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  • 1
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    Isostaticity and elasticity of colloidal gels [Applied Physical Sciences] (2012)
    National Academy of Sciences
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    Publication Date: 2012-10-03
    Description: We report a simple correlation between microstructure and strain-dependent elasticity in colloidal gels by visualizing the evolution of cluster structure in high strain-rate flows. We control the initial gel microstructure by inducing different levels of isotropic depletion attraction between particles suspended in refractive index matched solvents. Contrary to previous ideas...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 2
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    Solar EUV and XUV energy input to thermosphere on solar rotation time scales derived from photoelectron observations (2012)
    Wiley
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    Publication Date: 2012-05-18
    Description: Solar radiation below ∼100 nm produces photoelectrons, a substantial portion of the F region ionization, most of the E region ionization, and drives chemical reactions in the thermosphere. Unquantified uncertainties in thermospheric models exist because of uncertainties in solar irradiance models used to fill spectral and temporal gaps in solar irradiance observations. We investigate uncertainties in solar energy input to the thermosphere on solar rotation time scales using photoelectron observations from the FAST satellite. We compare observed and modeled photoelectron energy spectra using two photoelectron production codes driven by five different solar irradiance models. We observe about 1.7% of the ionizing solar irradiance power in the escaping photoelectron flux. Most of the code/model pairs used reproduce the average escaping photoelectron flux over a 109-day interval in late 2006. The code/model pairs we used do not completely reproduce the observed spectral and solar rotation variations in photoelectron power density. For the interval examined, 30% of the variability in photoelectron power density with equivalent wavelengths between 18 and 45 nm was not captured in the code/model pairs. For equivalent wavelengths below ∼16 nm, most of the variability was missed. This result implies that thermospheric model runs based on the solar irradiance models we tested systematically underestimate the energy input from ionizing radiation on solar rotation time scales.
    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 summer evening anomaly and conjugate effects (2011)
    Wiley
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    Publication Date: 2011-01-22
    Description: The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) GPS occultation data have been analyzed in this study to provide a better understanding of the Weddell Sea Anomaly (WSA) and to place it in the wider context of a general phenomenon that occurs near dusk in summer, which we are calling the summer evening anomaly to better capture its global nature. The terminator and the magnetically conjugate points for the terminator in the other hemisphere have been plotted on top of global maps of COSMIC NmF2 and hmF2 for 2 months either side of the December and June solstices for 2006–2008. These plots show that there are distinct enhancements of NmF2 and increases in hmF2 as soon as the conjugate footprint of the field line on the winter terminator is seen at middle latitudes in the summer hemisphere. This effect is most pronounced where the WSA is formed, but it also occurs across the South Pacific Ocean in the southern summer and across much of the North Atlantic Ocean, Siberia, and Kamchatka during the northern summer. An hmF2 increase occurs between the two terminators even at locations where there is no increase in NmF2. A similar, but reversed, effect occurs in hmF2 near dawn. This behavior appears to be most consistent with upward and poleward ion drifts in the evening, but neutral wind and downward precipitation may make important contributions to this effect.
    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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    Heating of the Sunlit Polar Cap Ionosphere by Reflected Photoelectrons (2014)
    Wiley
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    Publication Date: 2014-10-02
    Description: Photoelectrons escape from the ionosphere on sunlit polar cap field lines. In order for those field lines to carry zero current without significant heavy ion outflow or cold electron inflow, field-aligned potential drops must form to reflect a portion of the escaping photoelectron population back to the ionosphere. Using a 1-D ionosphere-polar wind model and measurements from the Resolute Bay Incoherent Scatter Radar (RISR-N), this paper shows that these reflected photoelectrons are a significant source of heat for the sunlit polar cap ionosphere. The model includes a kinetic suprathermal electron transport solver, and it allows energy input from the upper boundary in three different ways: thermal conduction, soft precipitation, and potentials that reflect photoelectrons. The simulations confirm that reflection potentials of several 10s of eV are required to prevent cold electron inflow and demonstrate that the flux tube integrated change in electron heating rate (FTICEHR) associated with reflected photoelectrons can reach 10 9 eVcm − 2 s − 1 . Soft precipitation can produce FTICEHR of comparable magnitudes, but this extra heating is divided among more electrons as a result of electron impact ionization. Simulations with no reflected photoelectrons and with downward field-aligned currents (FAC) primarily carried by the escaping photoelectrons have electron temperatures which are ~ 250 − 500 K lower than the RISR-N measurements in the 300-600 km region; however, simulations with reflected photoelectrons, zero FAC, and no other form of heat flux through the upper boundary can satisfactorily reproduce the RISR-N data.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    On the Solar Cycle Variation of the Winter Anomaly (2014)
    Wiley
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    Publication Date: 2014-05-24
    Description: Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Ionosonde and Global Ultraviolet Imager (GUVI) data have been used to investigate the solar cycle changes in the winter anomaly (the winter anomaly is defined as the enhancement of the F 2 peak electron density in the winter hemisphere over that in the summer hemisphere) in the last solar cycle. There is no winter anomaly in solar minimum, and an enhancement of about 50 % in winter over summer ones on the same day of the year at solar maximum. This solar cycle variation in the winter anomaly is primarily due to greater winter to summer differences of [O]/[N 2 ] in solar maximum than in solar minimum, with a secondary contribution from the effects of temperature on the recombination coefficient between O + and the molecular neutral gas. The greater winter increases in electron density in the northern hemisphere than in the southern hemisphere appear to be related to the greater annual variation of [O]/[N 2 ] in the north than in the south.
    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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    Mercury's complex exosphere: results from MESSENGER's third flyby (2010)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2010-07-22
    Description: During MESSENGER's third flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer detected emission from ionized calcium concentrated 1 to 2 Mercury radii tailward of the planet. This measurement provides evidence for tailward magnetospheric convection of photoions produced inside the magnetosphere. Observations of neutral sodium, calcium, and magnesium above the planet's north and south poles reveal altitude distributions that are distinct for each species. A two-component sodium distribution and markedly different magnesium distributions above the two poles are direct indications that multiple processes control the distribution of even single species in Mercury's exosphere.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Vervack, Ronald J Jr -- McClintock, William E -- Killen, Rosemary M -- Sprague, Ann L -- Anderson, Brian J -- Burger, Matthew H -- Bradley, E Todd -- Mouawad, Nelly -- Solomon, Sean C -- Izenberg, Noam R -- New York, N.Y. -- Science. 2010 Aug 6;329(5992):672-5. doi: 10.1126/science.1188572. Epub 2010 Jul 15.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. Ron.Vervack@jhuapl.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20647427" 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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  • 7
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    The major-element composition of Mercury's surface from MESSENGER X-ray spectrometry (2011)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2011-10-01
    Description: X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Nittler, Larry R -- Starr, Richard D -- Weider, Shoshana Z -- McCoy, Timothy J -- Boynton, William V -- Ebel, Denton S -- Ernst, Carolyn M -- Evans, Larry G -- Goldsten, John O -- Hamara, David K -- Lawrence, David J -- McNutt, Ralph L Jr -- Schlemm, Charles E 2nd -- Solomon, Sean C -- Sprague, Ann L -- New York, N.Y. -- Science. 2011 Sep 30;333(6051):1847-50. doi: 10.1126/science.1211567.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA. lnittler@ciw.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21960623" 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
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  • 8
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    The global magnetic field of Mercury from MESSENGER orbital observations (2011)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2011-10-01
    Description: Magnetometer data acquired by the MESSENGER spacecraft in orbit about Mercury permit the separation of internal and external magnetic field contributions. The global planetary field is represented as a southward-directed, spin-aligned, offset dipole centered on the spin axis. Positions where the cylindrical radial magnetic field component vanishes were used to map the magnetic equator and reveal an offset of 484 +/- 11 kilometers northward of the geographic equator. The magnetic axis is tilted by less than 3 degrees from the rotation axis. A magnetopause and tail-current model was defined by using 332 magnetopause crossing locations. Residuals of the net external and offset-dipole fields from observations north of 30 degrees N yield a best-fit planetary moment of 195 +/- 10 nanotesla-R(M)(3), where R(M) is Mercury's mean radius.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Anderson, Brian J -- Johnson, Catherine L -- Korth, Haje -- Purucker, Michael E -- Winslow, Reka M -- Slavin, James A -- Solomon, Sean C -- McNutt, Ralph L Jr -- Raines, Jim M -- Zurbuchen, Thomas H -- New York, N.Y. -- Science. 2011 Sep 30;333(6051):1859-62. doi: 10.1126/science.1211001.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. brian.anderson@jhuapl.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21960627" 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
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  • 9
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    Radioactive elements on Mercury's surface from MESSENGER: implications for the planet's formation and evolution (2011)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2011-10-01
    Description: The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 +/- 220 parts per million), thorium (Th, 220 +/- 60 parts per billion), and uranium (U, 90 +/- 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Peplowski, Patrick N -- Evans, Larry G -- Hauck, Steven A 2nd -- McCoy, Timothy J -- Boynton, William V -- Gillis-Davis, Jeffery J -- Ebel, Denton S -- Goldsten, John O -- Hamara, David K -- Lawrence, David J -- McNutt, Ralph L Jr -- Nittler, Larry R -- Solomon, Sean C -- Rhodes, Edgar A -- Sprague, Ann L -- Starr, Richard D -- Stockstill-Cahill, Karen R -- New York, N.Y. -- Science. 2011 Sep 30;333(6051):1850-2. doi: 10.1126/science.1211576.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. patrick.peplowski@jhuapl.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21960624" 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
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  • 10
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    Gravity field and internal structure of Mercury from MESSENGER (2012)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2012-03-23
    Description: Radio tracking of the MESSENGER spacecraft has provided a model of Mercury's gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury's northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR(2) = 0.353 +/- 0.017, where M and R are Mercury's mass and radius, and a ratio of the moment of inertia of Mercury's solid outer shell to that of the planet of C(m)/C = 0.452 +/- 0.035. A model for Mercury's radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Smith, David E -- Zuber, Maria T -- Phillips, Roger J -- Solomon, Sean C -- Hauck, Steven A 2nd -- Lemoine, Frank G -- Mazarico, Erwan -- Neumann, Gregory A -- Peale, Stanton J -- Margot, Jean-Luc -- Johnson, Catherine L -- Torrence, Mark H -- Perry, Mark E -- Rowlands, David D -- Goossens, Sander -- Head, James W -- Taylor, Anthony H -- New York, N.Y. -- Science. 2012 Apr 13;336(6078):214-7. doi: 10.1126/science.1218809. Epub 2012 Mar 21.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22438509" 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
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