33,323 research outputs found
Plasma Relaxation and Topological Aspects in Hall Magnetohydrodynamics
Parker's formulation of isotopological plasma relaxation process in
magnetohydrodynamics (MHD) is extended to Hall MHD. The torsion coefficient
alpha in the Hall MHD Beltrami condition turns out now to be proportional to
the "potential vorticity." The Hall MHD Beltrami condition becomes equivalent
to the "potential vorticity" conservation equation in two-dimensional (2D)
hydrodynamics if the Hall MHD Lagrange multiplier beta is taken to be
proportional to the "potential vorticity" as well. The winding pattern of the
magnetic field lines in Hall MHD then appears to evolve in the same way as
"potential vorticity" lines in 2D hydrodynamics
Observation of PT phase transition in a simple mechanical system
If a Hamiltonian is PT symmetric, there are two possibilities: Either the
eigenvalues are entirely real, in which case the Hamiltonian is said to be in
an unbroken-PT-symmetric phase, or else the eigenvalues are partly real and
partly complex, in which case the Hamiltonian is said to be in a
broken-PT-symmetric phase. As one varies the parameters of the Hamiltonian, one
can pass through the phase transition that separates the unbroken and broken
phases. This transition has recently been observed in a variety of laboratory
experiments. This paper explains the phase transition in a simple and intuitive
fashion and then describes an extremely elementary experiment in which the
phase transition is easily observed.Comment: 9 pages, 9 figure
Statistical Study of the Reconnection Rate in Solar Flares Observed with YOHKOH/SXT
We report a statistical study of flares observed with the Soft X-ray
Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical
parameters of 77 flares, such as the temporal scale, the size, and the magnetic
flux density and find that the sizes of flares tend to be distributed more
broadly as the GOES class becomes weaker and that there is a lower limit of
magnetic flux density that depends on the GOES class. We also examine the
relationship between these parameters and find weak correlation between
temporal and spatial scales of flares. We estimate reconnection inflow
velocity, coronal Alfven velocity, and reconnection rate using above observed
values. The inflow velocities are distributed from a few km/s to several tens
km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4
km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the
reconnection rate in a flare tends to decrease as the GOES class of the flare
increases. This value is within one order of magnitude from the theoretical
maximum value predicted by the Petschek model, although the dependence of the
reconnection rate on the magnetic Reynolds number tends to be stronger than
that in the Petschek model.Comment: 21 pages, 8 figures, accepted for publication in Ap
Spatial curvature effects on molecular transport by diffusion
For a substance diffusing on a curved surface, we obtain an explicit relation
valid for very small values of the time, between the local concentration, the
diffusion coefficient, the intrinsic spatial curvature and the time. We recover
the known solution of Fick's law of diffusion in the flat space limit. In the
biological context, this result would be useful in understanding the variations
in the diffusion rates of integral proteins and other molecules on membranes.Comment: 10 page
Buffalo National River Ecosystems - Part II
The priorities were established for the Buffalo National River Ecosystem Studies through meetings and correspondence with Mr. Roland Wauer and other personnel of the Office of Natural Sciences, Southwest Region of the National Park Service. These priorities were set forth in the appendix of contract no. CX 700050443 dated May 21, 1975
Effects of high-fat diet withdrawal on behavior and striatal opioid gene expression [abstract]
Abstract only availableFaculty Mentor: Dr. Matthew Will, Psychological SciencesThe central opioid systems play a critical role in certain aspects of food intake, particularly with regard to the rewarding impact of calorically dense food such as fat and sugar. The striatal opioid enkephalin may be a key component of this system. Infusions of mu opiate agonists into this region greatly increase feeding, while infusions of opiate antagonists decrease food intake. Only recently has enkephalin gene expression in relation to differing motivational states been explored. Recent evidence suggests that expression of striatal preproenkephalin mRNA responds to short-term food motivational states, but not to long-term metabolic responses such as diet restriction. The following study will expand on this and other findings by examining preproenkephalin expression patterns during another important motivational state: withdrawal from a high-fat diet. While it has previously been shown that preproenkephalin expression down regulates following intermittent exposure to a chronic high-fat liquid, the current study will examine the influence of withdrawal from a high-fat diet on preproenkephalin expression, while assessing the motivational state of the subject immediately prior to sacrifice. Rats will be placed in automated behavioral testing chambers and given 2.5 hr limited access to either a high-fat diet or chow for 14 consecutive days. On the 15th day, all rats will again be placed in the same chambers, yet half of the rats from each group will receive their respective diet, while the other half will receive no food (withdrawal state). During all days of the experiment, behavioral measures of both appetitive (approach and seeking), consummatory (amount eaten), and behavioral activity will be assessed. At the end of the 2.5 hr feeding session on the 15th day, all rats will be sacrificed and their brains will be prepared for analysis of preproenkephalin expression in the striatum and other feeding related regions
Evidence for a singularity in ideal magnetohydrodynamics: implications for fast reconnection
Numerical evidence for a finite-time singularity in ideal 3D
magnetohydrodynamics (MHD) is presented. The simulations start from two
interlocking magnetic flux rings with no initial velocity. The magnetic
curvature force causes the flux rings to shrink until they come into contact.
This produces a current sheet between them. In the ideal compressible
calculations, the evidence for a singularity in a finite time is that the
peak current density behaves like for a range of
sound speeds (or plasma betas). For the incompressible calculations consistency
with the compressible calculations is noted and evidence is presented that
there is convergence to a self-similar state. In the resistive reconnection
calculations the magnetic helicity is nearly conserved and energy is
dissipated.Comment: 4 pages, 4 figure
Magnetic Field Structure and Stochastic Reconnection in a Partially Ionized Gas
We consider stochastic reconnection in a magnetized, partially ionized
medium. Stochastic reconnection is a generic effect, due to field line
wandering, in which the speed of reconnection is determined by the ability of
ejected plasma to diffuse away from the current sheet along magnetic field
lines, rather than by the details of current sheet structure. We consider the
limit of weak stochasticity, so that the mean magnetic field energy density is
greater than either the turbulent kinetic energy density or the energy density
associated with the fluctuating component of the field. We consider field line
stochasticity generated through a turbulent cascade, which leads us to consider
the effect of neutral drag on the turbulent cascade of energy. In a
collisionless plasma, neutral particle viscosity and ion-neutral drag will damp
mid-scale turbulent motions, but the power spectrum of the magnetic
perturbations extends below the viscous cutoff scale. We give a simple physical
picture of the magnetic field structure below this cutoff, consistent with
numerical experiments. We provide arguments for the reemergence of the
turbulent cascade well below the viscous cut-off scale and derive estimates for
field line diffusion on all scales. We note that this explains the persistence
of a single power law form for the turbulent power spectrum of the interstellar
medium, from scales of tens of parsecs down to thousands of kilometers. We find
that under typical conditions in the ISM stochastic reconnection speeds are
reduced by the presence of neutrals, but by no more than an order of magnitude.Comment: Astrophysical Journal in pres
The Research Excellence Framework (REF): Assessing the impact of social work research on society
This paper reviews one aspect, impact, of the forthcoming assessment of research in UK universities, the Research Excellence Framework (REF), and examines its meaning and potential for enhanced partnerships between social work practice and academia in the context of the current economic crisis. Examples of case studies being developed to show how research has societal impact are described and some of the complexities of what, on the surface appears to echo social work 19s desire to make a positive difference to the lives of people in society, are drawn out. The importance of the REF for the integration of social work practice and academia have been rehearsed many times. This paper argues that making an impact is everybody 19s concern and practitioners and those who use social work services and their carers have a role to play in its creation and identification
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