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.

Stellar wind bow shocks are structures created when stellar winds with supersonic relative velocities interact with the local interstellar medium (ISM). They can be studied to understand the properties of stars as well as the ISM. Since bow shocks are asymmetric, light becomes polarized by scattering in the regions of enhanced density they create. We use a Monte Carlo radiative transfer code calle SLIP to simulate the polarization signatures produced by both resolved and unresolved bow shocks with analytically derived shapes and density structures. When electron scattering is the polarizing mechanism, we find that optical depth plays an important role in the polarization signatures. While results for low optical depths reproduce theoretical predictions, higher optical depths produce higher polarization and position angle rotations at specific viewing angles. This is due to the geometrical properties of the bow shock along with multiple scattering effects. For dust scattering, we find that the polarization signature is strongly affected by wavelength, dust size, dust composition, and viewing angle. Depending on the viewing angle, the polarization magnitude may increase or decrease as a function of wavelength. We will present results from these simulations and preliminary comparisons with observational data.

The bow shock is formed due to the abrupt deceleration of the supersonic solar wind in front of the terrestrial magnetic field. The solar wind plasma and the Interplanetary Magnetic Field (IMF) are both compressed across the shock, and according to Ampère's law a current thus flows on the bow shock at all times. The Bow Shock Current (BSC) is suggested to play an important role in solar wind-magnetosphere coupling, but there is still an open debate about its closure path. For predominantly east-west IMF, the BSC has been suggested to close to Earth's high latitude ionosphere as Field-Aligned Currents (FACs). Since the bow shock is magnetically connected to the solar wind, it must do so via open field lines through the magnetosheath. For southwards IMF with a significant east-west component, the R0 FAC flows into the ionosphere in one hemisphere, and out of it in the other. The R0 current flows on open field lines, and is thus a potential candidate to close the BSC. While a few studies have already found evidence in favour of this idea, the majority have been based on simulations. Additional observational evidence is required to confirm these findings. We used OMNI data for the IMF at the bow shock, and Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data for the FACs, to make simultaneous observations of the IMF at the bow shock and the northern hemisphere FACs, including the R0 current. We successfully identified 15 events of southwards but predominantly east-west IMF (Bz<0, |By|>|Bz|) at the bow shock, for which the northern hemisphere R0 current could be observed both in the AMPERE and DMSP data. In each of these events, the R0 current was of the correct polarity to connect to the BSC. Moreover, using Defense Meteorological Satellite Program (DMSP) and Super Dual Auroral Radar Network (SuperDARN) data, we were able to verify that part of the R0 current was flowing on open field lines. Collectively, the 15 events presented here constitute an argument in favour of at least a partial BSC closure to Earth's high latitude ionosphere as R0 FACs, for predominantly east-west IMF. Additional investigation is required to reveal the details of BSC closure.

Une nouvelle approche, en utilisant un 3D code électromagnétique (PIC), est présentée pour étudier la sensibilité de la magnétosphère de la terre à la variabilité du vent solaire. Commençant par un vent solaire empiétant sur une terre magnétisée, le temps a été laissé au système ainsi une structure d'état d'équilibre de la magnétosphère a été atteinte. Une perturbation impulsive a été appliquée au système par changeant la vitesse du vent solaire pour simuler une dépression en sa pression dynamique, pour zéro, au sud et du nord du champ magnétique interplanétaire(IMF). La perturbation appliquée, un effet de trou d'air qui pourrait être décrit comme espace ~15Re est formé pour tous les cas d'état de IMF. Dès que le trou d'air a frappé le choc d'arc initial de la magnétosphère régulière, une reconnexion entre le champ magnétique de la terre et le IMF sud a été notée à la coté jour magnétopause(MP). Pendant la phase d'expansion du système, la frontière externe de la coté jour du MP a enfoncé quand IMF=0, et pourtant elle sa forme de balle quand un IMF au sud et nordique étaient inclus. La relaxation de temps du MP pour les trois cas de IMF a été étudiée. Le code est alors appliqué pour étudier l'événement d'Halloween de l'octobre 2003. Notre simulation a produit un nouveau genre de trou d'air, un espace raréfié qui a été produit après un gradient fort en IMF d'empiétement. Un tel dispositif est tout à fait semblable aux anomalies chaudes observées d'écoulement et peut avoir la même origine

The solar wind is a supersonic flow of protons and electrons emitted in all directions from the sun. As the supersonic solar wind encounters Earth’s magnetic field, it creates the Earth’s bow shock, which increases the kinetic entropy of electrons passing through it. In this study, the aim is to analyze shock crossings of Earth’s bow shock in order to draw conclusions of which shock parameters that are important forkinetic entropy generation. Due to knowledge gained from an earlier study by M. Lindberg et al. [1], the shock crossings of interest in this study are quasi-perpendicular shocks with a low electron plasma beta. The data used is measured with the NASA MMS spacecraft and accessed through IRF Uppsala. As a result,a database with 13 shock crossings was created and the entropy change was related to, among other parameters, temperature and density change, shock angle, Alfv´en Mach number, ion ram pressure and upstream magnetic field. We found that a highAlfv´en Mach number related nearly proportionally to a large change in electron entropy for low electron plasma beta quasiperpendicularcollisionless shock crossing. / Solvinden består av protoner och elektroner som emitteras ut från solen i alla riktningar med enorma hastigheter. När dessa partiklar, med en hastighet som överstiger signalhastigheten, träffar Jordens magnetfält uppstår Jordens bågchock. Bågchocken ökar den kinetiska entropin hos elektroner som färdas genom den. För den här studien är målet att analysera chockkorsningar vid Jordens bågchock för att kunna dra slutsatser om vilka chockparametrar som är viktiga för generering av kinetisk entropi. Till följd av en tidigare studie av M. Lindberg et al. [1] är det endast kvasi-vinkelräta chockkorsningar med ett lågt plasma beta som denna studie avser. Den uppmätta datan erhålls från NASAs MMS satelliter och kan nås genom IRF Uppsala. Resultatet blev en databas med 13 chocker där entropiförändringen plottades mot bl. a. temperaturoch densitetsändring, chockvinkel, Alfve´n Machtal, jontrycket och magnetfältet uppströms. Det upptäcktes då att ett högt Alfve´n Mach-tal indikerade på en stor entropiökning hos kollisionslösa, kvasi-vinkelräta chockkorsningar med låga elektronplasmabeta. / Kandidatexjobb i elektroteknik 2022, KTH, Stockholm

Shock-shock interactions are among the most basic gas-dynamic problem, and are almost unavoidable in any high speed light, where shock waves generating from different sources crosses each other paths. These interactions when present very close to the solid surface lead to very high pressure and thermal loads on the surface. The related practical problem is that experienced at the cowl lip of a scramjet engine, where the interfering shock waves leads to high heat transfer rates which may also lead to the damage of the material. The classification by Edney (1968) on the shock-shock interaction patterns based on the visualization has since then served the basis for such studies. Though the problem of high heating on the surface in the vicinity of the shock-shock interactions has been studied at length at supersonic Mach numbers, the study on the topic at the hypersonic Mach numbers is little sparse. Even in the studies at hypersonic Mach numbers, the high speeds are not simulated, which is the measure of the kinetic energy of the ow. Very few experimental studies have addressed this problem by simulating the energy content of the ow. Also, some of the numerical studies on the shock-shock interactions suggest the presence of unsteadiness in the shock-shock interaction patterns as observed by Edney (1968), though this observation is not made very clearly in the experimental studies undertaken so far. In the present study, experiments are carried out in a conventional shock tunnel at Mach number of 5.62 (total enthalpy of 1.07 MJ/kg; freestream velocity of 1361 m/s), with the objective of mapping the surface pressure distribution and surface convective heat transfer rate distribution on the hemispherical body in the presence of the shock-shock interactions. A shock generator which is basically a wedge of angle = 25 , is placed at some dis-dance in front of the hemispherical body such that the planar oblique shock wave from the shock generator hits the bow shock wave in front of the hemi-spherical body. The relative distance between the wedge tip and the nose of the hemispherical body is allowed to change in di erent experiments to capture the whole realm of shock-shock interaction by making the planar oblique shock wave interact with the bow shock wave at different locations along its trajectory. The study results in a bulk of data for the surface pressure and heat transfer rates which were obtained by placing 5 kulites pressure transducers, 1 PCB pressure transducer and 21 platinum thin lm gauges along the surface of the hemispherical body in a plane normal to the freestream velocity direction. Along with the measurement of the surface pressure and the surface heat transfer rates, the schlieren visualization is carried out to capture the shock waves, expansion fans, slip lines, present in a certain shock-shock interaction pattern and the measured values were correlated with the captured schlieren images to evaluate the ow build up and steady and useful test time thereby helping in understanding the ow physics in the presence of the shock-shock interactions. From the present study it has been observed that in the presence of Edney Type-I and Edney Type-II interaction, the heat transfer rates on the hemi-spherical body are symmetrical about the centerline of the body, with the peak heating at the centerline which drops towards the shoulder. For Edney Type-III, Edney Type-IV, Edney Type-V and Edney Type-VI interaction pattern, the distribution in not symmetrical and shifts in peak heat transfer rates being on the side of the hemispherical from which planar oblique shock wave is incident. Also, it is observed that for the interactions which appear within the sonic circle, Edney Type-III and Edney Type-IV, the heat transfer rates observe an unsteadiness, such that the gauges located close to the interaction region experiencing varying heat transfer rates during the useful test time of the shock tunnel. Few experiments were conducted at Mach 8.36 (total enthalpy of 1.29 MJ/kg; freestream velocity of 1555.25 m/s) and Mach 10.14 (total enthalpy of 2.67 MJ/kg; freestream velocity of 2258.51 m/s) for the con gurations representing Edney Type-III interaction pattern to further evaluate the unsteady nature observed at Mach 5.62 ows. The unsteadiness was evident in both the cases. It is realized that the short test times in the shock tunnels pose a constraint in the study of unsteady flow fields, and the use of tailored mode operation of shock tunnel can alleviate this constraint. Also, limited number of experiments in the present study, which are carried out in a Free Piston Shock Tunnel, helps to understand the need to conduct such study in high enthalpy test conditions.

Collisionless shocks are widespread phenomena in the universe, and understanding the mechanisms behind their energy dissipation, with a rare number of collisions between particles, remains a significant unresolved question. The Earth’s bow shock provides an excellent opportunity to study this phenomena in situ. For high Mach number shocks, the shock cannot be sustained without partial reflection of the incoming ions. At higher Mach numbers, the shock surface starts to exhibit non-stationary behaviours, meaning that the shock surface starts evolving. One such behaviour is known as shock reformation, where a new shock forms upstream of an existing one. This study aims to investigate shock reformation using data obtained from NASA’s MMS mission, which offers precise measurements with high spatial and temporal resolutions through its constellation of four spacecraft. Using the MMS shocks database (Lalti et al., 2022), the gradient of the magnetic field magnitude is computed to infer non-stationary behaviour and identify potential instances of shock reformation and other shock behaviours. Through the analysis of the MMS measurements, some insight into the non-stationary characteristics of shocks is obtained using the gradient of the magnetic field. However, further analysis is needed in order to refine the method of identifying non-stationary behaviour of shocks, for future applications. / Kollisionsfria chocker är ett vanligt fenomen som förekommer i universum, och att förstå hur energidissipation inträffar i chocker med ett fåtal kollisioner mellan partikar är ett olöst problem. Jordens bogchock utger en bra möjlighet att studera detta på plats med mätningar från rymdfarkoster. Detta projekt försöker studera delar av jordens bogchock och undersöka dess dynamic. För chocker med högt machtal, måste en del av jonerna från solvinden reflekteras för att chocken ska skunna upprätthållas. Vid högre machtal kan chockytan visa icke-stationära beteenden, vilket innebär att den börjar förändras. Ett exempel på sådant beteende är chockreformation, där en ny chock formas framför en befintlig chock. Denna studie har som mål att undersöka chockreformation med hjälp av data som erhållits från NASA:s MMS-uppdrag, vilket erbjuder precisa mätningar med hög rumslig och tidsmässig upplösning genom sin konstellation av fyra rymdfarkoster. Genom användning av MMS-shockdatabasen (Lalti et al., 2022) beräknades gradienten av magnetfältets magnitud för att härleda icke-stationärt beteende och identifierade potentiella fall av chockreformation och andra beteenden. Genom analys av MMS-mätningarna erhölls viss insikt i de icke-stationära egenskaperna hos chocker med hjälp av gradienten av magnetfältet, men ytterligare analys krävs för att förbättra metoden för framtida tillämpningar.

The interaction between the solar wind and Earth’s magnetic field creates the Earth’s bow shock. It is an ideal region for space probes like MMS, THEMIS or Clusters to study the collisionless shock phenomenon in space plasma. More specifically the project focuses on the topic of wave-particle interactions in the space plasma environment, which allows irreversible energy dissipation and entropy production at the event of a shock when there are a lack of collisions between particles. Research is still ongoing regarding the topic of wave-particle interactions in plasma and this project aims to contribute to our understanding of this topic. To do this, measurement data of a total of 249 shock crossing events from NASA’s Magnetospheric Multiscale (MMS) mission are used to conduct a statistical study. The study aims to analyse the correlation between the electric- and magnetic field measured close to shock-crossing events, and their respective macroscopic shock parameters in different shock regions, and at three different frequency bands for the attempt of further our understanding of the dynamics of collisionless shocks. Through scatter plots, negative correlations are found between both the electric- and magnetic field power, and the different macroscopic shock parameters at various shock regions and at various frequency ranges. This leads to the suggestion of potential dependencies between the occurrence of electrostatic and electromagnetic waves and those shock parameters. However, there is still room for improvement of the statistical method used for the correlation studies. / Interaktionen mellan solvinden och jordens magnetfält skapar jordens bogchock. Det är en idealisk region för rymdfarkoster som MMS, THEMIS eller Clusters att studera kollisionsfria chocker i rymdplasma. Mer specifikt fokuserar detta projekt på vågpartikelinteraktioner i rymdplasma, vilket möjliggör irreversibel energidissipation och entropiproduktion vid en chock när det råder brist på kollisioner mellan partiklar. Forskning pågår fortfarande inom området vågpartikelinteraktioner i plasma och detta projekt syftar till att bidra till vår förståelse av ämnet. För att göra detta används mätdata från totalt 249 chocker från NASA:s Magnetospheric Multiscale (MMS)-uppdrag för att genomföra en statistisk studie. Studien syftar till att analysera korrelationen mellan de elektriska och magnetiska fälten som mäts nära chocker och deras respektive makroskopiska chockparametrar i olika chockregioner och vid tre olika frekvensband, i ett försök att vidare förstå dynamiken hos kollisionslösa chocker. Genom spridningsdiagram hittas negativa korrelationer både mellan de elektriska och magnetiska fältstyrkan och de olika makroskopiska chockparametrarna vid olika chockregioner och frekvensband. Detta leder till förslaget om potentiella samband mellan förekomsten av elektrostatiska och elektromagnetiska vågor och dessa chockparametrarna. Det finns dock fortfarande utrymme för förbättring av den statistiska metoden som används för korrelationsstudierna.