<p>The Earth's bow shock is a boundary where the solar wind becomes decelerated from supersonic to subsonic speed before being deflected around the Earth. This thesis presents measurements by the Cluster spacecraft upstream and at the Earth's quasi-parallel bow shock where the angle between the upstream magnetic field and the bow shock normal is less than 45 degrees. An intrinsic feature of quasi-parallel shocks is the ability of ions, that are reflected off the shock in a specular manner, to propagate far upstream and to interact with the incident solar wind. This leads to the generation of a variety of plasma waves, e.g., Ultra-Low Frequency (ULF) waves, which in their turn interact with the different ion populations. Some of the ULF waves are thought to steepen into so-called Short Large-Amplitude Magnetic Structures (SLAMS). </p><p>This thesis studies the impact of SLAMS on the incident solar wind. SLAMS are thought to play an important role in terms of 1) returning shock-reflected ions back to the shock where they can eventually contribute to downstream thermalisation and 2) local pre-dissipation of the solar wind. </p><p>The first electric field measurements of SLAMS showed a strong electric field rotation over SLAMS in association with the rotation of the magnetic field. This often leads to a local change from quasi-parallel to quasi-perpendicular conditions. In addition, short-scale electric field features were observed, e.g., spiky electric field structures associated with the leading edge of SLAMS and solitary electric field structures on Debye length scales, which are suggested to represent ion phase space holes. </p><p>Using the abilitiy of the four Cluster satellites to obtain propagation vectors of SLAMS and the high-resolution electric field measurements, the electric potential over SLAMS was studied. These structures are associated with a significant potential on the order of a few hundred to thousand Volt. Comparing these findings with data from the ion spectrometer, it was found that the bulk flow is locally significantly decelerated and moderately deflected and heated. In addition, SLAMS reflect incident ions on both the leading and trailing edge. The flux of so-called gyrating ions show a clear maximum in association with SLAMS. This indicates that SLAMS indeed play an important role for pre-dissipation of the solar wind upstream of the shock.</p>

Waves are a fundamental feature in many parts of physics, since they transport energy without transporting matter. This is the case also in space physics. Waves are responsible for energy transport both between different parts of space and between different particles in the space plasma. They are also useful for diagnostics of the space plasma itself. The present thesis considers two different parts of the large subject of space plasma waves: Lower hybrid cavities (LHCs) and simple-pole particle distribution functions. The LHCs are localised density depletions that have been observed by several spacecraft. They have increased wave activity in the lower hybrid frequency range, and was previously found on altitudes up to 1750 km. New observations by the Viking and Cluster satellites show that they are common magnetospheric features, at least up to an altitude of 35,000 km. Theoretical results, assuming a cylindrically symmetric density depletion, show that even though the density depletion may decrease slowly with increasing radial distance, and thus be essentially infinite in extent, there is a maximum distance within which a trapped mode, with given wave number kz parallel to the geomagnetic field, may propagate. Furthermore, there is a local relation between the plasma density gradient and the lowest possible frequency that the trapped waves can have, for any monotonic density and given kz. The combined theoretical and observational results indicate that the length of the cavities is larger than the width by a factor of at least 200. Simple-pole particle distribution functions are introduced because they can model high velocity tails of the particle distribution in a way that is not possible to do with Maxwellian distribution functions. These distributions also simplify the calculations. This gives new possibilities for the physical understanding, as well as the numerical calculations, of the dispersion relations of real space plasmas. The dispersion relations of plasmas described by simple-pole distributions are examined, both for unmagnetised and for magnetised plasmas. These examples show how particle populations with the same density and mean particle energy, but with somewhat different distribution functions, have different wave propagation properties that should be observable by existing spacecraft.

The Earth's bow shock is a boundary where the solar wind becomes decelerated from supersonic to subsonic speed before being deflected around the Earth. This thesis presents measurements by the Cluster spacecraft upstream and at the Earth's quasi-parallel bow shock where the angle between the upstream magnetic field and the bow shock normal is less than 45 degrees. An intrinsic feature of quasi-parallel shocks is the ability of ions, that are reflected off the shock in a specular manner, to propagate far upstream and to interact with the incident solar wind. This leads to the generation of a variety of plasma waves, e.g., Ultra-Low Frequency (ULF) waves, which in their turn interact with the different ion populations. Some of the ULF waves are thought to steepen into so-called Short Large-Amplitude Magnetic Structures (SLAMS). This thesis studies the impact of SLAMS on the incident solar wind. SLAMS are thought to play an important role in terms of 1) returning shock-reflected ions back to the shock where they can eventually contribute to downstream thermalisation and 2) local pre-dissipation of the solar wind. The first electric field measurements of SLAMS showed a strong electric field rotation over SLAMS in association with the rotation of the magnetic field. This often leads to a local change from quasi-parallel to quasi-perpendicular conditions. In addition, short-scale electric field features were observed, e.g., spiky electric field structures associated with the leading edge of SLAMS and solitary electric field structures on Debye length scales, which are suggested to represent ion phase space holes. Using the abilitiy of the four Cluster satellites to obtain propagation vectors of SLAMS and the high-resolution electric field measurements, the electric potential over SLAMS was studied. These structures are associated with a significant potential on the order of a few hundred to thousand Volt. Comparing these findings with data from the ion spectrometer, it was found that the bulk flow is locally significantly decelerated and moderately deflected and heated. In addition, SLAMS reflect incident ions on both the leading and trailing edge. The flux of so-called gyrating ions show a clear maximum in association with SLAMS. This indicates that SLAMS indeed play an important role for pre-dissipation of the solar wind upstream of the shock.

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. / QC 20101019

This thesis investigates the performance and improves a double color interferometer setup, absolutely calibrates a line radiation Balmer H-alpha measurement setup, and uses measurements from both setups to estimate the particle confinement time of a plasma. The double colour interferometer at the magnetic confinement plasma device EXTRAP T2R measures the line integrated electron density of the plasma. Electron density is an important parameter in fusion plasma diagnostics but the interferometer at EXTRAP T2R have had several problems. The interferometer setup was changed as follows: A piezo phase shifter was added, the beam expander was adjusted with the help of thermal image plates, and the electronics setup was rewired to remove interferences. The setup for Balmer H-alpha line radiation measurements was calibrated and characterized. The particle confinement time was estimated using Abel inversion to produce radial profiles of electron density, electron temperature and H-alpha irradiance. The interferometer upgrades did not solve all the problems, but the electron density measurements are now reliable up to around 10 – 20 ms. Since the interferometer only has one channel the electron density profile could not be determined reliably. However, the particle confinement time was estimated for two possible electron density profiles and the results agree with previous studies. / Fusionsvetenskap strävar efter att producera en ny, effektiv energikälla. I och med den ökande energikonsumtionen får fusionsvetenskap en allt viktigare roll i samhället. Kärnfusion har stor potential som energikälla, men att utvinna dess energi kommer med lika stora tekniska utmaningar. I det här projektet tacklas en av dessa utmaningar; att mäta elektrontätheten och joniseringshastigheten i ett plasma. Detta utfördes på EXTRAP T2R, ett magnetiskt inneslutningssystem för plasma på Alfvén laboratoriet, Kungliga Tekniska högskolan, Stockholm. Projektet behandlar två olika mätinstrument: En interferometer som mäter elektrontätheten i plasmat och en H-alphaexperimentuppställning som mäter joniseringshastigheten i plasmat. Interferometern har uppgraderats och justerats för att ge mer tillförlitliga mätningar. Den behöver förbättras ytterligare men kan ger nu tillförlitliga täthetsmätningar i början av plasma-skott. H$\alpha$-experimentuppställningen har karakteriserats och kalibrerats. Genom att mäta elektrontätheten och joniseringshastigheten kan partikelinneslutningstiden uppskattas. Partikelinneslutningstiden är den genomsnittliga tiden innan en partikel lämnar plasmat via en av många processer. Denna uppskattning baserades på två möjliga täthetsprofiler i plasmat eftersom en fullständig mätning skulle kräva flera interferometrar. Trots detta så stämmer uppskattningen väl överens med tidigare studier.

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.

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 &lt; 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.

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.

Recent missions to solar system comets, such as ESA's Rosetta mission, raise interest for models and descriptions of their plasma environment. The interaction with various space phenomena such as stellar wind make the construction of an analytical description difficult. Instead, a simplified view of the comet environment is considered where the effects of magnetism and departures from radial symmetry are neglected. This is done in an effort to construct an approximation of the comet plasma behaviour later to be compared against observational accounts to find which plasma features are dependent on more complex phenomena and which plasma features arise as a result of the simpler comet view. Several attempts are made to construct an analytical description of comet plasma as based on the description within another branch of plasma physics: fusion. Previous work regarding the vacuum expansion of plasma after a stationary target is rapidly ablated via high-intensity lasers appears promising for adaptation to the comet environment. Before the comet environment can be considered the different natures of the two problems have to be considered. For example, the comet case is a stationary expansion problem as opposed to fast-ignition fusion where the expansion is treated as an initial value problem. Having accounted for the problems' inherent differences, a few methods are proposed to convert solutions of lab fusion distribution functions to the comet case. Additionally, a numerical approach to calculate the distribution function of comet electrons is presented employing ergodic invariance. Lastly, a toy-model simulation of the timescale for variations in the potential show that the error in the ergodic invariance may in practice have a faster convergent timescale dependence than theoretical bounds suggest. Optimistically, this suggest the possibility of future use in numerical attempts at modelling comet plasma.

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.