http://hdl.handle.net/123456789/11 http://hdl.handle.net/123456789/5802 Statistics of Langmuir Waves Associated with Type III Solar Radio Bursts Vidojević, Sonja Interplanetary electron beams, produced by solar flares, are unstable in the solar wind and generate Langmuir waves at the local plasma frequency, fp. These waves are then converted into the so-called type III radio bursts which are freely propagating electromagnetic emissions at fp or its harmonic. The type IIIs are therefore observed as drifting emissions from high to low frequencies, in the kilometric wavelengths range. Since the first theoretical explanation by Ginzburg and Zhelezniakov (1958a), several refined models have attempted to describe in details the physical processes at the origin of type III bursts. The mechanisms of “Bump-on-tail” instabilities, Langmuir waves generations, conversion of these Langmuir waves into radio emissions throughout nonlinear wave-wave interactions etc, have been studied in detail. Of particular interest from the observational point of view are the so called in situ type III bursts for which the electron beam, at the origin of the emission and traveling along open interplanetary magnetic field lines, is observed directly in situ by a spacecraft, together with the local Langmuir waves and the resulting radio emissions. Until now only a few of these in situ type IIIs have been reported in the literature. The first research study performed in this thesis was to examine the first 16 years of radio, waves and particles data recorded by the Wind spacecraft in the Solar Wind and to look for in situ type IIIs. Applying rigorous and careful criteria, this examination has yielded to a data set of 36 high-quality events. With such a numerous data set, which is statistically representative of the studied phenomenon, it is now possible to constrain obser- vationally and with a better confidence the type III generation models. After having built our statistical dataset, we have studied, for each of the events, the precise shapes of the Langmuir wave power distributions, observed in the spectral domain. We have fitted these observed distributions by a Pearson’s system of probability distributions and have shown that the probability distributions of the logarithm of the Langmuir waves power spectral density belong to three “main” types of Pearson’s probability distributions: type I, type IV and type VI. In addition we have modeled the effects of the instrumental integration time of the Wind radio receivers on the observed Langmuir wave power distributions. By combining our observations with our models we have shown that it was not possible to con- clude definitively, that the distribution of the Langmuir waves energy in the real temporal domain is lognormal, as it is predicted in some theories as the Stochastic Growth Theory by Robinson (1992). In the last part of the thesis, we have shown how our high-quality data set of 36 in situ type III events can be used for further studies that could allow to constrain the theoretical models even better. For instance we have investigated the correlation between the Langmuir waves power and the energy of impulsive electron or with the power of the radio emissions themselves. Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/123456789/5795 UTICAJ INTERAKCIJA UDALJENIH GALAKSIJA NA NJIHOVO NETERMALNO RADIO ZRAČENJE Pavlović, Mrina The studyofgalaxiesthroughhighredshiftsarekeytounderstandingthe evolutionofgalaxiesthroughcosmictimes.Assuchobjectsareverydifficultto observedirectly,theyaremainlyexaminedusingempiricallyderivedtoolssuchas the numerouscorrelationsbetweentheirdifferentparametriccharacteristics,one of thembeingthelinearrelationshipbetweenfar-infraredandradioemissionin star-forming galaxies,namedtheFarInfrared-Radio(FIR)Correlation.Although the correlationwasconsideredtobestableintermsoflinearity,recentworks, whichincludegalaxiesathighredshifts(0 < z< 6), showedalargedeviation from thecorrelationinthesesystems.Thegoalofthisdoctoraldissertationis an examinationofthephysicalprocessesthatleadtothiskindofbehavior.As a possiblecauseofthisevolution,wewillassumeforthefirsttime,andexamine interactionsbetweengalaxies(collisionsandcloseapproaches).Interactionsbe- tweengalaxiesleadtotheformationofshockwavesonlargescalesthatcanlead to changesintherelationshipbetweeninfraredandradioemissions.Ourhypoth- esis wastestedinseveralstadiumsandthemainresultsareasfollows:1.We developedmodelsoftheevolutionoftheFIRcorrelationwithredshiftasfunctions of thegalaxyinteractionrate.Wetestedthemodelsonasampleofgalaxieswith an alreadydeterminedmorphologyseparatelyfordiscgalaxiesandforgalaxies that haverecentlybeenorarecurrentlyinteracting-irregulargalaxies. 2. Inasmallsampleof34galaxiesthatwetookfrompaperMiettinenetal.(2017), it wasshownthatthereisanindicationthattheinteractionbetweengalaxiescan beresponsiblefortheevolutionofthecorrelationwiththeredshift. 3. Thenextanalysiswasperformedonamuchlargersampleofstar-forming galaxies takenfromCOSMOSfield,wherewedidnotfindanyevolutionofcorre- lation withtheredshift.Also,itwasshownthatthemeanvalueofthecorrelation parameter islowerinirregulargalaxiesthanindiskgalaxies. Although recentobservationsindicatedanevolutionoftheFIRcorrelation with redshift,theresultsofthisresearchfailedtoreproducethatevolutionand showedthattheFIRcorrelationisstablewithredshift.However,itwasalso shownthatduetotheinteractionofgalaxies,theevolutionoftheFIRcorrelation is possibleiftherepresentationofinteractingsystemsinthesampleishigher. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/123456789/5793 IN- SITU DUST DETECTION USING RADIO ANTENNAS OF DIFFERENT SPACECRAFT - SPACE OBSERVATIONS AND MODELING Racković Babić, Kristina Interplanetary dust grains contain important information about the Solar System. Analyzing these particles is an important aspect of the heliosphere study. Dust impacts have been observed using radio and wave instruments onboard the spacecraft since the 1980s. The interaction between the impact-generated plasma cloud and antenna – spacecraft system elements generates the characteristic signal waveform. The present work focuses on the detection and interpretation of the dust generated signals from radio instruments onboard various spacecraft orbiting at 1 AU. In the first part of the thesis, we aim to develop a model which links the observed electric signals to the dust impact properties.We propose a new model which takes into account the effect of impact - ionization - charge collection and electrostatic-influence. Our model provides an analytical expression for the pulse. It allows us to measure the amount of total ion charge, the fraction of escaping charge, the rise timescale, and the relaxation timescale. The proposed model is simple and convenient for large data fitting. To validate the model, we use the Time Domain Sampler (TDS) subsystem of the STEREO/WAVES instrument, which generates high-cadence time series of voltage pulses for each monopole. Since the beginning of the STEREO mission in 2007, we have collected all the dust events detected by S/WAVES/TDS simultaneously on all three monopoles at 1 AU. Our study confirms that the rise time vastly exceeds the spacecraft’s short timescale of electron collection by the spacecraft. Aside from electron dynamics, we also obtained interesting results regarding the cloud’s electron temperature. The presented model provides an effective tool for analyzing vii dust waveforms, and is applicable for different space missions which investigate the distribution of dust particles, e.g., Solar Orbiter and Parker Solar Probe. In the second part of the thesis, we focus on the interstellar dust (ISD). Interplanetary and interstellar dust are the two main dust populations at 1 AU. Our objective is to search for interstellar dust by analyzing the data sets collected by STEREO and Wind, starting from the beginning of the missions. Between 2007 and 2012, while being at the solar minimum with a solar dipole pointing southward, all three spacecraft recorded ISD flux at 1 AU. However, before and after that period, the disappearance of the interstellar component was noticeable. The observed change of the impact rate suggests that the flux of interstellar dust at 1 AU varies with the solar cycle. Each time the magnetic dipole field changes its polarity during the solar cycle, small interstellar grains experience focusing or defocusing. Consequently, the dust grains are systematically deflected either towards, or away from the solar magnetic equator plane by the solar wind magnetic field which thus affects the dust dynamics and the total interstellar dust flux in the inner heliosphere. Our study provides the first quantitative description of the time variation of ISD flux at 1 AU. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/123456789/5790 PROMENA V I R MAGNITUDA IZABRANIH KVAZARA I POVEZIVANJE SISTEMA GAIA SA SISTEMOM ICRF Jovanović, Miljana One of the main objectives of the Gaia mission of the European Space Agency is to construct a celestial reference frame at the wavelengths of the optical domain, Gaia CRF. This frame needs a link to the International Celestial Reference Frame – ICRF which is fixed with respect to distant objects (quasars). The objects serving for the purpose of linking are required to be visible in both domains (optical and radio). A set of 47 such objects has been proposed and included which in the radio domain have no detected extended emission. The mentioned objects are active galactic nuclei (AGN) the brightness of which varies over the whole electromagnetic spectrum. The brightness change may be due to activity in different AGN regions, but also to external factors. Such variations can lead to changes in the photocentre position and, consequently, to changes of the object coordinates. In order to establish which objects are suitable for linking these two frames we have examined the brightness variation in the optical domain. The objects have been observed from 2013 in the V and R bands. We have analysed the brightness, colour (V − R) and optical spectral index (α). It has been established that for the majority of objects the brightness is variable, or possibly variable. Almost 15% of all objects have significant changes in their brightness (more than 1 mag), only ∼10% are stable with minor brightness changes of ∼0.3 mag. The results concerning the change analysis of the colour and α are also presented. Based on these results 17 objects are chosen as suitable for linking ICRF to Gaia CRF. The results of the analysis, as well as the observed values, are essential for the examination of these objects because of their importance in astrometry, also in astrophysics. These data are relevant to a better understanding of formation and evolution of galaxies. Mon, 01 Jan 2024 00:00:00 GMT