3,837 research outputs found
Differences in the trophic ecology of micronekton driven by diel vertical migration.
Many species of micronekton perform diel vertical migrations (DVMs), which ultimately contributes to carbon export to the deep sea. However, not all micronekton species perform DVM, and the nonmigrators, which are often understudied, have different energetic requirements that might be reflected in their trophic ecology. We analyze bulk tissue and whole animal stable nitrogen isotopic compositions (δ 15N values) of micronekton species collected seasonally between 0 and 1250 m depth to explore differences in the trophic ecology of vertically migrating and nonmigrating micronekton in the central North Pacific. Nonmigrating species exhibit depth-related increases in δ 15N values mirroring their main prey, zooplankton. Higher variance in δ 15N values of bathypelagic species points to the increasing reliance of deeper dwelling micronekton on microbially reworked, very small suspended particles. Migrators have higher δ 15N values than nonmigrators inhabiting the epipelagic zone, suggesting the consumption of material during the day at depth, not only at night when they migrate closer to the surface. Migrating species also appear to eat larger prey and exhibit a higher range of variation in δ 15N values seasonally than nonmigrators, likely because of their higher energy needs. The dependence on material at depth enriched in 15N relative to surface particles is higher in migratory fish that ascend only to the lower epipelagic zone. Our results confirm that stark differences in the food habits and dietary sources of micronekton species are driven by vertical migrations
Effective potential at finite temperature in a constant magnetic field I: Ring diagrams in a scalar theory
We study symmetry restoration at finite temperature in the theory of a
charged scalar field interacting with a constant, external magnetic field. We
compute the finite temperature effective potential including the contribution
from ring diagrams. We show that in the weak field case, the presence of the
field produces a stronger first order phase transition and that the temperature
for the onset of the transition is lower, as compared to the case without
magnetic field.Comment: Expanded comments, 4 figures added. Conclusions unchanged. Version to
match published pape
Double-containment coil with enhanced winding mounting for transcranial magnetic stimulation with reduced acoustic noise
Objective: This work aims to reduce the acoustic noise level of transcranial
magnetic stimulation (TMS) coils. TMS requires high currents (several thousand
amperes) to be pulsed through the coil, which generates a loud acoustic impulse
whose peak sound pressure level (SPL) can exceed 130 dB(Z). This sound poses a
risk to hearing and elicits unwanted neural activation of auditory brain
circuits. Methods: We propose a new double-containment coil with enhanced
winding mounting (DCC), which utilizes acoustic impedance mismatch to contain
and dissipate the impulsive sound within an air-tight outer casing. The coil
winding is potted in a rigid block, which is mounted to the outer casing by its
acoustic nodes that are subject to minimum vibration during the pulse. The rest
of the winding block is isolated from the casing by an air gap, and sound is
absorbed by foam within the casing. The casing thickness under the winding
center is minimized to maximize the coil electric field output. Results:
Compared to commercial figure-of-eight TMS coils, the DCC prototype has 10-33
dB(Z) lower SPL at matched stimulation strength, whilst providing 22% higher
maximum stimulation strength than equally focal commercial coils. Conclusion:
The DCC design greatly reduces the acoustic noise of TMS while increasing the
achievable stimulation strength. Significance: The acoustic noise reduction
from our coil design is comparable to that provided by typical hearing
protection devices. This coil design approach can enhance hearing safety and
reduce auditory co-activations in the brain and other detrimental effects of
TMS sound.Comment: 8 pages, 5 figure
Cyclic cocycles on twisted convolution algebras
We give a construction of cyclic cocycles on convolution algebras twisted by
gerbes over discrete translation groupoids. For proper \'etale groupoids, Tu
and Xu provide a map between the periodic cyclic cohomology of a gerbe-twisted
convolution algebra and twisted cohomology groups which is similar to a
construction of Mathai and Stevenson. When the groupoid is not proper, we
cannot construct an invariant connection on the gerbe; therefore to study this
algebra, we instead develop simplicial techniques to construct a simplicial
curvature 3-form representing the class of the gerbe. Then by using a JLO
formula we define a morphism from a simplicial complex twisted by this
simplicial curvature 3-form to the mixed bicomplex computing the periodic
cyclic cohomology of the twisted convolution algebras. The results in this
article were originally published in the author's Ph.D. thesis.Comment: 39 page
The distribution of oceanic cirripedes in the north-east Atlantic in summer 1983 and the connotations of the results to the problems of Conchoderma fouling
A review of biological processes within oceanic water columns relevant to the assessment of the safety of disposal of waste, notably radioactive isotopes on or within the seabed
Two-mirror Schwarzschild aplanats. Basic relations
It is shown that the theory of aplanatic two-mirror telescopes developed by
Karl Schwarzschild in 1905 leads to the unified description both the prefocal
and the postfocal systems. The class of surfaces in the ZEMAX optical program
has been properly extended to ascertain the image quality in exact
Schwarzschild aplanats. A comparison of Schwarzschild aplanats with approximate
Ritchey-Chretien and Gregory-Maksutov aplanatic telescopes reveals a noticeable
advantage of the former at fast focal ratio of the system.Comment: 19 page
Effective potential at finite temperature in a constant hypermagnetic field: Ring diagrams in the Standard Model
We study the symmetry breaking phenomenon in the standard model during the
electroweak phase transition in the presence of a constant hypermagnetic field.
We compute the finite temperature effective potential up to the contribution of
ring diagrams in the weak field, high temperature limit and show that under
these conditions, the phase transition becomes stronger first order.Comment: 15 pages, 8 Postscript figure
Growth and forms of Laplacian aggregates
The shapes and general morphological properties of aggregates grown following the 'T/ rule
(Vsurface ex IEI") [L. Niemeyer, L. Pietronero, and H. J. Wiessmann, Phys. Rev. Lett. 52, 1033
(1984)] have been investigated. Vsurface is the velocity at the interface and E the electric field. The
fractal dimension decreases monotonically from its diffusion-limited aggregation value ('T/ = 1) to
a number indistinguishable from 1 at 'T/ ~ 4. Simultaneously, the multifractal properties become
independent of'T/. An alternative method to generate large clusters, and gain insight into the growth
process, is also presented. Various analytical approximations are discussed.
PACS number(s): 82.20.Wt, 75.10.Jm, 75.10.Lp, 75.30.DsCICyT (Spain) Project No. MAT90-0544 and the U.S. Department of Energy. L.M.S. is thankful to the NSF for financial support under Grant No. DMR 91-17249. F.G. and E.L. acknowledge support from CICyT (Spain).Publicad
- …