Evolving turbulence and magnetic fields in galaxy clusters

We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic fields in galaxy clusters. Three physically distinct regimes can be identified in the evolution of cluster turbulence and magnetic fields. Firstly, the fluctuation dynamo will produce microgauss-strong, random magnetic fields during cluster formation and major mergers. Turbulent velocity of about 300 km/s can be maintained at scales 100-200 kpc. The magnetic field is intermittent, has a smaller scale of 20-30 kpc and average strength of 2 microgauss. Secondly, when major mergers end, turbulent speed and magnetic field undergo a power-law decay, decreasing in strength but increasing in scale by a factor of about two. Thirdly, smaller-mass subclusters and cluster galaxies produce turbulent wakes, with turbulent speeds and magnetic field strengths similar to those quoted above. The velocity scales are about 200 kpc and 10 kpc respectively, and the magnetic field scale is about 6 times smaller. Although these wakes may fill only a small fraction of the cluster volume, their area covering factor can be close to unity. So one can potentially reconcile observations that indicate the coexistence of turbulence with ordered filamentary gas structures, as in the Perseus cluster. Random Faraday rotation measure is estimated to be typically 100-200 rad/m^2, in agreement with observations. We predict detectable synchrotron polarization from cluster radio halos at wavelengths 3-6 cm, if observed at sufficiently high resolution (abridged).Comment: 20 pages, 9 figures, Replaced to match version accepted by MNRA

Public choices between lifesaving programs : how important are lives saved?

In developing and industrial countries alike, there is concern that health and safety policy may respond to irrational fears - to the"disaster of the month"- rather than address more fundamental problems. In the United States, for example, some policymakers say the public worries about trivial risks while ignoring larger ones and that funding priorities reflect this view. Many public health programs with a low cost per life saved are underfunded, for example, while many environmental regulations with a high cost per life saved are issued each year. Does the existing allocation of resources reflect people's preoccupation with the qualitative aspects of risks, to the exclusion of quantitative factors (lives saved)? Or can observed differences in the cost per life saved of environmental and public health programs be explained by the way the two sets of programs are funded? The authors examine the preferences of U.S. citizens for health and safety programs. They confronted a random sample of 1,000 U.S. adults with choices between environmental health and public health programs, to see which they would choose. The authors then examined what factors (qualitative and quantitative) seem to influence these choices. Respondents were asked about pairs of programs, among them: smoking education or industrial pollution control programs, industrial pollution control or pneumonia vaccine programs, radon eradication or a program to ban smoking in the workplace, and radon eradication or programs to ban pesticides. The survey results, they feel, have implications beyond the United States. They find that, while qualitative aspects of the life-saving programs are statistically significant in explaining people's choices among them, lives saved matter, too. Indeed, for the median respondent in the survey, the rate of substitution between most qualitative risk characteristics and lives saved is inelastic. But for a sizable minority of respondents, choice among programs appears to be insensitive to lives saved. The interesting question for public policy is what role the latter group plays in the regulatory process.Health Monitoring&Evaluation,Public Health Promotion,Insurance&Risk Mitigation,Insurance Law,Water Conservation,Health Monitoring&Evaluation,Insurance&Risk Mitigation,Insurance Law,Water Conservation,Environmental Economics&Policies

Global logistics indicators, supply chain metrics, and bilateral trade patterns

Past research into the determinants of international trade highlighted the importance of the basic spatial gravity model augmented by additional variables representing sources of friction. Studies modeled many sources of friction using various proxies, including indices based on expert judgment in some cases. This paper focuses on logistics friction and draws on a data set recently compiled by the World Bank with specific quantitative metrics of logistics performance interms of time, cost, and variability in time. It finds that the new variables that relate directly to logistics performance have a statistically significant relationship with the level of bilateral trade. It also finds that a single logistics index can capture virtually all of the explanatory power of multiple logistics indicators. The findings should spur public and private agencies that have direct or indirect power over logistics performance to focus attention on reducing sources of friction so as to improve their country's ability to compete in today's global economy. Moreover, since the logistics metrics are directly related to operational performance, countries can use these metrics to target actions to improve logistics and monitor their progress.Common Carriers Industry,Transport and Trade Logistics,Economic Theory&Research,Free Trade,Trade Policy

Family planning methods among women in a vaginal microbicide feasibility study in rural KwaZulu-Natal, South Africa

This study investigated contraceptive use among women in rural KwaZulu-Natal, South Africa. Of 866 sexually active women not intending pregnancy and screened for a microbicide feasibility study, 466 (54%) reported currently using modern contraceptives: injectables (31%), condoms (12%), sterilization (60%) and pills (4%). Multivariable logistic regression analyses revealed statistically significantly higher odds of current contraceptive use among married vs. engaged/unmarried women (aOR 1.64), multiparous vs. nulliparous (aOR 4.45) and women who completed secondary education or above vs. primary or less (aOR 1.64). Significantly lower odds of use were observed among women aged 40+ vs. age 15-19 (aOR 0.38). Age, marital status, education level and parity were associated with different contraceptive method choices. Among 195 women followed longitudinally for 9 months, contraceptive use increased significantly from 56% to 70%, largely due to increased condom use (15% to 28%). Results highlight the importance of integrating family planning and HIV/STI prevention counseling and informing promotion of further contraceptive uptake among women not intending pregnancy

Quasirandom Load Balancing

We propose a simple distributed algorithm for balancing indivisible tokens on graphs. The algorithm is completely deterministic, though it tries to imitate (and enhance) a random algorithm by keeping the accumulated rounding errors as small as possible. Our new algorithm surprisingly closely approximates the idealized process (where the tokens are divisible) on important network topologies. On d-dimensional torus graphs with n nodes it deviates from the idealized process only by an additive constant. In contrast to that, the randomized rounding approach of Friedrich and Sauerwald (2009) can deviate up to Omega(polylog(n)) and the deterministic algorithm of Rabani, Sinclair and Wanka (1998) has a deviation of Omega(n^{1/d}). This makes our quasirandom algorithm the first known algorithm for this setting which is optimal both in time and achieved smoothness. We further show that also on the hypercube our algorithm has a smaller deviation from the idealized process than the previous algorithms.Comment: 25 page

Recent applications of the transonic wing analysis computer code, TWING

An evaluation of the transonic-wing-analysis computer code TWING is given. TWING utilizes a fully implicit approximate factorization iteration scheme to solve the full potential equation in conservative form. A numerical elliptic-solver grid-generation scheme is used to generate the required finite-difference mesh. Several wing configurations were analyzed, and the limits of applicability of this code was evaluated. Comparisons of computed results were made with available experimental data. Results indicate that the code is robust, accurate (when significant viscous effects are not present), and efficient. TWING generally produces solutions an order of magnitude faster than other conservative full potential codes using successive-line overrelaxation. The present method is applicable to a wide range of isolated wing configurations including high-aspect-ratio transport wings and low-aspect-ratio, high-sweep, fighter configurations

Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters

Using three-dimensional hydrodynamic simulations, we investigate heating and turbulence driving in an intracluster medium (ICM) by orbital motions of galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and isotropic orbits through an ICM typical of rich clusters. An introduction of the galaxies immediately produces gravitational wakes, providing perturbations that can potentially grow via resonant interaction with the background gas. When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11} Msun, the perturbations are in the linear regime and the resonant excitation of gravity waves is efficient to generate kinetic energy in the ICM, resulting in the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 > 100, on the other hand, nonlinear fluctuations of the background ICM destroy galaxy wakes and thus render resonant excitation weak or absent. In this case, the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s. The angle-averaged velocity power spectra of turbulence driven in our models have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of resonant excitation, none of the cooling models considered are able to halt cooling catastrophe, suggesting that the galaxy motions alone are unlikely to solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ

A Unified treatment of small and large- scale dynamos in helical turbulence

Helical turbulence is thought to provide the key to the generation of large-scale magnetic fields. Turbulence also generically leads to rapidly growing small-scale magnetic fields correlated on the turbulence scales. These two processes are usually studied separately. We give here a unified treatment of both processes, in the case of random fields, incorporating also a simple model non-linear drift. In the process we uncover an interesting plausible saturated state of the small-scale dynamo and a novel analogy between quantum mechanical (QM) tunneling and the generation of large scale fields. The steady state problem of the combined small/large scale dynamo, is mapped to a zero-energy, QM potential problem; but a potential which, for non-zero mean helicity, allows tunneling of bound states. A field generated by the small-scale dynamo, can 'tunnel' to produce large-scale correlations, which in steady state, correspond to a force-free 'mean' field.Comment: 4 pages, 1 figure, Physical Review Letters, in pres