DK-1D: a drift-kinetic simulation tool for modelling the shear Alfvén wave and its interaction with collisionless plasma

We present a highly accurate tool for the simulation of shear Alfven waves (SAW) in collisionless plasma. SAW are important in space plasma environments because for small perpendicular scale lengths they can support an electric field parallel to the ambient magnetic field. Electrons can be accelerated by the parallel electric field and these waves have been implicated as the source of vibrant auroral displays. However, the parallel electric field carried by SAW is small in comparison to the perpendicular electric field of the wave, making it difficult to measure directly in the laboratory, or by satellites in the near-Earth plasma environment. In this paper, we present a simulation code that provides a means to study in detail the SAW-particle interaction in both space and laboratory plasma. Using idealised, small-amplitude propagating waves with a single perpendicular wavenumber, the simulation code accurately reproduces the damping rates and parallel electric field amplitudes predicted by linear theory for varying temperatures and perpendicular scale lengths. We present a rigorous kinetic derivation of the parallel electric field strength for small-amplitude SAW and show that commonly-used inertial and kinetic approximations are valid except for where the ratio of thermal to Alfv\'{e}n speed is between 0.7 and 1.0. We also present nonlinear simulations of large-amplitude waves and show that in cases of strong damping, the damping rates and parallel electric field strength deviate from linear predictions when wave energies are greater than only a few percent of the plasma kinetic energy, a situation which is often observed in the magnetosphere. The drift-kinetic code provides reliable, testable predictions of the parallel electric field strength which can be investigated directly in the laboratory, and will help to bridge the gap between studies of SAW in man-made and naturally occuring plasma.

A spectral view of nonlinear fluxes and stationary-transient interaction in the atmosphere

Nonlinear spectral transfers of kinetic energy and enstrophy, and stationary-transient interaction, are studied using global FGGE data for January 1979. It is found that the spectral transfers arise primarily from a combination, in roughly equal measure, of pure transient and mixed stationary-transient interactions. The pure transient interactions are associated with a transient eddy field which is approximately locally homogeneous and isotropic, and they appear to be consistently understood within the context of two-dimensional homogeneous turbulence. Theory based on spatial wale separation concepts suggests that the mixed interactions may be understood physically, to a first approximation, as a process of shear-induced spectral transfer of transient enstrophy along lines of constant zonal wavenumber. This essentially conservative enstrophy transfer generally involves highly nonlocal stationary-transient energy conversions. The observational analysis demonstrates that the shear-induced transient enstrophy transfer is mainly associated with intermediate-scale (zonal wavenumber m > 3) transients and is primarily to smaller (meridional) scales, so that the transient flow acts as a source of stationary energy. In quantitative terms, this transient-eddy rectification corresponds to a forcing timescale in the stationary energy budget which is of the same order of magnitude as most estimates of the damping timescale in simple stationary-wave models (5 to 15 days). Moreover, the nonlinear interactions involved are highly nonlocal and cover a wide range of transient scales of motion.

Buyer–seller interaction in experimental spatial markets

We report results from experimental spatial markets with endogenous buyer location on a discrete version of Hotelling's linear city. Buyer locations favor more often the hypothesis of transportation cost minimization than that of strategic location aimed at increasing price competition between sellers. However, the latter of the two hypotheses receives systematic support too. Differentiation by seller-subjects is substantially less than the theory would predict for the specific framework used. Our results suggest that location strategies adopted by subjects can be seen as a rational process favoring conservative product design and spatial agglomeration of economic activities.

The interaction of iron pyrite with oxygen, nitrogen and nitrogen oxides: a first-principles study

Sulphide materials, in particular MoS2, have recently received great attention from the surface science community due to their extraordinary catalytic properties. Interestingly, the chemical activity of iron pyrite (FeS2) (the most common sulphide mineral on Earth), and in particular its potential for catalytic applications, has not been investigated so thoroughly. In this study, we use density functional theory (DFT) to investigate the surface interactions of fundamental atmospheric components such as oxygen and nitrogen, and we have explored the adsorption and dissociation of nitrogen monoxide (NO) and nitrogen dioxide (NO2) on the FeS2(100) surface. Our results show that both those environmentally important NOx species chemisorb on the surface Fe sites, while the S sites are basically unreactive for all the molecular species considered in this study and even prevent NO2 adsorption onto one of the non-equivalent Fe–Fe bridge sites of the (1 1)–FeS2(100) surface. From the calculated high barrier for NO and NO2 direct dissociation on this surface, we can deduce that both nitrogen oxides species are adsorbed molecularly on pyrite surfaces.

Shared authority: courts and legislatures in legal theory

This book advances a fresh philosophical account of the relationship between the legislature and courts, opposing the common conception of law, in which it is legislatures that primarily create the law, and courts that primarily apply it. This conception has eclectic affinities with legal positivism, and although it may have been a helpful intellectual tool in the past, it now increasingly generates more problems than it solves. For this reason, the author argues, legal philosophers are better off abandoning it. At the same time they are asked to dismantle the philosophical and doctrinal infrastructure that has been based on it and which has been hitherto largely unquestioned. In its place the book offers an alternative framework for understanding the role of courts and the legislature; a framework which is distinctly anti-positivist and which builds on Ronald Dworkin’s interpretive theory of law. But, contrary to Dworkin, it insists that legal duty is sensitive to the position one occupies in the project of governing; legal interpretation is not the solitary task of one super-judge, but a collaborative task structured by principles of institutional morality such as separation of powers which impose a moral duty on participants to respect each other's contributions. Moreover this collaborative task will often involve citizens taking an active role in their interaction with the law.

The importance of anisotropic Coulomb interaction in LaMnO3

In low-temperature anti-ferromagnetic LaMnO3, strong and localized electronic interactions among Mn 3d electrons prevent a satisfactory description from standard local density and generalized gradient approximations in density functional theory calculations. Here we show that the strong on-site electronic interactions are described well only by using direct and exchange corrections to the intra-orbital Coulomb potential. Only DFT+U calculations with explicit exchange corrections produce a balanced picture of electronic, magnetic and structural observables in agreement with experiment. To understand the reason, a rewriting of the functional form of the +U corrections is presented that leads to a more physical and transparent understanding of the effect of these correction terms. The approach highlights the importance of Hund’s coupling (intra-orbital exchange) in providing anisotropy across the occupation and energy eigenvalues of the Mn d states. This intra-orbital exchange is the key to fully activating the Jahn-Teller distortion, reproducing the experimental band gap and stabilizing the correct magnetic ground state in LaMnO3. The best parameter values for LaMnO3 within the DFT(PBEsol)+U framework are determined to be U = 8 eV and J = 1.9 eV.

Effective interaction between monolayers of block copolymer compatiblizer in a polymer blend

The stability of ternary blends of two immiscible homopolymers and a block copolymer compatiblizer depends crucially on the effective interaction between the copolymermonolayers that form between the unlike homopolymer domains. Here, the interaction is calculated for blends involving A and B homopolymers of equal size with ABABdiblock copolymers of symmetric composition using both self-consistent field theory (SCFT) and strong-segregation theory (SST). If the homopolymers are larger than the copolymer molecules, an attractive interaction is predicted which would destroy the blend. This conclusion coupled with considerations regarding the elastic properties of the monolayer suggests that the optimum size of the homopolymer molecules is about 80% that of the copolymer molecule. A detailed examination of the theory demonstrates that the attraction results from the configurational entropy loss of the homopolymer molecules trapped between the copolymermonolayers. We conclude by suggesting how the monolayers can be altered in order to suppress this attraction and thus improve compatiblization.