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1
Ouahioune, Nedjma. „Čerenkov emission of whistler waves by electron holes“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-446395.
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Electron holes are positively charged nonlinear structures in which trapped electrons are supported by a positive electrostatic potential. These structure are regularly observed in space and laboratory plasmas by means of diverging bipolar electric field signatures. Recent observations and simulations have shown that fast moving electron holes can generate electromagnetic whistler waves via Čerenkov emission. The fast moving positive charge correspond to localised currents which can potentially excite waves. The aim of the project is to study both theoretically and numerically the properties leading to the Čerenkov emission of whistler waves by three-dimensional electron holes. In addition, efforts are dedicated to the derivation of a model providing the properties of emitted whistlers. The model is compared with the observational features of electromagnetic whistler waves generated by electron holes.
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Liléo, Sónia. „Auroral electrodynamics of plasma boundary regions“. Doctoral thesis, KTH, Rymd- och plasmafysik, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10446.
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The electrodynamic coupling between the auroral ionosphere and the magnetosphere is the main subject of this thesis. Satellite measurements of electric and magnetic fields and of charged particles are used to explore three distinct plasma boundaries, magnetically linked to the nightside auroral ionosphere. These boundaries are the inner edge of the plasma sheet (PS), and the inner and the outer edges of the plasma sheet boundary layer (PSBL). Strong ionospheric electric fields with amplitudes up to 400 mV/m may be observed in the subauroral ionosphere, in the vicinity of the ionospheric projection of the PS inner edge. Intense and dynamic auroral electric fields with local magnitudes up to 150 mV/m associated with upward ion beams and field-aligned currents are observed for the events treated here, at the inner and outer boundaries of the PSBL at an altitude of about 4-5 Earth radii, well above the acceleration region. Subauroral and auroral electric fields are the two main subjects of this thesis. Subauroral ion drifts (SAID) are associated with poleward electric fields, occurring predominantly in the premidnight region during the substorm recovery phase. The recently revealed abnormal subauroral ion drifts (ASAID) are associated with equatorward electric fields, occurring during extended periods of low auroral activity. The results indicate that the generation mechanism of SAID can neither be regarded as a pure voltage generator nor a pure current generator, but having certain characteristics of both generator types. Ionospheric feedback appears to play a major role for the development and maintenance of the SAID electric fields. The formation of ASAID is proposed to result from the proximity and interaction between different plasma boundaries of the innermost magnetosphere during extended periods of low auroral activity. The auroral electric fields observed in the upward current region at the PSBL inner and outer edges are associated with upward parallel electric fields, which partially decouple the high-altitude electric fields from the ionosphere. This is in contrast to the subauroral electric fields which are coupled. Multi-point measurements provided by the Cluster mission show that the observed electric fields are highly variable in space and time, revealing various types of acceleration processes. However, they appear to be tied to the boundary where they are originally formed. A case is presented where they are associated with large electromagnetic energy fluxes directed upward away from the ionosphere. The interaction between the magnetosphere and ionosphere, being more pronounced at plasma boundary regions, is important for the understanding of the formation and regulation of the highly structured auroral electric fields observed in the upward current region.QC 20100727
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Waara, Martin. „High altitude ion heating observed by the Cluster spacecraft“. Doctoral thesis, Umeå universitet, Institutionen för fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-43611.
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This thesis deals with heating of outowing oxygen ions at high altitude above the polar cap using data from the Cluster spacecraft. oInospheric plasma may flow up from the ionosphere but at velocities which are low enough that the ions are still gravitationally bound. For the ions to overcome gravity, further acceleration is needed. The cusp/polar cap is an important source of outowing oxygen ions. In the cusp/polar cap, transverse heating is more common than eld-aligned acceleration through a magnetic eld-aligned electric eld. It is thus believed that transverse heating of ions is important for ion outow and one of the probable explanations for transverse heating is wave-particle interaction. A general conclusion from our work on high altitude oxygen ion energization is that ion energization and outow occur in the high altitude cusp and mantle. The particles are often heated perpendicularly to the geomagnetic eld and resonant heating at the gyrofrequency is most of the time intense enough to explain the observed O+ energies measured in the high altitude (8 { 15 Earth radii, RE ) cusp/mantle region of the terrestrial magnetosphere. The observed average waves can explain the observed average O+ energies. At lower altitude only a few percent of the observed spectral density around the oxygen gyrofrequency needs to be in resonance with the ions to obtain the measured O+ energies. A difference as compared to low altitude measurements is that we must assume that almost all wave activity is due to waves which can interact with the ions, and of these we assume 50 % to be left-hand polarized. We also have shown a clear correlation between temperature and wave intensity at the gyrofrequency at each measurement point. We have described the average wave intensity and corresponding velocity diffusion oeffcients as a function of altitude in a format convenient for modelers. Furthermore we have shown that the wave activity observed in this high altitude region is consistent with Alfven waves, and inconsistent with static structures drifting past the spacecraft. We have also shown how large the variability of the observed spectral densities is, and how sporadic the waves typically are. Based on three cases we have found that the regions with enhanced wave activity and increased ion temperature are typically many ion gyro radii in perpendicular extent.
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Eriksson, Tommy. „Multi-point Measurements of Ultra Low Frequency Waves in the Terrestrial Magnetosphere“. Doctoral thesis, Stockholm, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4404.
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5
Eriksson, Tommy. „Resonant Waves in the Terrestrial Magnetosphere“. Licentiate thesis, Stockholm : Alfvén laboratory, Royal institute of technology, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-400.
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6
Ersmark, Tore. „Geant4 Monte Carlo Simulations of the International Space Station Radiation Environment“. Doctoral thesis, Stockholm, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4007.
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7
Dahlgren, Hanna. „Fine-scale morphology and spectral characteristics of active aurora“. Licentiate thesis, Stockholm : Electrical Engineering, Elektrotekniska system, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4745.
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8
Möller, Cecilia. „High Power Microwave Sources : design and experiments“. Licentiate thesis, KTH, Rymd- och plasmafysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-34072.
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High-Power Microwaves (HPM) can be used to intentionally disturb or destroy electronic equipment at a distance by inducing high voltages and currents. This thesis presents results from experiments with a narrow band HPM source, the vircator. The high voltages needed to generate HPM puts the vircator under great stress, especially the electrode materials. Several electrode materials have been tested for endurance and their influence on the characteristics of the microwave pulse. With the proper materials the shot-to-shot variations are small and the geometry can be optimized in terms of e.g. output power or frequency content. Experiments with a resonant cavity added to the vircator geometry showed that with proper tuning of the cavity, the frequency content of the microwave radiation is very narrow banded and the highest radiated fields are registred. Since HPM pulses are very short and have high field strengths, special field probes are needed. An HPM pulse may shift in frequency during the pulse so it is very important to be able to compensate for the frequency dependence of the entire measurement system. The development and use of a far-field measurement system is described.QC 20110616
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Azimi, Mohammad. „Study of the linear and nonlinear damping in plasma via simulation“. Thesis, Umeå universitet, Institutionen för fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-162714.
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10
Wanzambi, Ellinor, und Maja Gustafsson. „Parallel currents in the magnetotail and their connection to aurora“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-355621.
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We present an analysis from MMS satellites where we detected the strongest parallel currents from their data during the months May till November 2017. The strongest parallel current in July happened 2017-07-16 at 6 o’clock. At this event we found a change in the magnetic flow density at the locations: A. Svalbard, which would have resulted in an aurora borealis if it was not for the summer light. B. Antarctic, which visibly did result in an aurora australis. By examine the source of the parallel current, we looked near the Lagrange point L1 between Earth and the Sun and could observe how the flow pressure and magnetic field changed. This motivated the investigation by looking at the sun pictures by helioviewer, where we could observe an outburst the 2017-07-14, which turned out to be the requested source of this event.
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Eliasson, Henrik. „Modeling of carbon plasma discharges in high-power impulse magnetron sputtering“. Thesis, Linköpings universitet, Plasma och ytbeläggningsfysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-176319.
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Diamond like carbon (DLC) is a metastable state of amorphous carbon that has very important and wide-ranging thin film applications. DLC has a strong resemblance to pure diamond and exhibits many traits of real diamond, like mechanical hardness and chemical inertness, but with a drastically lower deposition cost. DLC is characterized by a high fraction of sp3 hybridization. To reach a high fraction of sp3 bonding by sputtering of a graphite target, an energetic ion population and a high ionized flux fraction (Fflux) is beneficial. High-power impulse magnetron sputtering (HiPIMS), an ionized physical vapour deposition technique (iPVD) based on magnetron sputtering, has been shown to produce significantly higher ionized fluxes and more energetic ions compared to the industry standard technique of direct current magnetron sputtering (dcMS). For carbon however, the ionized flux fraction is significantly lower than that of common metal targets like titanium and aluminium, even with HiPIMS. In this thesis the ionization region model is applied to experimental carbon-argon 50 µs HiPIMS discharges at peak current densities of 1, 2 and 3 A/cm2 to investigate why the fraction of sputtered carbon reaching the substrate as ions is so low. The ionized flux fraction of the experimental discharges was measured by an ion meter to be lower than 5 %. From the computational modeling we find that the ionization probability of a carbon neutral (α) increases with increased peak discharge current densities from 40 % at 1 A/cm2 to over 60 % at 3 A/cm2. However, the back attraction probability of carbon ions (β) is high or above 90 %. The model predicts a higher Fflux than measured for all cases. The modeled Fflux values were 6-8 %, 10-13 % and 13-15 % for peak discharge current densities of 1, 2 and 3 A/cm2, respectively. By the time evolution of the particle densities, it is clear that most of the ionization takes place at the end of the pulse and thus the afterglow plays a significant role, especially for shorter pulses. The main conclusion is that the HiPIMS carbon discharge is mainly governed by the argon working gas and shares many traits with a typical working gas recycling process
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Krämer, Eva. „Bow shock current closure to Earth's polar ionosphere - A statistical study using AMPERE and OMNI data“. Thesis, Umeå universitet, Institutionen för fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-184853.
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The bow shock current is hypothesized to connect to the Earth's magnetosphere on open field lines and to contribute as a field aligned current to the polar ionospheric currents. In order to investigate this, two years of AMPERE data are used to compute the average region 1 currents, and average currents within the polar cap boundary, under different interplanetary magnetic field (IMF) clock angles and IMF magnitudes. The average currents are studied for stable IMF conditions. Therefore data is selected if the IMF By and Bz components do not change sign in a 30 minutes time interval to ensure sufficiently stable conditions. In addition, the AMPERE current maps are renormalized and recentered such that the region 1 currents have a fixed extent. These averaged current maps are then used to discuss the sources of the currents in the polar cap and their possible connection to the bow shock current. The currents in the polar ionosphere are found to generally agree with the statistical convection pattern. For a strong east-west IMF there are some discrepancies for the region 0 currents which indicates a possible bow shock current closure. Furthermore there are currents in the polar cap of the northern hemisphere for a strong southward IMF, which however do not have the expected polarity for bow shock current closure. Therefore they must have another, but unknown, origin. In addition, SuperDARN data is used to confirm that currents on the dayside of the polar cap and polwards of the Region 1 currents are on open field lines for a strong eastward or westward IMF. Further investigations are needed to confirm the bow shock closure in the ionosphere. Additional investigations are needed to identify the source of the polar cap currents for a strong southward IMF.
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Norenius, Linus. „Geoeffectiveness of Magnetosheath Jets“. Thesis, Umeå universitet, Institutionen för fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-164846.
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In this report we present spacecraft and ground-based observations of magnetosheath jets impacting the magnetosphere, both as a case study and a statistical study. In the case study, jets were detected in the magnetosheath by the Magnetospheric MultiScale mission, MMS. By utilizing a data-based magnetosphericmodel (Tsyganenko T96 [29]), we estimated which jets were likely to impact the magnetopause and where they would do so. We examined ground based magnetometers, GMAGs, at the expected foot-point to the affected magnetic fieldline and compared this with the spacecraft observations. Theoretical transfertimes for a jet to be detectable by GMAGs have been estimated and compared with the observed time delay, from detection to GMAG response, and they were in good agreement for all cases. The times found for this geoeffective responsewere found to be around 1-2 min, and the response in the GMAGs was in the form of a pulse with an amplitude of around 50 nT. We suggest that jets of along enough time duration can be geoeffective in a way that they are detectable at ground level by GMAGs. It was also found that GMAGs fluctuate more during periods containing many detected jets. We performed our statistical study with the intention of comparing fluctuations in GMAG observations during Interplanetary Magnetic Field, IMF, configurations which is suggested to be favorable for jet creation. The IMF observationswas provided by the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. This was done by selecting periods of steady IMF with different orientations, and examining the GMAGs observed variations. GMAGs were selected based on a region where most of our foot-points were found in our previous case study. We performed this study over a three year interval, and found that GMAGs observe about 2 nT higher variation, according to their standard deviations, during radial IMF compared to northward IMF. During northward IMF we expect less effects from magnetopause phenomena, thus making it suitable to compare with radial IMF. Our statistical investigation support our findings that magnetosheath jets can be geoeffective in a waythat GMAGs can detect them.
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Zbinden, Jonas. „Full Physics Orbital Simulation around Comet Didymos for CubeSat MissionAPEX on HERA, ESA“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-427433.
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We investigate the dynamic environment of the double asteroid system (65803) Didymoswith its satellite Didymos B for stable trajectories of more than 30 days for a model cubesatcalled APEX, foreseen to visit Didymos with the HERA mission, to be launched in 2023.We find semi-stable orbits with lifetimes up to 15 days under the influence of solar radiationpressure. In comparison, in the absence of solar radiation pressure, orbit trajectories reachlifetimes of up to 90 days. We present the distribution of orbital stability for positionsof initial deployment and the related orbital lifetime for inclinations of i = 0, 6 and 30 to the ecliptic plane of the sun, to investigate expected seasonality of the Didymossystem. The influence on trajectories through solar radiation pressure on the arrangementof the solar panels of APEX is compared to trajectories without solar radiation pressure. Itbecomes clear that the influence of solar radiation pressure is the main variable for stabilityof trajectories and must be further investigated with great care. We also discuss briefly theoperation of APEX in relation to orbital corrections and the limitations for such corrections.
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Alinder, Simon. „Effect of the convective electric field on the ion number density around a low activity comet“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-337017.
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Vigren et al. (2015) presents an integral expression to calculate the ion number density around a low activity comet immersed in the solar wind's convective electric field. A certain parameter of the integral takes values of either 1 or 0 depending on whether a corresponding ion trajectory is feasible or not. The criteria used in the paper has been found not to be strict enough, yielding overestimated ion number densities in the cometary wake. The present project finds two new options for the criteria, one analytical and one numerical. The new numerical condition is tested in the same computations done in the original paper and compares the results of the old and new criteria. The new conditionis found to correct the previous error.Projektet gjort inom: Fördjupningskurs i fysik - projektkurs, 5.0 hp. Kurskod:1FA566.
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Chiaretta, Marco. „Numerical modelling of Langmuir probe measurements for the Swarm spacecraft“. Thesis, Uppsala universitet, Rymd- och plasmafysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-148633.
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This work studies the current collected by the spherical Langmuir probes to be mounted on the ESA Swarm satellites in order to quantify deviations from idealized cases caused by non-ideal probe geometry. The finite-element particle-in-cell code SPIS is used to model the current collection of a realistic probe, including the support structures, for two ionospheric plasma conditions with and without drift velocity. SPIS simulations are verified by comparing simulations of an ideal sphere at rest to previous numerical results by Laframboise parametrized to sufficient accuracy. It is found that for probe potentials much above the equivalent electron temperature, the deviations from ideal geometry decrease the current by up to 25 % compared to the ideal sphere case and thus must be corrected if data from this part of the probe curve has to be used for plasma density derivations. In comparison to the non-drifting case, including a plasma ram flow increases the current for probe potentials around and below the equivalent ion energy, as the contribution of the ions to the shielding is reduced by their high flow energy.
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Collier, Andrew. „VLF and ULF Waves Associated with Magnetospheric Substorms“. Doctoral thesis, KTH, Alfvénlaboratoriet, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3939.
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A magnetospheric substorm is manifested in a variety of phenomena observed both in space and on the ground. Two electromagnetic signatures are the Substorm Chorus Event (SCE) and Pi2 pulsations. The SCE is a Very Low Frequency (VLF) radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into an observer’s field of view from an injection site around midnight. The ascending frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts and the modification of the resonance conditions resulting from the radial component of the E × B drift. Two numerical models have been developed which simulate the production of a SCE. One accounts for both radial and azimuthal electron drifts but treats the wave-particle interaction in an approximate fashion, while the other retains only the azimuthal drift but rigorously calculates both the electron anisotropy and the wave growth rate. Results from the latter model indicate that the injected electron population should have an enhanced high-energy tail in order to produce a realistic SCE. Pi2 are damped Ultra Low Frequency (ULF) pulsations with periods between 40 and 150 s. The impulsive metamorphosis of the nightside inner magnetosphere during the onset of the substorm expansion phase is accompanied by a broad spectrum of magnetohydrodynamic (MHD) waves. Over a limited range of local times around midnight these waves excite field line resonances (FLRs) on field lines connected with the auroral zone. Compressional waves propagate into the inner magnetosphere, where they generate cavity mode resonances. The uniform frequency of Pi2 pulsations at middle and low latitudes is a consequence of these cavity modes. A number of Pi2 events were identified at times when the Cluster constellation was located in the nightside inner magnetosphere. Electric and magnetic field data from Cluster were used to establish the existence of both cavity and field line resonances during these events. The associated Poynting flux indicated negligible radial or field-aligned energy flow but an appreciable azimuthal flux directed away from midnight.QC 20100920
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Kvarnström, Joakim. „Searching for Titan's tail“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-356306.
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We have used Cassini measurements of electron density from Saturn's magnetosphere to search for a plasma tail behind the moon Titan. Such a plasma tail would consist of plasma that manage to escape Titan's gravitational pull and leave Titan's ionosphere to contribute to the plasma distribution in the Saturn system. The Cassini spacecraft was in orbit around Saturn for 13 years and performed 127 close flybys of Titan as well as many passes through Titan's orbit within the planets plasma-filled magnetosphere. We have used measurements of electron density from the Langmuir probe instrument, built by the Swedish Institute of Space Physics in Uppsala to search for such a tail. The data was analyzed in terms of looking at the spatial distribution of plasma around Titan and Saturn by examining the plasma density in Titan’s orbit in comparison to the rest of system, as well as comparisons of plasma density in front of Titan and behind Titan. The analysis provided no evidence of an extended plasma tail or torus.
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Al-Naseri, Haidar. „Quantum kinetic relativistic theory of linearized waves in magnetized plasmas“. Thesis, Umeå universitet, Institutionen för fysik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-150292.
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In this work we have studied linear wave propagation in magnetized plasmas using a fully relativistic kinetic equation of spin-1/2 particles in the long scale approximation. The linearized kinetic equation is very long and complicated, hence we worked with restricted geometries in order to simplify the calculations. The dispersion relation of the relativistic model was calculated and compared with a dispersion relation from a previous work at the semi-relativistic limit. Moreover, a new mode was discovered that survives in the zero temperature limit. The origin of the mode in the kinetic equation was discussed and derived from a non-relativistic kinetic equation from a previous work.
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Hahlin, Axel. „Coupled waves as a model to describe chaotic turbulence pumped by radio waves in the ionosphere“. Thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-356265.
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Experimental results concerning plasma turbulence pumped in theionosphere by powerful radio waves suggest that the turbulence is due todeterministic chaos. To investigate the possibility of deterministic chaosin the ionosphere coupled wave systems have been studied to see chaoticdynamics. If coupled waves can exhibit chaos it is a possible way tomodel ionospheric chaos. The result showed that chaos was present inboth wave systems studied which means that they could possibly explainthe chaos, to verify this more studies needs to be done on theparameters relevant to the coupled wave systems in the ionosphere andfind if they are in a regime where chaos developsStudier av plasmaturbulens i jonosfären som pumpas av kraftfulla radiovågor antyder att turbulensen är kopplat till deterministiskt kaos. För att undersöka möjligheten för deterministiskt kaos i jonosfären studeras kopplade vågsystem om de kan innehålla kaotiska regimer. Om dessa system visar kaotiskt beteende skulle de kunna användas för att beskriva kaos i jonosfären. Resultatet visade att kaos var närvarande i de kopplade vågsystem som studerats, för att verifiera om de kan användas för att beskriva kaos i jonosfären måste närmare studier av de parametrar som modellen använder sig av göras för att se om de faller inom ett intervall där kaos uppstår.
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Alinder, Simon. „Electron cooling in a cometary coma“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-324842.
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The ESA Rosetta spacecraft investigated comet 67P/Churyumov-Gerasimenko duringtwo years from August 2014 to the end of September 2016. The dual Langmuir probewas used to measure plasma parameters including the thermal energy of theelectrons. The observed thermal energy (or temperature) of the electrons was ratherhigh, in the range 5-10 eV almost throughout the mission. However, near perihelionthe Langmuir probe measurements indicated the prevalence of two electronpopulations with distinct temperatures, one hot (5-10 eV) and one cold (less than 1eV). It has been hypothesized that the electrons of the colder population wereformed relatively close to the nucleus and that they subsequently cooled by inelasticcollisions with the neutral gas. In this project work we develop a model for studyingelectron cooling in a cometary coma. The model takes into account collisions withwater molecules as well as the influence of a radial ambipolar electric field.Rymdsonden Rosetta från ESA undersökte kometen 67P/Churyumov Gerasimenkounder mer än två år, från augusti 2014 till slutet av september 2016.En Langumirprob användes för att undersökta plasmamiljön runt kometen, tillexempel elektronernas termiska energi. Den observerade termiska energin förelektronerna (eller elektrontemperaturen) var ganska hög, ca 5-10 eV undernästan hela uppdraget, men när kometen var nära perihelium detekterade instrumentenäven kalla elektroner, med en energi under 1 eV, distinkta från devarma. En hypotes är att dessa kalla elektroner bildas nära kärnan av att varmaelektroner genomgår inelastiska kollisioner med den neutrala gasen och tapparsin energi. I detta projekt utvecklar vi en modell för att studera elektronernasbeteende i koman. Modellen tar hänsyn till kollisioner med neutrala vattenmolekylersåväl som påverkan av ett radiellt ambipolärt elektriskt fält.
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Eriksson, Jacob. „Fuel ion densities and distributions in fusion plasmas : Modeling and analysis for neutron emission spectrometry“. Licentiate thesis, Uppsala universitet, Tillämpad kärnfysik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-192530.
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Neutrons produced in fusion reactions in a magnetically confined plasma carry information about the distributions and densities of the fuel ions in the plasma. This thesis presents work where various theoretical models of different fuel ion distributions in the plasma are used to calculate modeled components of the neutron energy spectrum. The calculated components can then be compared with measured data, either to benchmark and validate the model or to derive various plasma parameters from the experimental data. Neutron spectra measured with the spectrometers TOFOR and the MPR, which are both installed at the JET tokamak in England, are used for this purpose. The thesis is based on three papers. The first paper presents the analysis of TOFOR data from plasmas heated with neutral beams and radio frequency waves tuned to the third harmonic of the deuterium cyclotron frequency, which creates fast (supra thermal) ions in the MeV range. It is found that effects of the finite Larmor radii of the fast ions need to be included in the modeling in order to understand the data. These effects are important for fast ion measurements if there is a gradient in the fast ion distribution function with a scale length that is comparable to - or smaller than - the width of the field of view of the measuring instrument, and if this scale length is comparable to - or smaller than - the Larmor radii of the fast ions. The second paper presents calculations of the neutron energy spectrum from the T(t,n)4He reaction, for JET relevant fuel ion distributions. This is to to form a starting point for the investigation of the possibility to obtain fast ion information from the t-t neutron spectrum, in a possible future deuterium-tritium campaign at JET. The t-t spectrum is more challenging to analyze than the d-d and d-t cases, since this reaction has three (rather than two) particles in the final state, which results in a broad continuum of neutron energies rather than a peak. However, the presence of various final state interactions - in particular between the neutron and the 4He - might still allow for spectrometry analysis. Finally, in Paper III, a method to derive the fuel ion ratio, nt/nd, is presented and applied to MPR data from the JET d-t campaign in 1997. The trend in the results are consistent with Penning trap measurements of the fuel ion ratio at the plasma edge, but the absolute numbers are not the same. Measuring the fuel ion ratio in the core plasma is an important task for fusion research, and also a very complicated one. Future work should aim at measuring this quantity in several independent ways, which should then be cross checked against each other.
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Löfås, Henrik. „Ionospheric modification by powerful HF-waves : Underdense F-region heating by X-Mode“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-121898.
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Observations of modifications of the electron temperature in the F-region produced by powerful high-frequency waves transmitted in X-mode are presented. The experiments were performed during quiet nighttime conditions with low ionospheric densities so no reflections occurred. Nevertheless temperature enhancements of the order of 300-400K were obtained. The modifications found can be well described by the theory of Ohmic heating by the pump wave and both temporal and spatial changes are reproduced. A brief overview of several different experimental campaigns at EISCAT facilities in the period from October 2006 to February 2008 are also given pointing out some interesting features from the different experiments. The main focus is then on the campaign during October 2006 and modifications of the electron temperature in the F-region.
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Johansson, Tommy. „Investigations of auroral electric fields and currents“. Doctoral thesis, Stockholm : Elektro- och systemteknik, Kungliga Tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4512.
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Daldorff, Lars Kristen Selberg. „Numerical Simulation as a Tool for Studying Waves and Radiation in Space“. Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-9517.
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Vigstrand, Oscar. „Development of an absorption model for gas discharge lamp simulation“. Thesis, Malmö universitet, Malmö högskola, Institutionen för materialvetenskap och tillämpad matematik (MTM), 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:mau:diva-42518.
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Ultraviolet (UV) light has been used for disinfection purposes for over 100 years. Irradiation by UV light is a method to disinfect surfaces in order to prevent microbiological growth. At Tetra Pak this is of great importance as they are manufacturer of filling machines. Those filling machines must ensure a certain level of sterility on all packages produced. The irradiation process can be simulated using Geant4 which is a software package that tracks particles through matter. The simulation model used today does not consider the absorption of photons inside of medium-pressure UV lamps. By understanding the absorption that takes place in the lamp, one can quantify how changes in the design would impact the emitter output. In this master's thesis, the aim is to develop a model that can describe the interaction of photons with a medium-pressure UV lamp. An absorption model was suggested and developed with the assumption of local thermodynamical equilibrium and existing Hg radiative data. A simulation including the collision process in Geant4 was used. In this collision process the non-radiative transition probabilities were assumed to be the same as that of the radiative, this was done in order to demonstrate how it can be done. It resulted in collisions populating other states allowing more transitions to be present in the final output spectrum. The collision process and a method for computing the Einstein's emission coefficient with the software package General Relativistic Atomic Structure Package is proposed as future work.I över 100 år har ultraviolet (UV) ljus använts till desinficering. UV bestrålning är en metod för att desinficera ytor med målet att förhindra mikrobiologisk tillväxt. För Tetra Pak som är ledande inom tillverkning av fyllmaskiner är det extra viktigt. Förpackningarna inuti fyllningsmaskinerna måste garantera en viss nivå av sterilitet för alla förpackningar. Dagens simuleringar av medeltrycks UV lampa utförs i Geant4 som är ett mjukvarupaket som möjliggör följandet av partiklar genom olika medium. Detta görs utan att ta hänsyn till absorptionen av fotoner. Genom att förstå absorptionen som sker i lampans gas kan man kvantifiera hur förändringar i design skulle påverka emittorns utgående effekt. I detta examensarbete är målet att utveckla en modell som kan beskriva hur fotoner växelverkar med gasen i en medeltrycks UV lampa. En modell utvecklas och föreslås med antagandet att lokalt termodynamisk jämvikt råder och att enbart Hg strålnings data används. En simulering med en kollisionsprocess i Geant4 inkluderades. I denna kollisionsprocess antas den icke-optiska övergångssannolikheten vara densamma som för de optiska övergångarna. Detta inkluderades för att demonstrera hur en sådan process kan gå till. Detta resulterade i att kollisionerna populerade andra tillstånd vilket gjorde att dessa övergångar visade sig i utgående spektrum. Kollisionsprocessen och en metod för att beräkna Einsteins emissions koefficient med mjukvarupaketet General Relativistic Atomic Structure Package föreslås även som framtida arbete.
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Liljeblad, Elisabet. „Structures and processes in the Mercury magnetosphere“. Doctoral thesis, KTH, Rymd- och plasmafysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-207173.
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The mechanisms involved in the transfer of mass and energy from the solar wind to any planetary magnetosphere is considered an important topic in space physics. With the use of the Mercury spacecraft MESSENGER's data, it has been possible to study these processes in an environment different, yet similar, to Earth's. These data have resulted in new knowledge advancing not only the extraterrestrial space plasma research, but also the general space physics field. This thesis aims to investigate mechanisms for the transfer of mass and energy into Mercury’s magnetosphere, and magnetospheric regions affected by, and processes directly driven by, these. The work includes the Kelvin-Helmholtz instability (KHI) at the magnetopause, which is one of the main drivers for mass and energy transfer on Earth, the low-latitude boundary layer (LLBL), which is in direct connection to the magnetopause and proposed to be affected by the KHI, magnetospheric ultra-low frequency (ULF) waves driven by the KHI, and isolated magnetic field structures in the magnetosheath as possible analogues to the Earth magnetosheath plasmoids and jets. Kelvin-Helmholtz waves (KHW) and the LLBL are identified and characterized. The KHWs are observed almost exclusively on the duskside magnetopause, something that has not been observed on Earth. In contrast, the LLBL shows an opposite asymmetry. Results suggest that the KHI and LLBL are connected, possibly by the LLBL creating the asymmetry observed for the KHWs. Isolated changes of the total magnetic field strength in the magnetosheath are identified. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggest that they are analogues to diamagnetic plasmoids found on Earth. No clear analogues to paramagnetic plasmoids are found. Distinct magnetospheric ULF wave signatures are detected frequently in close connection to KHWs. Results from the polarization analysis on the dayside ULF waves indicate that the majority of these are most probably driven by the KHI. In general, likely KHI driven ULF waves are observed frequently in the Hermean magnetosphere. Although similar in many aspects, Mercury and Earth show fundamental differences in processes and structures, making Mercury a highly interesting planet to study to increase our knowledge of Earth-like planets.QC 20170519
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Borälv, Eva. „Substorm Features in the High-Latitude Ionosphere and Magnetosphere : Multi-Instrument Observations“. Doctoral thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3478.
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The space around Earth, confined in the terrestrial magnetosphere, is to some extent shielded from the Sun's solar wind plasma and magnetic field. During certain conditions, however, strong interaction can occur between the solar wind and the magnetosphere, resulting in magnetospheric activity of several forms, among which substorms and storms are the most prominent. A general framework for how these processes work have been outlayed through the history of research, however, there still remain questions to be answered. The most striking example regards the onset of substorms, where both the onset cause and location in the magnetosphere/ionosphere are still debated. These are clearly not easily solved problems, since a substorm is a global process, ideally requiring simultaneous measurements in the magnetotail and ionosphere. Investigated in this work are temporal and spatial scales for substorm and convection processes in the Earth's magnetosphere and ionosphere. This is performed by combining observations from a number of both ground-based and spacecraft-borne instruments. The observations indicate that the magnetotail's cross-section is involved to a larger spatial extent than previously considered in the substorm process. Furthermore, convection changes result in topological changes of the magnetosphere on a fast time scale. The results show that the magnetosphere is, on a global magnetospheric scale, highly dynamic during convection changes and ensuing substorms.
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Lindblom, Ville. „Heavy Ion Temperature Anisotropies in the Venus Plasma Environment“. Thesis, Luleå tekniska universitet, Institutionen för system- och rymdteknik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-85096.
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In a plasma environment particles and plasma waves have complex interactions and can affect each other significantly. The velocity distribution functions (VDFs) can effectively be used to try and understand these interactions. This study uses VDFs to investigate heavy ion temperatures in the Venusian plasma environment. A Maxwellian fitting methodology previously used to obtain proton plasma parameters is used to obtain plasma parameters for heavy ions instead. Ion and magnetic-field data are gathered from the ion mass analyser IMA and the magnetometer MAG which were on board the European spacecraft Venus Express. The temperature anisotropies are analysed to see if they may affect the observed plasma waves around Venus. Spatial maps of the obtained plasma parameters are presented, and the average values are shown. The temperature ratio T⊥/T∥ is calculated to look for anisotropies. Case studies were made to investigate how well the methodology worked with heavy ion data. The methodology is shown to work well in the magnetotail, where heavy ions are expected, and less well in the magnetosheath and solar wind, where heavy ions were not expected. No statistically significant anisotropies were found for the heavy ions in the Venusian plasma environment.
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Östman, Sara. „The Changing Character of Mars’ Bow Shock“. Thesis, Umeå universitet, Institutionen för fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-179264.
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The aim of the project is to investigate the characteristics and causes of two different types of bow shocks at Mars. We define Type 1 as an undefined, drawn out ramp, and Type 2 as a shorter duration ramp that has clearer characteristics and behaves more like a step increase in the magnetic field. A total of forty five events of the two different types were investigated using data from 2014-2015 from NASA’s MAVEN spacecraft. The Power Spectral Density of the magnetic field is calculated for downstream/ramp/upstream intervals. The normal is calculated with a mixed-mode coplanarity model. Proton, alpha particle and atomic oxygen density are also calculated. Results show higher frequencies for nose events of Type 1, and lower for flank events of Type 1. No such pattern can be seen in Type 2 events. Proton and alpha-particles are shown to be shocked, and their densities are slightly higher at the flanks as compared to the nose of the bow shock. Atomic oxygen density stays constant before and after the bow shock, likely due to the fact that the oxygen originates mostly from the exosphere rather than from the solar wind. Ion densities seem not to be affected by whether the event is Type 1 or 2.
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Dogurevich, Pavel. „Kinetic simulation of spherically symmetric collisionless plasma in the inner part of a cometary coma“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-389449.
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Walter, Erwin. „Field-Aligned Currents and Flow Bursts in the Earth’s Magnetotail“. Thesis, Umeå universitet, Institutionen för fysik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-148525.
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We use electric and magnetic field data from MMS spacecraft between 2016 and 2017 tostatistically investigate earthward propagating plasma flow bursts and field-aligned currents(FACs) inside the plasma sheet of the geomagnetic tail. We observe that the occurrence rateof flow burst peaks around the midnight region with decreasing trend towards Earth and theplasma sheet flanks. Further, we distinguish between long and short FACs. Long FACs laston average 6 sec and have a magnitude of 5-20 nA/m 2 . Short FACs last on average 10 timesshorter and have an magnitude of 10-50 nA/m 2 . Both, long and short FACs occur on averageone time per flow burst, on minimum 0 times and on maximum 4 times per flow burst. Intotal, 43 % of the observed FACs are located in a flow burst, 40 % before and 17 % right after aflow burst.
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Håkansson, Marcus. „Back-tracing of water ions at comet 67P/Churyumov–Gerasimenko“. Thesis, Luleå tekniska universitet, Rymdteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-67050.
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This paper examines the neutral coma of comet 67P/Churyumov–Gerasimenko by using measurements of charged particles (water ions) and tracing them back to their place of ionisation. The measurements were taken from Rosetta’s Ion Composition Analyser. The simulations made use of an existing program which traces particles forward, which was changed to trace particles backwards, with new conditions for terminating the simulation. Two types of simulations were made. The first type is referred to as ”one-day simulations”. In these, simulations are made using data from a single occasion, with nine occasions studied per selected day. The days were selected so that the spacecraft was in different positions in relation to the comet. The second is referred to as the ”full-hemisphere” simulation. In this simulation, data from all usable days are used to produce an image of the hemisphere facing the Sun. The full-hemisphere simulation suffers from lack of simultaneous measurements, and indeed it is impossible to obtain in-situ measurements at all positions at once. Both simulations could be improved using more precise models, which could not be done within the allotted time of this work.
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Gullman-Strand, Johan. „Turbulence and scalar flux modelling applied to separated flows“. Doctoral thesis, Stockholm : Department of Mechanics, Royal Institute of Technology, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-92.
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Semrén, Philip. „On the Interaction Between Electromagnetic, Gravitational, and Plasma Related Perturbations on LRS Class II Spacetimes“. Thesis, Umeå universitet, Institutionen för fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-184078.
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In this thesis, we investigate the interaction between electromagnetic, gravitational, and plasma related perturbations on homogeneous and hypersurface orthogonal Locally Rotationally Symmetric (LRS) class II spacetimes. By using these spacetimes, which allow for the inclusion of a non-zero magnetic field, as backgrounds in a perturbative approach, we are able to see interactions between the electromagnetic and gravitational variables already to first order in the perturbations. This is in contrast to earlier works using isotropic Friedmann-Lemaı̂tre-Robertson-Walker (FLRW) backgrounds, where one is usually faced with going to second order in the perturbations. To get the equations governing our perturbations, we use a 1+1+2 covariant approach and gather relations from the Ricci and Bianchi identities, Maxwell’s equations, particle conservation, and energy-momentum conservation for the individual plasma components. After linearising these equations around a LRS background, performing a harmonic decomposition, and using the Magnetohydrodynamic (MHD) approximation for a cold plasma, we then arrive at a closed system for the first order perturbations. This system, consisting of ordinary differential equations in time and a set of constraints, is then reduced to two separate subsectors, containing seven and nine variables respectively. These variables include quantities related to the Weyl tensor, the vorticity, and the electromagnetic fields, as well as perturbations in the plasma velocity and energy density. Through numerical calculations, we use the equations for these variables to show that perturbations in the magnetic field can be sourced by perturbations in both the plasma velocity and the gravitational variables. We also observe beat-like interference patterns for large values of the Alfvén velocity. These results can be of interest when considering the large scale cosmic magnetic fields, as their origin still seems to elude us. However, since we neglect thermal pressures and dissipative fluxes, it should be noted that our results are mainly applicable in the limit of low temperature and in cases where the thermal pressure is smaller than the pressure due to the electromagnetic fields.
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Khotyaintsev, Yuri. „Alfvén Waves and Energy Transformation in Space Plasmas“. Doctoral thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3264.
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This thesis is focused on the role of Alfvén waves in the energy transformation and transport in the magnetosphere. Different aspects of Alfvén wave generation, propagation and dissipation are considered. The study involves analysis of experimental data from the Freja, Polar and Cluster spacecraft, as well as theoretical development. An overview of the linear theory of Alfvén waves is presented, including the effects of fnite parallel electron inertia and fnite ion gyroradius, and nonlinear theory is developed for large amplitude Alfvén solitons and structures. The methodology is presented for experimental identification of dispersive Alfvén waves in a frame moving with respect to the plasma, which facilitates the resolution of the space-time ambiguity in such measurements. Dispersive Alfvén waves are identified on field lines from the topside ionosphere up to the magnetopause and it is suggested they play an important role in magnetospheric physics. One of the processes where Alfvén waves are important is the establishment of the field aligned current system, which transports the energy from the reconnection regions at the magnetopause to the ionosphere, where a part of the energy is dissipated. The main mechanism for the dissipation in the top-side ionosphere is related to wave-particle interactions leading to particle energization/heating. An observed signature of such a process is the presence of parallel energetic electron bursts associated with dispersive Alfvén waves. The accelerated electrons (electron beams) are unstable with respect to the generation of high frequency plasma wave modes. Therefore this thesis also demonstrates an indirect coupling between low frequency Alfvén wave and high frequency oscillations.
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Dreyer, Joshua. „A detailed study of auroral fragments“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-388546.
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Aurora occurs in various shapes, one of which is the hitherto unreported phenomenon of auroral fragments. For three periods of occurrence of these fragments their properties were studied in detail during this master’s thesis, using mainly ground-based instrumentation located near Longyearbyen on Svalbard, Norway. A base dataset was constructed from 103 all-sky camera images, manually marking 305 fragments for further analysis. This thesis reports and describes the fragment observations during the observed events, including the auroral and geomagnetic context. Fragments generally seem to fall into two categories, the first being singular, apparently randomly distributed fragments, and the second being periodic fragments that occur in groups with a regular spacing close to auroral arcs. A typical fragment has a small horizontal size below 20 km, a short lifetime of less than a minute and shows no field-aligned extent in the emission. The fragments appear mainly west of zenith (73%) during the three observation nights, whereas their north-south distribution is symmetric around the zenith. Almost all of them exhibit westward drift, the estimated speed for one of the fragments passing the field of view of ASK is ∼1 km/s. A spectral signature can be seen in the green auroral wavelength of O at 557.7 nm and red emission line of N2 at 673.0 nm, but no emission enhancement was observed in the blue wavelengths. One fragment passing the EISCAT Svalbard radar’s field of view shows a local ion temperature increase in a small altitude range of ∼15 km, whereas there is no visible increase in electron density. This could be explained by fragment generation due to locally strong horizontal electric fields. A potential mechanism for this might be electric fields of atmospheric waves superposing with the converging electric fields of auroral arcs created by particle precipitation and the corresponding field-aligned currents. The resulting field would be perpendicular to the magnetic field and the auroral arcs, leading to wave-like density variations of excited plasma close to the arcs. Further study is required to verify this hypothesis and improve the understanding of fragment properties determined from the limited dataset used for this thesis.
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Möslinger, Anja. „Particle Trajectory Simulations for SCIENA-N : Conversion surface design for an ENA sensor head“. Thesis, Luleå tekniska universitet, Rymdteknik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-83034.
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This thesis serves as a preliminary design study for the combination of a flight-proven ion optics system (SWIM) with a conversion surface to create a small energetic neutral atom (ENA) sensor. It is planned to use this sensor as ENA sensor for the DFP-SCIENA instrument on Comet Interceptor. Due to the nature of the Comet Interceptor mission (ESA F-class mission with a maximum launch mass of 1000 kg) the development time for a new sensor that meets the size and weight restrictions is limited. The proposed combination of SWIM with a conversion surface is based on a proven ion optics design and should result in a compact sensor design. The main goal of this thesis was to simulate different conversion surface designs and evaluate their compatibility with the SWIM instrument. During this process the different designs were optimised based on the intermediate simulation results. The simulation process was performed by using SIMION to calculate particle trajectories. In the end, two different conversion surface designs yielded promising results. With both designs detailed simulations and data analysis were conducted to determine the different properties of the two designs. One of these designs was chosen to be further investigated for use on the Comet Interceptor mission.
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Bader, Alexander. „Ion Temperature Anisotropies in the Venus Plasma Environment“. Thesis, Luleå tekniska universitet, Institutionen för system- och rymdteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-63909.
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Velocity distributions are a key to understanding the interplay between particles and waves in a plasma. Any deviation from a Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer MAG on-board Venus Express, ion distributions in the plasma environment of Venus are studied. The focus lies on temperature anisotropy, that is, the difference between the ion temperature parallel and perpendicular to the background magnetic field. This study presents spatial maps of the average ratio between the perpendicular temperature
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Lannér, Viktor. „Incoherent Scattering of Twisted Radar Beams from the Ionosphere“. Thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-325768.
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In the search for natural orbital angular momentum (OAM) effects, some of the first incoherent scatter experiments with twisted radar beams during aurora were conducted at Poker Flat Incoherent Scatter Radar (PFISR), Alaska, USA, in October 2012. Experimental data of scatter from beam configurations with opposite twists were investigated. By the use of hypothesis tests in combination with Monte Carlo simulations together with traditional estimations of the mean and confidence interval, asymmetries between scatter of radar beams with opposite twists were identified for an integration time of at least 30 minutes. Asymmetries were detected in the internal radar noise too, but not necessarily with the same signs as for the asymmetries from the ionospheric signals. The asymmetries identified could be due to amplified noise for signals coming from intense aurora or perhaps the rectangular-shaped antenna array used at PFISR. These two possible causes need to be ruled out before suggesting that the asymmetries identified are an outcome of OAM effects present in the ionosphere.
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Ramstad, Robin. „Ion escape from Mars : measurements in the present to understand the past“. Doctoral thesis, Umeå universitet, Institutionen för fysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-141892.
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Present-day Mars is a cold and dry planet with a thin CO2-dominated atmosphere comprising only a few mbar pressure at low altitudes. However, the Martian surface is marked with valley networks, hydrated mineral clays, carbonates and the remains of deltas and meandering rivers, i.e. traces of an active hydrological cycle present early in the planet's geological history. A strong greenhouse effect, and thus a thicker atmosphere, would have been required to sustain a sufficiently warm environment, particularly under the weaker luminosity of the early Sun. The fate of this early atmosphere is currently unknown. While several mechanisms can remove atmospheric mass over time, a prominent hypothesis suggests that the lack of an intrinsic Earth-like global magnetic dipole has allowed the solar wind to erode the early Martian atmosphere by imparting energy to the planet's ionosphere which subsequently flows out as ion escape, over time depleting the greenhouse gasses and collapsing the ancient hydrological cycle. Previous studies have found insignificant ion escape rates under present-day conditions, however, the young Sun emitted significantly stronger solar wind and photoionizing radiation flux compared to the present. The geological record establishes the time of collapse of the hydrological cycle, estimated to have occurred in the mid-late Hesperian period (~3.3 billion years ago) at latest. To constrain the amount of atmosphere lost through ion escape since, we use the extensive database of ion flux measurements from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) particles package on the Mars Express orbiter (2004-present) to quantify the ion escape rate dependence on upstream solar wind and solar radiation conditions. The Martian ion escape rate is shown to be insensitive to solar wind parameters with a weak inverse dependence on solar wind dynamic pressure, and linearly dependent on solar ionizing photon flux, indicating efficient screening of the bulk ionosphere by the induced magnetic fields. The impact of an extreme coronal mass ejection is studied and found to have no significant effect on the ion escape rate. Instead, intense solar wind is shown to only increase the escaping energy flux, i.e. total power of escaping ions, without increasing the rate by accelerating already escaping ions. The orientation of the strongest magnetized crustal fields are shown to modulate the ion escape rate, though to have no significant time-averaged effect. We also study the influence of solar wind and solar radiation on the major Martian plasma boundaries and discuss factors that might limit the ion escape rate, including solar wind-ion escape coupling, which is found to be ≲1% and decreasing with increased solar wind dynamic pressure. The significant escape rate dependencies found are extrapolated back in time, considering the evolution of solar wind and ionizing radiation, and shown to account for only 4.8 ± 1.1 mbar equivalent surface pressure loss since the time of collapse of the Martian hydrosphere in the Hesperian, with ~6 mbar as an upper estimate. Extended to the late Noachian period (3.9 billion years ago), the found dependencies can only account for ≲10 mbar removed through ion escape, an insignificant amount compared to the ≳1 bar surface pressure required to sustain a warm climate on early Mars.
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Arvelius, Sachiko. „Energization and Acceleration of Dayside Polar Outflowing Oxygen“. Doctoral thesis, Kiruna : Swedish Institute of Space Physics, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-647.
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Skan, Moa. „Reconstructing ICMEs with the toroidal Grad-Shafranov method“. Thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-382398.
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The main objective of this thesis is to model the magneticstructure of interplanetary coronal mass ejections (ICME) measuredin-situ from the WIND spacecraft positioned at L1. The modeling isdone by a magnetohydrodynamic reconstruction technique based onthe GS equation with a toroidal geometry. The purpose has been toextend the application of the reconstruction program to real dataand to test its performance when different input parameters arechanged. Two events are presented; 16-17 May 2012 and 15-16 May2005 ICMEs have been successfully reconstructed with this model. The main achievements of the study are that a) the code now worksfor real data b) the important parameters that can be changed fordifferent reconstructions in the code are the number of iterationsused to find the optimal Z-axis, the plasma pressure and the orderof the polynomial fitting of the flux functional, c) if all crosssection reconstructions for different variations of theseparameters strongly resembles each other then this is anindication that the model approximation is good and that the fluxrope exists. The results have been compared and verified withpreviously published studies of these events. Using a toroidal geometry for the GS reconstruction method weobtain very similar results to the one obtained with differentreconstruction techniques.This implies that at L1, the ICMEs haveexpanded so much that a cylindrical geometry is sufficient todescribe the flux rope geometry. The toroidal Grad-Shafranovreconstruction technique is best suited for circular, or slightlyelongated, flux rope cross section profiles but have been provento work for one complex ejecta consisting of two merged fluxropes. The toroidal model might become an important asset in thefuture when data from spacecraft closer to the Sun, such as ParkerSolar Probe and Solar Orbiter, is public. When the major radius ofthe flux rope is smaller the choice of geometry will most likelyhave a larger role than for measurements at L1 and so, thetoroidal Grad-Shafranov reconstruction technique will probably bethe better alternative of the models that exists today.
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Svensson, Martin. „Electron heating in collisionless shocks observed by the MMS spacecraft“. Thesis, Luleå tekniska universitet, Rymdteknik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-67892.
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Shock waves are ubiquitous in space and astrophysics. Shocks transform directed particle flow energy into thermal energy. As the major part of space is a collisionless medium, shocks in space physics arises through wave-particle interactions with the magnetic field as the main contributor.The heating processes are scale dependent. The large scale processes governs the ion heating and is well described by magnetohydrodynamics. The small scale processes governs the electron heating lies within the field of kinetic plasma theory and is still today remained disputed. A step towards the answer for the small scale heating would be to measure the scale, in order to relate it to a known instability or other small scale processes.The multi-spacecraft NASA MMS spacecraft carries several high resolute particle and field instruments enabling almost instantaneous 3D particle measurements and accurate measurements of the magnetic field. Also the separation between the four MMS spacecraft is as small as < 8km for a certain mission phase. This allows for new approaches when determining the scale which for shocks has not been possible before when using data from previous multi-spacecraft missions with spacecraft separation much larger. The velocity of the shock is large compared to the spacecraft,thus the shock width cannot be directly measured by each spacecraft. Either a constant velocity has to be estimated or we could use gradients of a certain parameter between the spacecraft as the shock flows over them. The usage of gradients is only possible with MMS as all the spacecraft could for MMS be within the shock simultaneously. The change for a parameter within the shockis assumed to be linear between the spacecraft and measurements. It is also assumed that the gradient of the parameter maximizes in the shock normal direction. Using these assumptions two methods have been developed. They have the same working principles but are using two or four spacecraft for linear estimation at each measurement point. From the gradient and parametric data the shock ramp width could then be found. The parameter used in this thesis is the electron temperature. The methods using one, two and four spacecraft were tested using electron temperature data from different shock crossings. Two problems with the gradient methods were found from the results, giving false data for certain time spans. To avoid these problems, the scale of the electron temperature gradient was determined for roughly half the shock ramp. It was found using the two and four spacecraft methods that an assumption of constant velocity for the shock speed is an uncertain assumption. The shock speed varies over short time scales and in the shock crossings analysed the constant velocity estimations were generally overestimated. From the two and four spacecraft methods roughly half of the temperature rise in the shock ramp occurred over 10.8km or 12.4 lpe. This is almost a factor of two greater than previous scale estimates using Cluster data and a multi-spacecraft timing method for shock speed estimation.From the results it is concluded that the methods when using gradients between spacecraft has some restrictions. They can only be used for MMS data, requires quasi-perpendicular high Mach number and will give false results if the temperature is disturbed by interacting hot plasma clouds. However, even though we have these limitations for the tested gradient methods, they were found better and more reliable compared to previous methods for shock scaling.
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Görtz, Stefan. „Realistic simulations of delta wing aerodynamics using novel CFD methods“. Doctoral thesis, KTH, Aeronautical and Vehicle Engineering, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-125.
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The overall goal of the research presented in this thesis is to extend the physical understanding of the unsteady external aerodynamics associated with highly maneuverable delta-wing aircraft by using and developing novel, more efficient computational fluid dynamics (CFD) tools. More specific, the main purpose is to simulate and better understand the basic fluid phenomena, such as vortex breakdown, that limit the performance of delta-wing aircraft. The problem is approached by going from the most simple aircraft configuration - a pure delta wing - to more complex configurations. As the flow computations of delta wings at high angle of attack have a variety of unusual aspects that make accurate predictions challenging, best practices for the CFD codes used are developed and documented so as to raise their technology readiness level when applied to this class of flows.
Initially, emphasis is put on subsonic steady-state CFD simulations of stand-alone delta wings to keep the phenomenon of vortex breakdown as clean as possible. For half-span models it is established that the essential characteristics of vortex breakdown are captured by a structured CFD code. The influence of viscosity on vortex breakdown is studied and numerical results for the aerodynamic coefficients, the surface pressure distribution and breakdown locations are compared to experimental data where possible.
In a second step, structured grid generation issues, numerical aspects of the simulation of this nonlinear type of flow and the interaction of a forebody with a delta wing are explored.
Then, on an increasing level of complexity, time-accurate numerical studies are performed to resolve the unsteady flow field over half and full-span, stationary delta wings at high angle of attack. Both Euler and Detached Eddy Simulations (DES) are performed to predict the streamwise oscillations of the vortex breakdown location about some mean position, asymmetry in the breakdown location due to the interaction between the left and right vortices, as well as the rotation of the spiral structure downstream of breakdown in a time-accurate manner. The computed flow-field solutions are visualized and analyzed in a virtual-reality environment.
Ultimately, steady-state and time-dependent simulations of a full-scale fighter-type aircraft configuration in steady flight are performed using the advanced turbulence models and the detached-eddy simulation capability of an edge-based, unstructured flow solver. The computed results are compared to flight-test data.
The thesis also addresses algorithmic efficiency and presents a novel implicit-explicit algorithm, the Recursive Projection Method (RPM), for computations of both steady and unsteady flows. It is demonstrated that RPM can accelerate such computations by up to 2.5 times.
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Al, Moulla Khaled. „Turbulence at MHD and sub-ion scales in the magnetosheath of Saturn : a comparative study between quasi-perpendicular and quasi-parallel bow shocks using in-situ Cassini data“. Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-354775.
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The purpose of this project is to investigate the spectral properties of turbulence in the magnetosheath of Saturn, using in-situ magnetic field measurements from the Cassini spacecraft. According to models of incompressible, turbulent fluids, the energy spectrum in the inertial range scales as the frequency to the power of -5/3, which has been observed in the near-Earth Solar wind but not in the Terrestrial magnetosheath unless close to the magnetopause. 120 time intervals for when Cassini is inside the magnetosheath are identified — 40 in each category of behind quasi-perpendicular bow shocks, behind quasi-parallel bow shocks, and inside the middle of the magnetosheath. The power spectral density is thereafter calculated for each interval, with logarithmic regressions performed at the MHD and sub-ion scales separated by the ion gyrofrequency. The results seem to indicate similar behaviour as in the magnetosheath of Earth, without significant difference between quasi-perpendicular and quasi-parallel cases except somewhat steeper exponents at the MHD scale for the former. These observations confirm the role of the bow shock in destroying the fully developed turbulence of the Solar wind, thus explaining the absence of the inertial range.Syftet med detta projekt är att undersöka de spektrala egenskaperna hos turbulens i Saturnus magnetoskikt, med in-situ-mätningar av magnetfältet från Cassini-rymdsonden. Enligt modeller av inkompressibla, turbulenta fluider, är energispektrumet i det intertiala omfånget proportionellt mot frekvensen upphöjd i -5/3, vilket har observerats i den jordnära Solvinden men inte i det jordiska magnetoskiktet förutom nära magnetopausen. 120 tidsintervall för när Cassini befinner sig inuti magnetoskiktet identifieras — 40 styck i kategorierna bakom kvasi-vinkelräta bogchockar, bakom kvasi-parallella bogchockar, och inuti mellersta delen av magnetoskiktet. Effektspektraltätheten beräknas därefter för varje intervall, med logaritmiska regressioner på MHD- och subjon-skalorna som separeras av jongyrofrekvensen. Resultaten verkar tyda på liknande beteende som i Jordens magnetoskikt, utan märkvärdig skillnad mellan kvasi-vinkelräta och kvasi-parallella fall förutom något brantare exponenter på MHD-skalan för de förnämnda. Dessa observationer bekräftar bogchokens roll i förstörandet av den fullt utvecklade turbulensen i Solvinden, därmed förklarande avsaknaden av det inertiala omfånget.
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Backrud, Marie. „Cluster Observations and Theoretical Explanations of Broadband Waves in the Auroral Region“. Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-5809.
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Shebanits, Oleg. „Titan’s ionosphere and dust : – as seen by a space weather station“. Doctoral thesis, Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-329490.
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Titan, the largest moon of Saturn, is the only known moon with a fully developed nitrogen-rich atmosphere, its ionosphere is detectable as high as 2200 km above its surface and hosts complex organic chemistry. Titan’s atmosphere and ionosphere has striking similarities to current theories of these regions around Earth 3.5 billion years ago. The Cassini spacecraft has been in orbit around Saturn since 2004 and carries a wide range of instruments for investigating Titan’s ionosphere, among them the Langmuir probe, a “space weather station”, manufactured and operated by the Swedish Institute of Space Physics, Uppsala. This thesis presents studies of positive ions, negative ions and negatively charged dust grains (also called aerosols) in Titan’s ionosphere using the in-situ measurements by the Cassini Langmuir probe, supplemented by the data from particle mass spectrometers. One of the main results is the detection of significant (up to about 4000 cm-3) charge densities of heavy (up to about 13800 amu/charge) negative ions and dust grains in Titan’s ionosphere below 1400 km altitude. The dust is found to be the main negative charge carrier below about 1100 km on the nightside/terminator ionosphere, forming a dusty plasma (also called “ion-ion” plasma). A new analysis method is developed using a combination of simultaneous observations by multiple instruments for a case study of four flybys of Titan’s ionosphere, further constraining the ionospheric plasma charge densities. This allows to predict a dusty plasma in the dayside ionosphere below 900 km altitude (thus declaring it a global phenomenon), as well as to empirically estimate the average charge of the negative ions and dust grains to between -2.5 and -1.5 elementary charges. The complete Cassini dataset spans just above 13 years, allowing to study effects of the solar activity on Titan’s ionosphere. From solar minimum to maximum, the increase in the solar EUV flux increases the densities by a factor of ~2 in the dayside ionosphere and, surprisingly, decreases by a factor of ~3-4 in the nightside ionosphere. The latter is proposed to be an effect of the ionospheric photochemistry modified by higher solar EUV flux. Modelling photoionization also reveals an EUV trend (as well as solar zenith angle and corotational plasma ram dependencies) in the loss rate coefficient.
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Villegas, Prados David. „Analysis of Hall effect thrusters using Hybrid PIC simulations and coupling to EP plume“. Thesis, Luleå tekniska universitet, Rymdteknik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-80641.
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In the last 30 years, numerical models have been developed to properly analyze Hall eect thrusters (HET),leading to a bridge between analytical prediction/empirical intuition and experiments. For companies in thespace sector, these codes serve to much more than simply simulating the thruster, but it provides a fast, cheapand reliable tool for processes such as validation and verication procedures, as well as for technical developmentof the thruster. During the testing of the thruster, mostly measurements upstream from the thruster exhaustare obtained since the high density plasma inside the channel disturbs any measurement inside the channel. Thisresults in the company knowing about the output of the thruster performance, but having little knowledge aboutthe processes and behavior of the thruster itself. The purpose of this study is to help reduce the uncertainty,using existing software to eectively analyze and understand HETs. Because of the physical nature of theproblem, HET simulations follow a multi-scale approach where the thruster is divided into two regions: insidechannel/near-plume region and far-plume region. To study each zone dierent softwares are typically used.This thesis aims to nd a common ground between both software, coupling them and creating a line of analysisto follow when studying HETs.The present thesis will focus on the analysis of the famous SPT-100. The design of this work can be divided intotwo: an hybrid-PIC simulation with a software focusing on the inside channel and near-plume region, Hallis; andanother hybrid-PIC simulation regarding the plasma plume expansion performed with SPIS-EP. During thisproject both software were mastered. Hallis is investigated, emphasizing the empirical modelling of the electronanomalous transport inside the thruster and its consequences on the output results. A sensitivity analysis isperformed to obtain a good set of the empirical parameters that drive the overall performance of the thrusterand the plasma behavior. Once a good match persist between Hallis and nominal operating conditions, theoutput is used to construct the input injection distributions needed by the plasma expansion software (SPIS).Finally, the plasma plume is simulated and results are compared to in-house experimental data. In this way,one is able to control and understand the nal output directly from the behavior of the thruster. It is importantto mention that due to condentiality reasons, the testing data cannot be fully shown and sometimes only thetrend can be analyzed.As a results of the analysis, it is found that establishing the coupling between softwares is feasible, but Halliscode needs to include some characteristics to fully take advantage of its potential. It is determined that theion denition followed by Hallis is enough to perfectly dene the ion energy distribution as well as generalperformance parameters of the SPT-100 (thrust, ionization eciency, power...), but the poor electron modelgenerates some deviation in the results. SPIS simulations and comparison with testing data suggest that Hallisoutput is not enough to properly match the experimental measurements, especially regarding the ion angledistribution function. According to Hallis, such distribution is too narrow compared to the observed plasmaplume. This problem is found to be caused by the small simulation domain of Hallis. Hence, although couplingof the software is easy, more functionalities of Hallis would allow for a better study and more accurate results.
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Behlke, Rico. „Dissipation at the Earth's Quasi-Parallel Bow Shock“. Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Universitetsbiblioteket [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6123.
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