1,390 research outputs found
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>10^{11}
cm^{-3}) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.Comment: 19 pages, 9 figures, accepted for publication in Ap
Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in the MAST spherical tokamak and the solar corona
Twisted magnetic flux ropes are ubiquitous in space and laboratory plasmas,
and the merging of such flux ropes through magnetic reconnection is an
important mechanism for restructuring magnetic fields and releasing free
magnetic energy. The merging-compression scenario is one possible start up
scheme for spherical tokamaks, which has been used on the Mega Amp Spherical
Tokamak MAST. Two current-carrying plasma rings, or flux ropes, approach each
other through the mutual attraction of their like currents, and merge, through
magnetic reconnection, into a single plasma torus, with substantial plasma
heating. 2D resistive MHD and Hall MHD simulations of this process are
reported, and new results for the temperature distribution of ions and
electrons are presented. A model of the based on relaxation theory is also
described, which is now extended to tight aspect ratio geometry. This model
allows prediction of the final merged state and the heating. The implications
of the relaxation model for heating of the solar corona are also discussed, and
a model of the merger of two or more twisted coronal flux ropes is presented,
allowing for different senses of twist
Morphology of Poly(styrene-co-butadiene) Random Copolymer Thin Films and Nanostructures on a Graphite Surface
We
studied the morphology of polyÂ(styrene-<i>co</i>-butadiene)
random copolymers on a graphite surface. Polymer solutions were spin
coated onto graphite, at various concentrations and molecular weights.
The polymer films and nanostructures were imaged using atomic force
microscopy. Above the overlap concentration, thin films formed. However,
total wetting did not occur, despite the polymers being well above
their <i>T</i><sub>g</sub>. Instead, dewetting was observed,
suggesting the films were in a state of metastable equilibrium. At
lower concentrations, the polymers formed networks, nanoislands, and
nanoribbons. Ordered nanopatterns were observed on the surface; the
polymers orientated themselves due to ÏâÏ stacking
interactions reflecting the crystalline structure of the graphite.
At the lowest concentration, this ordering was very pronounced. At
higher concentrations, it was less defined but still statistically
significant. Higher degrees of ordering were observed with polyÂ(styrene-<i>co</i>-butadiene) than polystyrene and polybutadiene homopolymers
as the copolymerâs aromatic rings are distributed along a flexible
chain, which maximizes ÏâÏ stacking. At the two
lowest concentrations, the size of the nanoislands and nanoribbons
remained similar with varying molecular weight. However, at higher
concentrations, the polymer network features were largest at the lowest
molecular weight, indicating that in this case, a large proportion
of shorter chains stay on top of the adsorbed ones. The contact angles
of the polymer nanostructures remained mostly constant with size,
which is due to the strong polymer/graphite adhesion dominating over
line tension and entropic effects
Collisional damping rates for plasma waves
The distinction between the plasma dynamics dominated by collisional
transport versus collective processes has never been rigorously addressed until
recently. A recent paper [Yoon et al., Phys. Rev. E 93, 033203 (2016)]
formulates for the first time, a unified kinetic theory in which collective
processes and collisional dynamics are systematically incorporated from first
principles. One of the outcomes of such a formalism is the rigorous derivation
of collisional damping rates for Langmuir and ion-acoustic waves, which can be
contrasted to the heuristic customary approach. However, the results are given
only in formal mathematical expressions. The present Brief Communication
numerically evaluates the rigorous collisional damping rates by considering the
case of plasma particles with Maxwellian velocity distribution function so as
to assess the consequence of the rigorous formalism in a quantitative manner.
Comparison with the heuristic ("Spitzer") formula shows that the accurate
damping rates are much lower in magnitude than the conventional expression,
which implies that the traditional approach over-estimates the importance of
attenuation of plasma waves by collisional relaxation process. Such a finding
may have a wide applicability ranging from laboratory to space and
astrophysical plasmas.Comment: 5 pages, 2 figures; Published in Physics of Plasmas, volume/Issue
23/6. Publisher: AIP Publishing LLC. Date: Jun 1, 2016. URL:
http://aip.scitation.org/doi/10.1063/1.4953802 Rights managed by AIP
Publishing LL
Fast electron slowing-down and diffusion in a high temperature coronal X-ray source
Finite thermal velocity modifications to electron slowing-down rates may be important for the deduction of solar flare total electron energy. Here we treat both slowing-down and velocity diffusion of electrons in the corona at flare temperatures, for the case of a simple, spatially homogeneous source. Including velocity diffusion yields a consistent treatment of both "accelerated" and "thermal" electrons. It also emphasises that one may not invoke finite thermal velocity target effects on electron lifetimes without simultaneously treating the contribution to the observed X-ray spectrum from thermal electrons. We present model calculations of the X-ray spectra resulting from injection of a power-law energy distribution of electrons into a source with finite temperature. Reducing the power-law distribution low-energy cutoff to lower and lower energies only increases the relative magnitude of the thermal component of the spectrum, because the lowest energy electrons simply join the background thermal distribution. Acceptable fits to RHESSI flare data are obtained using this model. These also demonstrate, however, that observed spectra may in consequence be acceptably consistent with rather a wide range of injected electron parameters
The psychological impact of a colorectal cancer diagnosis following a negative fecal occult blood test result.
- Background: Screening using fecal occult blood testing (FOBt) reduces colorectal cancer (CRC) mortality, but the test has low sensitivity. A 'missed' cancer may cause psychological harms in the screened population that partially counteract the benefits of early detection.
- Methods: 311 people diagnosed with CRC: i) after a negative FOBt result (interval cancer), ii) a positive result (screen-detected cancer), or iii) in regions where screening was not offered, completed questions on quality of life (FACT-C), depression (CES-D), perceived diagnostic delay, and trust in the results of FOBt screening. 15 withheld consent to data matching with medical records, leaving a sample size of 296.
- Results: Controlling for demographic and clinical variables, patients with an interval cancer reported poorer quality of life (difference in means = 6.16, p = 0.03) and more diagnostic delay (OR: 0.37, p = 0.02) than patients with screen-detected disease, with no differences in depression. No differences were observed between the interval cancer group and the group not offered screening on these measures. Patients with an interval cancer reported the lowest levels of trust in FOB testing.
Conclusions: An interval cancer has adverse effects on trust in FOBt but does not result in worse psychological outcomes compared with people diagnosed in areas with no screening programme. People with an interval cancer report poorer quality of life than people with screen-detected disease.
- Impact: Improvements in test sensitivity could improve quality of life among people who complete an FOB test over and above any benefits already conferred by earlier detection
Serum concentrations of the biomarkers CA125, CA15-3, CA72-4, tPSA and PAPP-A in natural and stimulated ovarian cycles
Objective
Biomarkers associated with cancer screening (CA125, CA15â3, CA72â4, total prostate specific antigen [tPSA]) and the monitoring of pregnancy (pregnancy associated plasma proteinâA [PAPPâA]) were measured during natural and stimulated ovarian cycles in diseaseâfree non-pregnant women to determine if they could reflect normal events relating to ovulation and/or endometrial changes.
Methods
A total of 73 blood samples (10 women) collected throughout the natural menstrual cycle, and 64 blood samples (11 women) taken during stimulated ovarian cycles, were analysed on the Roche Cobas e411 automated analyser.
Results
Detectable levels of tPSA were measured in at least one point in the cycle in 6/10 of women in the natural cycle and 10/11 of women in stimulated cycles, and CA72-4 was detected in only 12/21 women tested. Concentrations of CA125, tPSA, CA15â3 and CA72â4 showed no significant difference between the natural and stimulated ovarian cycle groups. On average the mean PAPPâA of the natural group was (2.41±0.58) mIU/L higher than the stimulated group (t=4.10, P< 0.001). CA125 and CA15â3 results were both significantly influenced by the stage of the cycle (P<0.0001), whereas tPSA and PAPPâA concentrations revealed no significant changes (Pâ„0.65). CA72â4 was not affected by the stage of the cycle nor ovarian stimulation.
Conclusion
Ovarian stimulation reduced serum PAPPâA levels, CA125 and CA15â3 levels were generally unaffected by ovarian stimulation but displayed cyclical changes throughout both natural and stimulated cycles, whilst tPSA and CA72-4 were not affected by the stage of the cycle or ovarian stimulation
Stem cell-based approaches in cardiac tissue engineering: controlling the microenvironment for autologous cells
Cardiovascular disease is one of the leading causes of mortality worldwide. Cardiac tissue engineering strategies focusing on biomaterial scaffolds incorporating cells and growth factors are emerging as highly promising for cardiac repair and regeneration. The use of stem cells within cardiac microengineered tissue constructs present an inherent ability to differentiate into cell types of the human heart. Stem cells derived from various tissues including bone marrow, dental pulp, adipose tissue and umbilical cord can be used for this purpose. Approaches ranging from stem cell injections, stem cell spheroids, cell encapsulation in a suitable hydrogel, use of prefabricated scaffold and bioprinting technology are at the forefront in the field of cardiac tissue engineering. The stem cell microenvironment plays a key role in the maintenance of stemness and/or differentiation into cardiac specific lineages. This review provides a detailed overview of the recent advances in microengineering of autologous stem cell-based tissue engineering platforms for the repair of damaged cardiac tissue. A particular emphasis is given to the roles played by the extracellular matrix (ECM) in regulating the physiological response of stem cells within cardiac tissue engineering platforms
The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra
Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms
of purely collisional electron beam propagation, ignoring spatial evolution and
collective effects. In this paper we present self-consistent numerical
simulations of the spatial and temporal evolution of an electron beam subject
to collisional transport and beam-driven Langmuir wave turbulence. These
wave-particle interactions represent the background plasma's response to the
electron beam propagating from the corona to chromosphere and occur on a far
faster timescale than coulomb collisions. From these simulations we derive the
mean electron flux spectrum, comparable to such spectra recovered from high
resolution hard X-rays observations of solar flares with RHESSI. We find that a
negative spectral index (i.e. a spectrum that increases with energy), or local
minima when including the expected thermal spectral component at low energies,
occurs in the standard thick-target model, when coulomb collisions are only
considered. The inclusion of wave-particle interactions does not produce a
local minimum, maintaining a positive spectral index. These simulations are a
step towards a more complete treatment of electron transport in solar flares
and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal
energies maybe a better approximation instead of a sharp cut-off in the
injected electron spectrum.Comment: 6 pages, 5 figures, accepted by ApJ
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