177,659 research outputs found
Cyclotron amplification of whistler-mode waves: a parametric study relevant to discrete VLF emissions in the earth's magnetosphere
No full textWe study the non linear amplification of VLF waves in the earth's magnetosphere. Particular attention is paid to zero order distribution functions of electrons in which there is a sharp step with respect to parallel velocity. It is shown that such steps strongly favour both the linear and non linear amplification processes. The results obtained are discussed in the light of recent observations of VLF emissions, particularly those of Bell and co workers at Stanford University
Highly anisotropic distribution of energetic electrons and triggered VLF emissions
No full textThe work considers the highly anisotropic electron distribution functions observed by Bell and co-workers at Stanford in the magnetosphere. IOt is maintained that Bells' observations provide evidence for the existence of step like deformations in electron distribution function, and that his observations may be due to such steps. The origin of such steps at the top of hiss band emissions is explained, and the implications for triggering of emissions and chorus is explained
Cyclotron acceleration of radiation belt electrons by whistlers
No full textThe work considers the non linear scattering of energetic electrons in the earth's radiation belts due to cyclotron interaction with VLF whistlers. In particular we consdier the acceleration of electrons which may result from trapping in the inhomogeneous medium. It is shown that considerable electron heating may result, and that the very anisotorpic electron distribution functions observed by Bell etal may be explained
VLF emission triggering by a highly anisotropic electron plasma
No full textA recent paper by Bell et al (Bell et al,2000) reports observations from the POLAR spacecraft of highly anisotropic hot electron distribution functions in the equatorial region of the magnetosphere at L=3.4. The particle instrument HYDRA measures electron fluxes from 1-20 keV. VLF emissions triggered by pulses from Omega (Norway) are found to coincide with 'pancake' type electron distributions with average pitch angles >70 degrees, such distributions being effectively confined to the equatorial zone. We examine the linear and non linear wave particle interaction process between pancake distributions and CW ducted VLF signals. It is concluded that the pitch angle range 67-76 degrees dominates the interaction process, and that with in duct wave saturation amplitudes of 6pT strong non linear trapping occurs for these particles. It is difficult to avoid the impression that highly anisotropic pitch angle distributions don’t have a great effect on resonant particle dynamics. High anisotropy has raised the pitch angle of maximum non linear contribution from 61->72 degrees, and reduced particle non linearity somewhat, in that the onset of trapping occurs at 2pT rather than 1.6pT. Using this data a 1D Vlasov Hybrid Simulation (VHS) VLF code was run to numerically simulate risers triggered by a 1 s Omega pulse. The VHS algorithm defines a time varying phase space simulation box covering the trans-equatorial nonlinear trapping region and a segment of parallel velocity space centred on the local resonance velocity. The simulation particles have F defined as a constant on their trajectories by Liouville's theorem. At each time step F is interpolated from the particles onto the fixed phase space grid, allowing resonant particle current to be calculated. The VHS method is extremely efficient since at each step particles leaving the phase box are discarded, and fresh particles are embedded into the phase fluid where the latter flows into the phase box. Successful numerical triggering of emissions by Omega is shown, and examples of risers, fallers and hooks are shown. The integrated linear trans-equatorial amplification of ~10dB agreed well with figures calculated by Bell from HYDRA data. These successful simulations of Omega emissions with highly anisotropic distribution functions confirm that non linear trapping of cyclotron resonant electrons in the geomagnetic field is the root plasma physical mechanism behind the triggering of VLF emissions
A parametric study of the numerical simulations of triggered VLF emissions
No full textThis work is concerned with the numerical modelling of VLF emissions triggered in the equatorial region of the Earth’s magnetosphere, using a well established 1D Vlasov Hybrid Simulation (VHS) code. Although this code reproduces observed ground based emissions well there is some uncertainty regarding the magnitude of simulation parameters such as saturation wave amplitude, cold plasma density, linear growth rate and simulation bandwidth. Concentrating on emissions triggered by pulses of VLF radio waves from the transmitter at Siple Station, Antarctica (L=4.2), these parameters, as well as triggering pulse length and amplitude, are systematically varied. This parametric study leads to an understanding of the physics of the triggering process and also of how the properties of these emissions, particularly their frequency time profile, depend upon these parameters. The main results are that weak power input tends to generate fallers, intermediate power input gives stable risers and strong growth rates give fallers, hooks or oscillating tones. The main factor determining the frequency sweep rate - of either sign - turns out to be the cold plasma density, lower densities giving larger sweep rates
A realistic evaluation : the case of protocol-based care
No full textBackground
'Protocol based care' was envisioned by policy makers as a mechanism for delivering on the service improvement agenda in England. Realistic evaluation is an increasingly popular approach, but few published examples exist, particularly in implementation research. To fill this gap, within this paper we describe the application of a realistic evaluation approach to the study of protocol-based care, whilst sharing findings of relevance about standardising care through the use of protocols, guidelines, and pathways.
Methods
Situated between positivism and relativism, realistic evaluation is concerned with the identification of underlying causal mechanisms, how they work, and under what conditions. Fundamentally it focuses attention on finding out what works, for whom, how, and in what circumstances.
Results
In this research, we were interested in understanding the relationships between the type and nature of particular approaches to protocol-based care (mechanisms), within different clinical settings (context), and what impacts this resulted in (outcomes). An evidence review using the principles of realist synthesis resulted in a number of propositions, i.e., context, mechanism, and outcome threads (CMOs). These propositions were then 'tested' through multiple case studies, using multiple methods including non-participant observation, interviews, and document analysis through an iterative analysis process. The initial propositions (conjectured CMOs) only partially corresponded to the findings that emerged during analysis. From the iterative analysis process of scrutinising mechanisms, context, and outcomes we were able to draw out some theoretically generalisable features about what works, for whom, how, and what circumstances in relation to the use of standardised care approaches (refined CMOs).
Conclusions
As one of the first studies to apply realistic evaluation in implementation research, it was a good fit, particularly given the growing emphasis on understanding how context influences evidence-based practice. The strengths and limitations of the approach are considered, including how to operationalise it and some of the challenges. This approach provided a useful interpretive framework with which to make sense of the multiple factors that were simultaneously at play and being observed through various data sources, and for developing explanatory theory about using standardised care approaches in practice
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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A scientific career launched at the start of the space age: Michael Rycroft at 80
No full textThe scientific career of Michael Rycroft (born in 1938) spans the space age, during which significant changes have occurred in how scientists work, experiment, and interact. Here, as part of his 80th birthday celebrations, we review his career to date in terms of the social and structural changes in collaborative international science. His contributions to research, teaching, and management across solar–terrestrial and ionospheric physics as well as atmospheric and space science are also discussed
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