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  • 1
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    Analysis of Individual Terrestrial Gamma‐Ray Flashes With Lightning Leader Models and Fermi Gamma‐Ray Burst Monitor Data (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The Gamma‐ray Burst Monitor (GBM) onboard the Fermi spacecraft has observed many tens of sufficiently bright events, which are suitable for individual analysis. In our previous study, we fit individual, bright terrestrial gamma‐ray flashes (TGFs) with Relativistic Runaway Electron Avalanche (RREA) models for the first time. For relativistic‐feedback‐based models, the TGF‐producing electrons, which are seeded internally by a positive feedback effect, are usually accelerated in a large‐scale field with fully developed RREAs. Alternatively, lightning leader models may apply to either a large‐scale thunderstorm fields with fully developed RREAs or to inhomogeneous fields in front of lightning leaders where RREAs only develop partially. The predictions of the latter, inhomogeneous models for the TGF‐beaming geometry show some differences from estimations of the relativistic feedback models in homogeneous fields. In this work, we analyze a large sample of 66 bright Fermi GBM TGFs in the framework of lightning leader models, making comparisons with previous results from the homogeneous‐field RREA models. In most cases, the spectral analysis does not strongly favor one mechanism over the other, with 59% of the TGF events being best fit with the fully developed RREA mechanism, which corresponds to high‐potential leader models. The majority of the GBM‐measured TGFs can be best fit if the source altitude is below 15 km and 70% of events best fit by leader models cannot be satisfactorily modeled unless a tilted photon beam is used. For several spectrally soft TGFs, the tilted beam low‐potential leader model can best fit the data.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    The Energy Spectrum of X-Rays from Rocket-triggered Lightning (2015)
    Wiley
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    Publication Date: 2015-09-30
    Description: Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last ten years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pile-up in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven NaI(Tl)/photomultiplier tube (PMT) scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than of lead placed over the detector (all the detectors are in a thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with RREA. It has a power-law dependence on the energies of the photons, and the power index, λ , is between 2.5 to 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    The Effect of Direct Electron-Positron Pair Production on Relativistic Feedback Rates (2015)
    Wiley
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    Publication Date: 2015-01-10
    Description: Runaway electron avalanches developing in thunderclouds in high electric field become self-sustaining due to relativistic feedback via the production of backward propagating positrons and back-scattered x-rays. To date, only positrons created from pair production by gamma rays interacting with the air have been considered. In contrast, direct electron-positron pair production, also known as “trident process”, occurs from the interaction of energetic runaway electrons with atomic nuclei, and so it does not require the generation of a gamma ray mediator. The positrons produced in this process contribute to relativistic feedback and become especially important when the feedback factor value approaches unity. Then, the the steady state flux of runaway electrons increases significantly. In certain cases, when the strong electrostatic field forms in a narrow area, the direct positrons become one of processes dominating relativistic feedback. Calculations of the direct positron production contribution to relativistic feedback are presented for different electric field configurations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Bipolar Atlantic deepwater circulation in the middle-late Eocene: effects of Southern Ocean gateway openings (2014)
    Wiley
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    Publication Date: 2014-03-29
    Description: We present evidence for Antarctic Circumpolar Current (ACC)-like effects on Atlantic deepwater circulation beginning in the late middle Eocene. Modern ocean circulation is characterized by a thermal differentiation between Southern Ocean and North Atlantic deepwater formation regions. In order to better constrain the timing and nature of the initial thermal differentiation between Northern Component Water (NCW) and Southern Component Water (SCW), we analyze benthic foraminiferal stable isotope (δ 18 O bf and δ 13 C bf ) records from Ocean Drilling Program Site 1053 (upper deepwater, western North Atlantic). Our data, compared with published records and interpreted in the context of ocean circulation models, indicate that progressive opening of Southern Ocean gateways and initiation of a circum-Antarctic current caused a transition to a modern-like deep ocean circulation characterized by thermal differentiation between SCW and NCW beginning ~38.5 Ma, in the initial stages of Drake Passage opening. In addition, the relatively low δ 18 O bf values recorded at Site 1053 show that the cooling trend of the middle-late Eocene was not global, because it was not recorded in the North Atlantic. The timing of thermal differentiation shows that NCW contributed to ocean circulation by the late middle Eocene, ~1-4 Myr earlier than previously thought. We propose that early NCW originated in the Labrador Sea, based on tectonic reconstructions and changes in foraminiferal assemblages in this basin. Finally, we link further development of meridional isotopic gradients in the Atlantic and Pacific in the late Eocene with the Tasman Gateway deepening (~34 Ma) and the consequent development of a circumpolar proto-ACC.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Late Cretaceous–Neogene trends in deep ocean temperature and continental ice volume: Reconciling records of benthic foraminiferal geochemistry (δ18O and Mg/Ca) with sea level history (2011)
    Wiley
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    Publication Date: 2011-12-16
    Description: We reconstruct trends in ice volume and deep ocean temperature for the past 108 Myr, resolving variations on timescales of ∼2 Myr and longer. We use a sea level record as a proxy for ice volume, a benthic foraminiferal Mg/Cabf record as a proxy for temperature, and a benthic foraminiferal δ18Obf record as a proxy for both. This allows us to construct dual estimates of temperature and ice volume variations for the interval 10–60 Ma: extracting temperature from δ18Obf by using sea level as a proxy for ice volume to constrain the δ18Osw component, and extracting seawater δ18Osw (which reflects ice volume) from δ18Obf by using Mg/Cabf to constrain the temperature component. Each of these approaches requires numerous assumptions, but the range of plausible solutions are concordant on timescales 〉2 Myr and within an uncertainty of ±2°C temperature and ±0.4‰ δ18Osw. The agreement between the two approaches for the last 50 Myr provides empirical justification for the use of δ18Obf, Mg/Cabf, and sea level records as robust climate proxies. Our reconstructions indicate differences between deep ocean cooling and continental ice growth in the late Cenozoic: cooling occurred gradually in the middle–late Eocene and late Miocene–Pliocene while ice growth occurred rapidly in the earliest Oligocene, middle Miocene, and Plio-Pleistocene. These differences are consistent with climate models that imply that temperatures, set by the long-term CO2 equilibrium, should change only gradually on timescales 〉2 Myr, but growth of continental ice sheets may be rapid in response to climate thresholds due to feedbacks that are not yet fully understood.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Estimating Earthquake Magnitudes from Reported Intensities in the Central and Eastern United States (2014)
    Seismological Society of America (SSA)
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    Publication Date: 2014-08-08
    Description: A new macroseismic intensity prediction equation is derived for the central and eastern United States and is used to estimate the magnitudes of the 1811–1812 New Madrid, Missouri, and 1886 Charleston, South Carolina, earthquakes. This work improves upon previous derivations of intensity prediction equations by including additional intensity data, correcting magnitudes in the intensity datasets to moment magnitude, and accounting for the spatial and temporal population distributions. The new relation leads to moment magnitude estimates for the New Madrid earthquakes that are toward the lower range of previous studies. Depending on the intensity dataset to which the new macroseismic intensity prediction equation is applied, mean estimates for the 16 December 1811, 23 January 1812, and 7 February 1812 mainshocks, and 16 December 1811 dawn aftershock range from 6.9 to 7.1, 6.8 to 7.1, 7.3 to 7.6, and 6.3 to 6.5, respectively. One-sigma uncertainties on any given estimate could be as high as 0.3–0.4 magnitude units. We also estimate a magnitude of 6.9±0.3 for the 1886 Charleston, South Carolina, earthquake. We find a greater range of magnitude estimates when also accounting for multiple macroseismic intensity prediction equations. The inability to accurately and precisely ascertain magnitude from intensities increases the uncertainty of the central United States earthquake hazard by nearly a factor of two. Relative to the 2008 national seismic hazard maps, our range of possible 1811–1812 New Madrid earthquake magnitudes increases the coefficient of variation of seismic hazard estimates for Memphis, Tennessee, by 35%–42% for ground motions expected to be exceeded with a 2% probability in 50 years and by 27%–35% for ground motions expected to be exceeded with a 10% probability in 50 years.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 7
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    An Analytical Approach for Calculating Energy Spectra of Relativistic Runaway Electron Avalanches in Air (2014)
    Wiley
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    Publication Date: 2014-08-20
    Description: Simplified equations describing the transport and energy spectrum of runaway electrons are derived from the basic kinematics of the continuity equations. These equations are useful in modeling the energy distribution of energetic electrons in strong electric fields, such as those found inside thunderstorms. Dwyer and Babich [2011] investigated the generation of low-energy electrons in relativistic runaway electron avalanches. The paper also developed simple analytical expressions to describe the detailed physics of Monte Carlo simulations of relativistic runaway electrons in air. In the current work, the energy spectra of the runaway electron population are studied in detail. Dependence of electron avalanche development on properties such asthe avalanche length, radiation length, and the effective M∅ller scattering efficiency factor, are discussed in detail. To describe the shapes of the electron energy spectra for a wide range of electric field strengths, the diffusion term responsible for random deviation of electron energy ionization loss from the mean value is added to the kinetic equation. We find that the diffusion in energy space helps maintain an exponential energy spectrum for electric fields that approach the runaway electron threshold field.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Impact of Antarctic Circumpolar Current development on late Paleogene ocean structure (2011)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2011-05-28
    Description: Global cooling and the development of continental-scale Antarctic glaciation occurred in the late middle Eocene to early Oligocene (~38 to 28 million years ago), accompanied by deep-ocean reorganization attributed to gradual Antarctic Circumpolar Current (ACC) development. Our benthic foraminiferal stable isotope comparisons show that a large delta(13)C offset developed between mid-depth (~600 meters) and deep (〉1000 meters) western North Atlantic waters in the early Oligocene, indicating the development of intermediate-depth delta(13)C and O(2) minima closely linked in the modern ocean to northward incursion of Antarctic Intermediate Water. At the same time, the ocean's coldest waters became restricted to south of the ACC, probably forming a bottom-ocean layer, as in the modern ocean. We show that the modern four-layer ocean structure (surface, intermediate, deep, and bottom waters) developed during the early Oligocene as a consequence of the ACC.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Katz, Miriam E -- Cramer, Benjamin S -- Toggweiler, J R -- Esmay, Gar -- Liu, Chengjie -- Miller, Kenneth G -- Rosenthal, Yair -- Wade, Bridget S -- Wright, James D -- New York, N.Y. -- Science. 2011 May 27;332(6033):1076-9. doi: 10.1126/science.1202122.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. katzm@rpi.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21617074" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    The rise of oxygen over the past 205 million years and the evolution of large placental mammals (2005)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2005-10-01
    Description: On the basis of a carbon isotopic record of both marine carbonates and organic matter from the Triassic-Jurassic boundary to the present, we modeled oxygen concentrations over the past 205 million years. Our analysis indicates that atmospheric oxygen approximately doubled over this period, with relatively rapid increases in the early Jurassic and the Eocene. We suggest that the overall increase in oxygen, mediated by the formation of passive continental margins along the Atlantic Ocean during the opening phase of the current Wilson cycle, was a critical factor in the evolution, radiation, and subsequent increase in average size of placental mammals.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Falkowski, Paul G -- Katz, Miriam E -- Milligan, Allen J -- Fennel, Katja -- Cramer, Benjamin S -- Aubry, Marie Pierre -- Berner, Robert A -- Novacek, Michael J -- Zapol, Warren M -- New York, N.Y. -- Science. 2005 Sep 30;309(5744):2202-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA. falko@imcs.rutgers.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/16195457" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; *Atmosphere ; *Biological Evolution ; Biomass ; Body Size ; Carbon/analysis ; Carbon Dioxide/analysis ; Carbon Isotopes/analysis ; Carbonates ; Fossils ; Geologic Sediments/chemistry ; *Mammals/anatomy & histology/physiology ; Oxidation-Reduction ; *Oxygen/analysis ; Photosynthesis ; Phytoplankton/physiology ; Placenta/physiology ; Regression Analysis ; Reproduction ; Sulfur Isotopes/analysis ; Temperature ; Time
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 10
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    The Phanerozoic record of global sea-level change (2005)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2005-11-29
    Description: We review Phanerozoic sea-level changes [543 million years ago (Ma) to the present] on various time scales and present a new sea-level record for the past 100 million years (My). Long-term sea level peaked at 100 +/- 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred. Sea level mirrors oxygen isotope variations, reflecting ice-volume change on the 10(4)- to 10(6)-year scale, but a link between oxygen isotope and sea level on the 10(7)-year scale must be due to temperature changes that we attribute to tectonically controlled carbon dioxide variations. Sea-level change has influenced phytoplankton evolution, ocean chemistry, and the loci of carbonate, organic carbon, and siliciclastic sediment burial. Over the past 100 My, sea-level changes reflect global climate evolution from a time of ephemeral Antarctic ice sheets (100 to 33 Ma), through a time of large ice sheets primarily in Antarctica (33 to 2.5 Ma), to a world with large Antarctic and large, variable Northern Hemisphere ice sheets (2.5 Ma to the present).〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Miller, Kenneth G -- Kominz, Michelle A -- Browning, James V -- Wright, James D -- Mountain, Gregory S -- Katz, Miriam E -- Sugarman, Peter J -- Cramer, Benjamin S -- Christie-Blick, Nicholas -- Pekar, Stephen F -- New York, N.Y. -- Science. 2005 Nov 25;310(5752):1293-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854, USA. kgm@rci.rutgers.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/16311326" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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