1,720,999 research outputs found
Trimpi Perturbations from Large Ionisation Enhancement Patches
No full textA number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of enhanced electron density in the D-region caused by lightning induced electron precipitation (LIEs). These LIEs can result in phase and amplitude perturbations on received VLF signals that are referred to as Trimpis. It is important, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum ?Ne values assumed have often been too large and patches have been incorrectly modeled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum ?Ne, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian profile in all coordinates. Results for a large elliptical LIE ~ 200 x 600 kms show that significant Trimpis, ~-0.4dB in amplitude and ~+4 degrees in phase are predicted, using modest maximum ?Ne values ~ 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed fluxes of electron precipitation following wave particle interactions with whistler-mode waves. This shows the continued viability of the original explanation of whistler-induced electron precipitation as the mechanism for the “Classical Trimpi”
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Determining scintillation effects on GPS receivers
No full textAlthough formulae are available to determine tracking jitter (variance of the phase tracking error at the output of the Global Navigation Satellite Systems receiver phase‐locked loop) resulting from scintillation for GPS/SBAS C/A code processing and semicodeless GPS L1 and L2 Y‐code, these require input of the spectral parameters p (inverse power law of the phase power spectral density (PSD)) and T (spectral strength of the phase PSD at 1 Hz) which will not generally be available. It would certainly be more convenient if tracking jitter could be determined just from scintillation indices (S4 and σ) enabling determination when spectral parameters are not readily available and permitting tracking jitter for all the simultaneously observed satellites to be easily determined and used in a scintillation mitigation scheme. The main difficulty is that the Fresnel frequency, fF, which is an important feature of the amplitude PSD, should be known. Here a method is proposed which uses both scintillation indices (σ and S4) to give an additional relation to find both p and T. This makes use of the known general fading frequency behavior of the PSD spectrum which is different between amplitude and phase scintillation. This difference is exploited, utilizing approximate models of the PSD for both amplitude and phase, to define equations that can be solved for p and T for any given fF. Even when fF is not known, it is shown that by taking account of the range of physically realistic values of fF, the tracking jitter can generally be determined to a reasonable degree of accuracy
Determining spectral parameters (p and T) and GPS receiver tracking jitter from scintillation indices
No full textA basis for a method of mitigating the effect of ionospheric scintillation on GPS positioning is to weight the measurements from each satellite in the positioning calculation inversely according to the tracking jitter present [1]. Tracking jitter resulting from scintillation for GPS/SBAS C/A code processing and semicodeless GPS L1 and L2 Y-code can be determined from formulae presented by [2] but require input of the spectral parameters p (inverse power law of the phase PSD) and T (spectral strength of the phase PSD at 1 Hz) which will not generally be available, It is certainly more convenient if tracking jitter can be found just from phase and amplitude scintillation indices which are easier to determine than spectral parameters since they only require time-domain processing, obviating the requirement of applying FFT transforms to every minute of data. Thus it is desirable to find a method of transforming scintillation indices into spectral parameters. The main difficulty is that the Fresnel frequency, which is an important feature of the amplitude PSD, really needs to be known. It has been shown [3] that utilizing approximate models of the PSD for both amplitude and phase, it is possible to define equations, utilizing known values of both scintillation indices, that can be solved for p and T. However, although this method requires the Fresnel frequency, it is found shown that the tracking jitter can be determined to a reasonable degree of accuracy by restricting the solution to physically realistic values of the Fresnel frequency. The method has been used to determine the spectral parameters and tracking jitter for both high and low latitude GPS data and comparison made with tracking jitter determined using p and T values measured from phase PSDs. As would be expected, for the method to work successfully, the amplitude and phase spectra of the data must approximately match the model spectra employed in the calculations. This condition can fail either for too much noise (particularly for low scintillation levels) or for very strongest scintillation conditions when the spectra can become dual-slope or more Gaussian in shape. This is discussed together with problems in validating the method from experimental data; in particular experimental determination of the Fresnel frequency and the problem of measuring the slope for both the amplitude and phase PSDs
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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