25,215 research outputs found
Resonance bifurcations from robust homoclinic cycles
We present two calculations for a class of robust homoclinic cycles with
symmetry Z_n x Z_2^n, for which the sufficient conditions for asymptotic
stability given by Krupa and Melbourne are not optimal.
Firstly, we compute optimal conditions for asymptotic stability using
transition matrix techniques which make explicit use of the geometry of the
group action.
Secondly, through an explicit computation of the global parts of the Poincare
map near the cycle we show that, generically, the resonance bifurcations from
the cycles are supercritical: a unique branch of asymptotically stable period
orbits emerges from the resonance bifurcation and exists for coefficient values
where the cycle has lost stability. This calculation is the first to explicitly
compute the criticality of a resonance bifurcation, and answers a conjecture of
Field and Swift in a particular limiting case. Moreover, we are able to obtain
an asymptotically-correct analytic expression for the period of the bifurcating
orbit, with no adjustable parameters, which has not proved possible previously.
We show that the asymptotic analysis compares very favourably with numerical
results.Comment: 24 pages, 3 figures, submitted to Nonlinearit
Sputtering of Oxygen Ice by Low Energy Ions
Naturally occurring ices lie on both interstellar dust grains and on
celestial objects, such as those in the outer solar system. These ices are
continu- ously subjected to irradiation by ions from the solar wind and/or
cosmic rays, which modify their surfaces. As a result, new molecular species
may form which can be sputtered off into space or planetary atmospheres. We
determined the experimental values of sputtering yields for irradiation of
oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ and
O2+) charged ions with 4 keV kinetic energy. In these laboratory experiments,
oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber
at low temperature to simulate the environment of space. The number of
molecules removed by sputtering was observed by measurement of the ice
thickness using laser interferometry. Preliminary mass spectra were taken of
sputtered species and of molecules formed in the ice by temperature programmed
desorption (TPD). We find that the experimental sputtering yields increase
approximately linearly with the projectile ion mass (or momentum squared) for
all ions studied. No difference was found between the sputtering yield for
singly and doubly charged ions of the same atom within the experimental
uncertainty, as expected for a process dominated by momentum transfer. The
experimental sputter yields are in good agreement with values calculated using
a theoretical model except in the case of oxygen ions. Preliminary studies have
shown molecular oxygen as the dominant species sputtered and TPD measurements
indicate ozone formation.Comment: to be published in Surface Science (2015
Final state interactions and hadron quenching in cold nuclear matter
I examine the role of final state interactions in cold nuclear matter in
modifying hadron production on nuclear targets with leptonic or hadronic beams.
I demonstrate the extent to which available experimental data in
electron-nucleus collisions can give direct information on final state effects
in hadron-nucleus and nucleus-nucleus collisions. For hadron-nucleus
collisions, a theoretical estimate based on a parton energy loss model tested
in lepton-nucleus collisions shows a large effect on mid-rapidity hadrons at
fixed target experiments. At RHIC energy, the effect is large for negative
rapidity hadrons, but mild at midrapidity. This final state cold hadron
quenching needs to be taken into account in jet tomographic analysis of the
medium created in nucleus-nucleus collisions.Comment: 14 pages, 7 figure
Failed attempts to improve the reliability of the alcohol Visual Probe Task following empirical recommendations
Introduction: The Visual Probe Task (VPT) is a computerised task used to measure attentional bias to substance-related stimuli. Little research has examined the psychometric properties of the VPT, despite concerns it demonstrates poor test-retest reliability and internal consistency. These issues can reduce confidence in inferences based on VPT performance. As such, we attempted to identify parameters under which the reliability of the alcohol VPT might be improved by applying recent empirical recommendations for outlier handling, bias calculation and task design from the anxiety literature.
Methods: We reanalysed data from 3 previously published studies in our laboratory and 2 newly collected data-sets. We compared tasks which presented images on the left/right of the screen to above/below, whether participants responded to the location or content of the probe, and whether general alcohol-related images or images personalised to the individual were used. In each VPT we also applied a-priori outlier removal (2 and 3 standard deviations and median absolute difference) and data-driven outlier removal (Winsorising), in addition to calculating trial-level bias scores.
Results: Across all studies and tasks internal consistency and test-retest reliability of attentional bias measures were inadequate. There was no consistent improvement in internal consistency or test-retest reliability as a function of outlier removal methods.
Discussion: We were unable to demonstrate adequate reliability of the alcohol VPT, which further supports observations that these tasks may not yield reliable measures. Future research should focus on improving the reliability of these tasks or abandoning them in favour of more reliable alternatives
Derivation of the Lorentz Force Law, the Magnetic Field Concept and the Faraday-Lenz Law using an Invariant Formulation of the Lorentz Transformation
It is demonstrated how the right hand sides of the Lorentz Transformation
equations may be written, in a Lorentz invariant manner, as 4--vector scalar
products. This implies the existence of invariant length intervals analogous to
invariant proper time intervals. This formalism, making essential use of the
4-vector electromagnetic potential concept, provides a short derivation of the
Lorentz force law of classical electrodynamics, the conventional definition of
the magnetic field, in terms of spatial derivatives of the 4--vector potential
and the Faraday-Lenz Law. An important distinction between the physical
meanings of the space-time and energy-momentum 4--vectors is pointed out.Comment: 15 pages, no tables 1 figure. Revised and extended version of
physics/0307133 Some typos removed and minor text improvements in this
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The relationship between adult attachment style and post-traumatic stress symptoms: A meta-analysis
There is increasing evidence that adult attachment plays a role in the development and perseverance of symptoms of posttraumatic stress disorder (PTSD). This meta-analysis aims to synthesise this evidence and investigate the relationship between adult attachment styles and PTSD symptoms. A random-effects model was used to analyse 46 studies (N = 9268) across a wide range of traumas. Results revealed a medium association between secure attachment and lower PTSD symptoms (ρ =-.27), and a medium association, in the opposite direction, between insecure attachment and higher PTSD symptoms (ρ =.26). Attachment categories comprised of high levels of anxiety most strongly related to PTSD symptoms, with fearful attachment displaying the largest association (ρ =.44). Dismissing attachment was not significantly associated with PTSD symptoms. The relationship between insecure attachment and PTSD was moderated by type of PTSD measure (interview or questionnaire) and specific attachment category (e.g. secure, fearful). Results have theoretical and clinical significance
Aerosol-cloud interactions in mixed-phase convective clouds - Part 1: Aerosol perturbations
Changes induced by perturbed aerosol conditions in moderately deep mixed-phase convective clouds (cloud top height 5 km) developing along sea-breeze convergence lines are investigated with high-resolution numerical model simulations. The simulations utilise the newly developed Cloud-AeroSol Interacting Microphysics (CASIM) module for the Unified Model (UM), which allows for the representation of the two-way interaction between cloud and aerosol fields. Simulations are evaluated against observations collected during the COnvective Precipitation Experiment (COPE) field campaign over the southwestern peninsula of the UK in 2013. The simulations compare favourably with observed thermodynamic profiles, cloud base cloud droplet number concentrations (CDNC), cloud depth, and radar reflectivity statistics. Including the modification of aerosol fields by cloud microphysical processes improves the correspondence with observed CDNC values and spatial variability, but reduces the agreement with observations for average cloud size and cloud top height. Accumulated precipitation is suppressed for higher-aerosol conditions before clouds become organised along the sea-breeze convergence lines. Changes in precipitation are smaller in simulations with aerosol processing. The precipitation suppression is due to less efficient precipitation production by warm-phase microphysics, consistent with parcel model predictions. In contrast, after convective cells organise along the sea-breeze convergence zone, accumulated precipitation increases with aerosol concentrations. Condensate production increases with the aerosol concentrations due to higher vertical velocities in the convective cores and higher cloud top heights. However, for the highest-aerosol scenarios, no further increase in the condensate production occurs, as clouds grow into an upper-level stable layer. In these cases, the reduced precipitation efficiency (PE) dominates the precipitation response and no further precipitation enhancement occurs. Previous studies of deep convective clouds have related larger vertical velocities under high-aerosol conditions to enhanced latent heating from freezing. In the presented simulations changes in latent heating above the 0°C are negligible, but latent heating from condensation increases with aerosol concentrations. It is hypothesised that this increase is related to changes in the cloud field structure reducing the mixing of environmental air into the convective core. The precipitation response of the deeper mixed-phase clouds along well-established convergence lines can be the opposite of predictions from parcel models. This occurs when clouds interact with a pre-existing thermodynamic environment and cloud field structural changes occur that are not captured by simple parcel model approaches
Studying Attractor Symmetries by Means of Cross Correlation Sums
We use the cross correlation sum introduced recently by H. Kantz to study
symmetry properties of chaotic attractors. In particular, we apply it to a
system of six coupled nonlinear oscillators which was shown by Kroon et al. to
have attractors with several different symmetries, and compare our results with
those obtained by ``detectives" in the sense of Golubitsky et al.Comment: LaTeX file, 16 pages and 16 postscript figures; tarred, gzipped and
uuencoded; submitted to 'Nonlinearity
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