The Spatial and Temporal Characteristics of Pc3 Geomagnetic Activity over Canada in 2000, as a Guide to Planning the Times of Aeromagnetic Surveys     

Marc A. Vallée  Larry Newitt  Ian R. Mann  Mouhamed Moussaoui  Régis Dumont  Pierre Keating 《Pure and Applied Geophysics》2007,164(1):161-176

Geomagnetic activity affects aeromagnetic surveys. Geomagnetic variations are quite complex and can be quantified in different ways. A measure of geomagnetic activity that is useful for planning aeromagnetic surveys is the Pc3 pulsation index developed by the Australian Space Weather Agency. Purposeful to developing guidelines for planning aeromagnetic surveys in Canada, we study the variations in Pc3 index amplitude over Canada in 2000. This study shows distinct patterns associated with the sub-auroral zone, the auroral zone, and the polar cap. Average Pc3 index activity is higher during the months of February, July, September, and November in the auroral and sub-auroral zones. The station in the polar cap exhibits maximum activity near midday during the summer months. Detailed analysis of a magnetic storm shows that Pc3 index amplitude during the beginning of the solar storm is least important at the polar cap. The mean Pc3 index also relates to solar wind parameters such as the solar wind velocity and the vertical polarity of the interplanetary magnetic field. Analysis of the morning maximum of the Pc3 index observed in the auroral zone can be used to develop guidelines for planning aeromagnetic surveys in Canada and other areas of the world affected by auroral zones.  相似文献   

Large-eddy simulation of plant canopy flows using plant-scale representation     

Wusi Yue  Marc B. Parlange  Charles Meneveau  Weihong Zhu  René van Hout  Joseph Katz 《Boundary-Layer Meteorology》2007,124(2):183-203

Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model. A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy.  相似文献   

Concentration profiles of particles settling in the neutral and stratified atmospheric boundary layer     

Marcelo Chamecki  René van Hout  Charles Meneveau  Marc B. Parlange 《Boundary-Layer Meteorology》2007,125(1):25-38

An expression for the vertical equilibrium concentration profile of heavy particles, including the effects of canopy on the eddy diffusivity as well as corrections for atmospheric stability, is proposed. This expression is validated against measurements of vertical concentration profiles of corn pollen above a corn field. The fitted theoretical profiles show that particle settling is correctly accounted for. The sensitivity to variations in the turbulent Schmidt number, settling velocity and stability corrections are explicitly characterized. The importance of independent measurements of the surface flux of pollen in future experiments is noted.  相似文献   

Tracing the Electron Density from the Corona to 1 au     

Yolande Leblanc  George A. Dulk  Jean-Louis Bougeret 《Solar physics》1998,183(1):165-180

We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm–3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r–2+4.1×106 r–4+8.0×107 r–6 cm–3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by n _e (1 AU)/7.2.We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with radio models based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed.  相似文献   

Spatial Breakdown Point of Variogram Estimators     

Marc G. Genton 《Mathematical Geology》1998,30(7):853-871

In the context of robust statistics, the breakdown point of an estimator is an important feature of reliability. It measures the highest fraction of contamination in the data that an estimator can support before being destroyed. In geostatistics, variogram estimators are based on measurements taken at various spatial locations. The classical notion of breakdown point needs to be extended to a spatial one, depending on the construction of most unfavorable configurations of perturbation. Explicit upper and lower bounds are available for the spatial breakdown point in the regular unidimensional case. The difficulties arising in the multidimensional case are presented on an easy example in IR² , as well as some simulations on irregular grids. In order to study the global effects of perturbations on variogram estimators, further simulations are carried out on data located on a regular or irregular bidimensional grid. Results show that if variogram estimation is performed with a 50% classical breakdown point scale estimator, the number of initial data likely to be contaminated before destruction of the estimator is roughly 30% on average. Theoretical results confirm the previous statement on data in IR^d , d 1.  相似文献   

Petrology and geochronology of basalt breccia from the 1996 earthquake swarm of Loihi seamount, Hawaii: magmatic history of its 1996 eruption     

Michael O. Garcia  Ken H. Rubin  Marc D. Norman  J. Michael Rhodes  David W. Graham  David W. Muenow  Khalil Spencer 《Bulletin of Volcanology》1998,59(8):577-592

 Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. ²¹⁰Po–²¹⁰Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify that they crystallized at moderate depths (∼8–9 km) within the volcano, which is approximately 1 km below the hypocenters for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current magma chamber is considerably deeper than the shallow magma chamber (∼3–4 km) in the adjoining active shield volcanoes. Received: 21 August 1997 / Accepted: 15 February 1998  相似文献   

The kinematics of the ionized gas in the Circinus galaxy     

Marc Elmouttie  Bärbel Koribalski  Scott Gordon  Keith Taylor  Sally Houghton  Tracy Lavezzi  Raymond Haynes  & Keith Jones 《Monthly notices of the Royal Astronomical Society》1998,297(1):49-68

The TAURUS-2 Fabry–Perot interferometer, mounted on the 3.9-m Anglo-Australian Telescope, has been used to observe the Circinus galaxy. We have mapped the intensity and velocity distribution of the ionized hydrogen in the galaxy using the Balmer series Hα spectral line.
The semiresolved core (observed with a seeing disc of 30 pc) appears amorphous in shape, which is commonly observed in Seyfert 2 galaxies. Its peak coincides with the core position measured in the radio continuum, suggesting that ionized gas surrounds a non-thermal source.
A circumnuclear ring or spiral of radius 220 pc and a rotational velocity of 350 km s⁻¹ (assuming circular motions) surrounds the core. The inclination angle of this feature, i =40°±10°, is less than that of the previously observed radio continuum disc. The velocity channel maps obtained for the Hα ring show that the kinematics resemble those of a rotating ring and the intensity displays a complex structure indicative of several, unresolved, H II regions. We believe the ring to represent a circumnuclear starburst.
Our Hα data also show the presence of the previously detected [O III ] ionization cone to the north-west of the core, measuring more than 400 pc in length. We suggest that the ionization cone lies in a different plane from that of the starburst ring and is directed away from us. Several kinematic components of the core are derived and we calculate an outflow velocity in excess of 150–200 km s⁻¹ for gas above the core of Circinus. We also present evidence for inflowing ionized gas at the centre of Circinus.
The correlation of the Hα and radio continuum features is discussed, as well as the possible presence of a starburst-driven superwind in the Circinus galaxy.  相似文献   

Outline of the Pan-African geology of adrar des Iforas (Republic of Mali)     

R. Black  H. Ba  E. Ball  J. M. Bertrand  A. M. Boullier  R. Caby  I. Davison  J. Fabre  M. Leblanc  L. I. Wright 《International Journal of Earth Sciences》1979,68(2):543-564

The Iforas (60 000 km²) falls within the Pan-African mobile belt bordering the West-African craton in north-eastern Mali Republic. It is characterized by major N-S shear belts parallel to the edge of the craton which delimit longitudinal blocks some of which have undergone considerable horizontal displacements. The central core of the Iforas which consists largely of reactivated pre-Pan-African basement injected by Pan-African syn- and post-tectonic intermediate and acid plutonic rocks, has behaved as a relatively rigid blocks during the Pan-African dividing the orogenic belt into a western Iforas and an eastern Iforas.Western Iforas displays W to E zonation: an ophiolitic suture (Timetrine); trench volcano-sedimentary deposits cut by gabbros diorites and acid granitoids (Tilemsi); and a late orogenic composite »coastal range batholith intruding the pre-Pan-African basement of Central Iforas and its overlying volcano-sedimentary deposits which here display a littoral facies and a tillite.Central Iforas consists of two major units: a polycyclic pre-Pan-African basement metamorphosed under high amphibolite facies conditions of presumed Eburnean age and the Iforas granulite block bound to the W, N and E by shear zones.Eastern Iforas was totally separated during metamorphism and deformation from the Iforas granulite block. From West to East, three lithological assemblages have been recognised separed by shear belts: a Quartzite Group, a Gneissic Group and a Pelitic Group the latter representing the southern prolongation of the central Hoggar Pharusian province.Shear zones are an essential feature of Pan-African tectonism East of the West-African craton. The superimposed stress fields have been recognised producing: early N20° trending sinistral shear zones, a north-south dextral shear zone (Andjour-Tamaradant shear zone) and late conjugating sinistral NNW and dextral ENE wrench faults.Late Pan-African events reflect the uplift and unroofing of the Pan-African composite batholith, the intrusion of circular granite plutons often located close to shear zones and alternating episodes of distension and compression.Lastly the simple model proposed for the closing stages of the Pan-African in the Iforas is that of an active continental plate margin separated from the West African craton by an oceanic domain. Subsequent continental collision to the South with a promotory of the West African craton led to the formation of the Dahomeyan thrust front and modified the stress field. Closure of the oceanic domain of western Iforas is thought to have taken place by continued eastward subduction of the oceanic plate and sinistral movement along an inferred north westerly trending transform fault coinciding with the future Cretaceous Gao trough and an alignement of strong positive gravity anomalies. It was accompanied by the northerly migration of central and western Iforas along the conjugating dextral N-S Andjour-Tamaradant shear zone. Further shortening led to folding of the arcuate Timetrine-Ydouban-Gourma fold belt overlying the deformed margin of the West African craton.

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Zusammenfassung Das Iforas-Gebiet (60 000 km²) gehört zur pan-afrikanischen Bewegungszone, die in Mali an das westafrikanische Kraton grenzt. Diese Zone wird von N-S Scherbewegungen parallel zum Kraton durchzogen, wobei größere horizontale Versetzungsbeträge langgestreckte Blöcke herausgetrennt haben. Der zentrale Teil von Iforas besteht im wesentlichen aus reaktiviertem prae-panafrikanischem Basement, das in pan-afrikanischer Zeit von syn- und posttektonischen, intermediären und sauren Plutoniten intrudiert wurde. Dieses Gebiet wirkt als relativ starrer Block, der während der pan-afrikanischen Orogenese den Orogengürtel in einen westlichen und einen östlichen Ast teilt. Das westliche Iforas-Gebiet zeigt eine E-W Zonierung: eine Ophiolith-Sutur, einen vulkano-sedimentären Gürtel und einen Rand-Batholithen.Zentral-Iforas wird aus zwei Einheiten aufgebaut: ein mehrfach metamorphisiertes Basement und einen Granitblock.In den überregionalen Scherzonen lassen sich drei Stress-Felder erkennen: eine ältere 20° streichende sinistrale Scherzone, eine N-S dextrale Scherzone und jüngere NNW und dextrale ENE Bruchzonen.Spät-pan-afrikanische Ereignisse sind durch Heraushebung und Abtrag, Granitintrusionen und wechselnden Dehnungs- und Kompressionsbewegungen gekennzeichnet.

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Résumé L'Adrar des Iforas (60 000 km²) fait partie de la zone mobile pan-africaine en marge du craton ouest-africain au Nord-Est de la République du Mali. La région est caractérisée par d'importants accidents mylonitiques parallèles à la bordure du craton qui délimitent des compartiments longitudinaux dont certains ont subi des déplacements horizontaux considérables. La zone dorsale des Iforas qui consiste essentiellement en un socle pré-pan-africain réactivé et injecté au Pan-Africain par des roches plutoniques intermédiaires et acides, syn- et post-tectoniques, s'est comportée en compartiments relativement rigides au cours du Pan-Africain, divisant la chaîne en un rameau occidental et un rameau oriental.Le rameau occidental présente une zonation d'Ouest en Est: une suture ophiolitique (Timetrine); des dépôts volcano-sédimentaires de fosse recoupés par des gabbros et des diorites; et un vaste batholite composite tardi-orogénique qui recoupe le socle pré-pan-africain de la zone dorsale des Iforas et sa couverture de dépôts volcanosédimentaires ici à faciès littoral.La zone dorsale des Iforas comprend deux unités majeures: un socle prépan-africain polycyclique métamorphisé dans le faciès amphibolite, d'âge éburnéen présumé et le môle granulitique des Iforas, délimité à l'W, au N et à l'E par des accidents mylonitiques.Le rameau oriental était séparé du môle granulitique des Iforas lors du métamorphisme et de la déformation. D'W en E, on trouve trois unités séparées par des zones mylonitiques: un Groupe de Quartzites, un Groupe de Gneiss et un Groupe de Pélites. Ce dernier représente le prolongement vers le Sud de la province pharusienne du centre Hoggar.Les grands accidents de cisaillement sont un fait marquant du tectonisme pan-africain à l'Est du craton ouest-africain. Trois champs de contraintes superposées ont produit des accidents précoces sénestres de direction N20, un accident N-S dextre (Andjour-Tamaradant), et des failles cisaillantes tardives conjuguées d'orientation NNW sénestres et ENE dextres.Les événements pan-africains tardifs sont marqués par la surrection et l'érosion des batholites pan-africains, la mise en place de plutons granitiques souvent à proximité des grands accidents et par des alternances de distensions et de compressions.Enfin un modèle simple est proposé pour les stades ultimes du Pan-Africain dans l'Adrar des Iforas: une marge continentale active séparée du craton ouest-africain par un domaine océanique; suite à une collision au Sud avec un promontoire du craton ouestafricain qui aurait produit le front de chevauchement dahomeyen et modifié le champ de contraintes, la fermeture du domaine océanique de l'Ouest Iforas se serait produite par subduction à l'E de la plaque océanique et une translation sénestre le long d'une faille transformante orientée NW et coincidant avec le fossé crétacé de Gao et un alignement d'anomalies gravimétriques positives. Elle aurait été accompagnée par le déplacement vers le N de l'Iforas occidental et central le long de l'accident cisaillant dextre d'Andjour-Tamaradant. Cette fermeture aurait provoqué les plissements de la chaîne du Timetrine-Ydouban-Gourma qui repose sur la bordure déformée du craton ouestafricain.

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Chromite growth,dissolution and deformation from a morphological view point: SEM investigations   总被引：6，自引：0，他引：6  

M. Leblanc 《Mineralium Deposita》1980,15(2):201-210

The SEM investigations on some chromite crystals show significant morphological differences between cumulate chromites (from stratiform complexes and from ophiolites) and chromites from podiform deposits (ophiolites). The later exhibit a rounded habit and abundant pits which are both the result of dissolution processes. Accordingly, the interpretation of the silicate inclusions within these chromites and the interpretration of the trace-elements distribution need great care. The nodules, a typical structure of the podiform chromites, are explained by a magmatic growth followed by a progressive recrystallization. The accessory chromites which define the mineral lineation in the tectonite peridotites from ophiolites have been deformed by processes of intracrystalline gliding, stretching and dissolution. In the harzburgites, the orthopyroxene has progressively exsolved at sub-solidus (symplektites) vermicular to massive chromite displaying euhedral forms. In the dunitic lenses, the idiomorphic chromites which are aligned parallel to the lineation may have also recrystallized in a solid state.  相似文献   

Sedimentation in Lake Biel,an eutrophic,hard-water lake in northwestern Switzerland   总被引：1，自引：0，他引：1  

Dr. Richard F. Wright  Prof. Albert Matter  Marc Schweingruber  Ulrich Siegenthaler 《Aquatic Sciences - Research Across Boundaries》1980,42(2):101-126

A 2-year study of sedimentation in Lake Biel reveals that three major factors can account for the spatial and temporal patterns of tripton sedimentation. Allochthonous inputs of particulate matter mostly derived from the Aare river during spring snowmelt and other high-water periods, provide about 50% of the carbonate and 50% of the siliceous matter collected in sediment traps. Phytoplankton activity in this eutrophic, hard-water lake accounts for the rest of the carbonate and siliceous matter, the latter being mostly diatom frustules. Sedimentation rates are thus highest during periods of maximum river flooding, maximum phytoplankton activity and following the breakdown of the thermocline, at which time trapped particles settle out. Resuspension of sediment is important in at least one basin. Resolution of carbonate appears to be minor.  相似文献