1,578 research outputs found
Modelling ion populations in astrophysical plasmas: carbon in the solar transition region
The aim of this work is to improve the modelling of ion populations in higher
density, lower temperature astrophysical plasmas, of the type commonly found in
lower solar and stellar atmospheres. Ion population models for these regions
frequently employ the coronal approximation, which assumes conditions more
suitable to the upper solar atmosphere, where high temperatures and lower
densities prevail. Using the coronal approximation for modelling the solar
transition region gives theoretical lines intensities for the Li-like and
Na-like isoelectronic sequences which can be factors of 2-5 times lower than
observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975)
showed the important part ions in excited levels play when included in the
modelling. Their models, however, used approximations for the atomic rates to
determine the ion balance. Presented here is the first stage in updating these
earlier models of carbon by using rates from up-to-date atomic calculations and
more recent photo-ionising radiances for the quiet Sun. Where such atomic rates
are not readily available, in the case of electron-impact direct ionisation and
excitation--auto-ionisation, new calculations have been made and compared to
theoretical and experimental studies. The effects each atomic process has on
the ion populations as density changes is demonstrated, and final results from
the modelling are compared to the earlier works. Lastly, the new results for
ion populations are used to predict line intensities for the solar transition
region in the quiet Sun, and these are compared with predictions from
coronal-approximation modelling and with observations. Significant improvements
in the predicted line intensities are seen in comparison to those obtained from
zero-density modelling of carbon.Comment: Draft accepted by A&A, 13 pages, 15 figure
Effects of density on the oxygen ionization equilibrium in collisional plasmas
The ion populations most frequently adopted for diagnostics in collisional plasmas are derived from the density independent coronal approximation. In higher density, lower temperature conditions, ionization rates are enhanced once metastable levels become populated, and recombination rates are suppressed if ions recombine into Rydberg levels. As a result, the formation temperatures of ions shift, altering the diagnostics of the plasma. To accurately model the effect of ionization from metastable levels, new electron impact ionization cross-sections have been calculated for oxygen, both for direct ionization and excitation–auto-ionization of the ground and metastable levels. The results have been incorporated into collisional radiative modelling to show how the ionization equilibrium of oxygen changes once metastable levels become populated. Suppression of dielectronic recombination has been estimated and also included in the modelling, demonstrating the shifts with density in comparison to the coronal approximation. The final results for the ionization equilibrium are used in differential emission measure modelling to predict line intensities for many lines emitted by O II–O VI in the solar transition region. The predictions show improved agreement by 15–40 per cent for O II, O VI, and the intercombination lines of O III–O V, when compared to results from coronal approximation modelling. While there are still discrepancies with observations of these lines, this could, to a large part, be explained by variability in the observations
The Deformation of an Elastic Substrate by a Three-Phase Contact Line
Young's classic analysis of the equilibrium of a three-phase contact line
ignores the out-of-plane component of the liquid-vapor surface tension. While
it has long been appreciated that this unresolved force must be balanced by
elastic deformation of the solid substrate, a definitive analysis has remained
elusive because conventional idealizations of the substrate imply a divergence
of stress at the contact line. While a number of theories of have been
presented to cut off the divergence, none of them have provided reasonable
agreement with experimental data. We measure surface and bulk deformation of a
thin elastic film near a three-phase contact line using fluorescence confocal
microscopy. The out-of-plane deformation is well fit by a linear elastic theory
incorporating an out-of-plane restoring force due to the surface tension of the
gel. This theory predicts that the deformation profile near the contact line is
scale-free and independent of the substrate elastic modulus.Comment: 4 pages, 3 figure
A minimal model for kinetic arrest
To elucidate slow dynamics in glassy materials, we introduce the {\it
Figure-8 model} in which hard blocks undergo Brownian motion around a
circuit in the shape of a figure-8. This system undergoes kinetic arrest at a
critical packing fraction , and for
long-time diffusion is controlled by rare, cooperative `junction-crossing'
particle rearrangements. We find that the average time between junction
crossings , and hence the structural relaxation time, does not
simply scale with the configurational volume \OmegaLow of transition states,
because also depends on the time to complete a junction crossing.
The importance of these results in understanding cage-breaking dynamics in
glassy systems is discussed.Comment: 4 pages, 4 figure
An Imaging System for Focusing Tests of Li Multiprism X‐ray Refractive Lenses
For rapid and efficient tests of novel X‐rays optics, such as lithium‐based compound refractive lenses, we have built a fast X‐ray sensitive CCD imaging system. We report on the linearity, response and resolution of the microscope‐based imaging system. For the low magnifications used here (X2‐X10), we find that a thinly doped YAG screen has a poorer resolution than a thick YAG screen. We provide an example of its use in testing a new 2D focusing multiprism X‐ray lens. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87661/2/780_1.pd
Scaling of Traction Forces with Size of Cohesive Cell Colonies
To understand how the mechanical properties of tissues emerge from
interactions of multiple cells, we measure traction stresses of cohesive
colonies of 1-27 cells adherent to soft substrates. We find that traction
stresses are generally localized at the periphery of the colony and the total
traction force scales with the colony radius. For large colony sizes, the
scaling appears to approach linear, suggesting the emergence of an apparent
surface tension of order 1E-3 N/m. A simple model of the cell colony as a
contractile elastic medium coupled to the substrate captures the spatial
distribution of traction forces and the scaling of traction forces with the
colony size.Comment: 5 pages, 3 figure
Impaired aerobic exercise adaptation in children and adolescents with craniopharyngioma is associated with hypothalamic involvement
OBJECTIVE: Many patients treated for craniopharyngioma (CP) complain of a relative incapacity for physical activity. Whether this is due to an objective decrease in adaptation to exercise is unclear. We assessed exercise tolerance in children with surgically treated CP and appropriate pituitary hormone replacement therapy compared with healthy controls and we examined the potential relationships with hypothalamic involvement, GH replacement, and the catecholamine deficiency frequently observed in these subjects. DESIGN AND METHODS: Seventeen subjects (12 males and five females) with CP and 22 healthy controls (14 males and eight females) aged 15.3+/-2.5 years (7.3-18 years) underwent a standardized cycle ergometer test. Maximum aerobic capacity was expressed as the ratio of VO(2max) to fat-free mass (VO(2max)/FFM), a measure independent of age and fat mass in children. RESULTS: VO(2max)/FFM was 20% lower in children with CP compared with controls (P<0.05), even after adjustment for gender. Children with hypothalamic involvement (n=10) had a higher percentage of fat mass (P<0.05) than those without hypothalamic involvement (n=7) and lower VO(2max)/FFM (P<0.05), whereas children without hypothalamic involvement had VO(2max)/FFM close to that of controls (P>0.05). GH treatment was associated with a significant positive effect on aerobic capacity (P<0.05) only in the absence of hypothalamic involvement. No relationship was found between exercise capacity parameters and daily urine epinephrine excretion or epinephrine peak response to insulin-induced hypoglycemia. CONCLUSIONS: Children with CP have a decrease in aerobic capacity mainly related to hypothalamic involvement. The hypothalamic factors altering aerobic capacity remain to be determined
Selective nanomanipulation using optical forces
We present a detailed theoretical study of the recent proposal for selective
nanomanipulation of nanometric particles above a substrate using near-field
optical forces [Chaumet {\it et al.} Phys. Rev. Lett. {\bf 88}, 123601 (2002)].
Evanescent light scattering at the apex of an apertureless near-field probe is
used to create an optical trap. The position of the trap is controlled on a
nanometric scale via the probe and small objects can be selectively trapped and
manipulated. We discuss the influence of the geometry of the particles and the
probe on the efficiency of the trap. We also consider the influence of multiple
scattering among the particles on the substrate and its effect on the
robustness of the trap.Comment: 12 pages, 17 figure
High resolution simulation of the South Asian monsoon using a variable resolution global climate model
International audienceThis study examines the feasibility of using a variable resolution global general circulation model (GCM), with telescopic zooming and enhanced resolution (~35 km) over South Asia, to better understand regional aspects of the South Asian monsoon rainfall distribution and the interactions between monsoon circulation and precipitation. For this purpose, two sets of ten member realizations are produced with and without zooming using the LMDZ (Laboratoire Meteorologie Dynamique and Z stands for zoom) GCM. The simulations without zoom correspond to a uniform 1° × 1° grid with the same total number of grid points as in the zoom version. So the grid of the zoomed simulations is finer inside the region of interest but coarser outside. The use of these finer and coarser resolution ensemble members allows us to examine the impact of resolution on the overall quality of the simulated regional monsoon fields. It is found that the monsoon simulation with high-resolution zooming greatly improves the representation of the southwesterly monsoon flow and the heavy precipitation along the narrow orography of the Western Ghats, the northeastern mountain slopes and northern Bay of Bengal (BOB). A realistic Monsoon Trough (MT) is also noticed in the zoomed simulation, together with remarkable improvements in representing the associated precipitation and circulation features, as well as the large-scale organization of meso-scale convective systems over the MT region. Additionally, a more reasonable simulation of the monsoon synoptic disturbances (lows and disturbances) along the MT is noted in the high-resolution zoomed simulation. On the other hand, the no-zoom version has limitations in capturing the depressions and their movement, so that the MT zone is relatively dry in this case. Overall, the results from this work demonstrate the usefulness of the high-resolution variable resolution LMDZ model in realistically capturing the interactions among the monsoon large-scale dynamics, the synoptic systems and the meso-scale convective systems, which are essential elements of the South Asian monsoon system
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