9,750 research outputs found
Fluctuation dynamo amplified by intermittent shear bursts in convectively driven magnetohydrodynamic turbulence
Intermittent large-scale high-shear flows are found to occur frequently and
spontaneously in direct numerical simulations of statistically stationary
turbulent Boussinesq magnetohydrodynamic (MHD) convection. The energetic
steady-state of the system is sustained by convective driving of the velocity
field and small-scale dynamo action. The intermittent emergence of flow
structures with strong velocity and magnetic shearing generates magnetic energy
at an elevated rate over time-scales longer than the characteristic time of the
large-scale convective motion. The resilience of magnetic energy amplification
suggests that intermittent shear-bursts are a significant driver of dynamo
action in turbulent magnetoconvection
Effect of Antiferromagnetic Interlayer Coupling on Current-Assisted Magnetization Switching
We compare magnetization switching in Co/Cu/Co nanopillars with uncoupled and
dipole-field coupled Co layers. In uncoupled nanopillars, current-driven
switching is hysteretic at low magnetic field H and changes to reversible,
characterized by telegraph noise, at high H. We show that dipolar coupling both
affects the switching current and causes the switching to become reversible at
small H. The coupling thus changes the switching to reversible, hysteretic, and
then reversible again as H increases. We describe our results in terms of
current-assisted thermal activation.Comment: 3 pages, 3 figure
COPTRAN - A method of optimum communication systems design
Single set of mathematical expressions describes system cost and probability of error of data transmission in terms of four basic parameters in the link equation. A Lagrange multiplier sets up equations whose solutions yield the optimum values for system design considerations and weight and cost values
Oxidized basalts on the surface of Venus: Compositional implications of measured spectral properties
Venera Lander reflectance data are compared with high temperature spectra of the same basaltic materials. The dark, flat unoxidized basalts are still inconsistent with the Venera data in the near-infrared. Basaltic material with a ferric component, however, would satisfy both the increase in reflectance beyond 0.7 microns as well as the dark, relatively colorless character in the visible. Therefore, it is concluded that besaltic surfaces of Venus represented by these measurements either contain minerals with uncommon characteristics, or, more likely, are relatively oxidized
Causality Violations in Cascade Models of Nuclear Collisions
Transport models have successfully described many aspects of intermediate
energy heavy-ion collision dynamics. As the energies increase in these models
to the ultrarelativistic regime, Lorentz covariance and causality are not
strictly respected. The standard argument is that such effects are not
important to final results; but they have not been seriously considered at high
energies. We point out how and why these happen, how serious of a problem they
may be and suggest ways of reducing or eliminating the undesirable effects.Comment: RevTeX, 23 pages, 9 (uuencoded) figures; to appear in Phys. Rev
A new record of Percursaria percursa (Ulvaceae, Ulvales) on the North Island, New Zealand
The filamentous green alga Percursaria percursa (Ulvaceae, Ulvales) was recorded for the first time on the North Island of New Zealand at mokoroa Estuary, Tauranga Harbour. This species is previously known within New Zealand from only two records, both from the South Island. In Tauranga Harbour, this species was restricted to anoxic estuarine sediments where mangrove forests had been mulched, and mulchate left in situ. Percursaria percursa was found intertwined with Ulva spp. and Rhizoclonium spp. Surveys of other North and South Island estuaries suggest that this alga, although occurring as part of nuisance green algal blooms in Tauranga Harbour, has only colonized human-impacted locations, and has not yet been observed in natural' estuarine ecosystems in New Zealand. As this species was found intertwined with other mat-forming filamentous green algae, it can easily be misidentified in the field, leading to both over- and under-reporting of species occurrence
Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection
We investigate the utility of the convex hull of many Lagrangian tracers to
analyze transport properties of turbulent flows with different anisotropy. In
direct numerical simulations of statistically homogeneous and stationary
Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD
Boussinesq convection a comparison with Lagrangian pair dispersion shows that
convex hull statistics capture the asymptotic dispersive behavior of a large
group of passive tracer particles. Moreover, convex hull analysis provides
additional information on the sub-ensemble of tracers that on average disperse
most efficiently in the form of extreme value statistics and flow anisotropy
via the geometric properties of the convex hulls. We use the convex hull
surface geometry to examine the anisotropy that occurs in turbulent convection.
Applying extreme value theory, we show that the maximal square extensions of
convex hull vertices are well described by a classic extreme value
distribution, the Gumbel distribution. During turbulent convection,
intermittent convective plumes grow and accelerate the dispersion of Lagrangian
tracers. Convex hull analysis yields information that supplements standard
Lagrangian analysis of coherent turbulent structures and their influence on the
global statistics of the flow.Comment: 18 pages, 10 figures, preprin
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