Collision frequency and transport properties of electrons in the ionosphere     

K.M. Aggarwal  Nardsder Nath  C.S.G.K. Setty 《Planetary and Space Science》1979,27(6):753-768

A unified ionospheric electron collision frequency model profile 〈ν_g〉 has been calculated in the height range 50–500 km. The computed profile accounts for the electron collisions with the neutral particles as well as the ions. Experimental values of momentum transfer cross-sections have been used for electron collisions with N₂, O₂ and Ar and theoretical values for N, O, He and H. It is observed that the electron-ion collisions 〈ν_ei〉 dominate over the electron-neutral collisions 〈ν_en〉 above 170 km. However, 〈ν_e?N〉 is of the same order of magnitude as 〈ν_e?O〉 in the height range 170–210 km. Above 360 km 〈ν_e?O〉 becomes more important among the neutrals. The temporal, seasonal and solar epoch variations of 〈ν_ei〉 are also shown. A typical electron collision frequency profile 〈ν_g〉 corresponding to the exospheric temperature of 1100 K has been compared with the available experimental results for D, E and F-regions obtained from different experimental techniques. This profile has been used to determine the electrical, thermal, heat flow and current flow conductivities, the mobility and the diffusivity of electrons. The results so obtained are found to be in good agreement with the earlier results.  相似文献   

One-dimensional hydrodynamic modeling of GLOF and impact on hydropower projects in Dhauliganga River using remote sensing and GIS applications     

Praveen K. Thakur  Suruchi Aggarwal  S. P. Aggarwal  S. K. Jain 《Natural Hazards》2016,80(2):1057-1081

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Evaluation of turbulent fluxes over Maitri,Antarctica     

Jaya Naithani  H. N. Dutta  P. K. Pasricha  B. M. Reddy  K. M. Aggarwal 《Boundary-Layer Meteorology》1995,74(1-2):195-208

Turbulent fluxes have been evaluated for clear sunny days over the Indian Antarctic station, Maitri, using the basic meteorological data recorded at four levels of a 28 m tower. The data are supplemented with radiation data. The surface layer over Maitri remains thermally stratified during the hours of minimum solar insolation, the so-called nighttime period. The surface winds during this period are generally very strong resulting in high momentum fluxes. In particular, for high winds (>12 m s^–1), the temperature gradient is found to be less positive than for moderate winds (4 to 7 m s^–1). Solar insolation provided the daytime heating necessary for the diurnal variation of atmospheric stability, and hence, for the turbulent fluxes. Thus, on clear days daytime conditions are marked by upward transport of heat with reduced momentum flux, while stable nighttime conditions are marked by a downward heat flux with increased momentum fluxes.  相似文献   

Contrastreaming instability in collisionless anisotropic plasmas     

S. S. Aggarwal  S. P. Talwar 《Astrophysics and Space Science》1973,23(2):315-323

The problem of contrastreaming instability in collisionless anisotropic plasmas is investigated both for electrostatic and electromagnetic perturbations propagatingalong the ambient magnetic field. Both electron-electron and ion-ion streams are considered. It is found that the electromagnetic instability may, under certain conditions, be characterised by larger growth rates than the electrostatic instability.  相似文献   

Extreme Ultraviolet Emission Lines of ca xv in Solar and Laboratory Spectra     

Keenan  F.P.  Aggarwal  K.M.  Katsiyannis  A.C.  Reid  R.H.G. 《Solar physics》2003,217(2):225-233

New R-matrix calculations of electron impact excitation rates in Caxv are used to derive theoretical electron density diagnostic emission line intensity ratios involving 2s ²2p ²–2s2p ³ transitions, specifically R ₁=I(208.70 Å)/I(200.98 Å), R ₂=I(181.91 Å)/I(200.98 Å), and R ₃=I(215.38 Å)/I(200.98 Å), for a range of electron temperatures (T _e=10^6.4–10^6.8 K) and densities (N _e=10⁹–10¹³ cm^–3) appropriate to solar coronal plasmas. Electron densities deduced from the observed values of R ₁, R ₂, and R ₃ for several solar flares, measured from spectra obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, are found to be consistent. In addition, the derived electron densities are in excellent agreement with those determined from line ratios in Caxvi, which is formed at a similar electron temperature to Caxv. These results provide some experimental verification for the accuracy of the line ratio calculations, and hence the atomic data on which they are based. A set of eight theoretical Caxv line ratios involving 2s ²2p ²–2s2p ³ transitions in the wavelength range 140–216 Å are also found to be in good agreement with those measured from spectra of the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides additional support for the accuracy of the theoretical line ratios and atomic data.  相似文献   

S xi Emission Lines in Active Region Spectra Obtained with the Solar euv Rocket Telescope and Spectrograph (Serts)     

Keenan  F.P.  Pinfield  D.J.  Mathioudakis  M.  Aggarwal  K.M.  Thomas  R.J.  Brosius  J.W. 《Solar physics》2000,197(2):253-262

Theoretical electron density sensitive emission line ratios involving a total of eleven 2s ²2p ²–2s2p ³ transitions in Sxi between 187 and 292 Å are presented. A comparison of these with solar active region observations obtained during rocket flights by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals generally good agreement between theory and experiment. However, the 186.87 Å line is masked by fairly strong Fexii emission at the same wavelength, while 239.83 Å is blended with an unknown feature, and 285.58 Å is blended with possibly Niv 285.56 Å. In addition, the 191.23 Å line appears to be more seriously blended with an Fexiii feature than previously believed. The presence of several new Sxi lines is confirmed in the SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 Å, in excellent agreement with laboratory measurements. In particular, the detection of the 2s ²2p ^{2 3} P ₁ –2s2p ^{3 3} P _0,1 transitions at 242.91 Å is the first time (to our knowledge) that this feature has been identified in the solar spectrum. The potential usefulness of the Sxi line ratios as electron density diagnostics for the solar transition region and corona is briefly discussed.  相似文献   

A study of atmospheric structures using sodar in relation to land and sea breezes     

S. K. Aggarwal  S. P. Singal  Ramesh K. Kapoor  B. B. Adiga 《Boundary-Layer Meteorology》1980,18(4):361-371

Simultaneous observations were made by an acoustic sounder and on a meteorological tower during the month of May 1978 at the Atomic Power Station Tarapur. The probing range of the acoustic sounder was 700 m. The meteorological tower could sense wind and temperature at various levels up to a height of 120 m.The site being close to the sea shore, the thermal environment of the lower atmosphere is controlled mostly by land and sea breeze circulations. Thermal convective structures were seen during the daytime and also at night. The frequency of plume formation and the height of the plumes were, however, low during the night. The convective boundary layer in the daytime ranged from 400–500 m while at night it was mostly under 200 m. The observation of thermals at night is explained by the presence of a naturally stable marine layer above 30 m at this site. In the morning hours, winds suddenly change their direction allowing advection of a land breeze which is responsible for the formation of surface-based shear echoes to a height of 200 m during the transition period and for the subsequent development of an elevated layer due to mixing of two different air masses. A marine layer was also seen over Tarapur for a few days during the early evening and night hours. Its height was mostly around 400 m. It may indicate the presence of a subsidence inversion at Tarapur. The need for collection of supporting meteorological data to a height of 500 m by tethered balloon or some other suitable in-situ technique is stressed.  相似文献   

Evidence of decay of flux ratio of Fe to Fe-Ni line features with electron temperature in solar flares     

Rajmal Jain  Malini Aggarwal  Raghunandan Sharma 《Journal of Astrophysics and Astronomy》2010,31(3):155-163

We report observational evidence of the decay of the flux ratio of Fe to Fe-Ni line features as a function of plasma electron temperature in solar flares in comparison to that theoretically predicted by Phillips (2004). We present the study of spectral analysis of 14 flares observed by the Solar X-ray Spectrometer (SOXS) — Low Energy Detector (SLD) payload. The SLD payload employs the state-of-the-art solid state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices. The sub-keV energy resolution of Si PIN detector allows us to study the Fe-line and Fe-Ni line features appearing at 6.7 and 8 keV, respectively, in greater detail. In order to best-fit the whole spectrum at one time in the desired energy range between 4 and 25 keV we considered Gaussian-line, the multi-thermal power-law and broken power-law functions. We found that the flux ratio of Fe to Fe-Ni line features decays with flare electron temperature by the asymptotic form of polynomial of inverse third order. The relative flux ratio is ∼30 at temperature 12 MK which drops to half, ∼15 at 20 MK, and at further higher temperatures it decreases smoothly reaching to ∼8 at ∼50 MK. The flux ratio, however, at a given flare plasma temperature, and its decrease with temperature is significantly lower than that predicted theoretically. We propose that the difference may be due to the consideration of higher densities of Fe and Fe-Ni lines in the theoretical model of Phillips (2004). We suggest revising the Fe and Fe-Ni line densities in the corona. The decay of flux ratio explains the variation of equivalent width and peak energy of these line features with temperature.  相似文献   

Reservoir Sedimentation Assessment Through Remote Sensing and Hydrological Modelling     

Rahimov Foteh  Vaibhav Garg  Bhaskar Ramchandra Nikam  Madhusudan Y. Khadatare  Shiv Prasad Aggarwal  A. Senthil Kumar 《Journal of the Indian Society of Remote Sensing》2018,46(11):1893-1905

Reservoir sedimentation is the gradual accumulation of incoming sediments from upstream catchment leading to the reduction in useful storage capacity of the reservoir. Quantifying the reservoir sedimentation rate is essential for better water resources management. Conventional techniques such as hydrographic survey have limitations including time-consuming, cumbersome and costly. On the contrary, the availability of high resolution (both spatial and temporal) in public domain overcomes all these constraints. This study assessed Jayakwadi reservoir sedimentation using Landsat 8 OLI satellite data combined with ancillary data. Multi-date remotely sensed data were used to produce the water spread area of the reservoir, which was applied to compute the sedimentation rate. The revised live storage capacity of the reservoir between maximum and minimum levels observed under the period of analysis (2015–2017) was assessed utilizing the trapezoidal formula. The revised live storage capacity is assessed as 1942.258 against the designed capacity of 2170.935 Mm³ at full reservoir level. The total loss of reservoir capacity due to the sediment deposition during the period of 41 years (1975–2017) was estimated as 228.677 Mm³ (10.53%) which provided the average sedimentation rate of 5.58 Mm³ year¹. As this technique also provides the capacity of the reservoir at the different elevation on the date of the satellite pass, the revised elevation–capacity curve was also developed. The sedimentation analysis usually provides the volume of sediment deposited and rate of the deposition. However, the interest of the reservoir authorities and water resources planner’s lies in sub-watershed-wise sediment yield, and the critical sub-watersheds upstream reservoir requires conservation, etc. Therefore, in the present study, Soil and Water Assessment Tool (SWAT) was used for the estimation of sediment yield of the reservoir. The average annual sediment yield obtained from the SWAT model using 36 years of data (1979–2014) was 13.144 Mm³ year^?1 with the density of the soil (loamy and clay) of 1.44 ton m^?3. The findings revealed that the rate of sedimentation obtained from the remote sensing-based methods is in agreement with the results of the hydrographic survey.  相似文献   

Studies of sodar‐observed dot echo structures     

S.P. Singal  B.S. Gera  S.K. Aggarwal 《大气与海洋》2013,51(3):304-312

Abstract

During moist weather under stably stratified and light wind conditions very often “dot” shaped echoes, either distributed randomly or arranged in a stratified layer, have been observed on sodar echograms. They last from a couple of hours to ten hours. Their horizontal widths are up to 200 m while their vertical sizes are up to 40 m. It is argued that dot echoes represent clusters of water vapour translated by the wind in the boundary layer, the back‐scattered acoustic energy being the contribution of correlated fluctuations in temperature and humidity (turbulent mixing) in the inertial subrange.  相似文献