1,111 research outputs found
On Nonlocal Energy Transfer via Zonal Flow in the Dimits Shift
The two-dimensional Terry-Horton equation is shown to exhibit the Dimits
shift when suitably modified to capture both the nonlinear enhancement of
zonal/drift-wave interactions and the existence of residual Rosenbluth-Hinton
states. This phenomenon persists through numerous simplifications of the
equation, including a quasilinear approximation as well as a four-mode
truncation. It is shown that the use of an appropriate adiabatic electron
response, for which the electrons are not affected by the flux-averaged
potential, results in an
nonlinearity that can efficiently transfer energy nonlocally to length scales
on the order of the sound radius. The size of the shift for the nonlinear
system is heuristically calculated and found to be in excellent agreement with
numerical solutions. The existence of the Dimits shift for this system is then
understood as an ability of the unstable primary modes to efficiently couple to
stable modes at smaller scales, and the shift ends when these stable modes
eventually destabilize as the density gradient is increased. This nonlocal
mechanism of energy transfer is argued to be generically important even for
more physically complete systems.Comment: 28 pages, 7 figures, 4 movie
Improved pulse shape discriminator for fast neutron-gamma ray detection system
Discriminator in nuclear particle detection system distinguishes nuclear particle type and energy among many different nuclear particles. Discriminator incorporates passive, linear circuit elements so that it will operate over a wide dynamic range
An Introduction to the Continental Record of the Laurentide Ice Sheet
The record of the Laurentide Ice Sheet is not well preserved in terrestrial deposits. Sediment sequences are incomplete and few record events older than the last ice retreat. Furthermore, in the absence of absolute dating methods for deposits older than the limit of radiocarbon dating, units are generally assigned a chronostratigraphic position by "counting from the top". As a result age estimates of many units can differ significantly between authors.Les dépôts abandonnés par la calotte laurentidienne relatent bien imparfaitement son évolution. Les séquences sédimentaires sont incomplètes et peu d'entre elles sont antérieures au retrait des derniers glaciers. De plus, en l'absence de méthodes de datation absolue des dépôts au-delà des limites du radiocarbone, la position chronostratigraphique d'un sédiment est généralement déterminée en comptant les unités de haut en bas. Conséquemment, les estimations de l'âge des unités varient de façon importante d'un auteur à l'autre
The Sangamonian Stage and the Laurentide Ice Sheet
This review of the most recent studies (up to June 1986) dealing with the Sangamonian in some key areas clearly indicates that, as yet, there is no definitive answer to the question : "When did the ice which eventually became the Laurentide Ice Sheet begin to accumulate?" In most areas the stratigraphic record simply identifies a probable interglacial period; the record yields no information on when ice growth may have started following that warm climatic interval. However the deltaic glacial lake sediments of the Scarborough Formation in the Toronto area and the Bécancour Till in the Trois-Rivières area are thought to possibly date from the Sangamonian (marine isotope sub-stages 5d-b). The Adam Till in the James Bay Lowland may be correlative. In Atlantic Canada, mostly in Cape Breton Island, plant fossils suggest a mid-Sangamonian climate roughly comparable to that which prevailed 11-12 ka ago. On Baffin Island a marine transgression of mid-Sangamonian age is thought to result from important ice accumulation in the area. These stratigraphic interpretations suggest significant glacier expansion in several areas of the North American continent during part of the Sangamonian Stage. Whether or not any of this ice survived a warmer climate period near the end of the Sangamonian to become part of the Laurentide Ice Sheet is a matter of speculation.La présente revue des travaux sur le Sangamonien (jusqu'à juin 1986) effectués dans des régions clés démontre qu'il n'y a pas encore de réponse satisfaisante à la question suivante: « À quel moment la glace, qui allait devenir la calotte glaciaire laurentidienne, a-t-elle commencer à s'accumuler? » Dans la plupart des régions, la séquence stratigraphique ne fait que signaler l'existence probable d'une période interglaciaire, sans toutefois permettre de déterminer le moment où la glace a commencé à s'accumuler après l'intervalle climatique chaud. Il existe toutefois quelques indices. Les sédiments d'un delta glaciolacustres de la Formation de Scarborough, dans la région de Toronto, et le Till de Bécancour, dans la région de Trois-Rivières, datent probablement du Sangamonien (sous-phases isotopiques marines 5d-b). Le Till d'Adam, dans les basses terres de la baie James, leur est probablement corrélatif. Dans les régions atlantiques du Canada, en particulier à l'île du Cap-Breton, des restes de plantes indiquent que le climat au cours du Sangamonien moyen aurait été très semblable à celui de la période de 11 000 à 12 000 ans BP. À l'île de Baffin, une transgression marine qui a eu lieu au Sangamonien moyen aurait pu être le résultat d'une importante accumulation de glace dans la région. Ces diverses interprétations régionales témoignent d'une importante croissance des glaciers dans plusieurs régions du continent nord-américain au cours du Sangamonien. On ne peut toutefois pas préciser si ces accumulations de glace ont survécu à la période plus chaude qui a prévalu à la fin du Sangamonien pour ensuite s'intégrer à la calotte glaciaire laurentidienne.Dieser Überblick über die neuesten Studien (bis Juni 1986) zur sangamonischen Zeit in einigen Schlüsselgebieten zeigt deutlich, dap es bis heute keine endgültige Antwort auf die Frage gibt: "Wann begann sich das Eis anzuhäufen, das schlieBlich zur laurentischen Eisdecke wurde?" In den meisten Gebieten zeugen die stratigraphischen Spuren von einem wahrscheinlichen interglazialen Zeitabschnitt; die Spuren geben keine Auskunft darüber, wann der Eiswuchs nach diesem warmen klimatischen Zwischenspiel begonnen haben mag. Die Sedimente der deltaförmigen EisSeen der ScarboroughFormation in der Gegend von Toronto und die Bécancour-Grundmoräne in der Gegend von Trois-Rivières stammen vermutlich beide aus der sangamonischen Phase (Meeres-lsotopische Unterstadien 5d-b). Die AdamGrundmoräne im James-Bay-Tiefland hängt damit möglicherweise zusammen. In OstKanada, vor allem auf der Cap-Breton-Insel, legen Pflanzen-Fossilien nahe, daB dort ein Klima der mittleren sangamonischen Phase herrschte, das dem von vor 11 bis 12 ka ungefähr glich. Auf der Baffinlnsel halt man die UferBewegung in der sangamonischen Zeit fur die Auswirkung groBer Eisanhäufung in der Gegend. Diëse Deutungen der Stratigraphie legen eine umfangreiche GletscherAusdehnung in mehreren Gegenden des nordamerikanischen Kontinents während eines Teils der sangamonischen Zeit nahe. Ob Teile dieses Eises eine wärmere Phase gegen Ende der sangamonischen Zeit überdauerten, um ein Teil der laurentischen Eisdecke zu werden, Oder ob dies nient der Fall war, bleibt often
Late Wisconsinan Morphosedimentary Sequences of the Lower Coppermine River Valley, Nunavut and Northwest Territories
A series of late Wisconsinan sedimentary sequences occupy parts of the Coppermine River valley, and are grouped into five morphosedimentary zones representing four environments at the time of last ice retreat: glacial, paraglacial, lacustrine and marine. The sequences commonly interfinger, and document episodic deposition in time-transgressive environments related to ice frontal positions. Glacial and paraglacial sediments are, in part, reworked in a glaciolacustrine environment extending from delta topsets of bouldery gravel to bottomset rhythmites of silt-clay. The glacial-lake rhythmites formed the matrix of a later sequence of debris-flow events, which emplaced a wedge of massive diamicton in the postglacial marine sediments. This sedimentary sequence, composed of a chaotic mass of heterogeneous material that is intimately mixed, and is accumulated in the form of a semi-fluid body in a marine environment, is possibly the best exposed Quaternary olistostrome. A general overview of the Quaternary history of the area east of Great Bear Lake and north of Point Lake shows: a) deglaciation proceeded by large-scale downwasting with no discernable readvance pulses, b) that the Coppermine River valley between Point Lake and Rocky Defile Rapids was ice-free by 10 250 14C years BP, and, c) that earth-shaping processes were intensely active for a short period at the time of deglaciation; since then the landscape has been mostly quiescent.SOMMAIREDes séquences sédimentaires de la fin du Wisconsinien qui occupent des sections de la vallée de la rivière Coppermine définissent cinq zones morphosédimentaires représentant quatre environnements différents du retrait glaciaire, soit les milieux glaciaire, paraglaciaire, lacustre et marin. Ces séquences qui s’interdigitent souvent, documentent des épisodes sédimentaires de milieux qui varient avec le temps et les changements de position du front glaciaire. Les sédiments glaciaires et paraglaciaires sont en partie remaniés dans des milieux glaciolacustres allant des lit topset deltaïques de gravier à blocs jusqu’à des rythmites de limon-argile. Les rythmites glacio-lacustres constituent la matrice d’une séquence subséquente de lave torrentielle, qui a déposé un énorme prisme de diamictons dans les sédiments marins postglaciaires. Cette séquence sédimentaire, composée d’une masse chaotique de matériaux hétérogènes intimement mélangés accumulée sous forme semi-fluide en milieu marin, est peut-être le meilleur affleurement d’olistostrome quaternaire. Un coup d’œil sur l’histoire du Quaternaire de la région à l’est du Grand lac de l’Ours et au nord du lac Point, montre : a) que la déglaciation s’est déroulée par fonte du glacier sans réavancée discernable, b) que la vallée de la rivière Coppermine entre le lac Point et les rapides de Rocky Defile  était libre de glace vers 10 250 14C BP, et c) que les mécanismes d’érosion ont été intenses pendant une courte période au moment de la déglaciation; depuis lors, le paysage est demeuré relativement statique
Glass transitions in 1, 2, 3, and 4 dimensional binary Lennard-Jones systems
We investigate the calorimetric liquid-glass transition by performing
simulations of a binary Lennard-Jones mixture in one through four dimensions.
Starting at a high temperature, the systems are cooled to T=0 and heated back
to the ergodic liquid state at constant rates. Glass transitions are observed
in two, three and four dimensions as a hysteresis between the cooling and
heating curves. This hysteresis appears in the energy and pressure diagrams,
and the scanning-rate dependence of the area and height of the hysteresis can
be described by power laws. The one dimensional system does not experience a
glass transition but its specific heat curve resembles the shape of the results in the supercooled liquid regime above the glass transition. As
increases, the radial distribution functions reflect reduced geometric
constraints. Nearest-neighbor distances become smaller with increasing due
to interactions between nearest and next-nearest neighbors. Simulation data for
the glasses are compared with crystal and melting data obtained with a
Lennard-Jones system with only one type of particle and we find that with
increasing crystallization becomes increasingly more difficult.Comment: 26 pages, 13 figure
Morphodynamics of Cold High Latitude Semiarid Regions: The Example of Ellef Ringnes Island, Nunavut
In this paper we use the example of Ellef Ringnes Island, Nunavut, to examine active geomorphic processes in a high latitude semiarid region. The spectacular landscape of Ellef Ringnes Island is principally the result of fluvial erosion controlled by geological structure. The nival regime concentrates flow in a brief late-spring period which provides strong erosive capabilities for rivers in spite of low annual precipitation. Other processes related to the cold environment produce a variety of minor landforms such as patterned ground, mud flows associated with ground ice, and solifluction lobes.Dans cet article, nous utilisons l’île Ellef Ringnes, au Nunavut, comme exemple en vue d’identifier les processus actifs pour cette région semi-aride de haute latitude. L’érosion fluviale contrôlée par la structure géologique est à l’origine du relief spectaculaire de l’île Ellef Ringnes. Le régime nival, qui concentre les débits sur une courte période à la fin du printemps, est caractérisé par une forte capacité érosive des cours d’eau en dépit des faibles précipitations annuelles. D’autres processus reliés aux milieux froids produisent une variété de formes mineures telles que les sols à figures géométriques, les coulées de boue associées à la présence de glace dans le sol et les lobes de solifluxion
Measuring Forest Canopy Height Using a Combination of LIDAR and Aerial Photography Data
It has been demonstrated that the height of forest canopies can be measured with a good accuracy using small footprint lidars. This is essentially accomplished by subtracting the last return altitude (ground) from the corresponding first return altitude (canopy surface). The technique is considered superior to photogrammetric methods mainly because the ground level, which is difficult to see on aerial photos of densely forested areas, can be well identified using small footprint lidars. However, lidar cannot be used to characterized past forest states, while these can be assessed, and photogrammetically measured, in the wealth of historical aerial photographs most developed countries possess. Our goal is to replace the first return lidar data by altitude models derived from aerial photos in order to map forest canopy height changes of the past decades. This paper presents the first methodological steps which consist in comparing canopy heights obtained from lidar data only to a combination of lidar and photogrammetry data. The lidar data was acquired over an area of the boreal forest in Quebec, Canada, in 1998, using Optech’s ALTM1020 flying at an altitude of 700 m. Two stereo-pairs of aerial black and white photographs were used: 1) a pair of 1:15,000 photos taken in 1994, and 2) a pair of 1:40,000 photos taken in 1998. A lidar canopy height model (CHM) was created by subtracting ground altitudes from canopy altitudes. Aerial photo altitude models were derived using the image correlation methods of Virtuozo 3.2 software. The ground level altitudinal fit between the aerial photo altitude model and the lidar data was checked on rock outcrops. A photo CHM was created by subtracting the lidar ground altitude model from the aerial photo altitude model. The photo CHM and the lidar CHM show a good degree of correlation
Les formes d’effondrement et le mode de déglaciation de la région du lac Qingaluk, Territoires-du-Nord-Ouest, Canada
Les formes et les sédiments glaciaires de la vallée de la rivière Qingaluk illustrent la complexité du recul du front du glacier du Keewatin, au Wisconsinien supérieur, à l'ouest de la vallée de la rivière Coppermine dans les Territoires-du-Nord-Ouest. Dans cette région à relief modéré (300 m), des retraits mineurs du front glaciaire (2-5 km) ont provoqué des changements importants dans le sens d'écoulement des eaux de fonte et ont modifié les environnements sédimentaires dans la vallée. La sédimentation rapide en bordure du front glaciaire fut responsable de l'ensevelissement de blocs de glace. De nombreuses formes d'affaissement ont résulté de la fonte de ceux-ci après que l'activité des eaux fluvio-glaciaires eut cessé. Ces formes, de même que les crêtes morainiques, peuvent être utilisées pour déterminer des positions successives du front glaciaire. Quatre de ces positions, associées à un haut niveau du lac glaciaire Coppermine (phase Kamut), ont été définies dans la région du lac Qingaluk. Ces positions frontales permettent d'expliquer des inversions du sens de l'écoulement des rivières et du relief dans les dépôts fluvio-glaciaires, ainsi que des étagements de terrasses dans l'exutoire de la phase Kamut du lac glaciaire Coppermine.Glacial and glaciofluvial sediments and associated landforms in the Qingaluk River valley illustrate the complexity of glacial retreat in the Coppermine River Valley region, N.W.T. In this area of moderate relief (300 m), minor retreat (2-5 km) of the ice front led to dramatic changes in melt-water flow patterns and in sedimentary environments within the valley. Near the ice-front, dead-ice blocks were buried by rapid deposition of coarse sediments from meltwater streams. Subsequent melting of these buried ice masses, after meltwater activity had ceased, created typical collapse landforms. These, along with moraine ridges, can be used to define the former ice margins. Four such positions have been recognized in the Qingaluk Lake region. The last of these held up a high-level Glacial Lake Coppermine (Kamut Phase). The sequence of déglaciation explains the reversals in river flow direction, the inverted relief in outwash, and the terracing along the outlet of the Kamut Phase of Glacial Lake Coppermine
A phase-shift-periodic parallel boundary condition for low-magnetic-shear scenarios
We formulate a generalized periodic boundary condition as a limit of the
standard twist-and-shift parallel boundary condition that is suitable for
simulations of plasmas with low magnetic shear. This is done by applying a
phase shift in the binormal direction when crossing the parallel boundary.
While this phase shift can be set to zero without loss of generality in the
local flux-tube limit when employing the twist-and-shift boundary condition, we
show that this is not the most general case when employing periodic parallel
boundaries, and may not even be the most desirable. A non-zero phase shift can
be used to avoid the convective cells that plague simulations of the
three-dimensional Hasegawa-Wakatani system, and is shown to have measurable
effects in periodic low-magnetic-shear gyrokinetic simulations. We propose a
numerical program where a sampling of periodic simulations at random
pseudo-irrational flux surfaces are used to determine physical observables in a
statistical sense. This approach can serve as an alternative to applying the
twist-and-shift boundary condition to low-magnetic-shear scenarios which, while
more straightforward, can be computationally demanding.Comment: 4 figures, 1 tabl
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