1,720,982 research outputs found
Galactic cosmic-ray recurrent and transient short-term variations: advanced data analysis and modeling
Full text linkA particle detector (PD) on board the European Space Agency LISA Pathfinder (LPF) mission allowed for the detection of galactic-cosmic rays (GCRs) above 70 MeV/n. The LPF spacecraft (S/ C) orbited around the L1 Lagrangian point from February 2016 through July 2017. Proton and helium nuclei (constituting roughly 98% of the GCR bulk in the inner heliosphere) were sampled at 0.067 Hz and hourly-averaged data allowed for the study of long-term and short-term GCR flux variations with a statistical uncertainty of 1%. Since LPF was in orbit for one year and a half during the descending phase of the solar cycle N. 24 in a positive polarity period, a corresponding long-term increase in the mean GCR count-rate during the mission elapsed time was observed. Short-term GCR flux variations observed by LPF are studied in detail. Some of those variations are found to be recurrent in time since they are associated with solar wind disturbances originated from long-living structure on the Sun surface. The quasi-periodicities related to Sun rotation are investigated through the Hilbert spectral analysis, using the Hilbert-Huang transform. The association of GCR recurrent short-term variations with solar wind disturbances is discussed. Another class of short-term variations consists of depressions or peaks in the GCR count rate associated with transient solar wind disturbances. The most intense transient phenomena of solar origin are represented by the interplanetary counterparts of coronal mass ejections (ICMEs). The typical signature on GCR observations of an ICME transit is a sudden intensity decrease and a gradual recovery, called Forbush decrease (FD). Three FDs were observed during the LPF mission elapsed time. The second part of this work focuses on the characterization of a FD observed on board LPF on August 2, 2016. A near-Earth ICME transit was observed in correspondence of this FD and a smooth rotating magnetic field accompanied by low values of temperature and plasma-beta allowed for the identification of a clear magnetic cloud (MC) structure. The Grad-Shafranov (GS) reconstruction is applied in order to retrieve the large-scale magnetic field configuration within the ICME. The GS reconstruction is an advanced data-analysis technique aiming to recover the configuration of a magnetic closed structure starting from single S/C in a quasi-3-D geometry. After performing the GS reconstruction, we combined this technique with a suited Monte Carlo simulation, in the test-particle approximation, in order to investigate the effect of the MC passage in terms of cosmic-ray modulation. In this simulation, the only physical process able to modulate the isotropic particle flux is the particle drift associated with gradient and curvature of the magnetic field. The simulation results show an excellent agreement with LPF data during the FD and a study of the energy dependence of the simulated decrease is found to be in agreement with ground-based observations. Our result suggests that for the analyzed MC a fraction of low-energy particles do leak into the MC, remaining trapped and/or pushed out by external closed field lines. Moreover, the small magnetic field fluctuations observed inside the MC does not favor a large-scale cross-field transport through diffusion process or random walk. Hence, we suggest that gradient and curvature drifts are mainly responsible for GCR FDs originated by large-scale closed MC structure
Low-energy electron emission at the separation of gold-platinum surfaces induced by galactic cosmic rays on board LISA Pathfinder
Full text linkGalactic cosmic rays and solar energetic particles will induce spurious Coulomb forces on free-falling, gold-platinum test-masses on board the future interferometers for low-frequency gravitational wave detection in space. The European Space Agency mission LISA Pathfinder (LPF) was aimed to test the performance of instruments that will be placed on board LISA (Laser Interferometric Space Antenna), expected to be launched in 2034. Free-falling test-mass charging was measured on board LPF in 2016–2017. Simulations of this charging process were carried out before the mission launch with GEANT4 and Fluka Monte Carlo tools. A very good agreement was found between net charging measurements and simulations, while the shot noise, associated with the charging process, resulted 3–4 times larger than expected. New dedicated simulations aiming to study in detail the low-energy electron emission from metal electrodes surrounding the test-masses reveal that the mismatch between measurements and former simulations may be due to the lack of propagation of electrons with energies smaller than the average ionization potential in the Monte Carlo tools mentioned above. Preliminary results are presented and discussed here
A New Method to Model Magnetic Cloud-driven Forbush Decreases: The 2016 August 2 Event
Full text linknterplanetary coronal mass ejections (ICMEs), generally containing magnetic clouds (MCs), are associated with galactic-cosmic ray (GCR) intensity depressions known as Forbush decreases (FDs). An ICME was observed at L1 between 2016 August 2 at 14:00 UT and August 3 at 03:00 UT. The MC region was identified and its magnetic configuration was retrieved by using the Grad-Shafranov (GS) reconstruction. A weak FD in the GCR count-rate was observed on 2016 August 2 by a particle detector on board the European Space Agency LISA Pathfinder mission. The spacecraft orbited around L1 and the particle detector allowed us to monitor the GCR intensity at energies above 70 MeVn(-1). A 9% decrease in the cosmic-ray intensity was observed during the ICME passage. The first structure of the ICME caused a 6.4% sharp decrease, while the MC produced a 2.6% decrease. A suited full-orbit test-particle simulation was performed on the MC configuration obtained through the GS reconstruction. The FD amplitude and time profile obtained through the simulation show an excellent agreement with observations. The test-particle simulation allows us to derive the energy dependence of the MC-driven FD providing an estimate of the amplitude at different rigidities, here compared with several neutron monitor observations. This work points out the importance of the large-scale MC configuration in the interaction between GCRs and ICMEs and suggests that particle drifts have a primary role in modulating the GCR intensity within the MC under study and possibly in at least all slowly expanding ICMEs lacking a shock/sheath region
A novel approach in magnetic cloud-driven Forbush decrease modeling
Full text linkInterplanetary coronal mass ejections (ICMEs) are large-scale solar wind disturbances propagating from the Sun and causing a depression of the galactic-cosmic ray (GCR) intensity known as Forbush decrease (FD). IC- MEs generally contain coherent plasma structures called magnetic clouds (MCs). A unique and powerful data analysis tool allowing for the study of the quasi-3-D configuration of a MC is the Grad-Shafranov (GS) recons - truction. The aim of this work is to investigate the role played by the MC configuration in the formation of a FD. A suited full-orbit test-particle simulation has been developed in order to evaluate FD amplitude and time pro- file produced by the MC obtained with the GS reconstruction. Particle trajectories are computed starting from an isotropic flux outside the MC region. In addition, particle diffusion has been modeled by superimposing a small-angle scattering over the unperturbed charged particle motion at each time step. The model allows us to investigate the MC effect on GCR propagation and to study the energy dependence of the physical processes in - volved, as it provides an estimate of ground-based GCR counts observations at different latitudes. A comparison between model results and both space-based cosmic-ray measurements in L1 and ground-based observations suggests a major role of drifts in producing the FD and a reduced contribution of GCR particle diffusion
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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