3,382 research outputs found
Turbulence and Mixing in the Intracluster Medium
The intracluster medium (ICM) is stably stratified in the hydrodynamic sense
with the entropy increasing outwards. However, thermal conduction along
magnetic field lines fundamentally changes the stability of the ICM, leading to
the "heat-flux buoyancy instability" when and the "magnetothermal
instability" when . The ICM is thus buoyantly unstable regardless of
the signs of and . On the other hand, these
temperature-gradient-driven instabilities saturate by reorienting the magnetic
field (perpendicular to when and parallel to when ), without generating sustained convection. We show that
after an anisotropically conducting plasma reaches this nonlinearly stable
magnetic configuration, it experiences a buoyant restoring force that resists
further distortions of the magnetic field. This restoring force is analogous to
the buoyant restoring force experienced by a stably stratified adiabatic
plasma. We argue that in order for a driving mechanism (e.g, galaxy motions or
cosmic-ray buoyancy) to overcome this restoring force and generate turbulence
in the ICM, the strength of the driving must exceed a threshold, corresponding
to turbulent velocities . For weaker driving, the ICM
remains in its nonlinearly stable magnetic configuration, and turbulent mixing
is effectively absent. We discuss the implications of these findings for the
turbulent diffusion of metals and heat in the ICM.Comment: 8 pages, 2 figs., submitted to the conference proceedings of "The
Monster's Fiery Breath;" a follow up of arXiv:0901.4786 focusing on the
general mixing properties of the IC
Cosmological MHD simulations of cluster formation with anisotropic thermal conduction
(abridged) The ICM has been suggested to be buoyantly unstable in the
presence of magnetic field and anisotropic thermal conduction. We perform first
cosmological simulations of galaxy cluster formation that simultaneously
include magnetic fields, radiative cooling and anisotropic thermal conduction.
In isolated and idealized cluster models, the magnetothermal instability (MTI)
tends to reorient the magnetic fields radially. Using cosmological simulations
of the Santa Barbara cluster we detect radial bias in the velocity and magnetic
fields. Such radial bias is consistent with either the inhomogeneous radial gas
flows due to substructures or residual MTI-driven field rearangements that are
expected even in the presence of turbulence. Although disentangling the two
scenarios is challenging, we do not detect excess bias in the runs that include
anisotropic thermal conduction. The anisotropy effect is potentially detectable
via radio polarization measurements with LOFAR and SKA and future X-ray
spectroscopic studies with the IXO. We demonstrate that radiative cooling
boosts the amplification of the magnetic field by about two orders of magnitude
beyond what is expected in the non-radiative cases. At z=0 the field is
amplified by a factor of about 10^6 compared to the uniform magnetic field
evolved due to the universal expansion alone. Interestingly, the runs that
include both radiative cooling and anisotropic thermal conduction exhibit
stronger magnetic field amplification than purely radiative runs at the
off-center locations. In these runs, shallow temperature gradients away from
the cluster center make the ICM neutrally buoyant. The ICM is more easily mixed
in these regions and the winding up of the frozen-in magnetic field is more
efficient resulting in stronger magnetic field amplification.Comment: submitted to ApJ, higher resolution figures available at:
http://www.astro.lsa.umich.edu/~mateuszr
The challenges and opportunities for professional societies in higher education in Australasia: a PEST analysis
Professional societies, established to support academic and professional staff in higher education, need to be vigilant of regional and international trends that affect their core business. In this paper, we provide an analysis of political, economic, social and technological factors that are impacting upon the Australasian higher education environment through considering the case of the Australasian Society for Computers in Learning in Tertiary Education (ascilite). Drawing on two ascilite membership surveys along with the relevant literature we identify significant challenges for professional societies and offer some strategic insights for similar regional societies and their executive teams.published_or_final_versio
Inherent noise can facilitate coherence in collective swarm motion
Among the most striking aspects of the movement of many animal groups are their sudden coherent changes in direction. Recent observations of locusts and starlings have shown that this directional switching is an intrinsic property of their motion. Similar direction switches are seen in self-propelled particle and other models of group motion. Comprehending the factors that determine such switches is key to understanding the movement of these groups. Here, we adopt a coarse-grained approach to the study of directional switching in a self-propelled particle model assuming an underlying one-dimensional Fokker–Planck equation for the mean velocity of the particles. We continue with this assumption in analyzing experimental data on locusts and use a similar systematic Fokker–Planck equation coefficient estimation approach to extract the relevant information for the assumed Fokker–Planck equation underlying that experimental data. In the experiment itself the motion of groups of 5 to 100 locust nymphs was investigated in a homogeneous laboratory environment, helping us to establish the intrinsic dynamics of locust marching bands. We determine the mean time between direction switches as a function of group density for the experimental data and the self-propelled particle model. This systematic approach allows us to identify key differences between the experimental data and the model, revealing that individual locusts appear to increase the randomness of their movements in response to a loss of alignment by the group. We give a quantitative description of how locusts use noise to maintain swarm alignment. We discuss further how properties of individual animal behavior, inferred by using the Fokker–Planck equation coefficient estimation approach, can be implemented in the self-propelled particle model to replicate qualitatively the group level dynamics seen in the experimental data
The relation between gas density and velocity power spectra in galaxy clusters: qualitative treatment and cosmological simulations
We address the problem of evaluating the power spectrum of the velocity field
of the ICM using only information on the plasma density fluctuations, which can
be measured today by Chandra and XMM-Newton observatories. We argue that for
relaxed clusters there is a linear relation between the rms density and
velocity fluctuations across a range of scales, from the largest ones, where
motions are dominated by buoyancy, down to small, turbulent scales:
, where
is the spectral amplitude of the density perturbations at wave number ,
is the mean square component of the velocity field,
is the sound speed, and is a dimensionless constant of order unity.
Using cosmological simulations of relaxed galaxy clusters, we calibrate this
relation and find . We argue that this value is set at
large scales by buoyancy physics, while at small scales the density and
velocity power spectra are proportional because the former are a passive scalar
advected by the latter. This opens an interesting possibility to use gas
density power spectra as a proxy for the velocity power spectra in relaxed
clusters, across a wide range of scales.Comment: 6 pages, 3 figures, submitted to ApJ Letter
Continuum limit of self-driven particles with orientation interaction
We consider the discrete Couzin-Vicsek algorithm (CVA), which describes the
interactions of individuals among animal societies such as fish schools. In
this article, we propose a kinetic (mean-field) version of the CVA model and
provide its formal macroscopic limit. The final macroscopic model involves a
conservation equation for the density of the individuals and a non conservative
equation for the director of the mean velocity and is proved to be hyperbolic.
The derivation is based on the introduction of a non-conventional concept of a
collisional invariant of a collision operator
Mind the message: Referral letter quality at a South African medical outpatient department
Background. Some patients need referral within the health system to achieve optimal care, and referral letters are an important part of this process. Healthcare practitioners often complain that referral letters lack information, are inaccurate, or direct patients to the wrong place. Poor communication affects patient experience and outcomes, has budgetary and service planning implications, and impacts on staff relationships and morale.Objectives. To investigate the quality and appropriateness of referral letters received by the medical outpatient department of a regional hospital in Eastern Cape Province, South Africa.Methods. Letters were collected by departmental staff as patients arrived at the clinic. Each letter was independently analysed by two healthcare workers for content and appropriateness, using defined criteria.Results. Of 100 letters collected between March and May 2017, 85 were suitable for analysis. Patient and clinician identifiers were present in >85%, but key clinical information was missing in 87%, and 48% did not state a reason for referral. It was possible to make triage decisions based on the letter in only 35% of cases. Nineteen percent of referrals were classified as inappropriate.Conclusions. Most letters lacked important clinical information, probably because of a combination of factors: gaps in clinical knowledge of referring clinicians who service a population with a high burden of disease and complex pathology; under-resourced peripheral healthcare clinics; inadequate staff-to-patient ratios; and time constraints. A suggested focus for improvement is education at undergraduate and postgraduate level, which should emphasise preparation for community service, specifically highlighting techniques for preparing good-quality referrals
Consensus of self-driven agents with avoidance of collisions
In recent years, many efforts have been addressed on collision avoidance of
collectively moving agents. In this paper, we propose a modified version of the
Vicsek model with adaptive speed, which can guarantee the absence of
collisions. However, this strategy leads to an aggregated state with slowly
moving agents. We therefore further introduce a certain repulsion, which
results in both faster consensus and longer safe distance among agents, and
thus provides a powerful mechanism for collective motions in biological and
technological multi-agent systems.Comment: 8 figures, and 7 page
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