11,978 research outputs found
Hall current effects in dynamic magnetic reconnection solutions
The impact of Hall current contributions on flow driven planar magnetic merging solutions is discussed. The Hall current is important if the dimensionless Hall parameter (or normalized ion skin depth) satisfies cH>η where η is the inverse Lundquist number for the plasma. A dynamic analysis of the problem shows, however, that the Hall current initially manifests itself, not by modifying the planar reconnection field, but by inducing a non-reconnecting perpendicular "separator" component in the magnetic field. Only if the stronger condition c2/H > η is satisfied can Hall currents be expected to affect the planar merging. These analytic predictions are then tested by performing a series of numerical experiments in periodic geometry, using the full system of planar magnetohydrodynamic (MHD) equations. The numerical results confirm that the nature of the merging changes dramatically when the Hall coupling satisfies c2/H > η. In line with the analytic treatment of sheared reconnection, the coupling provided by the Hall term leads to the emergence of multiple current layers that can enhance the global Ohmic dissipation at the expense of the reconnection rate. However, the details of the dissipation depend critically on the symmetries of the simulation, and when the merging is "head-on" (i.e., comprises fourfold symmetry) the reconnection rate can be enhanced
Relativistic magnetic reconnection at X-type neutral points
Relativistic effects in the oscillatory damping of magnetic disturbances near
two-dimensional X-points are investigated. By taking into account displacement
current, we study new features of extremely magnetized systems, in which the
Alfv\'en velocity is almost the speed of light. The frequencies of the
least-damped mode are calculated using linearized relativistic MHD equations
for wide ranges of the Lundquist number S and the magnetization parameter
. These timescales approach constant values in the large resistive
limit: the oscillation time becomes a few times the light crossing time,
irrespective of , and the decay time is proportional to and
therefore is longer for a highly magnetized system.Comment: 6 pages, 4 figure
Ferroelectricity in the Dion-Jacobson CsBiNbO from first principles
We have studied ferroelectricity in Dion-Jacobson CsBiNbO from first
principles. Using group-theoretical analysis and first-principles density
functional calculations of the total energy and phonons, we perform a
systematic study of the energy surface around a paraelectric prototypic phase.
Our results suggest that CsBiNbO is a ferroelectric with a polarization
of P=40C cm. We propose further experiments to clarify this
point
Visualization of flows in a motored rotary combustion engine using holographic interferometry
The use of holographic interferometry to view the small- and large-scale flow field structures in the combustion chamber of a motored Wankel engine assembly is described. In order that the flow patterns of interest could be observed, small quantities of helium were injected with the intake air. Variation of the air flow patterns with engine speed, helium flow rate, and rotor position are described. The air flow at two locations within the combustion chamber was examined using this technique
Family type and social support seeking among Nazarbayev University students
Social support positively influences health and well-being across cultural groups [1].
However, culture influences attitudes towards social support seeking, especially in developing countries
experiencing clashes of traditional and modern social norms [2]. Because university students who receive
social support have better academic performance and are less likely to drop out [3], the purpose of this
study was to examine social support from the perspectives of Nazarbayev University students
Family type and social support seeking among Nazarbayev University students
Social support positively influences health and well-being across cultural groups [1].
However, culture influences attitudes towards social support seeking, especially in developing countries
experiencing clashes of traditional and modern social norms [2]. Because university students who receive
social support have better academic performance and are less likely to drop out [3], the purpose of this
study was to examine social support from the perspectives of Nazarbayev University students
Physics of the Merging Clusters Cygnus A, A3667, and A2065
We present ASCA gas temperature maps of the nearby merging galaxy clusters
Cygnus A, A3667, and A2065. Cygnus A appears to have a particularly simple
merger geometry that allows an estimate of the subcluster collision velocity
from the observed temperature variations. We estimate it to be ~2000 km/s.
Interestingly, this is similar to the free-fall velocity that the two Cygnus A
subclusters should have achieved at the observed separation, suggesting that
merger has been effective in dissipating the kinetic energy of gas halos into
thermal energy, without channeling its major fraction elsewhere (e.g., into
turbulence). In A3667, we may be observing a spatial lag between the shock
front seen in the X-ray image and the corresponding rise of the electron
temperature. A lag of the order of hundreds of kiloparsecs is possible due to
the combination of thermal conduction and a finite electron-ion equilibration
time. Forthcoming better spatial resolution data will allow a direct
measurement of these phenomena using such lags. A2065 has gas density peaks
coincident with two central galaxies. A merger with the collision velocity
estimated from the temperature map should have swept away such peaks if the
subcluster total mass distributions had flat cores in the centers. The fact
that the peaks have survived (or quickly reemerged) suggests that the
gravitational potential also is strongly peaked. Finally, the observed specific
entropy variations in A3667 and Cygnus A indicate that energy injection from a
single major merger may be of the order of the full thermal energy of the gas.
We hope that these order of magnitude estimates will encourage further work on
hydrodynamic simulations, as well as more quantitative representation of the
simulation results.Comment: Corrected the Cyg-A figure (errors shown were 1-sigma not 90%); text
unchanged. ApJ in press. Latex, 5 pages, 3 figures (2 color), uses
emulateapj.st
Granular Rheology in Zero Gravity
We present an experimental investigation on the rheological behavior of model
granular media made of nearly elastic spherical particles. The experiments are
performed in a cylindrical Couette geometry and the experimental device is
placed inside an airplane undergoing parabolic flights to cancel the effect of
gravity. The corresponding curves, shear stress versus shear rate, are
presented and a comparison with existing theories is proposed. The quadratic
dependence on the shear rate is clearly shown and the behavior as a function of
the solid volume fraction of particles exhibits a power law function. It is
shown that theoretical predictions overestimate the experiments. We observe, at
intermediate volume fractions, the formation of rings of particles regularly
spaced along the height of the cell. The differences observed between
experimental results and theoretical predictions are discussed and related to
the structures formed in the granular medium submitted to the external shear.Comment: 10 pages, 6 figures to be published in Journal of Physics : Condensed
Matte
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