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Search Results: 1 - 10 of 151604 matches for " Gerald H. Share "
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Spatial and Temporal Variability of the Gamma Radiation from Earth's Atmosphere during a Solar Cycle
Michael J. Harris,Gerald H. Share,Mark D. Leising
Physics , 2003, DOI: 10.1029/2003JA009958
Abstract: The Solar Maximum Mission satellite's Gamma Ray Spectrometer observed Earth's atmosphere for most of the period 1980-1989. Its 28deg orbit ensured that a range of geomagnetic latitudes (geomagnetic cutoff rigidities) was sampled. We measured the variation with time and rigidity of albedo gamma-ray lines at 1.6 MeV, 2.3 MeV and 4.4 MeV which are diagnostic of Galactic cosmic radiation penetrating the cutoff and of the secondary neutrons produced in the atmosphere. We found that the gamma-ray line intensities varied inversely with solar activity and cutoff rigidity, as expected. The line ratio (1.6 MeV + 2.3 MeV)/4.4 MeV was remarkably constant (close to 0.39) at all times and rigidities; the former two lines are produced by 5-10 MeV secondary neutrons causing excitation and de-excitation of 14N, while the latter is produced by more energetic (>20 MeV) neutrons inducing spallation. We infer that the shape of the secondary neutron energy spectrum is virtually constant everywhere and at all times. We also measured the intensity of the 0.511 MeV electron-positron annihilation line. This line too varies with solar cycle and cutoff rigidity, but its fall-off from low to high rigidity is less marked than that of the nuclear lines. This results from the energy dependences of the cross sections for positron production and for the hadronic processes which which produce secondary neutrons.
Physics of Solar Neutron Production: Questionable Detection of Neutrons from the 2007 December 31 Flare
Gerald H. Share,Ronald J. Murphy,Allan J. Tylka,Benz Kozlovsky,James M. Ryan,Chul Gwon
Physics , 2010, DOI: 10.1029/2010JA015930
Abstract: Spacecraft observations in the inner heliosphere offer the first opportunity to measure 1-10 MeV solar neutrons. We discuss the physics of low-energy neutron production in solar flares and show that, even at interacting-particle energies of 2 MeV/nucleon, neutrons with energies >10 MeV are produced. On the other hand, a significant fraction of 1-10 MeV neutrons result from interactions of >10 MeV/nucleon ions in typical flare spectra. We calculate the escaping neutron spectra for mono-energetic and power-law particle spectra at the Sun for the location and observation angle of MESSENGER at the time of its reported detection of low-energy neutrons associated with the 2007 December 31 solar flare. We detail concerns about this questionable observation of solar neutrons: 1. the inferred number of accelerated protons at the Sun for this modest M2-class flare was 10X larger than any flare observed to date, 2. the onset and duration of the 'solar' neutron count rate was similar to that of the solar energetic particles (SEPs), and 3. the authors' argument that the SEPs were dominated by electrons and so could not have produced the neutron counts locally in the spacecraft. In contrast we argue that solar energetic protons and alpha particles, through local neutron production and accidental coincidences, were the source of most of the reported 'solar-neutron' counts.
High-Resolution Spectroscopy of Gamma-Ray Lines from the X-Class Solar Flare of 23 July, 2002
David M. Smith,Gerald H. Share,Ronald J. Murphy,Richard A. Schwartz,Albert Y. Shih,Robert P. Lin
Physics , 2003, DOI: 10.1086/378173
Abstract: The Reuven Ramaty High Energy Solar Spectroscopy Imager (RHESSI) has obtained the first high-resolution measurements of nuclear de-excitation lines produced by energetic ions accelerated in a solar flare, a GOES X4.8 event occurring on 23 July, 2002 at a heliocentric angle of 73 degrees. Lines of neon, magnesium, silicon, iron, carbon, and oxygen were resolved for the first time. They exhibit Doppler redshifts of 0.1--0.8% and broadening of 0.1--2.1% (FWHM), generally decreasing with mass. The measured redshifts are larger than expected for a model of an interacting ion distribution isotropic in the downward hemisphere in a radial magnetic field. Possible interpretations of the large redshifts include 1) an inclination of the loop magnetic field to the solar surface so that the ion distribution is oriented more directly away from the observer, and 2) extreme beaming of the ions downward along a magnetic field normal to the solar surface. Bulk downward motion of the plasma in which the accelerated ions interact can be ruled out.
Comment on "Detection and characterization of 0.5-8 MeV neutrons near Mercury: Evidence for a solar origin"
Gerald Share,Ronald Murphy,Allan Tylka,Brian Dennis,James Ryan
Physics , 2014, DOI: 10.1002/2014JA020663
Abstract: We argue that the hour-long neutron transient detected by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Neutron Spectrometer beginning at 15:45 UT on 2011 June 4 is due to secondary neutrons from energetic protons interacting in the spacecraft. The protons were probably accelerated by a shock that passed the spacecraft about thirty minutes earlier. We reach this conclusion after a study of data from the MESSENGER neutron spectrometer, gamma-ray spectrometer, X-ray Spectrometer, and Energetic Particle Spectrometer, and from the particle spectrometers on STEREO A. Our conclusion differs markedly from that given by Lawrence et al. [2014] who claimed that there is "strong evidence" that the neutrons were produced by the interaction of ions in the solar atmosphere. We identify significant faults with the authors' arguments that led them to that conclusion.
Directionality of Solar Flare Accelerated Protons and Alpha Particles from Gamma-Ray Line Measurements
G. H. Share,R. J. Murphy,J. Kiener,N. de Sereville
Physics , 2002, DOI: 10.1086/340595
Abstract: The energies and widths of gamma-ray lines emitted by ambient nuclei excited by flare-accelerated protons and alpha particles provide information on the ions directionality and spectra, and on the characteristics of the interaction region. We have measured the energies and widths of strong lines from de-excitations of 12C, 16O, and 20Ne in solar flares as a function of heliocentric angle. The line energies from all three nuclei exhibit ~1% redshifts for flares at small heliocentric angles, but are not shifted near the limb. The lines have widths of ~3% FWHM. We compare the 12C line measurements for flares at five different heliocentric angles with calculations for different interacting-particle distributions. A downward isotropic distribution (or one with a small upward component) provides a good fit to the line measurements. An angular distribution derived for particles that undergo significant pitch angle scattering by MHD turbulence in coronal magnetic loops provides comparably good fits.
Tumbling Motion of Elliptical Particles in Viscous Two-Dimensional Flow
Gerald H. Ristow
Physics , 2000, DOI: 10.1142/S0129183101001559
Abstract: The settling dynamics of spherical and elliptical particles in a viscous Newtonian fluid are investigated numerically using a finite difference technique. The terminal velocity for spherical particles is calculated for different system sizes and the extrapolated value for an infinite system size is compared to the Oseen approximation. Special attention is given to the settling and tumbling motion of elliptical particles where their terminal velocity is compared with the one of the surface equivalent spherical particle.
Transition to Centrifugal Particle Motion in Rotating Drums
Gerald H. Ristow
Physics , 1998,
Abstract: The dynamics and the transition to the centrifugal regime are studied analytically and numerically for particles in rotating drum. The importance of the particle-wall friction coefficient is demonstrated by studying first the motion of one non-rotating particle where three different regimes are found in the transition to the centrifugal motion. When a few rotating particles are considered, they behave similarly to one non-rotating particle in the low friction limit. A critical particle number is necessary to reach the centrifugal regime for which an analytic expression is derived in the limit of negligible inter-particle friction.
Critical Exponents for Granular Phase Transitions
Gerald H. Ristow
Physics , 1997, DOI: 10.1209/epl/i1997-00514-3
Abstract: The solid--fluid phase transition of a granular material shaken horizontally is investigated numerically. We find that it is a second-order phase transition and propose two order parameters, namely the averaged kinetic energy and the averaged granular temperature, to determine the fluidization point precisely. It scales with the acceleration of the external vibration. Using this fluidization point as critical point, we discuss the scaling of the kinetic energy and show that the kinetic energy and the granular temperature show two different universal critical point exponents for a wide range of excitation amplitudes.
Educa??o para a leitura crítica da mídia, democracia radical e a reconstru??o da educa??o
Kellner, Douglas;Share, Jeff;
Educa??o & Sociedade , 2008, DOI: 10.1590/S0101-73302008000300004
Abstract: this article explores the theoretical underpinnings of critical media education and analyzes contrasting approaches to teaching it. combining cultural studies with critical pedagogy, we argue for a critical media literacy that aims to expand the notion of literacy to include a wide range of forms of media culture, information and communication technologies and new media, as well as deepen the potential of literacy education to critically analyze relationships between media and audiences, information and power. a multiperspectivist approach addressing issues of gender, race, class and power is used to explore the interconnections of media literacy, cultural studies and critical pedagogy. our version of critical media literacy integrates analysis with production and aims at empowering students to participate fully in their society and thus promotes radical democracy and social justice.
The Role of Grandparents in Childcare in Ireland: Towards a Research Agenda
Michelle Share,Liz Kerrins
Irish Journal of Applied Social Studies , 2009,
Abstract: This paper discusses the role of grandparents in informal childcare in Ireland. It considers how recent demographic change and government policy on childcare have the potential to place greater pressures on the provision of grandparent childcare. It illustrates research literature that has examined the prevalence and intensity of grandparent care, factors influencing such care, and the role and needs of grandparents in childcare. We argue that there are significant gaps in Irish research evidence about the extent of, role and needs of grandparents as childcarers and outline a data and research agenda that builds on previous research in this area.
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