168 research outputs found
The various manifestations of collisionless dissipation in wave propagation
The propagation of an electrostatic wave packet inside a collisionless and
initially Maxwellian plasma is always dissipative because of the irreversible
acceleration of the electrons by the wave. Then, in the linear regime, the wave
packet is Landau damped, so that in the reference frame moving at the group
velocity, the wave amplitude decays exponentially with time. In the nonlinear
regime, once phase mixing has occurred and when the electron motion is nearly
adiabatic, the damping rate is strongly reduced compared to the Landau one, so
that the wave amplitude remains nearly constant along the characteristics. Yet,
we show here that the electrons are still globally accelerated by the wave
packet, and, in one dimension, this leads to a non local amplitude dependence
of the group velocity. As a result, a freely propagating wave packet would
shrink, and, therefore, so would its total energy. In more than one dimension,
not only does the magnitude of the group velocity nonlinearly vary, but also
its direction. In the weakly nonlinear regime, when the collisionless damping
rate is still significant compared to its linear value, this leads to an
effective defocussing effect which we quantify, and which we compare to the
self-focussing induced by wave front bowing.Comment: 23 pages, 6 figure
The telltale heart: a non-invasive method to determine the energy expenditure of incubating Great Cormorants Phalacrocorax carbo carbo
We studied the energetics of incubating Great Cormorants Phalacrocorax carbo carbo via heart rate and respirometric measurements performed in captive and free-living animals. We applied a modified heart beat frequency (HR) monitor built for use in human athletics as well as respirometry for measurements in four captive-bred cormorants at Neumuenster Zoo, Germany. The obtained data were used to model the relationship between HR and metabolic rate (MR). The resulting correlations were MR (W kg-0.723) = 4.76 + 0.01HR (bpm) during daytime and MR (W kg-0.723) = 2.33 + 0.03HR (bpm) at night. Furthermore, the heart beat frequencies of 5 free-living, incubating cormorants at the Chausey Islands, France, were measured acoustically using artificial eggs while the activities at the nest were observed via video. HR-MR models established in the captive animals were used to determine the activity-dependent energy expenditure in these free-living cormorants. The Median MR was 5.08 W kg-0.723 at night, 6.06 W kg-0.723 while resting and sleeping during daytime, 6.20 W kg-0.723 during preening, gular flutter and unrest and 6.47 W kg-0.723 during nest building. In resting birds we found a nocturnal reduction in the energy expenditure of 16 %. Our method for measurement of heart beat frequency appears promising as a technique for determination of HR with minimal restraint to the anima
The relative importance of physiological and behavioral adaptation in diving endotherms: a case study with great cormorants
Extensive morphological and physiological adjustments are assumed to underpin the adaptations of diving birds to
high thermoregulatory costs. However, the role of behavioural adaptations has received little consideration. We have assessed
the relative importance of physiological and behavioural adjustments in aquatic endotherms by studying the case of the poorly
insulated great cormorant (Phalacrocorax carbo) in two contrasting thermal environments: Normandy (water temperature
12°C) and Greenland (water temperature 5°C). Major differences were found in the feeding behaviour of birds breeding in the
two regions. Greenland birds showed a 70% reduction in time spent swimming relative to those in Normandy. Reduction in
Greenland was achieved first by reducing time spent on the surface between dives and secondly by returning to land in between
intensive bouts of diving. Total daily energy intake of cormorants was similar in both areas but prey capture rates in Greenland
were 150% higher than those in Normandy. Our study shows that in a cold foraging environment, poorly insulated great
cormorants significantly increase their foraging efficiency. To do this they rely on ecological adaptive patterns (minimization
of time spent swimming in cold water and increased prey capture rates) far more than physiological adaptations (minimizing
instantaneous costs). This finding supports predictions by Grémillet and Wilson (1999) that great cormorants can cope with
a wide range of abiotic parameters despite their morphological handicaps, provided they can adjust their distribution to exploit
dense prey patches
High-energy acceleration phenomena in extreme radiation-plasma interactions
We simulate, using a particle-in-cell code, the chain of acceleration
processes at work during the Compton-based interaction of a dilute electron-ion
plasma with an extreme-intensity, incoherent gamma-ray flux with a photon
density several orders of magnitude above the particle density. The plasma
electrons are initially accelerated in the radiative flux direction through
Compton scattering. In turn, the charge-separation field from the induced
current drives forward the plasma ions to near-relativistic speed and
accelerates backwards the non-scattered electrons to energies easily exceeding
those of the driving photons. The dynamics of those energized electrons is
determined by the interplay of electrostatic acceleration, bulk plasma motion,
inverse Compton scattering and deflections off the mobile magnetic fluctuations
generated by a Weibel-type instability. The latter Fermi-like effect notably
gives rise to a forward-directed suprathermal electron tail. We provide simple
analytical descriptions for most of those phenomena and examine numerically
their sensitivity to the parameters of the problem
A life in the fast lane: energetics and foraging strategies of the great cormorant
Body insulation is critically important for diving marine endotherms. However, cormorants have a wettable plumage, which leads to poor insulation. Despite this, these birds are apparently highly successful predators in most aquatic ecosystems. We studied the theoretical influence of water temperature, dive depth, foraging techniques, and prey availability on the energetic costs of diving, prey search time, daily food intake, and survival in foraging, nonbreeding great cormorants (Phalacrocorax carbo). Our model was based on field measurements and on data taken from the literature. Water temperature and dive depth influenced diving costs drastically, with predicted increases of up to 250% and 258% in males and females, respectively. Changes in water temperature and depth conditions may lead to an increase of daily food intake of 500-800 g in males and 440-780 g in females. However, the model predicts that cormorant foraging parameters are most strongly influenced by prey availability, so that even limited reduction in prey density makes birds unable to balance energy needs and may thus limit their influence on prey stocks. We discuss the ramifications of these results with regard to foraging strategies, dispersal, population dynamics, and intraspecific competition in this avian predator and point out the importance of this model species for our understanding of foraging energetics in diving endotherms
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Breakdown of Electrostatic Predictions for the Nonlinear Dispersion Relation of a Stimulated Raman Scattering-Driven Plasma Wave
The kinetic nonlinear dispersion relation, and frequency shift {delta}{omega}{sub srs}, of a plasma wave driven by stimulated Raman scattering (SRS) are presented. Our theoretical calculations are fully electromagnetic, and use an adiabatic expression for the electron susceptibility which accounts for the change in phase velocity as the wave grows. When k{lambda}{sub D} {approx}> 0.35 (k being the plasma wave number and {lambda}{sub D} the Debye length), {delta}{omega}{sub srs} is significantly larger than could be inferred by assuming that the wave is freely propagating. Our theory is in excellent agreement with 1-D Eulerian Vlasov-Maxwell simulations when 0.3 {le} k{lambda}{sub D} {le} 0.58, and allows discussion of previously proposed mechanisms for Raman saturation. In particular, we find that no 'loss of resonance' of the plasma wave would limit the Raman growth rate, and that saturation through a phase detuning between the plasma wave and the laser drive is mitigated by wave number shifts
Assessing the validity of the accelerometry technique for estimating the energy expenditure of diving double-crested cormorants Phalacrocorax auritus
Over the past few years, acceleration-data loggers have been used to provide calibrated proxies of energy expenditure: The accelerometry technique. Relationships between rate of oxygen consumption and a derivation of acceleration data termed "overall dynamic body acceleration" (ODBA) have now been generated for a range of species, including birds, mammals, and amphibians. In this study, we examine the utility of the accelerometry technique for estimating the energy expended by double-crested cormorants Phalacrocorax auritus to undertake a dive cycle (i.e., a dive and the subsequent pause at the surface before another dive). The results show that ODBA does not calibrate with energy expenditure in diving cormorants, where energy expenditure is calculated from measures of oxygen uptake during surface periods between dives. The possible explanations include reasons why energy expenditure may not relate to ODBA but also reasons why oxygen uptake between dives may not accurately represent energy expenditure during a dive cycle
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