7,289 research outputs found
Drones and Dirty Hands
The period known as the “War on Terror” has prompted a revival of interest in the idea of moral dilemmas and the problem of “dirty hands” in public life. Some contend that a policy of targeted killing of terrorist actors is (under specified but not uncommon circumstances) an instance of a dirty-handed moral dilemma – morally required yet morally forbidden, the least evil choice available in the circumstances, but one that nevertheless leaves an indelible moral stain on the character of the person who makes the choice. In this chapter we argue that, while dirty hands situations do exist as a persistent problem of political life, it is generally a mistake to classify policies of target killing (such as the current US policy) as examples of dirty hands. Instead, we maintain, such policies, if justified at all, must ordinarily be justified under the more exacting standards of just war theory and its provisions for justified killing – in particular the requirement that (with limited and defined exceptions) non-combatants be immune from intentional violence. Understanding this distinction both clarifies the significance of dirty hands as a moral phenomenon and also forestalls a set of predictable and all-too-easy appropriations of the concept to domains it was never intended to address
Land use, state and local users
The state and local and land use sessions involved a cross section of the expanding community of government managers who use remotely sensed information to make programmatic decisions. Problems that can be inferred from the presentations and resulting discussion are addressed. Recommendations are also given to facilitate utilization of remote sensing technology
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
Buoyant Bubbles in Intracluster Gas: Effects of Magnetic Fields and Anisotropic Viscosity
Recent observations by Chandra and XMM-Newton indicate there are complex
structures at the cores of galaxy clusters, such as cavities and filaments. One
plausible model for the formation of such structures is the interaction of
radio jets with the intracluster medium (ICM). To investigate this idea, we use
three-dimensional magnetohydrodynamic simulations including anisotropic
(Braginskii) viscosity to study the effect of magnetic fields on the evolution
and morphology of buoyant bubbles in the ICM. We investigate a range of
different initial magnetic field geometries and strengths, and study the
resulting x-ray surface brightness distribution for comparison to observed
clusters. Magnetic tension forces and viscous transport along field lines tend
to suppress instabilities parallel, but not perpendicular, to field lines.
Thus, the evolution of the bubble depends strongly on the initial field
geometry. We find toroidal field loops initially confined to the interior of
the bubble are best able reproduce the observed cavity structures.Comment: 1 table, 10 figures, 29 pages. Accepted at ApJ. Figures with better
quality are available at ApJ onlin
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
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
Letter from Parrish to Roach, January 11, 1974
A letter from James M. Parrish, Dean of the College of Business administration to W.J. Roach, Chairman, Department of Language and Literature. The letter congratulates Mr. Roach and the Halyard staff on the first edition of the campus newspaper
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