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Search Results: 1 - 10 of 650651 matches for " J. M. T. Thompson "
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The subsurface radial gradient of solar angular velocity from MDI f-mode observations
T. Corbard,M. J. Thompson
Physics , 2001, DOI: 10.1023/A:1014224523374
Abstract: We report quantitative analysis of the radial gradient of solar angular velocity at depths down to about 15 Mm below the solar surface for latitudes up to 75 degree using the Michelson Doppler Imager (MDI) observations of surface gravity waves (f modes) from the Solar and Heliospheric Observatory (SoHO). A negative outward gradient of around -400 nHz/R, equivalent to logarithmic gradient of the rotation frequency with respect to radius which is very close to -1, is found to be remarkably constant between the equator and 30 degree of latitude. Above 30 degree it decreases in absolute magnitude to a very small value at around 50 degree. At higher latitudes the gradient may reverse its sign: if so this reversal takes place in a thin layer extending only 5 Mm beneath the visible surface, as evidenced by the most superficial modes (with degrees l>250). The signature of the torsional oscillations is seen in this layer, but no other significant temporal variations of the gradient and value of the rotation rate there are found.
Climate tipping as a noisy bifurcation: a predictive technique
J. M. T. Thompson,Jan Sieber
Physics , 2010, DOI: 10.1093/imamat/hxq060
Abstract: It is often known, from modelling studies, that a certain mode of climate tipping (of the oceanic thermohaline circulation, for example) is governed by an underlying fold bifurcation. For such a case we present a scheme of analysis that determines the best stochastic fit to the existing data. This provides the evolution rate of the effective control parameter, the variation of the stability coefficient, the path itself and its tipping point. By assessing the actual effective level of noise in the available time series, we are then able to make probability estimates of the time of tipping. This new technique is applied, first, to the output of a computer simulation for the end of greenhouse Earth about 34 million years ago when the climate tipped from a tropical state into an icehouse state with ice caps. Second, we use the algorithms to give probabilistic tipping estimates for the end of the most recent glaciation of the Earth using actual archaeological ice-core data.
Dynamics of the Nearly Parametric Pendulum
B. Horton,J. Sieber,J. M. T. Thompson,M. Wiercigroch
Mathematics , 2008, DOI: 10.1016/j.ijnonlinmec.2010.11.003
Abstract: Dynamically stable periodic rotations of a driven pendulum provide a unique mechanism for generating a uniform rotation from bounded excitations. This paper studies the effects of a small ellipticity of the driving, perturbing the classical parametric pendulum. The first finding is that the region in the parameter plane of amplitude and frequency of excitation where rotations are possible increases with the ellipticity. Second, the resonance tongues, which are the most characteristic feature of the classical bifurcation scenario of a parametrically driven pendulum, merge into a single region of instability.
Quantified 'shock sensitivity' above the Maxwell load
J. M. T. Thompson,G. H. M. van der Heijden
Mathematics , 2013, DOI: 10.1142/S0218127414300092
Abstract: Using the static-dynamic analogy, work at Bath and Bristol has uncovered the vital organizing role of the Maxwell 'energy criterion' load in the advanced post-buckling of long-thin structures which exhibit severe shell-like imperfection sensitivity. It has become clear that above the Maxwell load, $P_M$, there are localized solutions offering an order-of-magnitude increase in sensitivity to lateral side-loads, whether static or dynamic. We propose to call this 'shock-sensitivity', and notice that so far only the seminal paper by Horak, Lord and Peletier in 2006 has quantified this in terms of an $E(P)$ energy-barrier versus load graph. In this paper we present three graphs of this nature for archetypal problems: the free twisted rod, the cylindrically constrained rod, and the strut on a softening elastic foundation. We find in all cases that the energy barrier of the localizing solution above $P_M$ is quite close to the energy of a single periodic wave. Now a single such wave is not kinematically admissible, and the corresponding periodic barrier must be for all the waves in the long structure, $N$, say. So in practice $N$ will be large, and does indeed tend to infinity with the length of the structure. Thus the sensitivity increases by a factor of a large $N$ as the Maxwell load is exceeded. This is important in its own right, and we do not seek to explain or fit curves to the scattered experimental buckling loads of shell structures.
Star formation in Galactic spiral arms and the inter-arm regions
D. J. Eden,T. J. T. Moore,L. K. Morgan,M. A. Thompson,J. S. Urquhart
Physics , 2013, DOI: 10.1093/mnras/stt279
Abstract: The line of sight through the Galactic Plane between longitudes l = 37.83 degr and l = 42.50 degr allows for the separation of Galactic Ring Survey molecular clouds into those that fall within the spiral arms and those located in the inter-arm regions. By matching these clouds in both position and velocity with dense clumps detected in the mm continuum by the Bolocam Galactic Plane Survey, we are able to look for changes in the clump formation efficiency (CFE), the ratio of clump-to-cloud mass, with Galactic environment. We find no evidence of any difference in the CFE between the inter-arm and spiral-arm regions along this line of sight. This is further evidence that, outside the Galactic Centre region, the large-scale structures of the Galaxy play little part in changing the dense, potentially star-forming structures within molecular clouds.
The Effect of Spiral Arms on Star Formation in the Galaxy
T. J. T. Moore,J. S. Urquhart,L. K. Morgan,M. A. Thompson
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.21740.x
Abstract: We have examined the ratio between the integrated luminosity of massive young stellar objects detected by the Red MSX Source (RMS) survey and the mass of molecular clouds in the Galactic Ring Survey region, as a function of Galactocentric radius. The results indicate that 60--80% of the observed increases in the star-formation rate density associated with spiral-arm features are due to source crowding within the arms. Of the remainder, most of the increase in the inner Sagittarius arm is due to an enhancement in the simple star-formation efficiency, i.e. in the number of RMS sources per unit molecular gas mass. In the inner Perseus arm, the residual increase is due to a higher than average mean source luminosity, which implies a top-heavy IMF, and this is entirely due to the presence, in the GRS region, of the W49 star-forming complex, which appears to be exceptional in its nature. The results also suggest that there is little or no increase in the star-formation efficiency on kiloparsec scales in the Scutum tangent region which includes W43. We discuss the possible role played by the spiral arms in influencing the star-formation efficiency and conclude that the most likely mechanisms are related to orbit crowding within the arms.
The statistics of triggered star formation: an overdensity of massive YSOs around Spitzer bubbles
M. A. Thompson,J. S. Urquhart,T. J. T. Moore,L. K. Morgan
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20315.x
Abstract: We present a detailed statistical study of massive star formation in the environment of 322 Spitzer mid-infrared bubbles by using the RMS survey for massive Young Stellar Objects (YSOs). Using a combination of simple surface density plots and a more sophisticated angular cross-correlation function analysis we show that there is a statistically significant overdensity of RMS YSOs towards the bubbles. There is a clear peak in the surface density and angular cross-correlation function of YSOs projected against the rim of the bubbles. By investigating the autocorrelation function of the RMS YSOs we show that this is not due to intrinsic clustering of the RMS YSO sample. RMS YSOs and Spitzer bubbles are essentially uncorrelated with each other beyond a normalised angular distance of two bubble radii. The bubbles associated with RMS YSOs tend to be both smaller and thinner than those that are not associated with YSOs. We interpret this tendency to be due to an age effect, with YSOs being preferentially found around smaller and younger bubbles. We find no evidence to suggest that the YSOs associated with the bubbles are any more luminous than the rest of the RMS YSO population, which suggests that the triggering process does not produce a top heavy luminosity function or initial mass function. We suggest that it is likely that the YSOs were triggered by the expansion of the bubbles and estimate that the fraction of massive stars in the Milky Way formed by this process could be between 14 and 30%.
Electronic Duality in Strongly Correlated Matter
T. Park,M. J. Graf,L. Boulaevskii,J. L. Sarrao,J. D. Thompson
Physics , 2008, DOI: 10.1073/pnas.0801873105
Abstract: Superconductivity develops from an attractive interaction between itinerant electrons that creates electron pairs which condense into a macroscopic quantum state--the superconducting state. On the other hand, magnetic order in a metal arises from electrons localized close to the ionic core and whose interaction is mediated by itinerant electrons. The dichotomy between local moment magnetic order and superconductivity raises the question of whether these two states can coexist and involve the same electrons. Here we show that the single 4f-electron of cerium in CeRhIn5 simultaneously produces magnetism, characteristic of localization, and superconductivity that requires itinerancy. The dual nature of the 4f-electron allows microscopic coexistence of antiferromagnetic order and superconductivity whose competition is tuned by small changes in pressure and magnetic field. Electronic duality contrasts with conventional interpretations of coexisting spin-density magnetism and superconductivity and offers a new avenue for understanding complex states in classes of materials.
STEREO/EUVI Event Catalog 2006-2012
Aschwanden,M. J.,Wuelser,J. P.,Nariaki,N. V.,Lemen,J. R.,Thompson,W. T
Physics , 2013, DOI: 10.1007/s11207-013-0378-5
Abstract: We generated an event catalog with an automated detection algorithm based on the entire EUVI image database observed with the two Solar Terrestrial Relations Observatory STEREO-A and -B spacecraft over the first six years of the mission (2006\,--\,2012). The event catalog includes the heliographic positions of some 20\,000 EUV events, transformed from spacecraft coordinates to Earth coordinates, and information on associated GOES flare events (down to the level of GOES A5-class flares). The 304 \ang\ wavelength turns out to be the most efficient channel for flare detection (79\,%), while the 171 \ang\ (4\,%), 195 \ang\ (10\,%), and the 284 \ang\ channel (7\,%) retrieve substantially fewer flare events, partially due to the suppressing effect of EUV dimming, and partially due to the lower cadence in the later years of the mission. Due to the Sun-circling orbits of STEREO-A and -B, a large number of flares have been detected on the farside of the Sun, invisible from Earth, or seen as partially occulted events. The statistical size distributions of EUV peak fluxes (with a power-law slope of $\alpha_P = 2.5\pm0.2$) and event durations (with a power-law slope of $\alpha_T=2.4\pm0.3$) are found to be consistent with the fractal-diffusive self-organized criticality model. The EUVI event catalog is available on-line \url{http://secchi.lmsal.com/EUVI/euvi__autodetection/euvi__events.txt} and may serve as a comprehensive tool to identify stereoscopically observed flare events for 3D reconstruction and to study occulted flare events.
School Adjustment and the Academic Success of Rural African American Early Adolescents in the Deep South
Farmer, T. W.,Irvin, M. J.,Thompson, J. H.,Hutchins, B. C.
Journal of Research in Rural Education , 2006,
Abstract: This study examined the relationship between end-of-year grades and the academic, behavioral, and social characteristics of rural African American youth. Participants included 392 7th and 8th grade students from 2 rural middle schools in the south. Participants were African American and were from 2 communities that have child poverty rates exceeding 50% for public school students. Girls were more likely to have positive characteristics than boys. Academic, behavioral, and social difficulties were linked to low end-of-year grades, and positive characteristics were linked to high grades. Implications for supporting low-achieving African American students from low-resource communities are discussed.
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