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Martinović, Mihailo (, 2016)[more][less]
Abstract: The quasithermal noise spectroscopy is an accurate method of determination of density and temperature in space plasmas. When an electric antenna is immersed into a plasma, it is able to measure electrostatic uctuations caused by the thermal motion of plasma particles. These uctuations are detected as the power spectral density at the antenna terminals, observing a spectrum at frequencies comparable to the electron plasma frequency for both electrons and protons, since the proton signal is strongly Dopplershifted towards higher frequencies due to the solar wind drift velocity. Beside inducing the uctuating electric eld, some of the electrons are impacting the antenna surface, causing disturbances of the antenna electric potential. The signal caused by this population is directly proportional to the ux of plasma electrons impacting the antenna and is dominant if the antenna has a large surface area. In this work, we use the orbit limited theory to calculate the incoming particle ux for a nonthermal plasma described by velocity distribution function, commonly measured in the solar wind. The increase in the particle collection by cylindrical and spherical objects is quanti ed and presented as a function of the surface electrostatic potential and the fraction of suprathermal particles. Including these results into the theory has turned out to be absolutely necessary for accurate measurements of the plasma parameters whenever the shot noise is the dominant component in the power spectrum. This is the case for STEREO because the impact noise is overwhelming on this probe, due to the presence of short and thick antennas. The comprehensive study of data on this mission is motivated by the fact that the electron analyzers are malfunctioning since launch and no information on thermal electrons is available. Results obtained are veri ed by comparing with the results from Wind, showing a good match between the values measured by the two spacecraft. Uncertainties of the measurements are determined by the uncertainties of the instruments used and are estimated to be around 40%. The nal outcome of this work will be establishing a database of the electron moments in both STEREO A and B that will be covering the entire duration of the mission. In the second part of the thesis, we use the kinetic approach to expand the theory of the quasithermal noise to plasmas where electronneutral collisions play a dominant role. This technique is able to measure the electron density, temperature and the collision frequency as independent parameters using the wide frequency range both below and above the plasma frequency, if the ratio of the collisional to plasma frequency is not smaller than 0.1. The results presented here have can be potentially applied in laboratory plasmas and unmagnetized ionospheres, while at the ionosphere of Earth their use is limited to low frequencies due to the presence of the magnetic field. URI: http://hdl.handle.net/123456789/4476 Files in this item: 1
MMartinovic_Thesis.pdf ( 16.42Mb ) 
Onić, Dušan (Beograd , 2013)[more][less]
Abstract: The evolution of supernova remnants is linked to the propagation of a collisionless shock wave, formed during the initial expansion of highvelocity supernova ejecta through the interstellar environment. Theoretical studies of shock waves are very important for the analysis of supernova remnants, as well as processes in the interstellar medium in general. In this doctoral dissertation, some theoretical results based on the magnetohydrodynamical theory of shock waves are presented, with special emphasis on ideal radiative magnetohydrodynamics for the optically thick case. Particularly, solutions for the case when jump in adiabatic index and/or ratio of gas to total pressure is allowed, are discussed. The main hypothesis of this dissertation is that thermal bremsstrahlung radiation at radio continuum frequencies can provide a signi cant contribution in the case of several Galactic supernova remnants. This hypothesis can give a natural explanation for nearly concave up radio continuum spectra of several Galactic supernova remnants that are expanding in the environment with higher than average density. In this context, it is important to identify the existence of the possible indicators of ensemble of thermal electrons at su ciently low temperatures and su ciently high densities so that the thermal bremsstrahlung radiation linked to a particular remnant could be observed at radio continuum frequencies (vicinity, interaction or expansion through the molecular cloud, presence of the cooled thermal Xray electrons during the post SedovTaylor phases, detection of lowfrequency turnovers associated with thermal absorption linked to the remnant, detection in H , identi cation of radio recombination lines linked to the remnant, etc). The signi cant presence of thermal component could theoretically explain radiospectral indices less than 0.5 measured for several evolutionary older supernova remnants, (mainly of mixedmorphology class) that expand in the high density region. Actually, these smaller radiospectral indices, under the assumption of simple power law, would represent a natural manifestation of a signi cant fraction of thermal emission at radio continuum frequencies. However, present knowledge of the radio continuum spectra of Galactic supernova remnants is still not determined precisely enough for any de nite conclusions to be made about the inherent thermal radioemission from supernova remnants. A thorough analysis is only possible in the case of three Galactic supernova remnants (3C396, IC443, 3C391) for which the thermal contribution is determined despite high associated uncertainties. New observations in the near future will lay the groundwork for making rmer conclusions about the existence of the socalled radio thermally active supernova remnants. This dissertation highlights the importance of observations of supernova remnants in X and rays, and multiwavelength analysis is general. Besides, it suggests a possible detection of rays from supernova remnant 3C434.1 based on the observations made by Fermi. URI: http://hdl.handle.net/123456789/3051 Files in this item: 1
Onic_Dusan_disertacija.pdf ( 10.67Mb ) 
Milić, Ivan (, 2014)[more][less]
Abstract: Multidimensional radiative transfer is an essential ingredient of modern ap proach to modeling of astrophysical objects. Realistic modeling calls for the as sumption of nonlocal thermodynamic equilibrium (NLTE), which, in turn requires selfconsistent solution of coupled equations of radiative transfer statistical equilib rium. This approach allows us to compute emergent spectrum from a given model of the object, which is, in principle, a necessary step in interpretation of observational results. Thanks to the highresolution and high signal to noise observations, it is often possible to measure not only intensity of the light but also its state of po larization. For interpretation of such observations it is necessary to solve radiative transfer problem for polarized radiation. This thesis deals with nonLTE transfer of (generally polarized) radiation in two dimensional media. Thesis can be divided in two parts. In the first part, we present a numerical method for the formal solution of the radiative transfer equation in 2D Cartesian coordinate system. This method allows us to explicitly account for the contribution of nonlocal source functions to the local specific intensity, and, hence, to the local scattering integral. The knowledge of these contributions is necessary for an iterative solution of coupled equations of radiative transfer and statistical equilibrium. Based on this formal solution we introduce two novel schemes for multidimensional NLTE radiative transfer which have so far been used only in 1D geometry: symmetric GaussSeidel iteration and “Sweepbysweep” implicit lambda iteration, latter one being based on “Forthandback” implicit lambda iteration. Both methods utilize implicit use of the local source function and the source func tion corrections each sweep of the computational grid (four times per iteration). “Sweepbysweep” implicit lambda iteration also uses the idea of iteration factors and achieves acceleration of about factor of seven with respect to the referent Ja cobi method. Both new methods also significantly surpass both Jacobi iteration and GaussSeidel iteration on problems with periodic boundary conditions. Also, it turns out that “Sweepbysweep” implicit lambda iteration scales with grid resolu tion better than the Jacobi iteration. The second part of the thesis deals with numerical polarized radiative transfer on 2D cylindrical grids. The method is based on the second order short characteristics for the formal solution and uses standard Jacobi iteration with Ng acceleration to solve the polarized nonLTE problem (Generalization to other iterative schemes is given in appendix A). This method allows for the selfconsistent solution of coupled equations of radiative transfer and statistical equilibrium equation for a two level atom model for polarized radiation. The method employs reduced intensity basis where intensity and source function are written as sixvectors and source function does not depend on direction which allows for significant saving in memory and computing time. It is applicable for modeling of axisymmetric objects such as as trophysical disks. The method has been tested on simple models of circumstellar and selfemitting disks. The most important conclusion of these computations is that the presence of the disk in the system introduces a significant degree of linear polarization due to the scattering processes and that one is able to model it in detail using our approach. Also, it is shown that the presence of rotation in selfemitting disks dramatically changes not only intensity, but also polarized spectral lines pro files. Interplay of nonLTE, multidimensional effects and rotation results in very complicated line profiles which are nontrivial for interpretation. However, the main effect is that the rotation decreases the amount of Stokes Q component and, de pending on the rotation velocity causes appearance of doublelobed U polarization profile. If these effects can be observed, this kind of modeling provides a useful tool for interpretation of the spectropolarimetric observations. URI: http://hdl.handle.net/123456789/4757 Files in this item: 1
IvanMilic_teza.pdf ( 1.075Mb ) 
Zeković, Vladimir (Beograd , 2020)[more][less]
Abstract: In the disertation, by the linear plasma theory we show that resonant electromagnetic (EM)microinstabilities are excited by the two colliding plasmas which are interpenetrating eachother with the superAlfv ́enic velocity, in the direction quasiparallel to the magnetic field.The expected RankineHugoniot shock conditions, naturally arise as a consequence of a highlyresonant interaction of ions with the instability. By using kinetic simulations, we find herethat such resonant instabilities appear in the linear stage, and we show how these instabilitiestrigger the shock formation during the nonlinear stage.By theoretical modeling, we show how a magnetosonic soliton forms and leads to the periodicshock reformation and initiation of the return current of ions, which drives the EM upstreaminstability. We find that ions are being preaccelerated by the upstream and shock instabilities ina mechanism that is similar to the diffusive shock acceleration (DSA). By our EM test particlesimulation runs we show that at quasiparallel shocks, EM instabilities highly contribute toelectron preacceleration, leading to the formation of a powerlaw electron spectra through theFermilike mechanism.By very long kinetic simulation runs, in this disertation we find that ions and electronsenter DSA in a similar number which is further applied in a model of nonlinear DSA. Withinthis model, we obtain the theoretical particle spectra and we find the electrontoproton ratioat high energies for the different Mach numbers. We show that the spectra of quasithermalparticles at the shock can be represented by a nonequilibriumκdistribution. We find thatthe level of modification decreases andκindex increases behind the shock, implying that theparticle distribution tends to become a Maxwellian. URI: http://hdl.handle.net/123456789/5094 Files in this item: 1
ZekovicVladimir.pdf ( 56.60Mb ) 
Jovanović, Predrag (Belgrade , 2005)[more][less]

Pavlović, Marina (Beograd , 2022)[more][less]
Abstract: The study of galaxies through high redshifts are key to understanding the evolution of galaxies through cosmic times. As such objects are very difficult to observe directly, they are mainly examined using empirically derived tools such as the numerous correlations between their different parametric characteristics, one of them being the linear relationship between farinfrared and radio emission in starforming galaxies, named the Far InfraredRadio (FIR) Correlation.Although the correlation was considered to be stable in terms of linearity, recent works, which include galaxies at high redshifts (0 < z < 6), showed a large deviation from the correlation in these systems. The goal of this doctoral dissertation is an examination of the physical processes that lead to this kind of behavior. As a possible cause of this evolution, we will assume for the first time, and examine interactions between galaxies (collisions and close approaches). Interactions be tween galaxies lead to the formation of shock waves on large scales that can lead to changes in the relationship between infrared and radio emissions. Our hypoth esis was tested in several stadiums and the main results are as follows: 1. We developed models of the evolution of the FIR correlation with redshift as functions of the galaxy interaction rate. We tested the models on a sample of galaxies with an already determined morphology separately for disc galaxies and for galaxies that have recently been or are currently interacting  irregular galaxies. 2. In a small sample of 34 galaxies that we took from paper Miettinen et al. (2017), it was shown that there is an indication that the interaction between galaxies can be responsible for the evolution of the correlation with the redshift. 3. The next analysis was performed on a much larger sample of starforming galaxies taken from COSMOS field, where we did not find any evolution of corre lation with the redshift. Also, it was shown that the mean value of the correlation parameter is lower in irregular galaxies than in disk galaxies. Although recent observations indicated an evolution of the FIR correlation with redshift, the results of this research failed to reproduce that evolution and showed that the FIR correlation is stable with redshift. However, it was also shown that due to the interaction of galaxies, the evolution of the FIR correlation is possible if the representation of interacting systems in the sample is higher. URI: http://hdl.handle.net/123456789/5460 Files in this item: 1
Disertacija_Marina_Pavlovic.pdf ( 8.624Mb ) 
Dimitrijević, Milan (Belgrade)[more][less]
Abstract: A detailed analysis of the motion of a charged particle under the influence of Coulomb, polarization, and quadrupolar potential has been performed, and analytical expressions for particle path parameters have been obtained. Also, conditions for existence of a critical impact parameter, which separates paths ending in the atom from those going to infinity, have been examined, and methods for its determination have been elaborated. Results of this investigations have been applied to the theory of Stark broadening (broadening of spectral lines under the influence of collisions with charged particles), where a special attention has been paid to the adiabatic limit, and to quasistatic and impact approximations. Modification of Stark broadening functions has been performed and it has been demonstrated that the biggest influence of path curvature on them, is in the vicinity of the critical impact parameter, which separates paths ending in the atom from those going to infinity. The change in the values of the Stark broadening functions will have the largest influence on Stark broadening parameters for lines for which, at the most probable velocity, influence of the contribution of inelastic collisions is dominant and effective polarizability large. The application of semiempirical approximative approach for calculation of Stark broadening parameters for multiply charged emitters, has been analyzed as well, and the modification of semiclassical theory has been performed. The averaged ratio of experimental and theoretical results is 0.75 for semiempirical, 1.47 for semiclassical, and 1.04 for modified semiclassical approach.У раду је извршена детаљна анализа кретања наелектрисане честице под утицајем Кулоновог, поларизационог и квадруполног потенцијала и добијени су аналитички изрази за параметре путање честице. Такође су испитани услови за егзистенцију критичног сударног параметра, који раздваја путање које завршавају у атому од оних које одлазе у бесконачност, и разрађене методе за његово налажење. Резултати овог испитивања су примењени на теорију Штарковог ширења (ширења спектралних линија под утицајем судара са наелектрисаним честицама), при чему је посебна пажња посвећена адијабатској граници и квазистатичкој и сударној апроксимацији. Изведена је модификација функција Штарковог ширења и показано је да је највећи утицај закривљености путање на њих, у близини критичног сударног параметра, који раздваја путање које завршавају у атому од оних које одлазе у бесконачност. Промена у величини функција Штарковог ширења ће највише утицати на параметре Штарковог ширења код линија за које је, при највероватнијој брзини, утицај доприноса нееластичних судара доминантан а ефективна поларизабилност велика. Такође је анализирана примена семиемпиријског апроксимативног прилаза за израчунавање параметара Штарковог ширења за вишеструко наелектрисане емитере, при чему је извршена модификација семикласичне теорије. Усредњени однос експерименталних и теоријских резултата је 0.75 за семиемпиријски, 1.47 за семикласични а 1.04 за модификовани семикласични прилаз. URI: http://hdl.handle.net/123456789/77 Files in this item: 1
Dimitrijevic.pdf ( 6.262Mb ) 
Vince, Ištvan (Belgrade)[more][less]

Lazović, Jovan (Belgrade , 1964)[more][less]
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