9,673 research outputs found
On the relationship between the velocity of E-region HF echoes and E B plasma drift
Full text linkInternational audienceIn this study, velocities of E-region HF echoes observed by the Stokkseyri HF radar are compared with ExB plasma drifts in the F-region measured by the DMSP satellites. Events were selected for which the DMSP track projected to the height of 110km was almost perpendicular to the central beams of the radar, resulting in a direct comparison of the cross-track component of the ExB drift and the line-of-sight HF velocity. We found that the typical ratio of HF velocity to the DMSP drift is ~0.35 in a range of DMSP drifts of 0-1700m/s. It is suggested that E-region HF velocities, observed both along the electrojet and at large flow angles, are significantly affected by scatter from the bottom of the electrojet layer where the irregularity phase velocity is expected to be strongly depressed with respect to the ExB flow
Characteristics of a CW HF chemical-laser calculated from a simplified two-dimensional model
Full text linkA two-dimensional simplified model of an HF chemical laser is introduced. Using an implicit finite difference scheme, the solution of two adjacent parallel streams with diffusion mixing and chemical reaction is generated. A contour of mixing and reaction boundary is obtained without presupposition. The distribution of the HF(v) concentrations, gas temperature and the optical small signal gain (alpha sub V, J) on the flowing plane (X, Y) are presented. Compared with the solution solved directly from a set of Navier-Stokes equations, the results of these two methods agree with each other qualitatively. The influences of the different velocity, temperature (T sub 0) and composition of the two streams on the small signal gain after the nozzle exit are investigated. It is interesting that for larger J with a fixed v, the peaks of alpha sub v-T sub 0 profiles move towards higher T sub 0. The computing method is simple and only a short computing time is needed
Investigation of the relationship between optical auroral forms and HF radar E region backscatter
Full text linkInternational audienceThe SuperDARN HF radars have been employed in the past to investigate the spectral characteristics of coherent backscatter from L-shell aligned features in the auroral E region. The present study employs all-sky camera observations of the aurora from Husafell, Iceland, and the two SuperDARN radars located on Iceland, Þykkvibær and Stokkseyri, to determine the optical signature of such backscatter features. It is shown that, especially during quiet geomagnetic conditions, the backscatter region is closely associated with east-west aligned diffuse auroral features, and that the two move in tandem with each other. This association between optical and radar aurora has repercussions for the instability mechanisms responsible for generating the E region irregularities from which radars scatter. This is discussed and compared with previous studies investigating the relationship between optical and VHF radar aurora. In addition, although it is known that E region backscatter is commonly observed by SuperDARN radars, the present study demonstrates for the first time that multiple radars can observe the same feature to extend over at least 3 h of magnetic local time, allowing precipitation features to be mapped over large portions of the auroral zone
Financial analysis, asset allocation, and portfolio construction: Theory & practice
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On financial statements modelling and fundamental analysis
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The mathematics of statistical modelling: Abstract to specific
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reproduced or transmitted in any forms or by any means now known or later devised, or sorted
in a data base or retrieval system, without the prior written permission of HF Consulting. This material has been shared with permission from the author and HF Consulting
Lahars in and around the Taipei basin: Implications for the activity of the Shanchiao fault
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High field strength element systematics and Lu-Hf & Sm-Nd garnet geochronology of orogenic eclogites
Full text linkConcerning the Bulk Silicate Earth (BSE), the depleted mantle and the continental crust are thought to balance the budget of refractory and lithophile elements, resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) Niob and Tantal appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion. The two HFSE Zr and Hf on the other hand seem not to be fractionated between the silicate reservoirs. They show more or less chondritic Zr/Hf ratios. In this study a series of orogenic eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question if eclogites could form a hidden reservoir to account for the mass imbalance of the BSE. The results show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show no fractionation of Nb/Ta ratios and no enrichment of Nb compared to e.g. MOR-basalts, the likely precursor of these rocks. With an average Nb/Ta ratio of 14.9 these eclogites could not balance the differences between BSE and chondrite. Additionally, with an average Nb/Ta ≈ MORB they also cannot balance the small differences in the Nb/Ta of the crust and the mantle. LA-ICPMS analyses of rutiles in these eclogites reveal a zonation of Nb/Ta ratios in this mineral, with rutile cores having higher Nb/Ta than rutile rims. As a consequence, Laser Ablation data of rutiles have to be evaluated carefully and cannot necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.Die kontinentale Kruste und der Mantel sind die beiden wichtigsten geochemischen Reservoire der Silikaterde (Bulk Silicate Earth, BSE). Dementsprechend verhalten sie sich bezüglich ihrer Gehalte an refraktären lithophilen Elementen relativ zu Chondrit in der Regel komplementär. Dies gilt z.B. für viele Seltene Erden (REE) und Sr-Nd Isotope (HOFMANN, 1988; HOFMANN, 1997). Die High Field Strength Elemente (HFSE) Zr, Nb, Hf und Ta werden weitgehend ebenfalls als Spurenelemente betrachtet, die in chondritischer Häufigkeit in der Silikaterde vertreten sind und ebensolche Elementverhältnisse aufzeigen. Es zeigt sich allerdings, dass sowohl die Nb/Ta Verhältnisse der kontinentalen Kruste und des verarmten Mantels, als auch die gesamte Silikaterde, subchondritisch sind, ein Phänomen, das als "Nb-Paradox" bekannt ist (BARTH et al., 2000; KAMBER and COLLERSON, 2000; RUDNICK et al., 2000; MÄNKER et al., 2003; RAPP et al., 2003). Nb und Ta sind zwei HFSE mit sehr ähnlichen geochemischen Eigenschaften und wurden lange Zeit als "geochemische Zwillinge" betrachtet, die sich während der Entwicklung des Kruste-Mantel Systems identisch verhalten. Erste Ideen um die Massenbilanz der Silikaterde auszugleichen legen ein superchondritisches Reservoir nahe, welches im Mantel zurückgehalten wird, und somit zu der heutigen Fraktionierung der Nb/Ta Verhältnisse zwischen BSE und dem chondritischen Wert geführt hat. Ein solches Reservoir kann in Form subduzierter ozeanischer Kruste vorliegen, deren Restit als Eklogit in den Mantel absinkt (MCDONOUGH, 1991; RUDNICK et al., 2000). Diese Betrachtungen basieren auf einem chondritischen Nb/Ta Verhältnis von 17.6, welches aus Analysen von Chondriten (Allende und Murchison) und Eukriten (Vesta) hergeleitet wurde (WEYER et al., 2002). MÄNKER et al. (2003) dagegen haben zur Berechnung des chondritischen Nb/Ta einen deutlich größeren Datensatz zugrundegelegt und kommen auf ein höheres Verhältnis von 19.9. Es wurden zudem von diesen Autoren einige kohlige Chondrite der CV-Gruppe (z.B. Allende) aussortiert, da diese eine ungewöhnliche Anreicherung von Nb-verarmten Einschlüssen zeigten. Dieser neue und höhere Wert für das - IV - chondritische Nb/Ta Verhältnis wird in dieser Arbeit verwendet, da er diesen durch den größeren Datensatz besser repräsentiert. ..
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