1,401 research outputs found

    A Brief History of CME Science

    Get PDF
    We present here a brief summary of the rich heritage of observational and theoretical research leading to the development of our current understanding of the initiation, structure, and evolution of Coronal Mass Ejections

    Spectral Analysis of the September 2017 Solar Energetic Particle Events

    Get PDF
    An interval of exceptional solar activity was registered in early September 2017, late in the decay phase of solar cycle 24, involving the complex Active Region 12673 as it rotated across the western hemisphere with respect to Earth. A large number of eruptions occurred between 4 and 10 September, including four associated with Xclass flares. The X9.3 flare on 6 September and the X8.2 flare on 10 September are currently the two largest during cycle 24. Both were accompanied by fast coronal mass ejections and gave rise to solar energetic particle (SEP) events measured by nearEarth spacecraft. In particular, the partially occulted solar event on 10 September triggered a groundlevel enhancement (GLE), the second GLE of cycle 24. A further, much less energetic SEP event was recorded on 4 September. In this work we analyze observations by the Advanced Composition Explorer (ACE) and the Geostationary Operational Environmental Satellites (GOES), estimating the SEP eventintegrated spectra above 300 keV and carrying out a detailed study of the spectral shape temporal evolution. Derived spectra are characterized by a lowenergy break at few/tens of MeV; the 10 September event spectrum, extending up to 1 GeV, exhibits an additional rollover at several hundred MeV. We discuss the spectral interpretation in the scenario of shock acceleration and in terms of other important external influences related to interplanetary transport and magnetic connectivity, taking advantage of multipoint observations from the Solar Terrestrial Relations Observatory. Spectral results are also compared with those obtained for the 17 May 2012 GLE event

    Thermal stability of C-S-H phases and applicability of Richardson and Grovesā€™ and Richardson C-(A)-S-H(I) models to synthetic C-S-H

    Get PDF
    Synthetic C-S-H samples prepared with bulk C/S ratios from 0.75 to 1.5 were analyzed by coupled TG/DSC/FTIR and in-situ XRD while heating, in order to correlate observed weight loss curves with the kinetics of evolved gases, and to investigate the transformations C-S-Hā†’Ī²-wollastoniteā†’Ī±-wollastonite. The temperature of the transformation to Ī²-wollastonite increased with increasing C/S. The temperature for the transformation from Ī²- to Ī±-wollastonite meanwhile decreased with increasing C/S; indicating that excess CaO stabilized the Ī±-polymorph. The transformation C-S-Hā†’Ī²-wollastonite was accompanied by the formation of Ī±`LC2S for C/S > 1. In the case of C-S-H with C/S = 1.5, both Ī²-C2S and rankinite were formed and then decomposed before the transformation to Ī²-wollastonite and Ī±`LC2S. C-S-H with low C/S was found to be more stable upon heating. The chemical structural models of Richardson and Grovesā€™ and Richardson C-A-S-H(I) were used to obtain the structural-chemical formulae

    Micro- and nano-structural evolutions in white Portland cement/pulverized fuel ash cement pastes due to deionized-water leaching

    No full text
    Thin slices of white Portland cement-low calcium pulverized fuel ash (pfa) blended cement pastes containing 30 or 50% pfa were leached progressively in de-ionized water. The paste with 50% pfa was aged 13 years prior to leaching and those with 30% pfa were aged 1 and 13 years. Pastes were leached for up to 75 days and were characterized using thermal analysis, X-ray diffraction, analytical scanning and transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. Leaching affected the pastes in the following sequence: (i) crystals of Ca(OH)2 large enough to be resolved by backscattered electron imaging were removed completely prior to any effect on C-A-S-H; (ii) the Ca/Si ratio of C-A-S-H reduced from ā‰ˆ1.4 to ā‰ˆ1.0 whilst the aluminosilicate structure was unaffected; (iii) further reduction in the Ca/Si ratio of C-A-S-H was accompanied by lengthening of the aluminosilicate chains; (iv) the Ca/Si ratio of C-A-S-H reduced ultimately to ā‰ˆ0.6
    • ā€¦
    corecore