1,720,984 research outputs found
Adaptive Local Iterative Filtering: A Promising Technique for the Analysis of Nonstationary Signals
No full textMany real-life signals and, in particular, in the space physics domain, exhibit variations across different temporal scales. Hence, their statistical momenta may depend on the time scale at which the signal is studied. To identify and quantify such variations, a time-frequency analysis has to be performed on these signals. The dependence of the statistical properties of a signal fluctuation on the space and time scales is the distinctive character of systems with nonlinear couplings among different modes. Hence, assessing how the statistics of signal fluctuations vary with scale will be of help in understanding the corresponding multiscale statistics of such dynamics. This paper presents a new multiscale data analysis technique, the adaptive local iterative filtering (ALIF), which allows to describe the multiscale nature of the geophysical signal studied better than via Fourier transform, and improves scale resolution with respect to discrete wavelet transform. The example of geophysical signal, to which ALIF has been applied, is ionospheric radio power scintillation on L band. ALIF appears to be a promising technique to study the small-scale structures of radio scintillation due to ionospheric turbulence
Role of the external drivers in the occurrence of low-latitude ionospheric scintillation revealed by multi-scale analysis
No full textIn the low latitude ionosphere, the formation of Equatorial Plasma Bubbles presents a regular behavior under quiet conditions ofthe geospace. The ionospheric irregularities embedded in the plasma bubbles may lead to amplitude scintillation of Global Navigation Satellite Systems signals. Solar events disturb the regular behavior of the magnetosphere-ionosphere system, leading to an intensification or a suppression of such ionospheric irregularities producing scintillations. During the same storm, inhibition and intensification ofthe ionospheric scintillations can both occur, depending on the local time ofthe storm arrival and on the storm features. Electric fields penetrating from the auroral latitudes and disturbing the ionospheric electrodynamics are commonly highlighted as the principal responsible for the inhibited/enhanced scintillations. Beside this mechanism, the disturbance dynamo is the concurrent key-physical phenomenon, being due to variations ofthe thermospheric winds induced by heating convecting from high towards equatorial latitudes and disturbing the electrodynamics ofthe Equatorial Electrojet [1]. In the present work, we analyze the scintillation over San Miguel de Tucumán (Argentina), located under the southern crest ofthe Equatorial Ionospheric Anomaly, focusing on the multi-scale variability and on the causal relationship between forcing factors from the geospace and the ionospheric response
Role of the external drivers in the occurrence of low-latitude ionospheric scintillation revealed by multi-scale analysis
Full text linkWe analyze the amplitude scintillation on L-band signals over San Miguel de Tucumán (Argentina), focusing on the multi-scale variability and speculating on the possible relationship between forcing factors from the geospace and the ionospheric response. The site is nominally located below the expected position of the southern crest of the Equatorial Ionospheric Anomaly (EIA). For this scope, we concentrate on the period 1-31 March 2011, during which one minor and one moderate storm characterize the first half of the month, while generally quiet conditions of the geospace stand for the second half. By leveraging on the Adaptive Local Iterative Filtering (ALIF) signal decomposition technique, we investigate the multi-scale properties of Global Navigation Satellite Systems (GNSS) amplitude scintillation and helio-geophysical parameters, looking for possible cause-effect mechanisms relating the former to the latter. Namely, we identify resonant modes in the Akasofu (ε) parameter as likely related to the frequency components in the time evolution found for the amplitude scintillation index, hence modulating the scintillation itself
NmF2 trends at low and mid latitudes for the recent solar minima and comparison with IRI-2012 model
No full textThe ionospheric electron density peak (NmF2) is analyzed for the recent minima of solar activity for two mid-latitude stations, Rome (41.8°N, 12.5°E, geomagnetic latitude 41.7°N, Italy) and Gibilmanna (37.9°N, 14.0°E, geomagnetic latitude 37.6°N, Italy), and for the low-latitude station of Tucumán (26.9°S, 294.6°E, geomagnetic latitude 17.2°S, Argentina), located in the south ridge of the equatorial ionization anomaly. An inter-minima comparison reveals that from an ionospheric point of view the last minimum of solar activity (minimum 23/24) was peculiar, with values of NmF2 lower than those recorded during the previous minima for all the stations and all the hours of the day. A more pronounced decrease is observed at Tucumán than at Rome and Gibilmanna. The study of the winter and semi-annual anomaly shows that at mid-latitude stations the winter anomaly is not visible only for the years 2008 and 2009, which represent the deeper part of the prolonged and anomalous last solar minimum. The same is for the semi-annual anomaly. A comparison with the version 2012 of the International Reference Ionosphere model (IRI) is also carried out. The results reveal that for low solar activity the model works better at mid latitudes than at low latitudes, confirming the problems of IRI in correctly representing the low-latitude ionosphere. Nevertheless, using as input updated values of the solar and geomagnetic indices, no loss of accuracy is detected in the IRI performances for the last solar minimum with respect to the previous ones, both at mid and low latitudes.Fil: Perna, L.. Universidad de Bologna; Italia. Istituto Nazionale di Geofisica e Vulcanologia; ItaliaFil: Pezzopane, M.. Istituto Nazionale di Geofisica e Vulcanologia; ItaliaFil: Ezquer, Rodolfo Gerardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Tecnológica Nacional. Facultad Regional Tucuman; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Ionosfera; ArgentinaFil: Cabrera, M.. Universidad Tecnológica Nacional. Facultad Regional Tucuman; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Electricidad, Electrónica y Computación. Laboratorio de Telecomunicaciones; ArgentinaFil: Baskaradas, J.A.. Sastra University; Indi
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
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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