application to
experimental data taken on the Fe/Cu3Au(001) system in order to highlight the
role of the various parameters included in the distribution function. A
specific model for the surface illumination has been developed as well as the
overlayer thickness and surface roughness have been considered.Comment: RevTex, nine pages with five eps figures; to be published in J.
Electron Spectrosc. Relat. Pheno
Coronal heating in coupled photosphere-chromosphere-coronal systems: turbulence and leakage
Coronal loops act as resonant cavities for low frequency fluctuations that
are transmitted from the deeper layers of the solar atmosphere and are
amplified in the corona, triggering nonlinear interactions. However trapping is
not perfect, some energy leaks down to the chromosphere, thus limiting the
turbulence development and the associated heating. We consider the combined
effects of turbulence and leakage in determining the energy level and
associated heating rate in models of coronal loops which include the
chromosphere and transition region. We use a piece-wise constant model for the
Alfven speed and a Reduced MHD - Shell model to describe the interplay between
turbulent dynamics in the direction perpendicular to the mean field and
propagation along the field. Turbulence is sustained by incoming fluctuations
which are equivalent, in the line-tied case, to forcing by the photospheric
shear flows. While varying the turbulence strength, we compare systematically
the average coronal energy level (E) and dissipation rate (D) in three models
with increasing complexity: the classical closed model, the semi-open corona
model, and the corona-chromosphere (or 3-layer) model, the latter two models
allowing energy leakage. We find that:
(i) Leakage always plays a role (even for strong turbulence), E and D are
systematically lower than in the line-tied model. (ii) E is close to the
resonant prediction, i.e., assuming effective turbulent correlation time longer
than the Alfven coronal crossing time (Ta). (iii) D is close to the value given
by the ratio of photospheric energy divided by Ta (iv) The coronal spectra
exibits an inertial range with 5/3 spectral slope, and a large scale peak of
trapped resonant modes that inhibit nonlinear couplings. (v) In the realistic
3-layer model, the two-component spectrum leads to a damping time equal to the
Kolmogorov time reduced by a factor u_rms/Va_coronaComment: 15 pages, 15 figures, Accepted for publication in A&
Turbulent Heating between 0.2 and 1 au: A Numerical Study
International audience; The heating of the solar wind is key to understanding its dynamics and acceleration process. The observed radial decrease of the proton temperature in the solar wind is slow compared to the adiabatic prediction, and it is thought to be caused by turbulent dissipation. To generate the observed 1/ R decrease, the dissipation rate has to reach a specific level that varies in turn with temperature, wind speed, and heliocentric distance. We want to prove that MHD turbulent simulations can lead to the 1/ R profile. We consider here the slow solar wind, characterized by a quasi-2D spectral anisotropy. We use the expanding box model equations, which incorporate into 3D MHD equations the expansion due to the mean radial wind, allowing us to follow the plasma evolution between 0.2 and 1 au. We vary the initial parameters: Mach number, expansion parameter, plasma ? , and properties of the energy spectrum as the spectral range and slope. Assuming turbulence starts at 0.2 au with a Mach number equal to unity, with a 3D spectrum mainly perpendicular to the mean field, we find radial temperature profiles close to 1/ R on average. This is done at the price of limiting the initial spectral extent, corresponding to the small number of modes in the inertial range available, due to the modest Reynolds number reachable with high Mach numbers
Surface Alfven Wave Damping in a 3D Simulation of the Solar Wind
Here we investigate the contribution of surface Alfven wave damping to the
heating of the solar wind in minima conditions. These waves are present in
regions of strong inhomogeneities in density or magnetic field (e. g., the
border between open and closed magnetic field lines). Using a 3-dimensional
Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping
contribution between 1-4 solar radii, the region of interest for both
acceleration and coronal heating. We consider waves with frequencies lower than
those that are damped in the chromosphere and on the order of those dominating
the heliosphere. In the region between open and closed field lines, within a
few solar radii of the surface, no other major source of damping has been
suggested for the low frequency waves we consider here. This work is the first
to study surface Alfven waves in a 3D environment without assuming a priori a
geometry of field lines or magnetic and density profiles. We determine that
waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In
quiet sun regions, surface Alfven waves are damped at further distances
compared to active regions, thus carrying additional wave energy into the
corona. We compare the surface Alfven wave contribution to the heating by a
variable polytropic index and find that it an order of magnitude larger than
needed for quiet sun regions. For active regions the contribution to the
heating is twenty percent. As it has been argued that a variable gamma acts as
turbulence, our results indicate that surface Alfven wave damping is comparable
to turbulence in the lower corona. This damping mechanism should be included
self consistently as an energy driver for the wind in global MHD models.Comment: Accepted to ApJ (scheduled September '09), 22 pages, 8 figure
Three-dimensional local anisotropy of velocity fluctuations in the solar wind
We analyse velocity fluctuations in the solar wind at magneto-fluid scales in
two datasets, extracted from Wind data in the period 2005-2015, that are
characterised by strong or weak expansion. Expansion affects measurements of
anisotropy because it breaks axisymmetry around the mean magnetic field.
Indeed, the small-scale three-dimensional local anisotropy of magnetic
fluctuations ({\delta}B) as measured by structure functions (SF_B) is
consistent with tube-like structures for strong expansion. When passing to weak
expansion, structures become ribbon-like because of the flattening of SFB along
one of the two perpendicular directions. The power-law index that is consistent
with a spectral slope -5/3 for strong expansion now becomes closer to -3/2.
This index is also characteristic of velocity fluctuations in the solar wind.
We study velocity fluctuations ({\delta}V) to understand if the anisotropy of
their structure functions (SF_V ) also changes with the strength of expansion
and if the difference with the magnetic spectral index is washed out once
anisotropy is accounted for. We find that SF_V is generally flatter than SF_B.
When expansion passes from strong to weak, a further flattening of the
perpendicular SF_V occurs and the small-scale anisotropy switches from
tube-like to ribbon-like structures. These two types of anisotropy, common to
SF_V and SF_B, are associated to distinct large-scale variance anisotropies of
{\delta}B in the strong- and weak-expansion datasets. We conclude that SF_V
shows anisotropic three-dimensional scaling similar to SF_B, with however
systematic flatter scalings, reflecting the difference between global spectral
slopes.Comment: accepted in MNRA
Investigating the Origin of the First Ionization Potential Effect With a Shell Turbulence Model
International audienceThe enrichment of coronal loops and the slow solar wind with elements that have low First Ionization Potential, known as the FIP effect, has often been interpreted as the tracer of a common origin. A current explanation for this FIP fractionation rests on the influence of ponderomotive forces and turbulent mixing acting at the top of the chromosphere. The implied wave transport and turbulence mechanisms are also key to wave-driven coronal heating and solar wind acceleration models. This work makes use of a shell turbulence model run on open and closed magnetic field lines of the solar corona to investigate with a unified approach the influence of magnetic topology, turbulence amplitude and dissipation on the FIP fractionation. We try in particular to assess whether there is a clear distinction between the FIP effect on closed and open field regions
Design and Studies of Ό-strip Stacked Module Prototypes for Tracking at Super-LHC
AbstractExperience at high luminosity hadrons collider experiments shows that tracking information enhances the trigger rejection capabilities while retaining high efficiency for interesting physics events. The design of a tracking based trigger for Super LHC (S-LHC), the already envisaged high luminosity upgrade of the LHC collider, is an extremely challenging task, and requires the identiïŹcation of high-momentum particle tracks as a part of the Level 1 Trigger.Simulation studies show that this can be achieved by correlating hits on two closely spaced silicon strip sensors. The progresses on the design and development of this micro-strip stacked prototype modules and the performance of few prototype detectors will be presented. The prototypes have been built with the silicon sensors and electronics used to equip the present CMS[1] Tracker.Preliminary results of a simulated tracker layout equipped with stacked modules are discussed in terms of pT resolution and triggering capabilities.The study of real prototypes in terms of signal over noise and tracking performance with cosmic rays and a dedicated beam test experiment will also be shown
Picosecond timescale tracking of pentacene triplet excitons with chemical sensitivity
Singlet fission is a photophysical process in which an optically excited singlet exciton is converted into two triplet excitons. Singlet fission sensitized solar cells are expected to display a greatly enhanced power conversion efficiency compared to conventional singlejunction cells, but the efficient design of such devices relies on the selection of materials capable of harvesting triplets generated in the fission chromophore. To this aim, the possibility of measuring triplet exciton ynamics with chemical selectivity paves the way for the rational design of complex heterojunctions, with optimized triplet conversion. Here we exploit the chemical sensitivity of X-ray absorption spectroscopy to track triplet exciton dynamics at the picosecond timescale in multilayer films of pentacene, the archetypal singlet fission material. We experimentally identify the signature of the triplet exciton in the Carbon K-edge absorption spectrum and measure its lifetime of about 300 ps. Our results are supported by state-of-the-art ab initio calculations
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