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Search Results: 1 - 10 of 438554 matches for " P. J. Spencer "
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P. J. Spencer
Journal of Venomous Animals and Toxins , 1996, DOI: 10.1590/s0104-79301996000200011
A three-dimensional time-dependent algorithm for ionospheric imaging using GPS
C. N. Mitchell,P. S. J. Spencer
Annals of Geophysics , 2003, DOI: 10.4401/ag-4373
Abstract: Global Positioning System (GPS) satellite receivers provide a world-wide network of phase and group delay measurements. The combination of two-frequency measurements can be used to derive the integral of the electron concentration along each satellite-to-receiver path, a parameter known as the Total Electron Content (TEC). At this stage these slant TEC data are diffi cult to interpret as they originate from a combination of a temporally changing ionosphere and spatially changing observation geometry. In this paper TEC data are inverted to evaluate the underlying distribution and time evolution of electron concentration. Accordingly, a new three-dimensional, time-dependent algorithm is presented here for imaging ionospheric electron concentration using GPS signals. The inversion results in a three-dimensional movie rather than a static image of the electron-concentration distribution. The technique is demonstrated using simulated ground-based GPS data from actual measurement geometry over Europe.
Imaging of fast moving electron-density structures in the polar cap
P. S. J. Spencer,C. N. Mitchell
Annals of Geophysics , 2007, DOI: 10.4401/ag-3074
Abstract: The imaging of fast-moving electron-density structures in the polar cap presents a unique set of challenges that are not encountered in other ionospheric imaging problems. GPS observations of total electron content in the polar cap are sparse compared to other regions in the Northern Hemisphere. Furthermore, the slow relative motion of the satellites across the sky complicates the problem since the velocity of the plasma can be large in comparison and traditional approaches could result in image blurring. This paper presents a Kalman-filter based method that incorporates a forward projection of the solution based on a model plasma drift velocity field. This is the first time that the plasma motion, rather than just integrations of electron density, has been used in an ionospheric imaging algorithm. The motion is derived from the Weimer model of the electric field. It is shown that this novel approach to the implementation of a Kalman filter provides a detailed view of the polar cap ionosphere under severe storm conditions. A case study is given for the October 2003 Halloween storm where verification is provided by incoherent scatter radars.
Localization of the Sensory Neurons and Mechanoreceptors Required for Stretch-Evoked Colonic Migrating Motor Complexes in Mouse Colon
Vladimir P. Zagorodnyuk,Nick J. Spencer
Frontiers in Physiology , 2011, DOI: 10.3389/fphys.2011.00098
Abstract: The pacemaker and pattern generator that underlies the cyclical generation of spontaneous colonic migrating motor complexes (CMMCs) has recently been identified to lie within the myenteric plexus and/or muscularis externa. Neither the mucosa, nor the release of substances from the mucosa were found to be required for the spontaneous generation of CMMCs. However, it is known that stretch applied to the colonic wall can also evoke CMMCs and since stretch of the gut wall is known to stimulate the mucosa, it is not clear whether release of substances from the mucosa and/or submucosal plexus are required for stretch-evoked CMMCs. Therefore, the aim of this study was to determine whether circumferential stretch-evoked CMMCs require the presence of the mucosa and/or submucosal plexus in isolated mouse colon. Spontaneous CMMCs were recorded from full length sheet preparations of colon in vitro. Graded circumferential stretch (at a rate of 100 μm/s) applied to a 15-mm segment of mid–distal colon reliably evoked a CMMC, which propagated to the oral recording site. Sharp dissection to remove the mucosa and submucosal plexus from the entire colon did not prevent spontaneous CMMCs and circumferential stretch-evoked CMMCs were still reliably evoked by circumferential stretch, even at significantly lower thresholds. In contrast, in intact preparations, direct stimulation of the mucosa (without accompanying stretch) proved highly inconsistent and rarely evoked a CMMC. These observations lead to the inescapable conclusion that the sensory neurons activated by colonic stretch to initiate CMMCs lie in the myenteric plexus, while the mechanoreceptors activated by stretch, lie in the myenteric ganglia and/or muscularis externa. Stretch activation of these mechanoreceptors does not require release of any substance(s) from the mucosa, or neural inputs arising from submucosal ganglia.
Air plasma for medical applications  [PDF]
Spencer P. Kuo
Journal of Biomedical Science and Engineering (JBiSE) , 2012, DOI: 10.4236/jbise.2012.59061
Abstract: The design and the electric and emission characteristics of two handheld air plasma spray generators are presented. The plasma is generated by 60 Hz periodic discharges between two concentrically cylindrical electrodes. A ring magnet is used to rotate arc discharges, which sprays outward by an air flow. The rotation of arc discharges keeps the generated plasma in non-equilibrium state and at relatively low temperature (<55°C). The plasma effluent yet contains high energy electrons which dissociate molecular oxygen into atomic oxygen. The emission spectroscopy of the plasma plume reveals that the plasma effluent, which carries abundant atomic oxygen, extends from the cap of the plasma spray by about 25 to 30 mm. Tests on blood droplets and smeared blood samples revealed the effectiveness and mechanism of low temperature air plasma on clotting blood. Tests on oral pathogens show that air plasma creates a zone of microbial growth inhibition in each of six treated samples, including those of grampositive bacteria and fungi, and on a cultivating biofilm sample of Streptococcus mutans UA159. The medical applications of the air plasma sprays for 1) bleeding control, 2) wound healing, and 3) dental disinfection, are then illustrated and discussed. As animal models, pigs were used in the tests of stopping wound bleeding and post-operative observation of wound healing by this air plasma spray. The results show that the bleeding from a cut to an ear artery is stopped swiftly; this air plasma spray also shortens wound healing time to about half (from 14 days to 8 days) after stopping the bleeding of a cross cut wound in the ham area. In-vitro tests demonstrate that the plasma effluent of the spray can prevent the formation of dental biofilms and further eliminate the mature biofilms.
Air Plasma Mitigation of Shock Wave  [PDF]
Spencer P. Kuo
Advances in Aerospace Science and Technology (AAST) , 2016, DOI: 10.4236/aast.2016.12006
Abstract: Shock wave is a detriment in the development of supersonic aircrafts; it increases flow drag as well as surface heating from additional friction; it also initiates sonic boom on the ground which precludes supersonic jetliner to fly overland. A shock wave mitigation technique is demonstrated by experiments conducted in a Mach 2.5 wind tunnel. Non-thermal air plasma generated symmetrically in front of a wind tunnel model and upstream of the shock, by on-board 60 Hz periodic electric arc discharge, works as a plasma deflector, it deflects incoming flow to transform the shock from a well-defined attached shock into a highly curved shock structure. In a sequence with increasing discharge intensity, the transformed curve shock increases shock angle and moves upstream to become detached with increasing standoff distance from the model. It becomes diffusive and disappears near the peak of the discharge. The flow deflection increases the equivalent cone angle of the model, which in essence, reduces the equivalent Mach number of the incoming flow, manifesting the reduction of the shock wave drag on the cone. When this equivalent cone angle exceeds a critical angle, the shock becomes detached and fades away. This shock wave mitigation technique helps drag reduction as well as eliminates sonic boom.
Shock Wave Mitigation by Air Plasma Deflector  [PDF]
Spencer P. Kuo
Advances in Aerospace Science and Technology (AAST) , 2018, DOI: 10.4236/aast.2018.34006
When the spacecraft flies much faster than the sound speed (~1200 km/h), the airflow disturbances deflected forward from the spacecraft cannot get away from the spacecraft and form a shock wave in front of it. Shock waves have been a detriment for the development of supersonic aircrafts, which have to overcome high wave drag and surface heating from additional friction. Shock wave also produces sonic booms. The noise issue raises environmental concerns, which have precluded routine supersonic flight over land. Therefore, mitigation of shock wave is essential to advance the development of supersonic aircrafts. A plasma mitigation technique is studied. A theory is presented to show that shock wave structure can be modified via flow deflection. Symmetrical deflection evades the need of exchanging the transverse momentum between the flow and the deflector. The analysis shows that the plasma generated in front of the model can effectively deflect the incoming flow. A non-thermal air plasma, generated by on-board 60 Hz periodic electric arc discharge in front of a wind tunnel model, was applied as a plasma deflector for shock wave mitigation technique. The experiment was conducted in a Mach 2.5 wind tunnel. The results show that the air plasma was generated symmetrically in front of the wind tunnel model. With increasing discharge intensity, the plasma deflector transforms the shock from a welldefined attached shock into a highly curved shock structure with increasing standoff distance from the model; this curved shock has increased shock angle and also appears in increasingly diffused form. In the decay of the discharge intensity, the shock front is first transformed back to a well-defined curve shock, which moves downstream to become a perturbed oblique shock; the baseline shock front then reappears as the discharge is reduced to low level again. The experimental observations confirm the theory. The steady of the incoming flow during the discharge cycle is manifested by the repeat of the baseline shock front.
Identification of the Visceral Pain Pathway Activated by Noxious Colorectal Distension in Mice
Vladimir P. Zagorodnyuk,Simon J. Brookes,Nick J. Spencer
Frontiers in Neuroscience , 2011, DOI: 10.3389/fnins.2011.00016
Abstract: In patients with irritable bowel syndrome, visceral pain is evoked more readily following distension of the colorectum. However, the identity of extrinsic afferent nerve pathway that detects and transmits visceral pain from the colorectum to the spinal cord is unclear. In this study, we identified which extrinsic nerve pathway(s) underlies nociception from the colorectum to the spinal cord of rodents. Electromyogram recordings were made from the transverse oblique abdominal muscles in anesthetized wild type (C57BL/6) mice and acute noxious intraluminal distension stimuli (100–120 mmHg) were applied to the terminal 15 mm of colorectum to activate visceromotor responses (VMRs). Lesioning the lumbar colonic nerves in vivo had no detectable effect on the VMRs evoked by colorectal distension. Also, lesions applied to the right or left hypogastric nerves failed to reduce VMRs. However, lesions applied to both left and right branches of the rectal nerves abolished VMRs, regardless of whether the lumbar colonic or hypogastric nerves were severed. Electrical stimulation applied to either the lumbar colonic or hypogastric nerves in vivo, failed to elicit a VMR. In contrast, electrical stimulation (2–5 Hz, 0.4 ms, 60 V) applied to the rectum reliably elicited VMRs, which were abolished by selective lesioning of the rectal nerves. DiI retrograde labeling from the colorectum (injection sites 9–15 mm from the anus, measured in unstretched preparations) labeled sensory neurons primarily in dorsal root ganglia (DRG) of the lumbosacral region of the spinal cord (L6-S1). In contrast, injection of DiI into the mid to proximal colon (injection sites 30–75 mm from the anus, measured in unstretched preparations) labeled sensory neurons in DRG primarily of the lower thoracic level (T6-L2) of the spinal cord. The visceral pain pathway activated by acute noxious distension of the terminal 15 mm of mouse colorectum is transmitted predominantly, if not solely, through rectal/pelvic afferent nerve fibers to the spinal cord. The sensory neurons of this spinal afferent pathway lie primarily in the lumbosacral region of the spinal cord, between L6 and S1.
Some Observations on the Accelerated Ageing of Thick-Film Resistors
F. N. Sinnadurai,P. E. Spencer,K. J. Wilson
Active and Passive Electronic Components , 1980, DOI: 10.1155/apec.6.241
Pseudo-Anosov subgroups of fibered 3-manifold groups
Spencer Dowdall,Richard P. Kent IV,Christopher J. Leininger
Mathematics , 2012,
Abstract: Let X be a hyperbolic surface and H the fundamental group of a hyperbolic 3-manifold that fibers over the circle with fiber X. Using the Birman exact sequence, H embeds in the mapping class group Mod(Y) of the surface Y obtained by removing a point from X. We prove that a subgroup G in H is convex cocompact in Mod(Y) if and only if G is finitely generated and purely pseudo-Anosov. We also prove a generalization of this theorem with H replaced by an arbitrary Gromov hyperbolic extension of the fundamental group of X, and an additional hypothesis of quasi-convexity of G in H. Along the way, we obtain a generalization of a theorem of Scott and Swarup on the geometric finiteness of subgroups of fibered 3-manifold groups.
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