39,631 research outputs found
Gamma Ray Bursts as Cosmological Probes
We discuss the prospects of using Gamma Ray Bursts (GRBs) as high-redshift
distance estimators, and consider their use in the study of two dark energy
models, the Generalized Chaplygin Gas (GCG), a model for the unification of
dark energy and dark matter, and the XCDM model, a model where a generic dark
energy fluid like component is described by the equation of state, . Given that the GRBs range of redshifts is rather high, it turns out that
they are not very sensitive to the dark energy component, being however, fairly
good estimators of the amount of dark matter in the Universe.Comment: 4 pages, 1 figure, talk presented by P.T.S. at the XV Encontro
Nacional De Astronomia e Astrofisica, Lisbon, Portugal, 28-30 July 200
Evaluation of a Pound Net Leader Designed to Reduce Sea Turtle Bycatch
Offshore pound net leaders in the southern portion of Chesapeake Bay in Virginia waters were documented to incidentally take protected loggerhead, Caretta caretta, and Kemp’s ridley, Lepidochelys kempii, sea turtles. Because of these losses, NOAA’s National Marine Fisheries Service (NMFS) in 2004 closed the area to offshore pound net leaders annually from 6 May to 15 July and initiated a study of an experimental leader design that replaced the top two-thirds of the traditional mesh panel leader with vertical ropes (0.95 cm) spaced 61 cm apart. This experimental leader was tested on four pound net sites on the eastern shore of Chesapeake Bay in 2004 and 2005. During the 2 trial periods, 21 loggerhead and Kemp’s ridley sea turtles were found interacting with the control leader and 1 leatherback turtle, Dermochelys coriacea, was found interacting with the experimental leader. Results of a negative binomial regression analysis comparing the two leader designs found the experimental leader significantly reduced sea turtle interactions (p=0.03).
Finfish were sampled from the pound nets in the study to assess finfish catch performance differences between the two leader designs. Although the conclusions from this element of the experiment are not robust, paired t-test and Wilcoxon signed rank test results determined no significant harvest weight difference between the two leaders. Kolmogorov-Smirnov tests did not reveal any substantive size selectivity differences between the two leaders
Electromagnetic field generation in the downstream of electrostatic shocks due to electron trapping
A new magnetic field generation mechanism in electrostatic shocks is found,
which can produce fields with magnetic energy density as high as 0.01 of the
kinetic energy density of the flows on time scales . Electron trapping during the shock formation process
creates a strong temperature anisotropy in the distribution function, giving
rise to the pure Weibel instability. The generated magnetic field is
well-confined to the downstream region of the electrostatic shock. The shock
formation process is not modified and the features of the shock front
responsible for ion acceleration, which are currently probed in laser-plasma
laboratory experiments, are maintained. However, such a strong magnetic field
determines the particle trajectories downstream and has the potential to modify
the signatures of the collisionless shock
Large angle magnetization dynamics measured by time-resolved ferromagnetic resonance
A time-resolved ferromagnetic resonance technique was used to investigate the
magnetization dynamics of a 10 nm thin Permalloy film. The experiment consisted
of a sequence of magnetic field pulses at a repetition rate equal to the
magnetic systems resonance frequency. We compared data obtained by this
technique with conventional pulsed inductive microwave magnetometry. The
results for damping and frequency response obtained by these two different
methods coincide in the limit of a small angle excitation. However, when
applying large amplitude field pulses, the magnetization had a non-linear
response. We speculate that one possible cause of the nonlinearity is related
to self-amplification of incoherence, known as the Suhl instabilities.Comment: 23 pages, 8 figures, submitted to PR
DC magnetic field generation in unmagnetized shear flows
The generation of DC magnetic fields in unmagnetized plasmas with velocity
shear is predicted for non relativistic and relativistic scenarios either due
to thermal effects or due to the onset of the Kelvin-Helmholtz instability
(KHI). A kinetic model describes the growth and the saturation of the DC field.
The predictions of the theory are confirmed by multidimensional
particle-in-cell simulations, demonstrating the formation of long lived
magnetic fields () along the full longitudinal
extent of the shear layer, with transverse width on the electron length scale
(), reaching magnitudes
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