1,721,864 research outputs found
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
The Connection between solar magnetic fields and photospheric dynamics
Full text linkLa convezione rappresenta il meccanismo principale di trasporto dell’energia negli strati sottostanti la superficie solare. La dinamica dei flussi fotosferici associati a tale meccanismo determina la formazione e l’evoluzione del campo magnetico globale e di una grande varietà di strutture presenti
nelle regioni più esterne del Sole. In particolare l’interazione tra i flussi di plasma e il campo magnetico determina la configurazione spaziale e l’evoluzione delle regioni attive e degli elementi magnetici superficiali, importanti ad esempio nel determinare la variabilità solare.
La convezione solare può essere studiata o mediante lo sviluppo di simulazioni di magnetoconvezione (simulazioni MHD) o attraverso osservazioni spettrali della superficie solare.
In questo lavoro il problema della connessione tra campi magnetici solari e dinamica fotosferica è stato affrontato seguendo un approccio sperimentale. In particolare abbiamo lavorato sui sistemi di
acquisizione per la spettroscopia solare bidimensionale, sulla pipeline di riduzione di dati spettroscopici solari e infine sull’analisi dei dati.
Uno degli strumenti principali della fisica solare sperimentale è la spettroscopia, che permette di derivare informazioni su molti parametri dell’atmosfera solare, quali velocità, temperatura e campo magnetico. Inoltre, l’analisi spettroscopica permette di ricavare la velocità verticale delle strutture emergenti sulla superficie solare. In questo modo, poiché ogni lunghezza d’onda può essere associata ad una determinata quota nell’atmosfera, è possibile trasformare un’immagine bidimensionale in un campo 3D. Al fine di studiare la dinamica dell’atmosfera solare, sono necessarie osservazioni ad alta risoluzione spettrale e spaziale. Inoltre, la rapida evoluzione delle strutture solari osservate richiede monocromatori con un’elevata trasparenza per acquisire spettri multi-riga in un tempo molto breve.
Uno strumento che soddisfa tutte queste richieste è IBIS (Interferometric BIdimensional Spectrometer), uno spettrometro bidimensionale installato presso il Dunn Solar Telescope-DST. IBIS produce dati con elevata risoluzione spaziale (0.2” al DST), spettrale (Dl/l~200000) e temporale (tempo di esposizione 10 ms, rate di acquisizione 5 immagini al secondo). Le immagini acuiqiste con IBIS sono registrare da un sensore CCD.
Il Capitolo 1 della tesi fornisce un’introduzione alla spettroscopia solare e all’uso delle immagini spettroscopiche per ottenere informazioni sulla dinamica degli strati fotosferici solari. Lo schema dello
strumento IBIS, utilizzato in questa tesi per l’acquisizione delle immagini spettroscopiche, è descritto.
Nel Capitolo 2 sono riportate le misure e le calibrazioni, effettuate in laboratorio attraverso la Tecnica della Photon Transfer, di due sensori: il sensore CMOS Si-1920-HD e il sensore EMCCD Andor Ixon
DV885. Il nostro interesse in questi sensori nasce dalla necessità di sostituire il sensore attualmente installato sul canale spettrale di IBIS, al fine di incrementare l’efficienza di acquisizione dei dati. In particolare, i miglioramenti al sistema di acquisizione di IBIS riguardano diversi aspetti: aumento della
sensibilità/efficienza quantica, riduzione del tempo di lettura, incremento della dimensione dell’array e aumento del guadagno del sensore.
Nel Capitolo 3 sono descritti i vari passi della pipeline di riduzione dei dati IBIS, che include sia una correzione standard delle immagini sia un software scritto in IDL per l’analisi di immagini solari ad
elevata risoluzione.
Nel Capitolo 4 riportiamo i risultati scientifici legati allo studio dell’emersione e dell’organizzazione
del campo magnetico sulla superficie solare sia come struttura isolata sia come cluster. Tipiche strutture magnetiche isolate sono le macchie solari e i “pore”.
E’ stata studiata la dinamica, su piccola scala, di una regione di intenso campo magnetico (pore), con struttura brillante interna. I pore rappresentano una delle tante strutture formate dall’emersione del
campo magnetico sulla superficie solare. Essi rappresentano un link tra i più piccoli elementi di flusso e
le regioni magnetiche associate alle macchie. I light bridge, in un pore o in una macchia, sono strutture brillanti che dividono la regione di ombra in una strutture più o meno complessa. Comunemente, i light bridge indicano la presenza di un processo in corso all’interno della regione attiva: l’emersione di regioni magnetiche o, al contrario, il disfacimento dell’intera struttura. In entrambi i casi ci si aspetta
una riconfigurazione topologica del campo magnetico emergente.
Un altro modo per studiare l’interazione del campo magnetico con i moti del plasma consiste nell’andare ad investigare le proprietà oscillatorie della cromosfera solare, sia quieta che attiva, in
relazione alla fotosfera sottostante, ponendo particolare riguardo alla topologia del capo magnetico. Nell’atmosfera solare esiste una frequenza di cut-off acustica che produce una riflessione delle onde a
bassa frequenza verso gli strati più bassi dell’atmosfera e la regione convettiva. Dunque solo le onde con frequenza maggiori della frequenza di cut-off possono propagarsi verso gli strati più alti
dell’atmosfera. Il campo magnetico modifica dunque le proprietà delle oscillazioni acustiche. In particolare, in presenza di un campo magnetico inclinato la frequenza di cut-off acustica si abbassa, permettendo così la propagazione verso l’alto di onde a frequenza maggiore.
Questo risultato è stato confermato dalle mappe dei picchi di potenza relative alla fotosfera e alla cromosfera, ottenute utilizzando i dati acquisiti con IBIS.
Lo studio della dinamica della fotosfera solare può essere intrapreso anche con metodi statistici, analizzando le proprietà topologiche degli elementi di origine convettiva e magnetica, come la
distribuzione spaziale delle strutture presenti nei magnetogrammi. A tal proposito è stato sviluppato un algoritmo in grado di determinare, in maniera automatica, i “vuoti” in una fissata distribuzione di
particelle. Questo metodo, applicato ad una serie temporale di magnetogrammi solari con un ampio campo di vista, ha permesso di identificare i vuoti tra strutture magnetiche e di studiarne la distribuzione sulla superficie solare.Convection is the chief mode of heat transport in the outer envelopes of cool stars such as the Sun. Convective effects are recognizable in large-scale features, such as the global differential rotation
and meridional circulation flows, as well as smaller scale phenomena such as granulation, mesogranulation, and supergranulation. Moreover, convective flows widely determine the evolution and organization of tiny magnetic elements observed in the solar surface responsible for small scale irradiance solar variations.
Our understanding of the solar convection derives from numerical simulations of compressible convection (MHD approach) and from spectral observations of the solar surface (velocity and center line maps, helioseismological data, etc.).
In this work we face the problem of connection between solar magnetic fields and photospheric dynamics through an experimental approach. In particular we worked on acquisitions systems for solar imaging spectroscopy, on a pipeline for the spectroscopic data reduction and on the data
analysis.
One of the basic tools of observational solar physics is spectroscopy, which allows us to derive information on several physical parameters of solar atmosphere such as velocity, temperature, magnetic field strength etc. Spectroscopic analysis allows us to determine the vertical velocity of solar surface structures. Moreover, as wavelength can be somehow associated to depth in the solar atmosphere, it is possible to transform a bidimensional image in a 3-D field.
In order to study solar atmosphere dynamics, observations of adequate spectral purity, together with high spatial resolution to resolve small-scale structures are necessary. Moreover, the rapid evolution of observed solar features requires monochromators with high transparency to acquire multiple-line
spectra in a comparatively short time. In order to meet all these requirements, suitable instruments and techniques have to be used. An instrument which satisfies all these constraints is IBIS, an Interferometric Bidimensional Spectrometer, installed at the Dunn Solar Telescope/NSO (Sac Peak, USA). IBIS produces data with high spectral (Dl/l~200000), spatial (0.2’’ at DST telescope) and temporal resolution (exposure time 10 ms; acquisition rate 5 frames s-1). Images acquired with IBIS
are currently recorded by a CCD camera.
Chapter 1 introduces the reader to the solar spectroscopy and to the use of spectroscopic imaging to retrieve information on solar photospheric layers dynamics. The basic concept and the layout of the IBIS spectrograph, used in this thesis to acquire spectroscopic images, is described.
Chapter 2 reports laboratory measurements and calibrations, derived through the application of the Photon Transfer Technique, of two sensors: the SI-1920 HD CMOS sensor and the Andor DV885 EMCCD sensor. Our interest in these sensors is related to the necessity to replace the CCD camera, now installed on the IBIS spectral channel. Improvements in the IBIS camera system concern an increased sensitivity/quantum efficiency, a decreased detector readout time, a larger array size and an increased full well/programmable detector gain.
Chapter 3 describes the various steps of the pipeline developed for the IBIS data reduction. The pipeline includes both the standard image processing and a high performance IDL software package
written specifically for high resolution solar images.
In Chapter 4 we report some results related to the study of the emergence and the organization of the magnetic field on the solar surface both as isolated structures and as clusters. More in detail, typical isolated magnetic features are pores or sunspots.
We investigated the small scale dynamics of a strong magnetic field region (pore) with a light bridge inside it, observed with the IBIS spectrometer. An analysis of the intensity and velocity maps
revealed the presence, inside the light bridge, of elongated structures showing a kind of reversal in intensity and velocity. More in detail, in the intensity images we observed a narrow central dark lane running along the axis of the light bridge, that we explain proposing an analytical model.
Regarding the velocity structure, its topology resembles a convective roll and may indicate a modification of the photospheric convective flows.
By adopting the IBIS dataset, we studied the oscillatory properties of the solar atmosphere, in the photosphere and the chromosphere, with particular regard to the influence of the magnetic topology.
In particular, we analyzed the propagation of waves in the atmosphere in correspondence of a pore, of a magnetic network area and of a quiet Sun region. Studying the generation and propagation of
waves in the solar atmosphere provides information about the atmospheric structure and dynamics and it helps to identify the key mechanism of chromospheric and coronal heating.
Finally, by using large FoV MDI magnetograms we analyzed the spatial distribution of reticular clusters of magnetic features, such as the magnetic network. For this purpose, we developed a
numerical algorithm able to detect voids between magnetic fragments. We computed Void
Probability Functions which describe, in a uniform and objective way, the assessment of the void structure of different magnetic elements distributions
Gambling on fairness: A fair scheduler for IIoT communications based on the shell game
No full textThe Industrial Internet of Things (IIoT) paradigm represents nowadays the cornerstone of the industrial automation since it has introduced new features and services for different environments and has granted the connection of industrial machine sensors and actuators both to local processing and to the Internet. One of the most advanced network protocol stack for IoT-IIoT networks that have been developed is 6LoWPAN which supports IPv6 on top of Low-power Wireless Personal Area Networks (LoWPANs). 6LoWPAN is usually coupled with the IEEE 802.15.4 low-bitrate and low-energy MAC protocol that relies on the time-slotted channel hopping (TSCH) technique. In TSCH networks, a coordinator node synchronizes all end-devices and specifies whether (and when) they can transmit or not in order to improve their energy efficiency. In this scenario, the scheduling strategy adopted by the coordinator plays a crucial role that impacts dramatically on the network performance. In this paper, we present a novel scheduling strategy for time-slot allocation in IIoT communications which aims at the improvement of the overall network fairness. The proposed strategy mimics the well-known shell game turning the totally unfair mechanics of this game into a fair scheduling strategy. We compare our proposal with three allocation strategies, and we evaluate the fairness of each scheduler showing that our allocator outperforms the others
Service Parameters Identification for Adaptive Networked Music Performance
No full textNetworked Music Performance (NMP) aims at revolutionizing musical interaction. NMP services need straight QoS (quality of service) requirements. So far, in best effort network, musicians adopt solution that try to reduce as much as possible the delay and jitter contributions under their control (i.e., at the peripheral part outside of the network). In this paper we will present a brief overview on NMP and some of its specific requirements that should be mapped on future networks able to provide adequate QoS performances such as the promised 'deterministic Internet' or proper slicing parametrization provided by 5G cellular networks and beyond. The aim of the paper is to presents an in-depth-study of latency issue in this domain underlining the most significant blocks that contribute to the overall end-to-end latency. The proposal presented in the paper is to identify control parameters useful to more flexibly smooth the network requirements, discussing about some possible practical implementations. Lastly, a low-cost measurement set-up is described, and some preliminary experimental results are shown
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
Nonlinear elasticity of nanostructured materials
No full textWe elaborate on a blended continuum/atomistic theoretical picture of the nonlinear elastic properties of nanostructured materials, looking at diverse aspects such as dispersions of inhomogeneities within a matrix, random or graded nanograined materials, two-dimensional atomic sheets. In particular, we discuss the possible onset of length-scale effects and we establish the limits and merits of continuum versus atomistics. While most situations here discussed correspond to model systems, the main conclusions have a paradigmatic relevance and indeed apply to most nanomaterials of current interest
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
- …
