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Search Results: 1 - 10 of 354685 matches for " R. E. Sheehan "
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Multistation digisonde observations of equatorial spread F in South America
R. E. Sheehan ,C. E. Valladares
Annales Geophysicae (ANGEO) , 2004,
Abstract: UHF scintillation measurements of zonal ionospheric drifts have been conducted at Ancon, Peru since 1994 using antennas spaced in the magnetic east-west direction to cross-correlate geo-synchronous satellite signals. An empirical model of average drift over a wide range of Kp and solar flux conditions was constructed from successive two-dimensional fits of drift vs. the parameters and day of year. The model exhibits the typical local time trend of maximum eastward velocity in the early evening with a gradual decrease and reversal in the early morning hours. As expected, velocities at all hours increase with the solar flux and decrease with Kp activity. It was also found that vertical drifts could contribute to the variability of drift measurements to the east of Ancon at a low elevation angle. The vertical drift at the ionospheric intersection to the east can be estimated when combined with nearly overhead observations at Ancon or a similar spaced-antenna site at Antofagasta, Chile. Comparisons on five days with nearly simultaneous measurements of vertical drift by the Julia radar at Jicamarca, Peru show varying agreement with the spaced-antenna estimates. Statistical results from 1997 to 2001 generally agree with radar and satellite studies.
Time evolution of high-altitude plasma bubbles imaged at geomagnetic conjugate points
C. E. Valladares, J. Villalobos, R. Sheehan,M. P. Hagan
Annales Geophysicae (ANGEO) , 2004,
Abstract: A latitudinal-distributed network of GPS receivers has been operating within Colombia, Peru and Chile with sufficient latitudinal span to measure the absolute total electron content (TEC) at both crests of the equatorial anomaly. The network also provides the latitudinal extension of GPS scintillations and TEC depletions. The GPS-based information has been supplemented with density profiles collected with the Jicamarca digisonde and JULIA power maps to investigate the background conditions of the nighttime ionosphere that prevail during the formation and the persistence of plasma depletions. This paper presents case-study events in which the latitudinal extension of GPS scintillations, the maximum latitude of TEC depletion detections, and the altitude extension of radar plumes are correlated with the location and extension of the equatorial anomaly. Then it shows the combined statistics of GPS scintillations, TEC depletions, TEC latitudinal profiles, and bottomside density profiles collected between September 2001 and June 2002. It is demonstrated that multiple sights of TEC depletions from different stations can be used to estimate the drift of the background plasma, the tilt of the plasma plumes, and in some cases even the approximate time and location of the depletion onset. This study corroborates the fact that TEC depletions and radar plumes coincide with intense levels of GPS scintillations. Bottomside radar traces do not seem to be associated with GPS scintillations. It is demonstrated that scintillations/depletions can occur when the TEC latitude profiles are symmetric, asymmetric or highly asymmetric; this is during the absence of one crest. Comparison of the location of the northern crest of the equatorial anomaly and the maximum latitude of scintillations reveals that for 90% of the days, scintillations are confined within the boundaries of the 50% decay limit of the anomaly crests. The crests of the anomaly are the regions where the most intense GPS scintillations and the deepest TEC depletions are encountered. In accord with early results, we observe that GPS scintillations/TEC depletions mainly occur when the altitude of the magnetic equator F-region is above 500km. Nevertheless, in many instances GPS scintillations and TEC depletions are observed to exist when the F-layer is well below 500km or to persist when the F-layer undergoes its typical nighttime descent. Close inspection of the TEC profiles during scintillations/depletions events that occur when the equatorial F-layer peak is below 500km altitude reveals that on these occasions the ratio of the crest-to-equator TEC is above 2, and the crests are displaced 10° or more from the magnetic equator. When the equatorial F-layer is above 500km, neither of the two requirements is needed, as the flux tube seems to be inherently unstable. We discuss these findings in terms of the Rayleigh-Taylor instability (RTI) mechanism for flux-tube integrated quantities. We advance the idea that
A comparison of satellite scintillation measurements with HF radar backscatter characteristics
S. E. Milan, S. Basu, T. K. Yeoman,R. E. Sheehan
Annales Geophysicae (ANGEO) , 2005,
Abstract: We examine the correspondence between high latitude ionospheric scintillation measurements made at 250MHz with the occurrence of 10MHz HF coherent radar backscatter, on 13 and 14 December 2002. We demonstrate that when the ionospheric intersection point of the scintillation measurements is co-located with significant HF radar backscatter, the observed scintillation, quantified by the S4 index, is elevated. Conversely, when the radar indicates that backscatter is observed away from the intersection point due to movements of the auroral zone, the observed scintillation is low. This suggests that scintillation is highly location-dependent, being enhanced in the auroral zone and being lower at sub-auroral latitudes. The coexistence of scintillation and HF radar backscatter, produced by ionospheric density perturbations with scale sizes of 100s of metres and ~15 m, respectively, suggests that a broad spectrum of density fluctuations is found in the auroral zone.
Accuracy of Stream Habitat Interpolations Across Spatial Scales  [PDF]
Kenneth R. Sheehan, Stuart A. Welsh
Journal of Geographic Information System (JGIS) , 2013, DOI: 10.4236/jgis.2013.56057

Stream habitat data are often collected across spatial scales because relationships among habitat, species occurrence, and management plans are linked at multiple spatial scales. Unfortunately, scale is often a factor limiting insight gained from spatial analysis of stream habitat data. Considerable cost is often expended to collect data at several spatial scales to provide accurate evaluation of spatial relationships in streams. To address utility of single scale set of stream habitat data used at varying scales, we examined the influence that data scaling had on accuracy of natural neighbor predictions of depth, flow, and benthic substrate. To achieve this goal, we measured two streams at gridded resolution of 0.33 × 0.33 meter cell size over a combined area of 934 m2 to create a baseline for natural neighbor interpolated maps at 12 incremental scales ranging from a raster cell size of 0.11 m2 to 16 m2. Analysis of predictive maps showed a logarithmic linear decay pattern in RMSE values in interpolation accuracy for variables as resolution of data used to interpolate study areas became coarser. Proportional accuracy of interpolated models (r2) decreased, but it was maintained up to 78% as interpolation scale moved from 0.11 m2 to 16 m2. Results indicated that accuracy retention was suitable for assessment and management purposes at various scales different from the data collection scale. Our study is relevant to spatial modeling, fish habitat assessment, and stream habitat management because it highlights the potential of using a single dataset to fulfill analysis needs rather than investing considerable cost to develop several scaled datasets

Simultaneous observation of traveling ionospheric disturbances in the Northern and Southern Hemispheres
C. E. Valladares, J. Villalobos, M. A. Hei, R. Sheehan, Su. Basu, E. MacKenzie, P. H. Doherty,V. H. Rios
Annales Geophysicae (ANGEO) , 2009,
Abstract: Measurements of total electron content (TEC) using 263 GPS receivers located in the North and South America continents are presented to demonstrate the simultaneous existence of traveling ionospheric disturbances (TID) at high, mid, and low latitudes, and in both Northern and Southern Hemispheres. The TID observations pertain to the magnetically disturbed period of 29–30 October 2003 also known as the Halloween storm. The excellent quality of the TEC measurements makes it possible to calculate and remove the diurnal variability of TEC and then estimate the amplitude, wavelength, spectral characteristics of the perturbations, and the approximate velocity of the AGW. On 29 October 2003 between 17:00 and 19:00 UT, there existed a sequence of TEC perturbations (TECP), which were associated with the transit of atmospheric gravity waves (AGW) propagating from both auroral regions toward the geographic equator. A marked difference was found between the northern and southern perturbations. In the Northern Hemisphere, the preferred horizontal wavelength varies between 1500 and 1800 km; the propagation velocity is near 700 m/s and the perturbation amplitude about 1 TEC unit (TECu). South of the geographic equator the wavelength of the TECP is as large as 2700 km, the velocity is about 550 m/s, and the TECP amplitude is 3 TECu. Concurrently with our observations, the Jicamarca digisonde observed virtual height traces that exhibited typical features that are associated with TIDs. Here, it is suggested that differences in the local conductivity between northern and southern auroral ovals create a different Joule heating energy term. The quality of these observations illustrates the merits of GPS receivers to probe the ionosphere and thermosphere.
PI3K Signaling and Stat92E Converge to Modulate Glial Responsiveness to Axonal Injury
Johnna Doherty,Amy E. Sheehan,Rachel Bradshaw,A. Nicole Fox,Tsai-Yi Lu,Marc R. Freeman
PLOS Biology , 2014, DOI: 10.1371/journal.pbio.1001985
Abstract: Glial cells are exquisitely sensitive to neuronal injury but mechanisms by which glia establish competence to respond to injury, continuously gauge neuronal health, and rapidly activate reactive responses remain poorly defined. Here, we show glial PI3K signaling in the uninjured brain regulates baseline levels of Draper, a receptor essential for Drosophila glia to sense and respond to axonal injury. After injury, Draper levels are up-regulated through a Stat92E-modulated, injury-responsive enhancer element within the draper gene. Surprisingly, canonical JAK/STAT signaling does not regulate draper expression. Rather, we find injury-induced draper activation is downstream of the Draper/Src42a/Shark/Rac1 engulfment signaling pathway. Thus, PI3K signaling and Stat92E are critical in vivo regulators of glial responsiveness to axonal injury. We provide evidence for a positive auto-regulatory mechanism whereby signaling through the injury-responsive Draper receptor leads to Stat92E-dependent, transcriptional activation of the draper gene. We propose that Drosophila glia use this auto-regulatory loop as a mechanism to adjust their reactive state following injury.
5β-Dihydroprogesterone and Human Preterm Labor  [PDF]
Penelope M. Sheehan, Gregory E. Rice, Shaun P. Brennecke
Open Journal of Endocrine and Metabolic Diseases (OJEMD) , 2014, DOI: 10.4236/ojemd.2014.45013

We previously investigated the progesterone metabolite 5β-dihydroprogesterone (5βDHP) in relation to human parturition at term, demonstrating that peripheral venous concentrations decrease in association with the onset of spontaneous labour. In this study our aim was to determine if 5βDHP concentrations were lower in women presenting in spontaneous preterm labour than in controls matched for gestational age. Blood samples were obtained from women presenting in spontaneous preterm labour (n = 20). The diagnosis was made on the presence of regular contractions and cervical effacement and dilatation of at least 3 cms. All women in the preterm labour group delivered before 37 weeks gestation. Blood samples were then obtained from controls, closely matched for gestational age with uncomplicated pregnancies. The preterm labour group was further stratified by cause into three groups, chorioamnionitis (n = 5), abruption (n = 4) and idiopathic (n = 11). Following organic solvent extraction, steroids were separated by HPLC and 5βDHP quantified by radioimmunoassay. Women in the idiopathic preterm labour group were found to have significantly lower circulating concentrations of 5βDHP than controls (p < 0.05). There were no significant differences found in either the chorioamnionitis or abruption groups. This result suggests that there may be a group of women in whom the normal endocrine process of labour is triggered early resulting in preterm labour.

Low Energy Electron Point Projection Microscopy of Suspended Graphene, the Ultimate "Microscope Slide"
J. Y. Mutus,L. Livadaru,J. T. Robinson,R. Urban,M. H. Salomons,M. Cloutier,P. E. Sheehan,R. A. Wolkow
Physics , 2011, DOI: 10.1088/1367-2630/13/6/063011
Abstract: Point Projection Microscopy (PPM) is used to image suspended graphene using low-energy electrons (100-200eV). Because of the low energies used, the graphene is neither damaged or contaminated by the electron beam. The transparency of graphene is measured to be 74%, equivalent to electron transmission through a sheet as thick as twice the covalent radius of sp^2-bonded carbon. Also observed is rippling in the structure of the suspended graphene, with a wavelength of approximately 26 nm. The interference of the electron beam due to the diffraction off the edge of a graphene knife edge is observed and used to calculate a virtual source size of 4.7 +/- 0.6 Angstroms for the electron emitter. It is demonstrated that graphene can be used as both anode and substrate in PPM in order to avoid distortions due to strong field gradients around nano-scale objects. Graphene can be used to image objects suspended on the sheet using PPM, and in the future, electron holography.
Microbial Ecology of the Hive and Pollination Landscape: Bacterial Associates from Floral Nectar, the Alimentary Tract and Stored Food of Honey Bees (Apis mellifera)
Kirk E. Anderson, Timothy H. Sheehan, Brendon M. Mott, Patrick Maes, Lucy Snyder, Melissa R. Schwan, Alexander Walton, Beryl M. Jones, Vanessa Corby-Harris
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0083125
Abstract: Nearly all eukaryotes are host to beneficial or benign bacteria in their gut lumen, either vertically inherited, or acquired from the environment. While bacteria core to the honey bee gut are becoming evident, the influence of the hive and pollination environment on honey bee microbial health is largely unexplored. Here we compare bacteria from floral nectar in the immediate pollination environment, different segments of the honey bee (Apis mellifera) alimentary tract, and food stored in the hive (honey and packed pollen or “beebread”). We used cultivation and sequencing to explore bacterial communities in all sample types, coupled with culture-independent analysis of beebread. We compare our results from the alimentary tract with both culture-dependent and culture-independent analyses from previous studies. Culturing the foregut (crop), midgut and hindgut with standard media produced many identical or highly similar 16S rDNA sequences found with 16S rDNA clone libraries and next generation sequencing of 16S rDNA amplicons. Despite extensive culturing with identical media, our results do not support the core crop bacterial community hypothesized by recent studies. We cultured a wide variety of bacterial strains from 6 of 7 phylogenetic groups considered core to the honey bee hindgut. Our results reveal that many bacteria prevalent in beebread and the crop are also found in floral nectar, suggesting frequent horizontal transmission. From beebread we uncovered a variety of bacterial phylotypes, including many possible pathogens and food spoilage organisms, and potentially beneficial bacteria including Lactobacillus kunkeei, Acetobacteraceae and many different groups of Actinobacteria. Contributions of these bacteria to colony health may include general hygiene, fungal and pathogen inhibition and beebread preservation. Our results are important for understanding the contribution to pollinator health of both environmentally vectored and core microbiota, and the identification of factors that may affect bacterial detection and transmission, colony food storage and disease susceptibility.
Simple Flight Simulator Model
Loay E.George,Suhad F.Sheehan
Asian Journal of Information Technology , 2012,
Abstract: Pilots are trained using computerized flight simulators. A flight simulator is a training system where pilots can acquire flying skills without need to practice on a real airplane. Simulators are used by professional pilots to practice flying strategies under emergency or hazardous conditions, or to train on new aircraft types. In this study a framework for flight simulation is presented and the layout of an implemented program is described. The calculations were based on simple theoretical approach. The implementation was based on utilizing some of utilities supported by ActiveX, DirectX and OpenGL written in Visual C++. The main design consideration is to build a simple flight simulation program can operate without need to high computer environment specifications.
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