bluuebunny/crossref_metadata_2025 huggingface dataset:
BETWEEN WORLD HISTORY AND STATE FORMATION: NEW PERSPECTIVES ON AFRICA'S CITIES
Authors: FOURCHARD LAURENT
Published: August 2011

The dramatic urban change taking place on the African continent has led to a renewed and controversial interest in Africa's cities within several academic and expert circles. Attempts to align a growing but fragmented body of research on Africa's urban past with more general trends in urban studies have been few but have nevertheless opened up new analytical possibilities. This article argues that to move beyond the traps of localism and unhelpful categorizations that have dominated aspects of urban history and the urban studies literature of the continent, historians should explore African urban dynamics in relation to world history and the history of the state in order to contribute to larger debates between social scientists and urban theorists. By considering how global socio-historical processes articulate with the everyday lives of urban dwellers and how city-state relationships are structured by ambivalence, this article will illustrate how historians can participate in those debates in ways that demonstrate that history matters, but not in a linear way. These illustrations will also suggest why it is necessary for historians to contest interpretations of Africa's cities that construe them as ontologically different from other cities of the world.
KWAME NKRUMAH, DISABILITY, AND REHABILITATION IN GHANA, 1957–66
Authors: GRISCHOW JEFF D.
Published: August 2011

This article examines a rehabilitation program for disabled Ghanaians developed by Kwame Nkrumah's government between 1961 and 1966. Arising at a time when Nkrumah was moving away from welfarism in favor of a ‘big push’ for industrialization, rehabilitation sought to integrate disabled citizens into the national economy as productive workers. Nkrumah's program was preceded by a colonial rehabilitation project during the 1940s for disabled African soldiers. The colonial initiative drew heavily on the British model of social orthopaedics, which equated citizenship with work. This philosophy resonated with Nkrumah's vision of national development based on full employment. Although its economic focus had troubling implications for citizenship and welfare, Nkrumah's rehabilitation program was unique among newly independent African states, and it arguably produced a positive legacy.
The Politics of Everyday Life in Twenty-First Century Myanmar
Authors: Thawnghmung Ardeth Maung
Published: August 2011

Myanmar has been conventionally regarded as one of the most repressive countries in the world. As a result, many scholars, journalists, and human rights organizations understandably focus their attention on the draconian policies of the Myanmarese military regime. When the country makes headlines, the story of events taking place is typically framed in terms of state oppression and direct popular opposition. This leads to a restrictive view of the “political” dimensions of life in Myanmar, an approach to the topic that deals with only a small number of admittedly important subjects: authoritarian governance, organized efforts to bring about systemic change, and the fate of Aung San Suu Kyi, the Nobel Peace Prize Laureate whose latest release from house arrest put Myanmarese politics back in the headlines in November 2010. What is left out of the picture—or given only glancing attention—are a host of social, economic, and cultural issues that also have political dimensions and implications, namely the efforts by Myanmarese citizens to carve out space for independent, meaningful action on a personal level. These actions, which have political aspects but stop short of being outright forms of dissent, will be my focus in this essay.
Ending War
Authors: Rodin David
Published: August 2011

In “The Ethics of America's Afghan War,” Richard W. Miller argues that reflecting on whether and how to end the war in Afghanistan exposes serious deficiencies in just war theory. I agree, though for different reasons than those canvassed by Professor Miller. Miller argues that by focusing on the traditional categories of just cause, proportionality, and necessity (or last resort), just war theory obscures the importance of broader geostrategic considerations that he believes are the most plausible—though ultimately for Miller insufficient—rationale for continuing with the strategy of large-scale counterinsurgency in Afghanistan. I doubt that geostrategic considerations can play the role in moral assessment that Miller believes they do. But the phenomena he is pointing to do illuminate important defects in traditional just war theory.
Libya and the Responsibility to Protect: The Exception and the Norm
Authors: Bellamy Alex J.
Published: August 2011

The Responsibility to Protect (RtoP) played an important role in shaping the world's response to actual and threatened atrocities in Libya. Not least, the adoption of Resolution 1973 by the UN Security Council on May 17, 2011, approving a no-fly zone over Libya and calling for “all necessary measures” to protect civilians, reflected a change in the Council's attitude toward the use of force for human protection purposes; and the role played by the UN's new Joint Office on the Prevention of Genocide and the Responsibility to Protect points toward the potential for this new capacity to identify threats of mass atrocities and to focus the UN's attention on preventing them. Given the reluctance of both the Security Council and the wider UN membership even to discuss RtoP in the years immediately following the 2005 World Summit—the High-level Plenary Meeting of the 60th Session of the General Assembly that gave birth to RtoP—these two facts suggest that significant progress has been made thanks to the astute stewardship of UN Secretary-General Ban Ki-moon, who is personally committed to the principle. Where it was once a term of art employed by a handful of like-minded countries, activists, and scholars, but regarded with suspicion by much of the rest of the world, RtoP has become a commonly accepted frame of reference for preventing and responding to mass atrocities.
RtoP Alive and Well after Libya
Authors: Weiss Thomas G.
Published: August 2011

With the exception of Raphael Lemkin's efforts on behalf of the 1948 Genocide Convention, no idea has moved faster in the international normative arena than “the responsibility to protect” (RtoP), which was formulated in 2001 by the International Commission on Intervention and State Sovereignty (ICISS). Friends and foes have pointed to the commission's conceptual contribution to reframing sovereignty as contingent rather than absolute, and to establishing a framework for forestalling or stopping mass atrocities via a three-pronged responsibility—to prevent, to react, and to rebuild. But until the international military action against Libya in March 2011, the sharp end of the RtoP stick—the use of military force—had been replaced by evasiveness and skittishness from diplomats, scholars, and policy analysts.
The Ethics of Humanitarian Intervention in Libya
Authors: Pattison James
Published: August 2011

Wars and interventions bring to the fore certain ethical issues. For instance, NATO's intervention in Kosovo in 1999 raised questions about the moral import of UN Security Council authorization (given that the Council did not authorize the action), and the means employed by interveners (given NATO's use of cluster bombs and its targeting of dual-use facilities). In what follows, I consider the moral permissibility of the NATO-led intervention in Libya and suggest that this particular intervention highlights three issues for the ethics of humanitarian intervention in general. The first issue is whether standard accounts of the ethics of humanitarian intervention, which draw heavily on just war theory, can capture the prospect of mission creep. The second issue is whether epistemic difficulties in assessing the intervention's likely long-term success mean that we should reject consequentialist approaches to humanitarian intervention. The third issue concerns selectivity. I outline an often overlooked way that selectivity can be problematic for humanitarian intervention.
Comment on “Populism, Paranoia, and the Politics of Free Silver”
Authors: Bensel Richard
Published: August 2011

Let me begin with the bottom line: Samuel DeCanio has addressed a very important topic, skillfully crafted an argument, and marshaled an impressive body of evidence behind his thesis. The result is a significant addition to the literature on American political development on several different levels. And that is so despite the fact that I believe his interpretation extends beyond the evidence in some respects. This comment addresses both these extensions and, as a collateral objective, suggests that the way in which we reconstruct political situations and causal relationships is inevitably an art, not a science.
A Survey in Mathematics for Industry: Two-timing and matched asymptotic expansions for singular perturbation problems
Authors: O'MALLEY R. E. , KIRKINIS E.
Published: August 2011

Following the derivation of amplitude equations through a new two-time-scale method [O'Malley, R. E., Jr. & Kirkinis, E (2010) A combined renormalization group-multiple scale method for singularly perturbed problems.Stud. Appl. Math.124, 383–410], we show that a multi-scale method may often be preferable for solving singularly perturbed problems than the method of matched asymptotic expansions. We illustrate this approach with 10 singularly perturbed ordinary and partial differential equations.
S'UNIR À L'INTELLECT, VOIR DIEU. AVERROÈS ET LA DOCTRINE DE LA JONCTION AU CŒUR DU THOMISME
Authors: Brenet Jean-Baptiste
Published: August 2011

The article examines the relation that Aquinas' theory of the beatific vision maintains with Averroes' noetics as presented in hisGreat Commentary on the De anima. Starting with hisCommentary on the Sentences, in which the young Thomas Aquinas offers an explicit transposition of the philosophical intellection of separate substances into the Christian theological order, through to his later works where no mention of it is found, we will endeavour to present the exact nature of these borrowings and to evaluate their accuracy by questioning the conceptual coherence of Aquinas' gesture: could Aquinas base his conception of a vision of God by essence on a noetic construction which was originally part of a system judged both erroneous and contrary to faith? Can one concede theologically, concerning the relation between divine essence and intellect, what one refuses philosophically, concerning the relation between the separate intellect and the body? Although Aquinas and his followers, in the incipient quarrel, assert it to be so, we will indicate how the original paradoxical borrowing maintains something conceptually problematic at the heart of Aquinas' thinking.
Philologia ancilla philosophiae
Authors: Montada Josep Puig
Published: August 2011

If anyone still raises doubts about the function of philology and the need for critical editions, this book should definitely put those doubts to rest. It shows that no history of philosophy is possible without intensive philological work with the original texts.
Corps et continuité. Remarques sur la “nouvelle” physique d'Averroès
Authors: Cerami Cristina
Published: August 2011

Dans l'horizon de l’étude de la philosophie naturelle d'Averroès, le nouveau travail de Ruth Glasner (RG) intituléAverroes’ Physics: a Turning Point in Medieval Natural Philosophyoccupera assurément une place de premier plan. Dans cet ouvrage, RG propose une étude analytique des trois commentaires d'Averroès à laPhysiqued'Aristote – l’Abrégé, leCommentaire Moyenet leGrand Commentaire. La force incontestable de son travail réside tout d'abord dans son approche double du texte d'Averroès, à la fois philologique et théorique. Tout au long de son analyse, ces deux aspects restent intimement liés et la mobilisation des instruments philologiques n'est jamais une fin en soi. Cette analyse a en effet pour but ultime de reconstruire le système du monde tel qu'Averroès le concevait. L'enjeu fondamental de cette étude est alors de supplanter la vision d'un Averroès interprète servile de la physique aristotélicienne et de présenter sa conception de la nature comme un système cohérent, organique et original.
Helioseismic Observations of Solar Convection Zone Dynamics
Authors: Hill Frank , Howe Rachel , Komm Rudi , Hernández Irene González , Kholikov Shukur , Leibacher John
Published: August 2011

The large-scale dynamics of the solar convection zone have been inferred using both global and local helioseismology applied to data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) on board SOHO. The global analysis has revealed temporal variations of the “torsional oscillation” zonal flow as a function of depth, which may be related to the properties of the solar cycle. The horizontal flow field as a function of heliographic position and depth can be derived from ring diagrams, and shows near-surface meridional flows that change over the activity cycle. Time-distance techniques can be used to infer the deep meridional flow, which is important for flux-transport dynamo models. Temporal variations of the vorticity can be used to investigate the production of flare activity. This paper summarizes the state of our knowledge in these areas.
Large-scale magnetic fields of low-mass dwarfs: the many faces of dynamo
Authors: Donati J.-F.
Published: August 2011

Magnetic field are ubiquitous to low-mass stars and can potentially impact their evolution and their internal structure; yet the physical processes (called dynamo) that succeed at generating them in the stellar convective zones of cool dwarfs are still enigmatic. Although theoretical modelling and numerical simulations of stellar dynamo action showed breathtaking progress in the last decade, they are not yet in the state of accurately predicting the various magnetic topologies that different low-mass stars can generate.

Thanks to the advent of new-generation instruments, spectropolarimetric observations can now reveal the large-scale magnetic topologies of cool dwarfs, from the brown dwarf threshold (spectral type M8) up to the limit beyond which outer convective zones get vanishingly small (spectral type F5). In particular, they can reconstruct through tomographic methods the poloidal and toroidal components of the large-scale field, hence offering a fresh option for guiding dynamo theories to a more mature state.

We review here the latest observational advances, showing in particular that magnetic topologies of low-mass dwarfs can drastically vary with mass and rotation rate, and discuss their implications for our understanding of dynamo processes.
Differential Rotation and Magnetism in Simulations of Fully Convective Stars
Authors: Browning Matthew K.
Published: August 2011

Stars of sufficiently low mass are convective throughout their interiors, and so do not possess an internal boundary layer akin to the solar tachocline. Because that interface figures so prominently in many theories of the solar magnetic dynamo, a widespread expectation had been that fully convective stars would exhibit surface magnetic behavior very different from that realized in more massive stars. Here I describe how recent observations and theoretical models of dynamo action in low-mass stars are partly confirming, and partly confounding, this basic expectation. In particular, I present the results of 3–D MHD simulations of dynamo action by convection in rotating spherical shells that approximate the interiors of 0.3 solar-mass stars at a range of rotation rates. The simulated stars can establish latitudinal differential rotation at their surfaces which is solar-like at “rapid” rotation rates (defined within) and anti-solar at slower rotation rates; the differential rotation is greatly reduced by feedback from strong dynamo-generated magnetic fields in some parameter regimes. I argue that this “flip” in the sense of differential rotation may be observable in the near future. I also briefly describe how the strength and morphology of the magnetic fields varies with the rotation rate of the simulated star, and show that the maximum magnetic energies attained are compatible with simple scaling arguments.
Dipolar and Quadrupolar Magnetic Field Evolution over Solar Cycles 21, 22, and 23
Authors: DeRosa M. L. , Brun A. S. , Hoeksema J. T.
Published: August 2011

Time series of photospheric magnetic field maps from two observatories, along with data from an evolving surface-flux transport model, are decomposed into their constituent spherical harmonic modes. The evolution of these spherical harmonic spectra reflect the modulation of bipole emergence rates through the solar activity cycle, and the subsequent dispersal, shear, and advection of magnetic flux patterns across the solar photosphere. In this article, we discuss the evolution of the dipolar and quadrupolar modes throughout the past three solar cycles (Cycles 21–23), as well as their relation to the reversal of the polar dipole during each solar maximum, and by extension to aspects of the operation of the global solar dynamo.
Modeling mass independent of anisotropy
Authors: Wolf Joe
Published: August 2011

By manipulating the spherical Jeans equation, Wolf et al. (2010) show that the mass enclosed within the 3D deprojected half-light radius r1/2 can be determined with only mild assumptions about the spatial variation of the stellar velocity dispersion anisotropy as long as the projected velocity dispersion profile is fairly flat near the half-light radius, as is typically observed. They find M1/2 = 3 G−1 〈σ2los〉 r1/2 ≃ 4 G−1 〈σ2los〉 Re, where 〈σ2los〉 is the luminosity-weighted square of the line-of-sight velocity dispersion and Re is the 2D projected half-light radius. This finding can be used to show that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent with having formed within a halo of mass approximately 3 × 109 M⊙, assuming a ΛCDM cosmology. In addition, the dynamical I-band mass-to-light ratio ϒI1/2 vs. M1/2 relation for dispersion-supported galaxies follows a U-shape, with a broad minimum near ϒI1/2 ≃ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to ϒI1/2 ≃ 3,200 for ultra-faint dSphs, and a more shallow rise to ϒI1/2 ≃ 800 for galaxy cluster spheroids.
Galaxy Dynamics: Secular Evolution and Accretion
Authors: Combes Francoise
Published: August 2011

Recent results are reviewed on galaxy dynamics, bar evolution, destruction and re-formation, cold gas accretion, gas radial flows and AGN fueling, minor mergers. Some problems of galaxy evolution are discussed in particular, exchange of angular momentum, radial migration through resonant scattering, and consequences on abundance gradients, the frequency of bulgeless galaxies, and the relative role of secular evolution and hierarchical formation.
Magnetic Fields in Galaxies
Authors: Zweibel Ellen G.
Published: August 2011

The origin and evolution of magnetic fields in the Universe is a cosmological problem. Although exotic mechanisms for magneotgenesis cannot be ruled out, galactic magnetic fields could have been seeded by magnetic fields from stars and accretion disks, and must be continuously regenerated due to the ongoing replacement of the interstellar medium. Unlike stellar dynamos, galactic dynamos operate in a multicomponent gas at low collisionality and high magnetic Prandtl number. Their background turbulence is highly compressible, the plasma β ~ 1, and there has been time for only a few large exponentiation times at large scale over cosmic time. Points of similarity include the importance of magnetic buoyancy, the large range of turbulent scales and tiny microscopic scales, and the coupling between the magnetic field and certain properties of the flow. Understanding the origin and maintenance of the large scale galactic magnetic field is the most challenging aspect of the problem.
The dual nature of the Milky Way stellar halo
Authors: Curir Anna , Murante Giuseppe , Poglio Eva , Villalobos Álvaro
Published: August 2011

The theory of the Milky Way formation, in the framework of the ΛCDM model, predicts galactic stellar halos to be built from multiple accretion events starting from the first structure to collapse in the Universe.

Evidences in the past few decades have indicated that the Galactic halo consists of two overlapping structural components, an inner and an outer halo. We provide a set of numerical N-body simulations aimed to study the formation of the outer Milky Way (MW) stellar halo through accretion events between a (bulgeless) MW-like system and a satellite galaxy. After these minor mergers take place, in several orbital configurations, we analyze the signal left by satellite stars in the rotation velocity distribution. The aim is to explore the orbital conditions of the mergers where a signal of retrograde rotation in the outer part of the halo can be obtained, in order to give a possible explanation of the observed rotational properties of the MW stellar halo.

Our results show that the dynamical friction has a fundamental role in assembling the final velocity distributions originated by different orbits and that retrograde satellites moving on low inclination orbits deposit more stars in the outer halo regions and therefore can produce the counter-rotating behavior observed in the outer MW halo.
The interstellar magnetic field near the Galactic center
Authors: Ferrière Katia
Published: August 2011

We review the observational knowledge that has built up over the past 25 years on the interstellar magnetic field within ~ 150 pc of the Galactic center. We also provide a critical discussion of the main observational findings and comment on their possible theoretical interpretations. To conclude, we propose a coherent view of the interstellar magnetic field near the Galactic center, which accounts at best for the vast body of observations.
Magnetic Fields in Molecular Clouds
Authors: Padoan Paolo , Lunttila Tuomas , Juvela Mika , Nordlund Åke , Collins David , Kritsuk Alexei , Normal Michael , Ustyugov Sergey
Published: August 2011

Supersonic magneto-hydrodynamic (MHD) turbulence in molecular clouds (MCs) plays an important role in the process of star formation. The effect of the turbulence on the cloud fragmentation process depends on the magnetic field strength. In this work we discuss the idea that the turbulence is super-Alfvénic, at least with respect to the cloud mean magnetic field. We argue that MCs are likely to be born super-Alfvénic. We then support this scenario based on a recent simulation of the large-scale warm interstellar medium turbulence. Using small-scale isothermal MHD turbulence simulation, we also show that MCs may remain super-Alfvénic even with respect to their rms magnetic field strength, amplified by the turbulence. Finally, we briefly discuss the comparison with the observations, suggesting that super-Alfvénic turbulence successfully reproduces the Zeeman measurements of the magnetic field strength in dense MC clouds.
Time-dependent Turbulence in Stars
Authors: Arnett W. David , Meakin Casey
Published: August 2011

Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning by Meakin & Arnett (2007b) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a global instability in the convective region, which has been suppressed in simulations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to random fluctuations in time. An extension of the Lorenz model, which includes Kolmogorov damping and nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those seen in the 3D simulations. A simple model of a convective layer (composed of multiple Lorenz cells) gives luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants, see Schwarzschild (1975). Details and additional discussion may be found in Arnett & Meakin (2011).

Apparent inconsistencies between Arnett, Meakin, & Young (2009) and Nordlund, Stein, & Asplund (2009) on the nature of convective driving have been resolved, and are discussed.
Dynamo coefficients from the Tayler instability
Authors: Arlt Rainer , Rüdiger Günther
Published: August 2011

Current-driven instabilities in stellar radiation zones, to which we refer as Tayler instabilities, can lead to complex nonlinear evolutions. It is of fundamental interest whether magnetically driven turbulence can lead to dynamo action in these radiative zones. We investigate initial-value simulations in a 3D spherical shell including differential rotation. The Tayler instability is connected with a very weak kinetic helicity, stronger current helicity, and a positive αφφ in the northern hemisphere. The amplitudes are small compared to the effect of the tangential cylinder producing an eddy with negative kinetic helicity and negative αφφ in the northern hemisphere. The αφφ from the Tayler instability reaches about 1% of the rms velocity.
3D Magnetic Reconnection
Authors: Parnell Clare E. , Maclean Rhona C. , Haynes Andrew L. , Galsgaard Klaus
Published: August 2011

Magnetic reconnection is an important process that is prevalent in a wide range of astrophysical bodies. It is the mechanism that permits magnetic fields to relax to a lower energy state through the global restructuring of the magnetic field and is thus associated with a range of dynamic phenomena such as solar flares and CMEs. The characteristics of three-dimensional reconnection are reviewed revealing how much more diverse it is than reconnection in two dimensions. For instance, three-dimensional reconnection can occur both in the vicinity of null points, as well as in the absence of them. It occurs continuously and continually throughout a diffusion volume, as opposed to at a single point, as it does in two dimensions. This means that in three-dimensions field lines do not reconnect in pairs of lines making the visualisation and interpretation of three-dimensional reconnection difficult.

By considering particular numerical 3D magnetohydrodynamic models of reconnection, we consider how magnetic reconnection can lead to complex magnetic topologies and current sheet formation. Indeed, it has been found that even simple interactions, such as the emergence of a flux tube, can naturally give rise to ‘turbulent-like’ reconnection regions.
Magnetic Cycles and Meridional Circulation in Global Models of Solar Convection
Authors: Miesch Mark S. , Brown Benjamin P. , Browning Matthew K. , Brun Allan Sacha , Toomre Juri
Published: August 2011

We review recent insights into the dynamics of the solar convection zone obtained from global numerical simulations, focusing on two recent developments in particular. The first is quasi-cyclic magnetic activity in a long-duration dynamo simulation. Although mean fields comprise only a few percent of the total magnetic energy they exhibit remarkable order, with multiple polarity reversals and systematic variability on time scales of 6-15 years. The second development concerns the maintenance of the meridional circulation. Recent high-resolution simulations have captured the subtle nonlinear dynamical balances with more fidelity than previous, more laminar models, yielding more coherent circulation patterns. These patterns are dominated by a single cell in each hemisphere, with poleward and equatorward flow in the upper and lower convection zone respectively. We briefly address the implications of and future of these modeling efforts.
Lack of universality in MHD turbulence, and the possible emergence of a new paradigm?
Authors: Pouquet Annick , Brachet Marc-Etienne , Lee Ed , Mininni Pablo , Rosenberg Duane , Uritsky Vadim
Published: August 2011

We review some of the recent results obtained in MHD turbulence, as encountered in many astrophysical objects. We focus attention on the lack of universality in such flows, including in the simplest case (no externally imposed magnetic field, no forcing, unit magnetic Prandtl number). Several parameters can foster such a breakdown of classical Kolmogorov scaling, such as the presence of velocity-magnetic field correlations, or of magnetic helicity and the role of the interplay between nonlinear eddies and Alfvén waves. A link with avalanche processes is also discussed. These findings have led to the conjecture of the emergence of a new paradigm for MHD turbulence, as a possibly unsettled competition between several dynamical phenomena.
Overshooting above a convection zone
Authors: Chan Kwing L. , Cai Tao , Singh Harinder P.
Published: August 2011

As compressible convection has inherent up/down asymmetry, overshooting above and below a convection zone behave differently. In downward overshooting, the narrow down-flow columns dynamically play an important role. It is customary, and reasonable, to use the downward flux of kinetic energy as a proxy for overshooting. In the upward situation, the flux of kinetic energy can take on different signs near the upper boundary of the convection zone, and its magnitude is generally small. It cannot make a good proxy for overshooting. This paper discusses the results of a set of numerical experiments that investigate the problem of overshooting above a convection zone. Particle tracing and color advection are used to follow the mixing process. The overshoot region above a convection zone is found to contain multiple counter cell layers.
Onward from solar convection to dynamos in cores of massive stars
Authors: Toomre Juri
Published: August 2011

We reflect upon a few of the research challenges in stellar convection and dynamo theory that are likely to be addressed in the next five or so years. These deal firstly with the Sun and continuing study of the two boundary layers at the top and bottom of its convection zone, namely the tachocline and the near-surface shear layer, both of which are likely to have significant roles in how the solar dynamo may be operating. Another direction concerns studying core convection and dynamo action within the central regions of more massive A, B and O-type stars, for the magnetism may have a key role in controlling the winds from these stars, thus influencing their ultimate fate. Such studies of the interior dynamics of massive stars are becoming tractable with recent advances in codes and supercomputers, and should also be pursued with some vigor.
Convection and dynamo action in B stars
Authors: Augustson Kyle C. , Brun Allan S. , Toomre Juri
Published: August 2011

Main-sequence massive stars possess convective cores that likely harbor strong dynamo action. To assess the role of core convection in building magnetic fields within these stars, we employ the 3-D anelastic spherical harmonic (ASH) code to model turbulent dynamics within a 10 M⊙ main-sequence (MS) B-type star rotating at 4 Ω⊙. We find that strong (900 kG) magnetic fields arise within the turbulence of the core and penetrate into the stably stratified radiative zone. These fields exhibit complex, time-dependent behavior including reversals in magnetic polarity and shifts between which hemisphere dominates the total magnetic energy.
Internal wave breaking and the fate of planets around solar-type stars
Authors: Barker Adrian J. , Ogilvie Gordon I.
Published: August 2011

Internal gravity waves are excited at the interface of convection and radiation zones of a solar-type star, by the tidal forcing of a short-period planet. The fate of these waves as they approach the centre of the star depends on their amplitude. We discuss the results of numerical simulations of these waves approaching the centre of a star, and the resulting evolution of the spin of the central regions of the star and the orbit of the planet. If the waves break, we find efficient tidal dissipation, which is not present if the waves perfectly reflect from the centre. This highlights an important amplitude dependence of the (stellar) tidal quality factor Q′, which has implications for the survival of planets on short-period orbits around solar-type stars, with radiative cores.
Hunting down giant cells in deep stellar convective zones
Authors: Bessolaz Nicolas , Brun Allan Sacha
Published: August 2011

3D high resolution simulations for the convective zone of a 4Myr old 0.7 M⊙ pre-main sequence star in gravitational contraction are carried out with different radial density contrast using the pseudo spectral ASH code (Brun et al. 2004). We extract giant cells signal from the complex surface convective patterns by using a wavelet analysis. We then characterize them by estimating their lifetime and rotation rate according to the density contrast.
The Effect of Small Scale Motion on an Essentially-Nonlinear Dynamo
Authors: Byington Benjamin M. , Brummell Nicholas H. , Tobias Steven M.
Published: August 2011

A dynamo is a process by which fluid motions sustain magnetic fields against dissipative effects. Dynamos occur naturally in many astrophysical systems. Theoretically, we have a much more robust understanding of the generation and maintenance of magnetic fields at the scale of the fluid motions or smaller, than that of magnetic fields at scales much larger than the local velocity. Here, via numerical simulations, we examine one example of an “essentially nonlinear” dynamo mechanism that successfully maintains magnetic field at the largest available scale (the system scale) without cascade to the resistive scale. In particular, we examine whether this new type of dynamo at the system scale is still effective in the presence of other smaller-scale dynamics (turbulence).
Large resistivity in numerical simulations of radially self-similar outflows
Authors: Čemeljić Miljenko , Vlahakis Nektarios , Tsinganos Kanaris
Published: August 2011

We investigate conditions in a radially self-similar outflow in the regime of large resistivity. Using the PLUTO code, we performed simulations with proper choice of boundary conditions, relaxed at the footpoints of critical surfaces in the flow. We investigate outflow propagation in a high-resistive disk corona, and compare it to the results with small or vanishing resistivity.
Vorticity from irrotationally forced flow
Authors: Sordo Fabio Del , Brandenburg Axel
Published: August 2011

In the interstellar medium the turbulence is believed to be forced mostly through supernova explosions. In a first approximation these flows can be written as a gradient of a potential being thus devoid of vorticity. There are several mechanisms that could lead to vorticity generation, like viscosity and baroclinic terms, rotation, shear and magnetic fields, but it is not clear how effective they are, neither is it clear whether the vorticity is essential in determining the turbulent diffusion acting in the ISM. Here we present a study of the role of rotation, shear and baroclinicity in the generation of vorticity in the ISM.
The star capture model for fueling quasar accretion disks
Authors: Kennedy Gareth F. , Miralda-Escudé Jordi , Kollmeier Juna A.
Published: August 2011

Although the powering mechanism for quasars is now widely recognized to be the accretion of matter in a geometrically thin disk, the transport of matter to the inner region of the disk where luminosity is emitted remains an unsolved question. Miralda-Escudé & Kollmeier (2005) proposed a model whereby quasars are fuelled when stars are captured by the accretion disk as they plunge through the gas. Such plunging stars can then be destroyed and deliver their mass to the accretion disk.

Here we present the first detailed calculations for the capture of stars originating far from the accretion disk near the zone of influence of the central black hole. In particular we examine the effect of adding a perturbing mass to a fixed stellar cusp potential on bringing stars into the accretion disk where they can be captured. The work presented here will be discussed in detail in an upcoming publication Kennedy et al. (2010).
Particle dynamics in the stellar magnetosphere by gravitational collapse
Authors: Kryvdyk Volodymyr
Published: August 2011

The particle dynamics and in the stellar magnetosphere during gravitational collapse is investigated. The formations of relativistic jets and the generation of the radiation bursts in the stellar magnetosphere by gravitational collapse are considered. As follows from results, the stars on the stage of gravitational collapse must be powerful sources of the relativistic jets and the non-thermal radiation. These jets will formed in the polar caps of collapsing stars magnetospheres, when the stellar magnetic field increases during collapse and the charged particles will be accelerate. These jets will generate the non-thermal radiation. The radiation flux grows with decreasing stellar radius and can be observed in the form of radiation burst in wide band wave- from radio to gamma-ray. These bursts radiation can be observed as gamma- and X- rays bursts.
Can turbulent reconnection be fast in 2D?
Authors: Kulpa-Dybeł K. , Kowal G. , Otmianowska-Mazur K. , Lazarian A. , Vishniac E.
Published: August 2011

Turbulent reconnection is studied by means of two-dimensional (2D) compressible magnetohydrodynamical numerical calculations. The process of homogeneous turbulence is set up by adding two-dimensional random forcing implemented in the spectral space at small wave numbers with no correlation between velocity and forcing. We apply the initial Harris current sheet configuration together with a density profile calculated from the numerical equilibrium of magnetic and gas pressures. We assume that there is no external driving of the reconnection. The reconnection develops as a result of the initial vector potential perturbation. We use open boundary conditions. Our main goal is to find the dependencies of reconnection rate on the uniform resistivity. We present that the reconnection speed depends on the Lindquist number in 2D in the case of low as well as high resolution. When we apply more powerful turbulence the reconnection is faster, however the speed of reconnection is smaller than in the case of our three-dimensional numerical simulations.
Cosmological magnetic field seeds produced by the Weibel instabilities
Authors: Lazar M. , Schlickeiser R. , Skoda T.
Published: August 2011

The source of the cosmological magnetic field is still unknown because the widely invoked dynamo processes are only able to regenerate and amplify some initial magnetic field seeds. In the hot and highly ionized intergalactic matter such magnetic field seeds can easily be produced by the (electro-)magnetic instabilities of Weibel type. Here we discuss suplementary mechanisms that can make these Weibel created fields to evolve at large scales presently observed in galaxies and clusters and can also enhance these magnetic field seeds after the dissipation.
Rapid mass segregation in young star clusters without substructure?
Authors: Olczak C. , Spurzem R. , Henning Th.
Published: August 2011

The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the initial conditions under which star clusters form and the impact of these often harsh environments on the formation and evolution of their stellar and substellar members.

We present recent results showing that mass segregation in realistic models of young star clusters occurs very quickly for subvirial spherical systems without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions. The rapid concentration of massive stars is usually associated with strong gravitational interactions early on during cluster evolution and the subsequent formation of multiple systems and ejection of stars.
Supergranulation and its Activity Dependence
Authors: Paniveni U. , Krishan V. , Singh J. , Srikanth R.
Published: August 2011

We study the complexity of supergranular cells using the intensity patterns obtained at the Kodaikanal solar observatory during the solar maximum. Our data consists of visually identified supergranular cells, from which a fractal dimension D is obtained according to the relation P ∝ AD/2 where A is the area and P is the perimeter of the cells. We find a difference in the fractal dimension between the active and the quiet region cells which is conjectured to be due to the magnetic activity level.
3D MHD modelling of a chromosphere above the sun's convective zone
Authors: Pinto Rui , Brun Sacha
Published: August 2011

We report on the extension of the ASH code to include an atmospheric stable layer (i.e not convective). This layer is meant to model the sun's chromosphere within the anelastic approximation limits while coping with the wide range of densities, time and spatial scales between r = 0.7 R⊙ and r = 1.03 R⊙. Convective overshoot into the stable atmospheric layer is observed in a region ~ 0.01 R⊙ thick, exciting waves which propagate upwards into the atmosphere.
Modelling turbulent fluxes due to thermal convection in rectilinear shearing flow
Authors: Smolec Radoslaw , Houdek Günter , Gough Douglas
Published: August 2011

We revisit a phenomenological description of turbulent thermal convection along the lines proposed originally by Gough (1965) in which eddies grow solely by extracting energy from the unstably stratified mean state and are subsequently destroyed by internal shear instability. This work is part of an ongoing investigation for finding a procedure to calculate the turbulent fluxes of heat and momentum in the presence of a shearing background flow in stars.
Variation in convective properties across the HR diagram
Authors: Tanner Joel , Basu Sarbani , Demarque Pierre , Robinson Frank
Published: August 2011

We perform 3D radiative hydrodynamic simulations to study convection in low-mass main-sequence stars with the aim of improving stellar models. Comparing models from a 0.90 M⊙ evolutionary track with 3D simulations reveals distinct differences between simulations and mixing length theory. The simulations show obvious structural differences throughout the superadiabatic layer where convection is inefficient at transporting energy. The discrepancy between MLT and simulation changes as the star evolves and the dynamical effects of turbulence increase. Further, the simulations reveal a T-tau relation that is sensitive to the strength of the turbulence, which is in contrast to 1D stellar models that use the same T-tau relation across the HR diagram.
Using simulations of solar surface convection as boundary conditions on global simulations
Authors: Trampedach Regner , Augustson Kyle
Published: August 2011

Direct numerical simulations of convective stellar envelopes, are divided between two different physical regimes, that are rather difficult to reconcile — at least with the computational power of present-day computers. This paper outlines an attempt at bridging the gap between surface and interior simulations of convection.
Numerical simulations of the circumstellar medium of massive binaries
Authors: van Marle Allard Jan , Keppens Rony
Published: August 2011

We have made 3-D models of the collision of binary star winds and followed their interaction over multiple orbits. This allows us to explore how the wind-wind interaction shapes the circumstellar environment. Specifically, we can model the highly radiative shock that occurs where the winds collide. We find that the shell that is created at the collision front between the two winds can be highly unstable, depending on the characteristics of the stellar winds.
Recurrent flux emergence from dynamo-generated fields
Authors: Warnecke Jörn , Brandenburg Axel
Published: August 2011

we investigate the emergence of a large-scale magnetic field. This field is dynamo-generated by turbulence driven with a helical forcing function. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Time series of the magnetic field structure show recurrent plasmoid ejections.
Full polarimetric beam‐forming algorithm for through‐the‐wall radar imaging
Authors: Zhang Wenji , Hoorfar Ahmad , Thajudeen Christopher , Ahmad Fauzia
Published: August 2011

Development of an imaging algorithm that accurately models the wave propagation physical process has become an important topic in through‐the‐wall radar imaging (TWRI). In this paper, a full polarimetric beam‐forming algorithm for through‐the‐wall radar imaging for a general multilayer wall case is presented and applied to various 2D and 3D simulated and measured scenarios. Polarimetric TWRI not only enhances the target characterization but also mitigates the wall ringing effect in cross polarizations. The far field layered medium Green's function is incorporated in the proposed TWRI algorithm for the quad‐polarization of the target returns, namely, VV, HV, VH and HH. Due to the incorporation of the layered medium Green's function, the imaging algorithm not only takes into account the wall reflection, bending, and delay effects but also accounts for the complex scattering mechanism due to the presence of the wall. Numerical and experimental results show that the proposed full polarimetric beam former can provide high quality focused images in various wall‐target scenarios, in particular, when the technique is combined with a wall parameter estimation technique.
Long‐term changes in light scattering in Chesapeake Bay inferred from Secchi depth, light attenuation, and remote sensing measurements
Authors: Gallegos Charles L. , Werdell P. Jeremy , McClain Charles R.
Published: August 2011

The relationship between the Secchi depth (ZSD) and the diffuse attenuation coefficient for photosynthetically active radiation (Kd(PAR)), and in particular the product of the two, ZSD · Kd(PAR), is governed primarily by the ratio of light scattering to absorption. We analyzed measurements of ZSD and Kd(PAR) at main stem stations in Chesapeake Bay and found that the ZSD · Kd(PAR) product has declined at rates varying from 0.020 to 0.033 yr−1 over the 17 to 25 years of measurement, implying that there has been a long‐term increase in the scattering‐to‐absorption ratio. Remote sensing reflectance at the green wavelength most relevant to ZSD and Kd(PAR) in these waters, Rrs(555), did not exhibit an increasing trend over the 10 years of available measurements. To reconcile the observations we constructed a bio‐optical model to calculate ZSD, Kd(PAR), ZSD · Kd(PAR), and Rrs(555) as a function of light attenuating substances and their mass‐specific absorption and scattering coefficients. When simulations were based exclusively on changes in concentrations of light attenuating substances, a declining trend in ZSD · Kd entailed an increasing trend in Rrs(555), contrary to observations. To simulate both decreasing ZSD · Kd(PAR) and stationary Rrs(555), it was necessary to allow for a declining trend in the ratio of backscattering to total scattering. Within our simulations, this was accomplished by increasing the relative proportion of organic detritus with high mass‐specific scattering and low backscattering ratio. An alternative explanation not explicitly modeled is an increasing tendency for the particulate matter to occur in large aggregates. Data to discriminate between these alternatives are not available.