1,651 research outputs found
Oblique Ion Two-Stream Instability in the Foot Region of a Collisionless Shock
Electrostatic behavior of a collisionless plasma in the foot region of high
Mach number perpendicular shocks is investigated through the two-dimensional
linear analysis and electrostatic particle-in-cell (PIC) simulation. The
simulations are double periodic and taken as a proxy for the situation in the
foot. The linear analysis for relatively cold unmagnetized plasmas with a
reflected proton beam shows that obliquely propagating Buneman instability is
strongly excited. We also found that when the electron temperature is much
higher than the proton temperature, the most unstable mode is the highly
obliquely propagating ion two-stream instability excited through the resonance
between ion plasma oscillations of the background protons and of the beam
protons, rather than the ion acoustic instability that is dominant for parallel
propagation. To investigate nonlinear behavior of the ion two-stream
instability, we have made PIC simulations for the shock foot region in which
the initial state satisfies the Buneman instability condition. In the first
phase, electrostatic waves grow two-dimensionally by the Buneman instability to
heat electrons. In the second phase, highly oblique ion two-stream instability
grows to heat mainly ions. This result is in contrast to previous studies based
on one-dimensional simulations, for which ion acoustic instability further
heats electrons. The present result implies that overheating problem of
electrons for shocks in supernova remnants is resolved by considering ion
two-stream instability propagating highly obliquely to the shock normal and
that multi-dimensional analysis is crucial to understand the particle heating
and acceleration processes in shocks.Comment: 20 pages, 9 figures, accepted for publication in Ap
Analysis of dynamic characteristics of fluid force induced by labyrinth seal
Flow patterns of the labyrinth seal are experimentally investigated for making a mathematical model of labyrinth seal and to obtain the flow induced force of the seal. First, the flow patterns in the labyrinth chamber are studied on the circumferential flow using bubble and on the cross section of the seal chamber using aluminum powder as tracers. And next, the fluid force and its phase angle are obtained from the measured pressure distribution in the chamber and the fluid force coefficients are derived from the fluid force and the phase angle. Those are similar to the expression of oil film coefficients. As a result, it is found that the vortices exist in the labyrinth chambers and its center moves up and down periodically. The pressure drop is biggest in the first stage of chambers and next in the last stage of chambers
Diffusion-weighted MR neurography for the assessment of brachial plexopathy in oncological practice
Background: To evaluate diffusion-weighted MR neurography (DW-MRN) for visualizing the brachial plexus and for the assessment of brachial plexopathy. Methods: 40 oncological patients with symptoms of brachial plexopathy underwent 1.5 T MRI using conventional MR sequences and unidirectional DW-MRN. The images were independently reviewed by two radiologists. Anatomic visualization of the brachial plexus was scored using a 5 point scale on conventional MR sequences and then combined with DW-MRN. A brachial plexus abnormality was also scored using a 5 point scale and inter-observer agreement determined by kappa statistics. Diagnostic accuracy for brachial plexopathy assessed by conventional MRI alone versus conventional MRI combined with DW-MRN was compared by ROC analysis using reference standards. Results: DW-MRN significantly improved visualization of the brachial plexus compared with conventional MRI alone (P <0.001). When assessing brachial plexopathy, inter-observer agreement was moderate for conventional MRI (kappa = 0.48) but good for conventional MRI with DW-MRN (kappa = 0.62). DW-MRN combined with conventional MRI significantly improved diagnostic accuracy in one observer (P <0.05) but was similar in the other observer. Conclusion: DW-MRN improved visualization of the brachial plexus. Combining DW-MRN with conventional MRI can improve inter-observer agreement and detection of brachial plexopathy in symptomatic oncological patients
Development of Simple Sequence Repeat (SSR) Markers and Their Use to Assess Genetic Diversity in Apomictic Guineagrass (\u3cem\u3ePanicum Maximum\u3c/em\u3e Jacq.)
Guineagrass is an important and widely grown tropical forage grass. Despite its importance and increasing popularity, only little is known about its genetic diversity (Ebina et al., 2001). Such information is useful for the selection of diverse parents in breeding programmes. Moreover, no simple sequence repeat (SSR) markers have been reported in any apomixis species. In this study SSR markers were developed and used to investigate genetic diversity in germplasm of apomictic guineagrass
On the Origin of the Highest Energy Cosmic Rays
We present the results of a new estimation of the photodisintegration and
propagation of ultrahigh energy cosmic ray (UHCR) nuclei in intergalactic
space. The critical interactions for photodisintegration and energy loss of
UHCR nuclei occur with photons of the infrared background radiation (IBR). We
have reexamined this problem making use of a new determination of the IBR based
on empirical data, primarily from IRAS galaxies, and also collateral
information from TeV gamma-ray observations of two nearby BL Lac objects. Our
results indicate that a 200 EeV Fe nucleus can propagate apx. 100 Mpc through
the IBR. We argue that it is possible that the highest energy cosmic rays
observed may be heavy nuclei.Comment: 2 pages revtex with one figure, submitted to Physical Review Letter
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