Electron depletion in the wake of ionospheric spacecraft—A comparison between results from Langmuir probes and antennas     

Uri Samir  H. Weil 《Planetary and Space Science》1973,21(6):993-1000

In situ observations of the electron depletion in the wakes of satellites and rockets in the ionosphere using Langmuir type probes and antennas are analyzed and compared. The quantitative degree of agreement between the results is demonstrated and discussed. One consequence is an improved interpretation of results previously presented for the OGO II Satellite wake.  相似文献   

Post-collision neogene volcanism of the Eastern Rif (Morocco): magmatic evolution through time   总被引：6，自引：0，他引：6  

Samir El Bakkali  Alain Gourgaud  Jean-Louis Bourdier  Herv Bellon  Niyazi Gundogdu 《Lithos》1998,45(1-4):523-543

Neogene volcanism in the Eastern Rif (Morocco) comprises a series of calc-alkaline, potassic calc-alkaline, shoshonitic and alkaline volcanic rocks. According to new stratigraphical, along with new and previous chronological and geochemical data, the orogenic volcanism was successively (1) calc-alkaline (basaltic andesites and andesites: 13.1 to 12.5 Ma, rhyolites: 9.8 Ma), (2) K-calc-alkaline (basaltic andesitic to rhyolitic lavas and granodiorites: 9.0 to 6.6 Ma), and (3) shoshonitic (absarokites, shoshonites, latites, trachytes: 7.0 to 5.4 Ma). The later Pliocene volcanism was basaltic and alkaline (5.6 to 1.5 Ma). The calc-alkaline and K-calc-alkaline series exhibit lower K₂O (0.7–5.3 wt.%), Nb (8–19 ppm) contents and higher ⁸⁷Sr/⁸⁶Sr (0.70773–0.71016) than the shoshonitic series (K₂O: 2.4–7.2 wt.%, Nb: 21–38 ppm, ⁸⁷Sr/⁸⁶Sr: 0.70404–0.70778). Pliocene alkaline basalts have a sodic tendency (Na₂O/K₂O: 1.7–3.5), high Nb content (up to 52 ppm), and low ⁸⁷Sr/⁸⁶Sr ratio (0.70360–0.70413). The variations through time of K₂O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-alkaline and shoshonitic series are derived from a mantle source modified by older subduction, (ii) alkaline basalts are derived mainly from an enriched mantle source. Through time, incompatible elements such as Nb increased while ⁸⁷Sr/⁸⁶Sr decreased, suggesting a decreasing influence of metasomatized mantle (inherited subduction). Such evolution is related to the post-collision regimes operating in this area, and could be linked to the succession of extensional, compressional and strike-slip fault tectonics.  相似文献   

Global detailed geoid computation and model analysis     

James G. Marsh  Samir Vincent 《Surveys in Geophysics》1974,1(4):481-511

This paper presents a survey of recent work on the gravimetric geoid. The gravity models considered are those published in the past few years by the Goddard Space Flight Center (GSFC), the Smithsonian Astrophysical Observatory (SAO) and the Ohio State University (OSU). Comparisons and analyses have been carried out through the ose of detailed gravimetric geoids which we have computed by combining the above-mentioned models with a set of 26 000, 1^ox1^o mean free air gravity anomalies. The accuracy of the detailed gravimetric geoid computed using the most recent Goddard Earth Model (GEM-6) in conjunction with the set 1^ox1^o mean free air gravity anomalies is assessed at 2 m on the continents of North America, Europe And Australia, 2 to 5 m in the North-East Pacific and North Atlantic areas and 5 to 10 m in other areas where surface gravity data are sparse. Rms differences between this detailed geoid and the detailed geoids computed using the other satellite gravity fields in conjunction with same set of surface data range from 3 to 7 m. The maximum differences in all cases occurred in the Southern Hemisphere where surface data and satellite observations are sparse. These differences exhibited wavelengths of approximately 30^o to 50^o in longitude. Detailed geoidal heights were also computed with models truncated to 12th degree and order as well as 8th degree and order. This truncation resulted in a reduction of the rms differences to a maximum of 5 m. Comparisons have been made with the astrogeodetic data of Rice (United States), Bomford (Europe), and Mather (Australia) and also with geoidal heights from satellite solutions for geocentric station coordinates in North America and the Caribbean.  相似文献   

The First Recorded Occurrences and the Distribution of Physalia physalis (Hydrozoa: Physaliidae) in Algerian Waters     

Bachouche  Samir  Ghribi  Tarek  Rouidi  Samir  Etsouri  Mohamed  Belkacem  Yasmine  Selmani  Rabah  Djellali  Mostapha  Hadj Aissa  Raouf  Grimes  Samir 《Ocean Science Journal》2022,57(3):411-419

Ocean Science Journal - Recently, there have been recorded findings of the pleustonic siphonophore Physalia physalis Linnæus, 1758, in different regions of Algeria. Several colonies washed...  相似文献   

The Role of Coffee in the Development of Southwest Ethiopia's Forests: Farmers' Strategies,Investor Speculation,and Certification Projects     

Samir El Ouaamari  Hubert Cochet 《社会与自然资源》2014,27(2):200-214

This article examines the role of coffee in the agriculture of southwest Ethiopian highlands, a context characterized by recent huge private coffee estate developments, coffee certification projects and smallholdings. The analysis shows that smallholder farmers prioritize staple crops, and allocate land and labor to coffee only once food production objectives have been reached. It also underscores the diversity of the local peasantry, often treated as a homogeneous group in development projects designed to boost farmers' income through coffee production. Based on this analysis, the article evaluates the effects of recent coffee-oriented development policies: state forest concessions for coffee investment, and two coffee certification projects (UTZ-Certified and Fair-Trade/Organic). Because they omit the differentiated farmers' strategies, these certification initiatives fail to improve local populations' livelihoods and accelerate incipient socioeconomic inequalities. Moreover, when applied to big private plantations, they participate in farmers' dispossession from critical forest resources essential to households' food self-sufficiency.  相似文献   

First record of palynofossils from the Fulra Limestone Formation (Middle Eocene) of Kachchh basin,Gujarat and their palaeoenvironmental implications     

Samir Sarkar  J. P. Mandal 《Journal of the Geological Society of India》2014,83(6):641-646

The present paper deals with the first record of a palynofloral assemblage recovered from the Fulra Limestone Formation exposed at the confluence of Fulra and Panandhro Nalas in the west of Babia hill, Kachchh basin, Gujarat. The recovered palynoflora consists of dinoflagellate cysts, fungal spores and ascostromata, pteridophyte spores, gymnosperm and angiosperm pollen. Some of the important constituents of the palynofloral assemblage are: Lygodiumsporites, Polypodiaceasporites, Polypodiisporites, Margocolporites, Tricolporopilites, Tricolporopollis, Graminidites, Aplanosporites, Phragmothyrites, Spiniferites, Operculodinium and Achomosphaera. Palynological data suggest that the Fulra Limestone Formation was mostly laid down in a shallow marine environment under a warm and humid tropical climate. Abundance of terrestrial palynofossils in some of the samples clearly points towards the relative proximity of the shore. A late middle Eocene age has been assigned for the Fulra Limestone Formation on the basis of recorded palynofossils.  相似文献   

Seasonal prediction of Indian summer monsoon in NCEP coupled and uncoupled model     

Hemantkumar S. Chaudhari  Samir Pokhrel  Sachiko Mohanty  Subodh K. Saha 《Theoretical and Applied Climatology》2013,114(3-4):459-477

This study has identified probable factors that govern ISMR predictability. Furthermore, extensive analysis has been performed to evaluate factors leading to the predictability aspect of Indian Summer Monsoon Rainfall (ISMR) using uncoupled and coupled version of National Centers for Environmental Prediction Coupled Forecast System (CFS). It has been found that the coupled version (CFS) has outperformed the uncoupled version [Global Forecast System (GFS)] of the model in terms of prediction of rainfall over Indian land points. Even the spatial distribution of rainfall is much better represented in the CFS as compared to that of GFS. Even though these model skills are inadequate for the reliable forecasting of monsoon, it imparts the capacious knowledge about the model fidelity. The mean monsoon features and its evolution in terms of rainfall and large-scale circulation along with the zonal and meridional shear of winds, which govern the strength of the monsoon, are relatively closer to the observation in the CFS as compared to the GFS. Furthermore, sea surface temperature–rainfall relation is fairly realistic and intense in the coupled version of the model (CFS). It is found that the CFS is able to capture El Niño Southern Oscillation ISMR (ENSO-ISMR) teleconnections much strongly as compared to GFS; however, in the case of Indian Ocean Dipole ISMR teleconnections, GFS has the larger say. Coupled models have to be fine-tuned for the prediction of the transition of El Niño as well as the strength of the mature phase has to be improved. Thus, to sum up, CFS tends to have better predictive skill on account of following three factors: (a) better ability to replicate mean features, (b) comparatively better representation of air–sea interactions, and (c) much better portrayal of ENSO-ISMR teleconnections. This study clearly brings out that coupled model is the only way forward for improving the ISMR prediction skill. However, coupled model’s spurious representation of SST variability and mean model bias are detrimental in seasonal prediction.  相似文献   

Identification of Shocks in the Spectra from Black Holes     

Samir Mandal  Sandip Kumar Chakrabarti 《Astrophysics and Space Science》2005,297(1-4):269-274

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Intercomparison among plasma wake models for plasmaspheric and ionospheric conditions     

U. Samir   《Planetary and Space Science》1987,35(12):1477-1487

Intercomparisons are made among the angular distributions of ions in the wake of a body moving through a space plasma as computed from three different expressions (models). Both subsonic and supersonic relative flows are considered in order to examine the wake current depletion ratios under conditions realistic for the topside ionosphere and plasmasphere. Results of these comparisons demonstrate the importance of including the thermal flux at low Mach numbers and of taking into account the angular acceptance of ion detectors in making theory-experiment comparisons. Gradients in the angular variations of the fluxes are found to be steeper near the wake-ambient interface than closer to the maximum rarefaction region of all models, although quantitatively there is considerable variation among the models. From examining the variations of the wake depletion ratio parametrically with Mach number and normalized potential over ranges characteristic of the plasmasphere and topside ionosphere, we find considerable variation with both parameters, with sensitivity to normalized potential increasing dramatically with Mach number. Overall, however, the Mach number variation appears to be the more significant over this range of parameters.  相似文献   

Comparison of theory and in situ observations for electron and ion distributions in the near wake of the explorer 31 and AE-C satellites     

Uri Samir  Ernest G. Fontheim 《Planetary and Space Science》1981,29(9):975-987

Measurements of electron density, plasma potential, and mean ion mass from the Explorer 31 satellite and measurements of ion current, plasma potential, and ion composition from the Atmosphere Explorer C (AE-C) satellite were used in a comparative study with theory regarding the charged particle distribution in the near wake of an ionospheric satellite. The theoretical wake model of Parker (1976) has been used in the study. It is shown that theory and experiment agree fairly well in the angle-of-attack range between 90 and 135°. In that angular range even the neutral approximation (which treats ions as if they were neutral particles thus ignoring the influence of the electric field) gives fair agreement with the measurements. In the maximum rarefaction zone (145 < θ < 180°), however, the theoretical model overestimates the measured ion depletion (AE-C measurements) by several orders of magnitude. A similar conclusion is drawn from the comparison between theory and the Explorer 31 electron measurements where the theory also significantly overestimates the electron depletion. The study indicates that the discrepancies are mainly due to the use of a steady-state theory and of a single ion equation (using a mean ion mass). It is recommended that improved agreement between theory and experiment be obtained by the use of the timedependent Vlasov-Poisson equations with separate equations for the various ion species.  相似文献