The transferability of the low-cost model to long-haul airline operations

Since their emergence in the US in the mid 1970's there has been significant growth in the low-cost airline sector but with a few notable exceptions low-cost airlines have operated on short-haul routes. This paper examines the extent to which the low-cost model is, or could be, applicable to long-haul operations and whether the recent emergence of long-haul low-cost carriers is a sustainable phenomenon. The authors explore the extent to which elements of the so-called low-cost model might be transferable to long-haul operations. The paper seeks to quantify the potential cost differentials that might be achievable on a long-haul service. The paper also speculates as to the development and sustainability of the low-cost long-haul operations

Screening of charged singularities of random fields

Many types of point singularity have a topological index, or 'charge', associated with them. For example the phase of a complex field depending on two variables can either increase or decrease on making a clockwise circuit around a simple zero, enabling the zeros to be assigned charges of plus or minus one. In random fields we can define a correlation function for the charge-weighted density of singularities. In many types of random fields, this correlation function satisfies an identity which shows that the singularities 'screen' each other perfectly: a positive singularity is surrounded by an excess of concentration of negatives which exactly cancel its charge, and vice-versa. This paper gives a simple and widely applicable derivation of this result. A counterexample where screening is incomplete is also exhibited.Comment: 12 pages, no figures. Minor revision of manuscript submitted to J. Phys. A, August 200

The Radiation Transfer at a Layer of Magnetized Plasma With Random Irregularities

The problem of radio wave reflection from an optically thick plane monotonous layer of magnetized plasma is considered at present work. The plasma electron density irregularities are described by spatial spectrum of an arbitrary form. The small-angle scattering approximation in the invariant ray coordinates is suggested for analytical investigation of the radiation transfer equation. The approximated solution describing spatial-and-angular distribution of radiation reflected from a plasma layer has been obtained. The obtained solution has been investigated numerically for the case of the ionospheric radio wave propagation. Two effects are the consequence of multiple scattering: change of the reflected signal intensity and anomalous refraction.Comment: 22 pages, 4 figure

Electromagnetic vortex lines riding atop null solutions of the Maxwell equations

New method of introducing vortex lines of the electromagnetic field is outlined. The vortex lines arise when a complex Riemann-Silberstein vector $({\bm E} + i{\bm B})/\sqrt{2}$ is multiplied by a complex scalar function $\phi$. Such a multiplication may lead to new solutions of the Maxwell equations only when the electromagnetic field is null, i.e. when both relativistic invariants vanish. In general, zeroes of the $\phi$ function give rise to electromagnetic vortices. The description of these vortices benefits from the ideas of Penrose, Robinson and Trautman developed in general relativity.Comment: NATO Workshop on Singular Optics 2003 To appear in Journal of Optics

A dynamical trichotomy for structured populations experiencing positive density-dependence in stochastic environments

Positive density-dependence occurs when individuals experience increased survivorship, growth, or reproduction with increased population densities. Mechanisms leading to these positive relationships include mate limitation, saturating predation risk, and cooperative breeding and foraging. Individuals within these populations may differ in age, size, or geographic location and thereby structure these populations. Here, I study structured population models accounting for positive density-dependence and environmental stochasticity i.e. random fluctuations in the demographic rates of the population. Under an accessibility assumption (roughly, stochastic fluctuations can lead to populations getting small and large), these models are shown to exhibit a dynamical trichotomy: (i) for all initial conditions, the population goes asymptotically extinct with probability one, (ii) for all positive initial conditions, the population persists and asymptotically exhibits unbounded growth, and (iii) for all positive initial conditions, there is a positive probability of asymptotic extinction and a complementary positive probability of unbounded growth. The main results are illustrated with applications to spatially structured populations with an Allee effect and age-structured populations experiencing mate limitation

The behaviour of commercial broilers in response to a mobile robot

1. Modern broiler production, in increasingly large sheds holding upwards of 50.000 birds, controls indoor climate based on a handful of fixed location sensors, often well above the bird occupied zone. Significant deviations within a shed from the optimal climate conditions for the birds are common, but installing a higher density grid of fixed sensors is not cost effective. A robotic platform, moving through the flock of birds, collecting detailed spacial information on a wide range of climate parameters at bird level, will enable accurate decisions to optimise the climate in large sheds being made in real time. 2. A preliminary study investigated the feasibility of running a mobile robotic platform among a flock of broiler chickens for an entire 6-week cycle. Bird behaviour in response to the robot was also studied

A Collision of Subclusters in Abell 754

We present direct evidence of a collision of subclusters in the galaxy cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC X-ray data reveal three collision signatures predicted by n-body/hydrodynamical simulations of hierarchical cluster evolution. First, there is strong evidence of a non-hydrostatic process; neither of the two major clumps in the galaxy distribution lies on the off-center peak of the X-ray emission from the intracluster gas. Second, the peak of the X-ray emission is elongated perpendicular to the collision axis defined by the centroids of the two galaxy clumps. Third, there is evidence of compression-heated gas; one of A754's two X-ray temperature components (Henry & Briel 1995) is among the hottest observed in any cluster and hotter than that inferred from the velocity dispersion of the associated galaxy clump. These signatures are consistent with the qualitative features of simulations (Evrard 1990a,b) in which two subclusters have collided in the plane of the sky during roughly the last Gyr. The detection of such collisions is crucial for understanding both the dynamics of individual clusters and the underlying cosmology. First, for systems like A754, estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium and isothermality is incorrect and may produce the discrepancies sometimes found between X-ray masses and those derived from gravitational lens models (Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which subclusters have collided in the last Gyr is especially sensitive to the mean mass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993; Lacey & Cole 1993). With a large, well-defined cluster sample, it will be possible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in ApJ Letter

Helicity, polarization, and Riemann-Silberstein vortices

Riemann-Silberstein (RS) vortices have been defined as surfaces in spacetime where the complex form of a free electromagnetic field given by F=E+iB is null (F.F=0), and they can indeed be interpreted as the collective history swept out by moving vortex lines of the field. Formally, the nullity condition is similar to the definition of "C-lines" associated with a monochromatic electric or magnetic field, which are curves in space where the polarization ellipses degenerate to circles. However, it was noted that RS vortices of monochromatic fields generally oscillate at optical frequencies and are therefore unobservable while electric and magnetic C-lines are steady. Here I show that under the additional assumption of having definite helicity, RS vortices are not only steady but they coincide with both sets of C-lines, electric and magnetic. The two concepts therefore become one for waves of definite frequency and helicity. Since the definition of RS vortices is relativistically invariant while that of C-lines is not, it may be useful to regard the vortices as a wideband generalization of C-lines for waves of definite helicity.Comment: 5 pages, no figures. Submitted to J of Optics A, special issue on Singular Optics; minor changes from v.

The Luminosity Function of Galaxies in Compact Groups

From R-band images of 39 Hickson compact groups (HCGs), we use galaxy counts to determine a luminosity function extending to M_R=-14.0, approximately two magnitudes deeper than previous compact group luminosity functions. We find that a single Schechter function is a poor fit to the data, so we fit a composite function consisting of separate Schechter functions for the bright and faint galaxies. The bright end is best fit with M^*=-21.6 and alpha=-0.52 and the faint end with M^*=-16.1 and alpha=-1.17. The decreasing bright end slope implies a deficit of intermediate luminosity galaxies in our sample of HCGs and the faint end slope is slightly steeper than that reported for earlier HCG luminosity functions. Furthermore, luminosity functions of subsets of our sample reveal more substantial dwarf populations for groups with x-ray halos, groups with tidal dwarf candidates, and groups with a dominant elliptical or lenticular galaxy. Collectively, these results support the hypothesis that within compact groups, the initial dwarf galaxy population is replenished by "subsequent generations" formed in the tidal debris of giant galaxy interactions.Comment: 26 pages, to be published in The Astrophysical Journal, 8 greyscale plates (figures 1 and 2) can be retrieved at http://www.astro.psu.edu/users/sdh/pubs.htm

Topological quantum memory

We analyze surface codes, the topological quantum error-correcting codes introduced by Kitaev. In these codes, qubits are arranged in a two-dimensional array on a surface of nontrivial topology, and encoded quantum operations are associated with nontrivial homology cycles of the surface. We formulate protocols for error recovery, and study the efficacy of these protocols. An order-disorder phase transition occurs in this system at a nonzero critical value of the error rate; if the error rate is below the critical value (the accuracy threshold), encoded information can be protected arbitrarily well in the limit of a large code block. This phase transition can be accurately modeled by a three-dimensional Z_2 lattice gauge theory with quenched disorder. We estimate the accuracy threshold, assuming that all quantum gates are local, that qubits can be measured rapidly, and that polynomial-size classical computations can be executed instantaneously. We also devise a robust recovery procedure that does not require measurement or fast classical processing; however for this procedure the quantum gates are local only if the qubits are arranged in four or more spatial dimensions. We discuss procedures for encoding, measurement, and performing fault-tolerant universal quantum computation with surface codes, and argue that these codes provide a promising framework for quantum computing architectures.Comment: 39 pages, 21 figures, REVTe