288 research outputs found
Chaetognatha of the Caribbean Sea and adjacent areas
This illustrated manual is a guide to the distribution and identification of the 6 genera and 28 species of benthic
and planktonic Chaetognatha known to occur in the Caribbean Sea, the Gulf of Mexlco, the Florida Straits, and the southwestern North Atlantic Ocean. As background, previous studies of chaetognaths in these areas are reviewed, gross morphology of the different forms is described, and instructions on methods of preserving and handling specimens preparatory to identification are provided. The key to genera and species is preceeded by a discussion of chaetognath taxonomy. A description of each species, consisting of an abbreviated synonymy, a summary of taxonomically important morphological features, and horizontal and vertical distribution follows the key. The occurrence of species in relation to water masses in the Caribbean and adjacent areas is noted. (PDF file contains 39 pages.
The effect of photo-electric absorption on space-charge limited flow in pulsars
Photo-electric absorption of blackbody photons is an important process which
limits the acceleration of ions under the space-charge limited flow boundary
condition at the polar caps of pulsars with positive corotational charge
density. Photo-electric cross-sections in high magnetic fields have been found
for the geometrical conditions of the problem, and ion transition rates
calculated as functions of the surface temperatures on both the polar cap and
the general neutron-star surface. The general surface temperature is the more
important and, unless it is below 10^5 K, limits the acceleration electric
field in the open magnetosphere to values far below those needed either for
electron-positron pair creation or slot-gap X-ray sources. But such ion beams
are unstable against growth of a quasi-longitudinal Langmuir mode at rates that
can be observationally significant as a source of coherent radio emission.Comment: 7 pages; to be published in Monthly Notices of the Royal Astronomical
Societ
Measuring change in vulnerable adolescents: Findings from a peer education evaluation in South Africa
Introduction: In the context of poverty and HIV and AIDS, peer education is thought to be capable of providing vulnerable youth with psychosocial support as well as information and decision-making skills otherwise limited by scarce social and material resources. As a preventative education intervention method, peer education is a strategy aimed at norms and peer group influences that affect health behaviours and attitudes. However, too few evaluations of peer-led programmes are available,and they frequently fail to reflect real differences between those who have been recipients of peer education and those who have not. This article reports on an evaluation of a pilot peer-led intervention, entitled Vhutshilo, implemented on principles agreed upon through a collaborative effort in South Africa by the Harvard School of Public Health and the Centre for theSupport of Peer Education (the Rutanang collaboration). Vhutshilo targeted vulnerable adolescents aged 14–16 years living in some of South Africa’s under-resourced communities. Methodology: The research design was a mixed-method (qualitative and quantitative), longitudinal, quasi-experimental evaluation. Tools used included a quantitative survey questionnaire (n ¼ 183) and semi-structured interviews (n ¼ 32) with beneficiaries of peer education. Surveys were administered twice forbeneficiaries of peer education (n ¼ 73), immediately after completion of the programme (post-test) and 4 months later (delayed post-test), and once for control group members (n ¼ 110). The three main methodological limitations in this study were the use of a once-off control group assessment as the baseline for comparison, without a pre-test, due to timing and resource constraints; a small sample size (n ¼ 183), which reduced the statistical power of the evaluation; and the unavailability of existing tested survey questions to measure the impact of peer education and its role in behaviour change. Findings: This article reports on the difficulties of designing a comprehensive evaluation within time and financial constraints, critically evaluates survey design with multi-item indicators, and discusses six statistically significant changes observed in Vhutshilo participants out of a 92-point survey. Youth struggling with poor quality education and living in economically fraught contexts with little social support, nonetheless, showed evidence of having greater knowledge of support networks and an expanded emotional repertoire by the end of the Vhutshilo programme, and 4 months later. At both individual and group level, many with low socio-economic status showed great improvement with regard to programme indicator scores. Conclusion: For the poorest adolescents, especially those living in the rural parts of South Africa, peer education has the potential to change future orientation, attitudes and knowledge regarding HIV and AIDS, including an intolerance for multiple concurrent partnerships. When well organised and properly supported, peer education programmes (and the Vhutshilo curriculum, in particular) provide vulnerable youth with opportunities to develop psychosocial skills and informational resources that contribute to the changing of norms, attitudes and behaviours. However, the article also flags the need for effective peer education evaluations that take into account limited financial resources and that possess tested indicators of programme effectiveness
A Tale of Two Current Sheets
I outline a new model of particle acceleration in the current sheet
separating the closed from the open field lines in the force-free model of
pulsar magnetospheres, based on reconnection at the light cylinder and
"auroral" acceleration occurring in the return current channel that connects
the light cylinder to the neutron star surface. I discuss recent studies of
Pulsar Wind Nebulae, which find that pair outflow rates in excess of those
predicted by existing theories of pair creation occur, and use those results to
point out that dissipation of the magnetic field in a pulsar's wind upstream of
the termination shock is restored to life as a viable model for the solution of
the "" problem as a consequence of the lower wind 4-velocity implied by
the larger mass loading.Comment: 17 pages, 6 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
Nulls subpulse drift and mode-switching in pulsars: the polar-cap surface
Little attention has so far been paid to the division of the observed
population between pulsars of the two spin directions that are possible. Almost
all pulsars with positive corotational charge density at the polar caps are
expected to satisfy space-charge limited flow boundary conditions. Charge
separation by blackbody photo-electric transitions in moving ions limits the
acceleration potential, analogously with the more usually considered pair
creation. But the limitation is more severe so that proton and ion energies can
be relativistic but not ultra-relativistic, and these allow the growth of
Langmuir-mode induced turbulence that couples directly with the radiation
field, as shown by Asseo, Pelletier & Sol. The consequences of this, and of the
several possible physical states of the polar cap, are described,
qualitatively, as possible explanations for the complex phenomena of nulls,
subpulse drift and mode-switching observed in subsets of pulsars.Comment: To be published in Monthly Notices of the Royal Astronomical society;
11 page
Nuclear physics of reverse electron flow at pulsar polar caps
Protons produced in electromagnetic showers formed by the reverse-electron
flux are usually the largest component of the time-averaged polar-cap open
magnetic flux-line current in neutron stars with positive corotational charge
density. Although the electric-field boundary conditions in the corotating
frame are time-independent, instabilities on both medium and short time-scales
cause the current to alternate between states in which either protons or
positrons and ions form the major component. These properties are briefly
discussed in relation to nulling and microstructure in radio pulsars, pair
production in an outer gap, and neutron stars with high surface temperatures.Comment: 13 pages; to be published in Monthly Notices of the Royal
Astronomical Societ
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
Instabilities nulls and sub-pulse drift in radio pulsars
This paper continues a previous study of neutron stars with positive
polar-cap corotational charge density in which free emission of ions maintains
the surface electric-field boundary condition E.B = 0. The composition of the
accelerated plasma on any subset of open magnetic flux-lines above the polar
cap alternates between two states; either protons or positrons and ions, of
which the proton state cannot support electron-positron pair creation at higher
altitudes. The two states coexist at any instant of time above different moving
elements of area on the polar cap and provide a physically consistent basis for
a description of pulse nulls and sub-pulse drift. In the latter case, it is
shown that the band separation P3 is determined not by the ExB drift velocity,
as is generally assumed, but by the diffusion time for protons produced in
reverse-electron showers to reach the region of the atmosphere from which they
are accelerated. An initial comparison is made with the survey of sub-pulse
drift published by Weltevrede et al.Comment: 12 pages; accepted for publication in Monthly Notices of the Royal
Astronomical Societ
An Empirical Model for the Radio Emission from Pulsars
A model for slow radio pulsars is proposed which involves the entire
magnetosphere in the production of the observed radio emission. It is argued
that observations of pulsar profiles suggest that a feedback mechanism exists
between the star surface and the null charge surface, requiring particle flow
in both directions. In their flow to and from the surface the particles execute
an azimuthal drift around the magnetic pole, thereby creating a ring of
discrete `emission nodes' close to the surface. Motion of the nodes is observed
as the well-known subpulse `drift', but is interpreted here as a small residual
component of the real particle drift. The nodes can therefore move in either
direction, or even remain stationary. A precise fit is found for the pulsar
PSR0943+10. Azimuthal interactions between different regions of the
magnetosphere depend on the angle between the magnetic and rotation axes and
influence the conal type, as observed. The requirement of intermittent weak
pair-production in an outergap suggests a natural evolutionary link between
radio and gamma-ray pulsars.Comment: 17 pages 8 figure
Field Evaluation of the Cepheid GeneXpert Chlamydia trachomatis assay for Detection of Infection in a Trachoma Endemic Community in Tanzania.
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To determine the sensitivity, specificity, and field utility of the Cepheid GeneXpert Chlamydia trachomatis (CT) Assay (GeneXpert) for ocular chlamydia infection compared to Roche Amplicor CT assay (Amplicor). In a trachoma-endemic community in Kongwa Tanzania, 144 children ages 0 to 9 were surveyed to assess clinical trachoma and had two ocular swabs taken. One swab was processed at Johns Hopkins University, Baltimore MD, using Amplicor, (Roche Molecular Diagnostics) and the other swab was processed at a field station in Kongwa using the GeneXpert Chlamydia trachomatis/Neisseria gonorrhoeae assay (Cepheid). The sensitivity and specificity of GeneXpert was compared to the Amplicor assay. Of the 144 swabs taken the prevalence of follicular trachoma by clinical exam was 43.7%, and by evidence of infection according to Amplicor was 28.5%. A total of 17 specimens (11.8%) could not be processed by GeneXpert in the field due to lack of sample volume, other specimen issues or electricity failure. The sensitivity of GeneXpert when compared to Amplicor was 100% and the specificity was 95%. The GeneXpert test identified more positives in individuals with clinical trachoma than Amplicor, 55% versus 52%. The GeneXpert test for C. trachomatis performed with high sensitivity and specificity and demonstrated excellent promise as a field test for trachoma control.\u
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