6,526 research outputs found
Buoyancy Instabilities in a Weakly Collisional Intracluster Medium
The intracluster medium of galaxy clusters is a weakly collisional, high-beta
plasma in which the transport of heat and momentum occurs primarily along
magnetic-field lines. Anisotropic heat conduction allows convective
instabilities to be driven by temperature gradients of either sign, the
magnetothermal instability (MTI) in the outskirts of non-isothermal clusters
and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We
employ the Athena MHD code to investigate the nonlinear evolution of these
instabilities, self-consistently including the effects of anisotropic viscosity
(i.e. Braginskii pressure anisotropy), anisotropic conduction, and radiative
cooling. We highlight the importance of the microscale instabilities that
inevitably accompany and regulate the pressure anisotropies generated by the
HBI and MTI. We find that, in all but the innermost regions of cool-core
clusters, anisotropic viscosity significantly impairs the ability of the HBI to
reorient magnetic-field lines orthogonal to the temperature gradient. Thus,
while radio-mode feedback appears necessary in the central few tens of kpc,
conduction may be capable of offsetting radiative losses throughout most of a
cool core over a significant fraction of the Hubble time. Magnetically-aligned
cold filaments are then able to form by local thermal instability. Viscous
dissipation during the formation of a cold filament produces accompanying hot
filaments, which can be searched for in deep Chandra observations of nearby
cool-core clusters. In the case of the MTI, anisotropic viscosity maintains the
coherence of magnetic-field lines over larger distances than in the inviscid
case, providing a natural lower limit for the scale on which the field can
fluctuate freely. In the nonlinear state, the magnetic field exhibits a folded
structure in which the field-line curvature and field strength are
anti-correlated.Comment: 20 pages, 20 figures, submitted to ApJ; Abstract abridge
Shaken and stirred: conduction and turbulence in clusters of galaxies
(abridged) Uninhibited radiative cooling in clusters of galaxies would lead
to excessive mass accretion rates contrary to observations. One of the key
proposals to offset radiative energy losses is thermal conduction from outer,
hotter layers of cool core clusters to their centers. However, conduction is
sensitive to magnetic field topology. In cool-core clusters the heat buoyancy
instability (HBI) leads to B-fields ordered preferentially in the direction
perpendicular to that of gravity, which significantly reduces the level of
conduction below the classical Spitzer-Braginskii value. However, the cluster
cool cores are rarely in perfect hydrostatic equilibrium. Sloshing motions due
to minor mergers, galaxy motions or AGN can significantly perturb the gas and
affect the level of thermal conduction. We perform 3D AMR MHD simulations of
the effect of turbulence on the properties of the anisotropic thermal
conduction in cool core clusters. We show that very weak subsonic motions, well
within observational constraints, can randomize the magnetic field and
significantly boost effective thermal conduction beyond the saturated values
expected in the pure unperturbed HBI case. We find that the turbulent motions
can essentially restore the conductive heat flow to the cool core to level
comparable to the theoretical maximum of 1/3 Spitzer for a highly tangled
field. Runs with radiative cooling show that the cooling catastrophe can be
averted and the cluster core stabilized. Above a critical Froude number, these
same turbulent motions also eliminate the tangential bias in the velocity and
magnetic field that is otherwise induced by the trapped g-modes. Our results
can be tested with future radio polarization measurements, and have
implications for efficient metal dispersal in clusters.Comment: submitted to ApJ, references added, expanded Section
An investigation of potential applications of OP-SAPS: Operational Sampled Analog Processors
The application of OP-SAP's (operational sampled analog processors) in pattern recognition system is summarized. Areas investigated include: (1) human face recognition; (2) a high-speed programmable transversal filter system; (3) discrete word (speech) recognition; and (4) a resolution enhancement system
An empirical analysis of the distribution of the duration of overshoots in a stationary gaussian stochastic process
This analysis utilizes computer simulation and statistical estimation. Realizations of stationary gaussian stochastic processes with selected autocorrelation functions are computer simulated. Analysis of the simulated data revealed that the mean and the variance of a process were functionally dependent upon the autocorrelation parameter and crossing level. Using predicted values for the mean and standard deviation, by the method of moments, the distribution parameters was estimated. Thus, given the autocorrelation parameter, crossing level, mean, and standard deviation of a process, the probability of exceeding the crossing level for a particular length of time was calculated
Real time flight simulation methodology
An example sensitivity study is presented to demonstrate how a digital autopilot designer could make a decision on minimum sampling rate for computer specification. It consists of comparing the simulated step response of an existing analog autopilot and its associated aircraft dynamics to the digital version operating at various sampling frequencies and specifying a sampling frequency that results in an acceptable change in relative stability. In general, the zero order hold introduces phase lag which will increase overshoot and settling time. It should be noted that this solution is for substituting a digital autopilot for a continuous autopilot. A complete redesign could result in results which more closely resemble the continuous results or which conform better to original design goals
Factors Relating to Diapause in the Alfalfa Weevil Parasite \u3cem\u3eBathyplectes curculionis\u3c/em\u3e (Thomson)
Biological and ecological factors regulating diapause in the ichneumonid parasite of the alfalfa weevil, Bathyplectes curculionis (Thomson), were studied. In most experiments, both parasites and weevil larvae were maintained in the environmental conditions under study from the emergence of the adult parasite until the evaluation of results.
The factors either preventing or inducing diapause in B. curculionis were found to be an interaction of environmental factors. A long scotophase with cool temperatures prevented diapause. The percentage of nondiapausing parasites increased as the scotophase was increased to 15 hours. Over 15 hours of scotophase, fewer nondiapausing parasites were produced. The optimum temperature during scotophase was 7.2° C for maximum nondiapausing. The temperature during the nine-hour photophase had to be raised. The greatest percentage of nondiapausing individuals occurred when the temperature was raised to 25.0° C. No effects of relative humidity were observed between 50-80 percent. Relative humidity held at near saturation produced high mortality, while below 20 percent increased the amount of diapause to over 58 percent. A 15-hour scotophase at 7.2° C, a nine-hour photophase at 25.0° C, and relative humidity between 50-80 percent, consistently more than 95 percent of the parasites did not enter diapause. At either a long photophase or failure to alternate temperatures, essentially 100 percent diapaused.
Continuous photophase or continuous scotophase, either with or without alternating the temperature cycle, produced 100 percent diapause.
Part, but not all, of the regulation of diapause was determined by the effects of the photoperiod plus temperature cycle on the adult parasite just prior to oviposition. The percentage of diapause in the offspring increased markedly when the adults were transferred directly from a diapausing regime, when compared to those undergoing a five-day period with ideal conditions prior to oviposition. In both cases oviposition was under the best temperature-light cycle.
No relationship to diapause was observed between offspring from diapausing and nondiapausing parents. No study was made related to parasites from different geographical areas.
The incidence of diapause did not vary based on the instar of the host. The results were conclusive with the first three instars, but not with the fourth, due to low acceptance by the parasite.
The tendency to produce diapausing offspring increased as the female parent grew older. The number of eggs produced in a given period of time decreased markedly after the first 10 days. Very little oviposition occurred after 15 days, although some individuals survived for more than 20 days. Female parasites which were not exposed to weevil larvae lived longer than ovipositioning females.
Temperatures in excess of 30.0° C for longer than 36 hours produced mortality in both diapausing and nondiapausing Bathyplectes larvae within cocoons
Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction
We perform a suite of simulations of cooling cores in clusters of galaxies in
order to investigate the effect of the recently discovered heat flux buoyancy
instability (HBI) on the evolution of cores. Our models follow the
3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture
the effects of anisotropic heat conduction along the lines of magnetic field,
but do not account for the cosmological setting of clusters or the presence of
AGN. Our model clusters can be divided into three groups according to their
final thermodynamical state: catastrophically collapsing cores, isothermal
cores, and an intermediate group whose final state is determined by the initial
configuration of magnetic field. Modeled cores that are reminiscent of real
cluster cores show evolution towards thermal collapse on a time scale which is
prolonged by a factor of ~2-10 compared with the zero-conduction cases. The
principal effect of the HBI is to re-orient field lines to be perpendicular to
the temperature gradient. Once the field has been wrapped up onto spherical
surfaces surrounding the core, the core is insulated from further conductive
heating (with the effective thermal conduction suppressed to less than 1/100th
of the Spitzer value) and proceeds to collapse. We speculate that, in real
clusters, the central AGN and possibly mergers play the role of "stirrers,"
periodically disrupting the azimuthal field structure and allowing thermal
conduction to sporadically heat the core.Comment: 16 pages, 3 tables, 17 figures, accepted to ApJ with minor revisions,
to appear in Volume 704, Oct 20, 2009 issu
A polymorphic reconfigurable emulator for parallel simulation
Microprocessor and arithmetic support chip technology was applied to the design of a reconfigurable emulator for real time flight simulation. The system developed consists of master control system to perform all man machine interactions and to configure the hardware to emulate a given aircraft, and numerous slave compute modules (SCM) which comprise the parallel computational units. It is shown that all parts of the state equations can be worked on simultaneously but that the algebraic equations cannot (unless they are slowly varying). Attempts to obtain algorithms that will allow parellel updates are reported. The word length and step size to be used in the SCM's is determined and the architecture of the hardware and software is described
Anticipating the species jump: surveillance for emerging viral threats.
Zoonotic disease surveillance is typically triggered after animal pathogens have already infected humans. Are there ways to identify high-risk viruses before they emerge in humans? If so, then how and where can identifications be made and by what methods? These were the fundamental questions driving a workshop to examine the future of predictive surveillance for viruses that might jump from animals to infect humans. Virologists, ecologists and computational biologists from academia, federal government and non-governmental organizations discussed opportunities as well as obstacles to the prediction of species jumps using genetic and ecological data from viruses and their hosts, vectors and reservoirs. This workshop marked an important first step towards envisioning both scientific and organizational frameworks for this future capability. Canine parvoviruses as well as seasonal H3N2 and pandemic H1N1 influenza viruses are discussed as exemplars that suggest what to look for in anticipating species jumps. To answer the question of where to look, prospects for discovering emerging viruses among wildlife, bats, rodents, arthropod vectors and occupationally exposed humans are discussed. Finally, opportunities and obstacles are identified and accompanied by suggestions for how to look for species jumps. Taken together, these findings constitute the beginnings of a conceptual framework for achieving a virus surveillance capability that could predict future species jumps
Could AGN Outbursts Transform Cool Core Clusters?
The origin of the bimodality in cluster core entropy is still unknown. At the
same time, recent work has shown that thermal conduction in clusters is likely
a time-variable phenomenon. We consider if time-variable conduction and AGN
outbursts could be responsible for the cool-core (CC), non cool-core (NCC)
dichotomy. We show that strong AGN heating can bring a CC cluster to a NCC
state, which can be stably maintained by conductive heating from the cluster
outskirts. On the other hand, if conduction is shut off by the heat-flux driven
buoyancy instability, then the cluster will cool to the CC state again, where
it is stabilized by low-level AGN heating. Thus, the cluster cycles between CC
and NCC states. In contrast with massive clusters, we predict the CC/NCC
bimodality should vanish in groups, due to the lesser role of conductive
heating there. We find tentative support from the distribution of central
entropy in groups, though firm conclusions require a larger sample carefully
controlled for selection effects.Comment: Slightly revised version, accepted for publication in MNRAS. 9 pages,
3 figure
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