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  • Wiley  (22)
  • Cell Press  (6)
  • American Institute of Physics (AIP)
  • 2015-2019  (28)
  • 1995-1999
  • 2017  (28)
  • 1
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    Permeability of porous media: role of the critical pore size (2017)
    Wiley
    Details
    Publication Date: 2017-08-18
    Description: The knowledge of the permeability of porous media is crucial to understand fluid flow in various natural and artificial materials. Due to the complex nature of pore structure, the pore characteristic (porosity, pore radius) determining the permeability has long been under discussion. Here, we determined the critical pore radius, which is the radius of the largest sphere that can freely pass through a porous medium, using the water-expulsion method, an experimental technique measuring the pressure at which gas passes through a water-saturated porous medium. We demonstrate that the critical pore radius correlates well with the permeability for a variety of porous granular media and volcanic products with an extensive range of porosities (0.71%–50%) and permeabilities (10 −20 –10 −10 m 2 ). We also obtained a porosity–critical pore radius–permeability relationship that provides a better prediction of the permeability compared with predictions obtained by previous correlations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Daytime zonal drifts in the ionospheric 150 km and E regions estimated using EAR observations (2017)
    Wiley
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    Publication Date: 2017-08-18
    Description: Multi-beam observations of the 150-km echoes made using the Equatorial Atmosphere Radar (EAR), located at Kototabang, Indonesia provide unique opportunity to study both vertical and zonal E x B plasma drifts in the equatorial ionosphere. In this paper, we focus on estimating daytime zonal drifts at the 150 km (140-160 km) and E (100-110 km) regions using multi-beam observations of 150-km- and E-region echoes made using the EAR and study the daytime zonal drifts covering all seasons, not studied before from Kototabang. Zonal drifts in the 150-km and E regions are found to be westward and mostly below -80 m s -1 and -60 m s -1 , respectively. While the zonal drifts in the 150-km and E regions do not go hand in hand on a case by case basis, the seasonal mean drifts in the two height regions are found to be in good agreement with each other. Zonal drifts at the 150 km region show seasonal variations with three maxima peaking around May, September and January. The zonal drifts at the 150 km region are found to be smaller than the F region drifts obtained from CINDI onboard C/NOFS by about 25 m s -1 consistent with the height variations of F region zonal drifts observed by the Jicamarca radar. These results constitute the first comprehensive study of zonal drifts at the 150 km and E regions from Kototabang, Indonesia and the results are discussed in the light of current understanding on the low latitude electrodynamics and coupling.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Low-latitude daytime F-region irregularities observed in two geomagnetically quiet days by the Hainan coherent scatter phased array radar (HCOPAR) (2017)
    Wiley
    Details
    Publication Date: 2017-02-19
    Description: Hainan coherent scatter phased array radar (HCOPAR) located at low-latitude of China has recorded the extremely rare daytime F -region irregularities at noon of 22 July 2013 and 23 May 2016. The two field-aligned irregularities (FAIs) appeared in the topside F2 -layer and presented small Doppler velocities and narrow spectral widths. The fan sector maps show that the FAIs moved northward with almost no zonal speed. The irregularities emerged in the geomagnetically quiet condition and were irrelevant to the storm induced eastward electric field as other daytime cases. More than two hours after the emergency of the daytime irregularities over Hainan, the Shaoyang digisonde situated ~870 km north to the HCOPAR recorded the spread- F in ionospheric F1 -layer. According to the echo altitudes, the spread- F may connect the daytime bubbles via magnetic field line. The strong photo-ionization after sunrise made it difficult to generate the plasma bubbles in the sunlit ionosphere. Consequently, the two midday FAIs over Hainan may drift along the magnetic field lines from higher altitudes in the south and are most likely the remnant of previous night's bubbles.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    A geomagnetic paleointensity record of 0.6 to 3.2 m.y. from sediments in the western equatorial Pacific and remanent magnetization lock-in depth (2017)
    Wiley
    Details
    Publication Date: 2017-09-27
    Description: Information of past geomagnetic intensity variations is important for better understanding of the geodynamo, and efforts to recover continuous paleointensity records continue, in particular for ages older than 2 Ma. In this study, a new relative paleointensity (RPI) record of 0.6 to 3.2 Ma was obtained from a sediment core in the western equatorial Pacific, which has good age control by the oxygen-isotope stratigraphy from 2.0 to 3.2 Ma. The RPI record could well be correlated to existing RPI templates despite the low sedimentation rate of ~5 m/m.y., and will be useful for future construction of a stacked paleointensity curve beyond 2 Ma. Magnetic minerals of the core consist mainly of biogenic and terrigenous oxidized magnetite. Some rock-magnetic contamination to long-term RPI changes is recognized; RPI inversely correlates with the ratio of anhysteretic remanent magnetization to saturation remanent magnetization, which may be caused by differences in remanence acquisition efficiency between the biogenic and terrigenous magnetic components. The lock-in depth of the studied core is estimated to be approximately zero from a line of evidence including no apparent offset between PRI and cosmogenic beryllium nuclide flux changes across some polarity boundaries. Lock-in depths vary locally from core to core or even within a single core, and have no obvious relation with sedimentation rates and carbonate contents. Lock-in depths may be controlled by factors that are currently difficult to be evaluated, such as variations in sediment flocculation sizes and biogeochemical remanent magnetization dependent to chemical conditions of sediments.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    The Solar flux dependence of ionospheric 150-km radar echoes and implications (2017)
    Wiley
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    Publication Date: 2017-11-14
    Description: Radar echoes from the daytime equatorial ionospheric F1 region, popularly known as ‘150-km echoes’, have challenged ionospheric plasma physicists for several decades. Recent theoretical simulations showed that enhanced photo-electron fluxes can amplify the amplitude of plasma waves, generating spectra similar to those of the radar echoes, implying that larger solar fluxes should produce more frequent and stronger 150-km echoes. Inspired by this proposal we studied the occurrence and intensity dependence of the echoes on the EUV flux observed by SOHO over several years. The occurrence and intensity of the echoes were found to have an inverse relationship with this EUV flux measurement. The multi-year trend is independent of the variability often observed over successive days with nearly identical EUV fluxes. These results imply that the relationship between the echoes and EUV flux is more complex. We propose that gravity waves modulate the amplitude of 150-km echoes through changes in the variations in plasma density and photoelectron fluxes associated with the gravity wave induced neutral density modulations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 6
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    A reconstruction of subtropical western North Pacific SST variability back to 1578, based on a Porites coral Sr/Ca record from the northern Ryukyus, Japan (2017)
    Wiley
    Details
    Publication Date: 2017-11-16
    Description: We present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally ~0.5°C, making Sr/Ca-derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser-ablation ICP-MS technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670-1700 during the Maunder Minimum (MM) and between 1766-1788 characterized by a negative phase of the North Atlantic Oscillation (NAO). Cold summers in 1783 and 1784 coincide with the long-lasting Laki eruption that had a profound impact on the northern hemisphere climate, including the severe 'Tenmei' famine in Japan. The decades between 1855-1900 are significantly cooler than the first half of the 20th century, while those between 1700-1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific decadal Oscillation (PDO), so that external forcing remains the main driver of low-frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio-Oyashio variability to the large-scale atmosphere at decadal timescale.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Equatorial and low-latitude ionospheric response to the 17-18 March 2015 great storm over South East Asia longitude sector (2017)
    Wiley
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    Publication Date: 2017-05-13
    Description: This study mainly investigates equatorial and low-latitude ionospheric response to a great geomagnetic storm occurred on the 17 March 2015. We found that there were some interesting ionospheric phenomena, e.g. short-term ionospheric positive effect, daytime spread F, and morning Equatorial Ionization Anomaly (EIA) in the topside ionosphere, emerged at equatorial and low-latitude region along the longitude of about 100oE. Ground-based ionosondes and in situ satellite (Swarm) were utilized to study the possible mechanisms for these ionospheric phenomena. We found that vertical downward transport of plasma or neutral induced by traveling ionospheric disturbances (TIDs) or traveling atmospheric disturbances (TADs) might make a contribution to the short-term ionospheric positive effect at the main stage of this great storm. Additionally, results suggested that the occurrence of daytime spread F at low latitudes might be due to the diffusion of equatorial ionospheric irregularities in the topside ionosphere along geomagnetic field lines. Moreover, observational evidence shows that TIDs also might be the main driver for morning EIA-like feature recorded by Swarm B satellite in the topside ionosphere. These ionospheric phenomena mentioned above could make us to gain a better understanding of ionospheric storm effects at equatorial and low-latitude region.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Vertical rise velocity of Equatorial Plasma Bubbles estimated from Equatorial Atmosphere Radar (EAR) observations and HIRB model simulations. (2017)
    Wiley
    Details
    Publication Date: 2017-05-25
    Description: The vertical rise velocity (V r ) and maximum altitude (H m ) of Equatorial Plasma Bubbles (EPBs) were estimated using the two dimensional fan sector maps of 47 MHz Equatorial Atmosphere Radar (EAR), Kototabang during May 2010 – April 2013. A total of 86 EPBs were observed out of which 68 were post-sunset EPBs and remaining 18 EPBs were observed around midnight hours. The vertical rise velocities of the EPBs observed around the midnight hours are significantly smaller (~26 – 128 m/s) compared to those observed in post-sunset hours (~45 – 265 m/s). Further, the vertical growth of the EPBs around midnight hours ceases at relatively lower altitudes, whereas the majority of EPBs at post-sunset hours found to have grown beyond the maximum detectable altitude of the EAR. The three dimensional numerical High Resolution Bubble (HIRB) model with varying background conditions are employed to investigate the possible factors that control the vertical rise velocity and maximum attainable altitudes of EPBs. The estimated rise velocities from EAR observations at both post-sunset and midnight hours are in general consistent with the nonlinear evolution of EPBs from the HIRB model. The smaller vertical rise velocities (V r ) and lower maximum altitudes (H m ) of EPBs during midnight hours are discussed in terms of weak polarization electric fields within the bubble due to weaker background electric fields and reduced background ion density levels.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Effects of intensification of the Indian Summer Monsoon on northern Andaman Sea sediments during the past 700 years (2017)
    Wiley
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    Publication Date: 2017-05-13
    Description: ABSTRACT Recent studies have revealed an intensification of the Indian Summer Monsoon (ISM) over the past 700 years, but its influence on the sediments in the northern Andaman Sea is unclear. To examine this influence, we used radiocarbon-dated sediment core StMY6, acquired 100 km offshore in the northern Andaman Sea from the Ayeyarwady River mouth, and obtained a 700-year-long record of the grain-size distribution and geochemistry of the sedimentary sequence. The ISM influences precipitation surrounding the Ayeyarwady River, and thus the river water and sediment discharges to the northern Andaman Sea near the river mouth in the Ayeyarwady River and the weathering intensity in the river catchment. Based on the system, we propose that higher sedimentation rates and larger modal grain size (the most abundant size in the grain-size distribution of sediments) and lower carbonate concentrations after about AD 1600 can be attributed to higher ISM intensity. Profiles of total organic carbon (TOC) content and carbon/nitrogen (C/N) ratios, however, did not show any trends after about AD 1600, which suggests that the ISM has had little influence on these proxies at this site.
    Print ISSN: 0267-8179
    Electronic ISSN: 1099-1417
    Topics: Geography , Geosciences
    Published by Wiley
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  • 10
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    Nitric oxide-enhanced Shiga toxin production was regulated by Fur and RecA in enterohemorrhagic Escherichia coli O157 (2017)
    Wiley
    Details
    Publication Date: 2017-03-16
    Description: Enterohemorrhagic Escherichia coli (EHEC) produces Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). Nitric oxide (NO), which acts as an antimicrobial defense molecule, was found to enhance the production of Stx1 and Stx2 in EHEC under anaerobic conditions. Although EHEC O157 has two types of anaerobic NO reductase genes, an intact norV and a deleted norV , in the deleted norV -type EHEC, a high concentration of NO (12–29 μmol/L, maximum steady-state concentration) is required for enhanced Stx1 production and a low concentration of NO (~12 μmol/L, maximum steady-state concentration) is sufficient for enhanced Stx2 production under anaerobic conditions. These results suggested that different concentration thresholds of NO elicit a discrete set of Stx1 and Stx2 production pathways. Moreover, the enhancement of Shiga toxin production in the intact norV -type EHEC required treatment with a higher concentration of NO than was required for enhancement of Shiga toxin production in the deleted norV -type EHEC, suggesting that the specific NorV type plays an important role in the level of enhancement of Shiga toxin production in response to NO. Finally, Fur derepression and RecA activation in EHEC were shown to participate in the NO-enhanced Stx1 and Stx2 production, respectively. Enterohemorrhagic Escherichia coli (EHEC) produces Stx1 and Stx2. A high concentration of nitric oxide (NO) enhanced Stx1 production in EHEC by Fur derepression under anaerobic conditions. A low concentration of NO enhanced Stx2 production by RecA activation. The specific NorV (NO reductase) type in EHEC plays an important role in the level of enhancement of Shiga toxin production in response to NO.
    Electronic ISSN: 2045-8827
    Topics: Biology , Medicine
    Published by Wiley
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