3,472 research outputs found
On the Spatial Distribution of Hard X-Rays from Solar Flare Loops
The aim of this paper is to investigate the spatial structure of the
impulsive phase hard X-ray emission from solar flares. This work is motivated
by the YOHKOH and the forthcoming HESSI observations. Summarizing past results,
it is shown that the transport effects can account for the observations by
inhomogeneous loops where there is a strong field convergence and/or density
enhancement at the top of the flaring loop. Scattering by plasma turbulence at
the acceleration site or pancake type pitch angle distribution of the
accelerated electrons can also give rise to enhanced emission at the loop tops.
These could be a natural consequence of acceleration by plasma waves. This
paper considers a general case of stochastic scattering and acceleration that
leads to an isotropic pitch angle distribution and an enhanced emission from
the loop tops or the acceleration site.
Following the formalism developed in earlier papers the strength and the
spectrum of the radiation expected from the acceleration site and the foot
points are evaluated and their dependence on the parameters describing the
acceleration process and the flare plasma are determined. The theoretical ratio
of these two intensities and relative values of their spectral indices are
compared with the YOHKOH observations, demonstrating that the above mentioned
parameters can be constrained with such observations. It is shown that future
high spatial and spectral resolution observations, for example those expected
from HESSI, can begin to distinguish between different models and constrain
their parameters.Comment: 37 pages with 20 figures. Accepted for publication in ApJ
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Nonlinear screening and stopping power in two-dimensional electron gases
We have used density functional theory to study the nonlinear screening
properties of a two-dimensional (2D) electron gas. In particular, we consider
the screening of an external static point charge of magnitude Z as a function
of the distance of the charge from the plane of the gas. The self-consistent
screening potentials are then used to determine the 2D stopping power in the
low velocity limit based on the momentum transfer cross-section. Calculations
as a function of Z establish the limits of validity of linear and quadratic
response theory calculations, and show that nonlinear screening theory already
provides significant corrections in the case of protons. In contrast to the 3D
situation, we find that the nonlinearly screened potential supports a bound
state even in the high density limit. This behaviour is elucidated with the
derivation of a high density screening theorem which proves that the screening
charge can be calculated perturbatively in the high density limit for arbitrary
dimensions. However, the theorem has particularly interesting implications in
2D where, contrary to expectations, we find that perturbation theory remains
valid even when the perturbing potential supports bound states.Comment: 23 pages, 15 figures in RevTeX
Physical role for the nucleus in cell migration
Cell migration is important for the function of many eukaryotic cells. Recently the nucleus has been shown to play an important role in cell motility. After giving an overview of cell motility mechanisms we review what is currently known about the mechanical properties of the nucleus and the connections between it and the cytoskeleton. We also discuss connections to the extracellular matrix and mechanotransduction. We identify key physical roles of the nucleus in cell migration
Superconducting gap anisotropy of LuNi2B2C thin films from microwave surface impedance measurements
Surface impedance measurements of LuNi2B2C superconducting thin films as a
function of temperature have been performed down to 1.5 K and at 20 GHz using a
dielectric resonator technique. The magnetic penetration depth closely
reproduces the standard B.C.S. result, but with a reduced value of the energy
gap at low temperature. These data provide evidence for an anisotropic s-wave
character of the order parameter symmetry in LuNi2B2C. From the evaluation of
the real part of complex conductivity, we have observed constructive (type II)
coherence effects in the electromagnetic absorption below Tc.Comment: 15 pages, 4 figure
Culture change in elite sport performance teams: Examining and advancing effectiveness in the new era
Reflecting the importance of optimizing culture for elite teams, Fletcher and Arnold (2011) recently suggested the need for expertise in culture change. Acknowledging the dearth of literature on the specific process, however, the potential effectiveness of practitioners in this area is unknown. The present paper examines the activity's precise demands and the validity of understanding in sport psychology and organizational research to support its delivery. Recognizing that sport psychologists are being increasingly utilized by elite team management, initial evidence-based guidelines are presented. Finally, to stimulate the development of ecologically valid, practically meaningful knowledge, the paper identifies a number of future research directions
Effect of synthesis conditions on formation pathways of metal organic framework (MOF-5) Crystals
Metal Organic Frameworks (MOFs) represent a class of nanoporous crystalline materials with far reaching potential in gas storage, catalysis, and medical devices. We investigated the effects of synthesis process parameters on production of MOF-5 from terephthalic acid and zinc nitrate in diethylformamide. Under favorable synthesis conditions, we systematically mapped a solid formation diagram in terms of time and temperature for both stirred and unstirred conditions. The synthesis of MOF-5 has been previously reported as a straightforward reaction progressing from precursor compounds in solution directly to the final MOF-5 solid phase product. However, we show that the solid phase formation process is far more complex, invariably transferring through metastable intermediate crystalline phases before the final MOF-5 phase is reached, providing new insights into the formation pathways of MOFs. We also identify process parameters suitable for scale-up and continuous manufacturing of high purity MOF-5
Understanding Health and Social Challenges for Aging and Long-Term Care in China
The second King’s College London Symposium on Ageing and Long-term Care in China was convened from 4 to 5th July 2019 at King’s College London in London. The aim of the Symposium was to have a better understanding of health and social challenges for aging and long-term care in China. This symposium draws research insights from a wide range of disciplines, including economics, public policy, demography, gerontology, public health and sociology. A total of 20 participants from eight countries, seek to identify the key issues and research priorities in the area of aging and long-term care in China. The results published here are a synthesis of the top four research areas that represent the perspectives from some of the leading researchers in the field
Solar interacting protons versus interplanetary protons in the core plus halo model of diffusive shock acceleration and stochastic re-acceleration
With the first observations of solar γ-rays from the decay of pions, the relationship of protons producing ground level enhancements (GLEs) on the Earth to those of similar energies producing the γ-rays on the Sun has been debated. These two populations may be either independent and simply coincident in large flares, or they may be, in fact, the same population stemming from a single accelerating agent and jointly distributed at the Sun and also in space. Assuming the latter, we model a scenario in which particles are accelerated near the Sun in a shock wave with a fraction transported back to the solar surface to radiate, while the remainder is detected at Earth in the form of a GLE. Interplanetary ions versus ions interacting at the Sun are studied for a spherical shock wave propagating in a radial magnetic field through a highly turbulent radial ray (the acceleration core) and surrounding weakly turbulent sector in which the accelerated particles can propagate toward or away from the Sun. The model presented here accounts for both the first-order Fermi acceleration at the shock front and the second-order, stochastic re-acceleration by the turbulence enhanced behind the shock. We find that the re-acceleration is important in generating the γ-radiation and we also find that up to 10% of the particle population can find its way to the Sun as compared to particles escaping to the interplanetary space
Structure, dynamical impact and origin of magnetic fields in nearby galaxies in the SKA era
Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their importance
for star formation, they govern the transport of cosmic rays, relevant to the launch and
regulation of galactic outflows and winds, which in turn are pivotal in shaping the structure of halo
magnetic fields. Mapping the small-scale structure of interstellar magnetic fields in many nearby
galaxies is crucial to understand the interaction between gas and magnetic fields, in particular
how gas flows are affected. Elucidation of the magnetic role in, e.g., triggering star formation,
forming and stabilising spiral arms, driving outflows, gas heating by reconnection and magnetising
the intergalactic medium has the potential to revolutionise our physical picture of the ISM
and galaxy evolution in general. Radio polarisation observations in the very nearest galaxies at
high frequencies (3 GHz) and with high spatial resolution (500) hold the key here. The galaxy
survey with SKA1 that we propose will also be a major step to understand the galactic dynamo,
which is important for models of galaxy evolution and for astrophysical magnetohydrodynamics
in general. Field amplification by turbulent gas motions, which is crucial for efficient dynamo
action, has been investigated so far only in simulations, while compelling evidence of turbulent
fields from observations is still lacking
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