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Observations of whistlers during quiet times made at low-latitude ground station Nainital (geomag. lat. 19 1 N) are used to deduce plasmasphere-ionosphere coupling fluxes. The whistler data from 3 magnetically quiet days are presented that show a smooth decrease in dispersion with time. This decrease in dispersion is interpreted in terms of a corresponding decrease in electron content of tubes of ionization. The electron densities, electron tube contents (1016 el/m2-tube) and coupling fluxes (10 el m–1 s–2) are computed by means of an accurate curve fitting method developed by Tarcsai (1975) and are in good agreement with the results reported by other workers. 相似文献
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Making use of currently available theory of wave absorption, an attempt has been made to estimate the refractive indices and absorption coefficients for different wave frequencies during day and night times in the Jovian ionosphere. The results obtained have striking similarity with the corresponding results in the case of the Earth's ionosphere. It is concluded that VLF signals can be observed more easily during night times. 相似文献
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This paper presents discrete chorus type emissions observed in January/July, 1970 during the routine recording of whistlers and VLF emissions at our low latitude ground station Gulmarg (geomag. lat., 24°26N; geomag. long., 147°09 E). The chorus type emissions are comprised of discrete, sometimes overlapping, tones of one or more spectral shapes (risers, falling tones, hooks, etc.). It is shown that these emissions are generated in the equatorial plane (L1.2) by cyclotron resonance between the propagating wave and gyrating electrons. 相似文献
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Muhammad Umer Altaf Arnold W. Heemink Martin Verlaan Ibrahim Hoteit 《Ocean Dynamics》2011,61(8):1093-1105
The Dutch continental shelf model (DCSM) is a shallow sea model of entire continental shelf which is used operationally in
the Netherlands to forecast the storm surges in the North Sea. The forecasts are necessary to support the decision of the
timely closure of the moveable storm surge barriers to protect the land. In this study, an automated model calibration method,
simultaneous perturbation stochastic approximation (SPSA) is implemented for tidal calibration of the DCSM. The method uses
objective function evaluations to obtain the gradient approximations. The gradient approximation for the central difference
method uses only two objective function evaluation independent of the number of parameters being optimized. The calibration
parameter in this study is the model bathymetry. A number of calibration experiments is performed. The effectiveness of the
algorithm is evaluated in terms of the accuracy of the final results as well as the computational costs required to produce
these results. In doing so, comparison is made with a traditional steepest descent method and also with a newly developed
proper orthogonal decomposition-based calibration method. The main findings are: (1) The SPSA method gives comparable results
to steepest descent method with little computational cost. (2) The SPSA method with little computational cost can be used
to estimate large number of parameters. 相似文献
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Mohd. Kafeel Ahmad Ansari Elif Oztetik Altaf Ahmad Shahid Umar Muhammad Iqbal Gary Owens 《洁净——土壤、空气、水》2013,41(8):789-796
The effectiveness of 10 Indian mustard (Brassica juncea L.) genotypes (viz. Agrani, BTO, Kranti, Pusa Bahar, Pusa Basant, Pusa Bold, Pusa Jai Kisan, Vaibhav, Vardhan, and Varuna) were evaluated for their potential to phytoremediate copper from contaminated waters under laboratory controlled conditions. The genotypes were grown for 20 days in aqueous solutions containing various concentrations of copper sulfate (0–50 µM) in a hydroponic chamber. Throughout plant development, changes in growth variables, biomass accumulation, and total Cu content were evaluated. The results suggested that Indian mustard cv. Pusa Jai Kisan possesses the best capacities of Cu sequestration and tolerance amongst all the genotypes studied. Thus, Pusa Jai Kisan has the greatest potential to become a viable candidate in the development of practical phytoremediation technologies for Cu contaminated sites. 相似文献
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Krishna Kumar Singh Ravindra Pratap Patel Balraj Kumar Mohamad Altaf Prince Ahmad Ganai Abhay Kumar Singh Ram Pal Singh Lalmani 《Acta Geophysica》2009,57(2):536-547
The observation of hisslers during daytime at low latitude station Jammu, India, is reported. The hissler elements are quasi-periodic
falling tones observed during the period of hiss activity and appear in minutelong sequences with average spacing between
individual elements of the order of 0.15 s. Hissler elements exhibit almost no dispersion and no complex internal structure
in slope and intensity, and successive hissler elements do not overlap in time. It seems that the reported hisslers might
have propagated in prolongitudinal mode. 相似文献
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M. U. Altaf E. S. Titi T. Gebrael O. M. Knio L. Zhao M. F. McCabe I. Hoteit 《Computational Geosciences》2017,21(3):393-410
We consider a recently introduced continuous data assimilation (CDA) approach for downscaling a coarse resolution configuration of the 2D Bénard convection equations into a finer grid. In this CDA, a nudging term, estimated as the misfit between some interpolants of the assimilated coarse-grid measurements and the fine-grid model solution, is added to the model equations to constrain the model. The main contribution of this study is a performance analysis of CDA for downscaling measurements of temperature and velocity. These measurements are assimilated either separately or simultaneously, and the results are compared against those resulting from the standard point-to-point nudging approach (NA). Our numerical results suggest that the CDA solution outperforms that of NA, always converging to the true solution when the velocity is assimilated as has been theoretically proven. Assimilation of temperature measurements only may not always recover the true state as demonstrated in the case study. Various runs are conducted to evaluate the sensitivity of CDA to noise in the measurements, the size, and the time frequency of the measured grid, suggesting a more robust behavior of CDA compared to that of NA. 相似文献
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Helene E. Unland Altaf M. Arain Chawn Harlow Paul R. Houser Jaime Garatuza-Payan Paul Scott Omer L. Sen W. James Shuttleworth 《水文研究》1998,12(4):527-542
Measurements of micrometeorological variables were made for a complete annual cycle using an automatic weather station and two energy budget–Bowen ratio systems at a field site adjacent to the Santa Cruz River in southern Arizona. These data were used to provide the basis of an estimate of the evaporation from a one-mile long losing reach of a riparian corridor in this semi-arid environment. A remotely sensed map of vegetation cover was used to stratify the corridor into five categories of surface cover. The total evaporation was calculated as the area-weighted average of the measured evaporation for sampled areas of the two most common covers, and appropriate estimates of evaporation for the less common covers. Measurements showed a substantial, seasonally dependent evaporation from the taller, deep-rooted riparian cover in the study reach, while the short, sparse vegetation provided little evaporation. In terms of the volume of water evaporated from the study reach, the evaporation from irrigated agriculture accounts for almost half of the total loss, while the majority of the remaining evaporation is from the taller riparian vegetation covers, with about one-quarter of the total loss estimated as coming from obligatory phreatophytes, primarily cottonwood. © 1998 John Wiley & Sons, Ltd. 相似文献