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Browsing by master's degree program "Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma (Particle Physics and Astrophysical Sciences)"

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  • Leppälä, Ari (2023)
    The thesis presents results for simulating an orbit determination process with least-squares fitting (LSF) of synthetic observations for non-cooperative satellites. Five satellites with different masses, sizes, and orbital inclinations are orbiting in Low Earth Orbit (LEO). Three different inclinations (53°, 85°, and 98°) are simulated using Orekit (Orbit Extrapolation Kit), an open-source astro- dynamical software library. Satellites are observed with four hypothetical radars. The thesis is a feasibility study. It addresses four main research questions: How reliable are predictions of the satellite position for 24 h, 48 h, 72 h, and 96 h after the first detection when using the observations from four consecutive overpasses and the LSF-estimated orbit? How accurately the Keplerian or- bital elements be modelled with LSF? How does the satellite’s mass affect the results’ reliability, and can the satellite’s drag, reflection, and absorption coefficients be estimated with Orekit? The simulation utilizes the Orekit software library in a Python environment, and the simulation incorporates several perturbing forces: Earth’s gravitational potential model EGM2008, an atmo- sphere model with space weather data NRLMSISE-00, tidal forces, and point-like masses of the Sun and the Moon. Synthetic observations were simulated by assuming Gaussian distribution for the uncertainties of the measurements. The initial estimate of the orbit is done with Gooding’s method, and the Levenberg–Marquardt algorithm is used for the LSF. The propagation of the actual satellite for 96 h was compared with the 96-h propagation of estimated orbital elements and with a satellite with estimated size and mass. Simulations were conducted with overestimated and underestimated initial masses. Each combination of a satellite, an observing radar, and an orbit was repeated 100 times. The results for reliability were promising. The likelihood of the sky-plane projected error of the position staying under the 3.0-km for up to 3 days was found to be 90% for two of the simulated radars, but only in cases of orbits having an inclination of 53°. Two other orbits had less successful results, but simulations revealed a potential increase in reliability by adjusting the observational strategy. There was a small bias in the results hinting that underestimating the satellite’s mass could result in less accurate orbit estimates than overestimating it. The satellite’s drag, absorption, and reflection coefficients could not be obtained due to the short observational time intervals simulated.
  • Yu, Sicheng (2024)
    Aims. In this thesis, we review the history in studying the evolution path of magnetic white dwarfs and explain the longstanding questions and debates over their magnetic origin. We intend to find magnetic white dwarfs in various forms (isolated or with companions) from the spectral database of LAMOST DR7 to complement the current magnetic white dwarf catalogue. We then move on to compare our results with some commonly accepted scenarios regarding their magnetic origin. Methods. Low-resolution spectra is the main source in this study, we intend to locate signs of Zeeman-splitting in spectra of isolated white dwarfs, measure separations of substructures due to Zeeman-splitting and estimate magnetic field strength. Magnetic white dwarfs in binary or multiple systems are found by seeking signs of cyclotron radiation due to mass transfer and particle movement in magnetic fields. Photometric survey from Transiting Exoplanet Survey Satellite (TESS) was used to fold periodic light curves for targets of interest, in order to further study the nature of our candidates, especially the ones that are believed to coexist with companions. Results. We identified 31 isolated magnetic white dwarfs in the LAMOST DR7 database by the discovery of Zeeman-splitting components. Their estimated magnetic field strength ranging from below 1 mega gauss (MG) to a scale ten times larger. Two Polars/Intermediate Polars were found with both Zeeman-splitting components and broad Balmer emissions usually seen in cataclysmic variables. We also discovered two candidates of detached magnetic binaries. These systems are believed to be the progenitors of polars or intermediate polars. Despite their rarity, these candidates serve as vital hints in clarifying the ongoing debates concerning the magnetic field origins in white dwarfs.
  • Khalil, Hossam (2024)
    Understanding the baryonic physics on the galaxy group level is a prerequisite for cosmological studies of large-scale structures. While the majority of baryons in galaxy groups are located in their intragroup medium (IGrM), one poorly understood aspect of galaxy groups is their hot intragroup X-ray emission. In this thesis, a new all-sky catalogue of X-ray detected groups (AXES-2MRS) is presented, based on the identification of large X-ray sources discovered in the ROSAT All-Sky Survey (RASS) with the 2MRS Bayesian Group Catalogue. In addition to X-ray luminosity coming from the shallow survey data of RASS, detailed X-ray properties of the groups have been obtained by matching the catalogue to archival X-ray observations conducted by XMM-Newton. The relationship between X-ray and optical properties of AXES-2MRS is explored through scaling relations, namely $\sigma_{v}-L_{X}$, $\sigma_{v}-kT$, $\sigma_{v}-M$, and $kT-L_{X}$ which denote (velocity dispersion vs. X-ray luminosity), (velocity dispersion vs. X-ray temperature), (velocity dispersion vs. hydrostatic mass), (X-ray temperature vs. X-ray luminosity), respectively. The scaling relations reveal similarities between our low-redshift catalogue and high-redshift studies implying that our knowledge about galaxy groups is redshift-invariant. This study enhances the representation of the underexplored low-z, low-luminosity galaxy groups, particularly in low-mass systems ($< 10^{14} M_{\odot}$). This enhances the completeness of galaxy group catalogs, addressing the persistent issue of missing faint, low-mass systems. Moreover, previous catalogues, based on detecting the peak of the X-ray emission preferentially sample the high dark matter (DM) halo-concentration groups, while AXES-2MRS includes many low DM halo-concentration groups.
  • Arvo, Jukka (2023)
    Tutkielmani käsittelee kullan pysyvän isotoopin Au-197 tuottamista neutroniaktivaatiolla luonnollisesta elohopeanäytteestä. Kokeen kannalta pääasiallinen reaktiomenetelmä oli Hg-196 neutronikaappaus. Kyseinen transmutaatio suoritettiin myös kokeellisesti. Elohopeaa sisältävänä näytteenä käytettiin Ardentin valmistamia Futura Standard -hammasamalgaamikapseleita. Turvallisuussyistä kapselit olivat koejärjestelyssä alkuperäisessä purkissaan. Kapseleita oli kaikkiaan 50 kappaletta, ja jokaisessa oli 400 mg elohopeaa. Yhteensä näytteessä oli siis 20 grammaa elohopeaa. Näytettä säteilytettiin STUKin tiloissa 14 vuorokauden ajan kolmen AmBe-neutronilähteen avulla. Valmistajan ilmoittamat neutronituotot käytetyille lähteille ovat 2.0E+7 n/s, 2.1E+6 n/s ja 6.7E+5 n/s. Lähteiden ilmoitetut aktiivisuudet ovat vastaavasti 333 GBq, 37.0 GBq ja 11.1 GBq. Neutronien hidastamiseen käytettiin HDPE-tankoa. Säteilytyksen jälkeen näytettä mitattiin STUKin gammaspektrometrian laboratorion B6 p-HPGe BE5030 -germaniumilmaisimella, ja kullan synty voitiin todentaa spektristä löytyvien karakterististen gamma- ja röntgenpiikkien avulla. Koejärjestely onnistui, ja työni osoittaa, että kullan pysyvän isotoopin Au-197 valmistaminen luonnollisesta elohopeasta havaittavissa määrin on mahdollista toteuttaa melko yksinkertaisella koejärjestelyllä käyttäen varsin pienitehoisia neutronilähteitä. Varsinaisen kokeen lisäksi käsittelen työssäni myös kullanteon historiaa sekä aiheeseen liittyvää teoriaa.
  • Vuori, Mikko (2023)
    A method for deriving the complex refractive index of a mm-sized single particle in a specific wavelength using laboratory measurements is presented. Laboratory measurements were done using the 4π scatterometer, which measures Mueller matrix elements of a particle suspended in air using acoustic levitation as a function of scattering angle. To obtain the complex refractive index of the particle, measurements were compared to simulations from a newly developed SIRIS4 Fixed Orientation (SIRIS4 FO) geometric optics simulation. The 4π scatterometer is a unique instrument which measures Mueller matrix elements from a particle using linear polarizers and a detector rotating about the particle on a rotational stage. The scatterometer uses an acoustic levitator as a sample holder which provides nondestructive measurements and full orientation control of the sample. To compare the measurement results to simulations, SIRIS4 single-particle geometric optics code was modified to handle particles in a fixed orientation. The original code is able to calculate the Mueller matrix elements for a given 3D model, but averages the results over the orientation of the particle. The modified SIRIS4 FO calculates the Mueller matrix elements over the full solid angle as functions of the two scattering angles, which give the direction of observation of the scattered wave compared to the direction of the incident wave. A 3D model of the shape of the measured particle was constructed using X-ray microtomography, and was translated to SIRIS4 FO. The complex refractive index was obtained with a nonlinear least squares analysis by minimizing the sum of squared residuals between the measurements and simulations with varying refractive index values. Finally, confidence regions were constrained for the results, by estimating the computed residuals between simulations and measurements as the random errors in the nonlinear model.
  • Blomqvist, Sofia (2024)
    The matter in neutron stars exist under extreme conditions, and the cores of these stars harbour densities unreachable in any laboratory setting. Therefore, this unique environment provides an exceptional opportunity to investigate high-density matter, described by the theory of Quantum Chromodynamics (QCD). This thesis centers on the exploration of twin stars, hypothetical compact objects that extend beyond the neutron star sequence. Originating from a first-order phase transition between hadronic matter and quark matter, our focus is on understanding the constraints on these phase transitions and their effect on the observable properties of twin stars. In our investigation of twin stars, we construct a large ensemble of possible equations of state featuring a strong first-order phase transition. We approximate the low- and high-density regions with polytropic form and connect them to chiral effective field theory results at nuclear densities and extrapolated perturbative QCD at high densities. The resulting equations of state are then subjected to astrophysical constraints obtained from high-mass pulsars and gravitational wave detections to verify their compatibility with observations. Within our simple study, we identify two distinct types of twin stars, each providing a clear signature in macroscopic observables. These solutions originate from separate regions in the parameter space, with both regions being relatively small. Twin stars in our approach generally obtain small maximum masses, while the part of the sequence corresponding to neutron stars extends to large radii, indicating that these solutions only marginally pass the astrophysical constraints. Finally, we find that all twin stars obtain sizable cores of quark matter.
  • Lipsanen, Veera (2024)
    The constant outflow of solar wind from the Sun and possible larger structures within it influence the Earth's magnetosphere. These large structures include interplanetary coronal mass ejections (ICMEs) and high speed streams (HSSs). They can contain substructures: fast enough ICMEs can have a turbulent sheath region in front of them, while a HSS can interact with the slower ambient solar wind and form a stream interaction region (SIR). Pc5 Ultra-low frequency (ULF) waves have a frequency range of 2–7 mHz and they are important in energy transfer from solar wind to the magnetosphere and they affect energetic electrons in the radiation belts. ULF waves in the magnetosphere are generated by multiple mechanisms. For example fluctuations in solar wind's dynamic pressure create waves on the dayside, Kelvin-Helmholtz instability, often caused by HSSs, on the magnetopause flanks and substorms on the nightside. This makes ULF waves MLT dependent. For this thesis a new ground-based ULF index that is MLT dependent and has a resolution of 1 min is constructed using the wavelet analysis method. The aim of this thesis is to study how this new index correlates with multiple solar wind parameters, geomagnetic indices and an already existing ULF index during substructures of four events: weak HSS and ICME and strong HSS and ICME. The ULF power is found to peak at the sheath–ejecta boundary during ICMEs and the stream interface during HSSs, primarily driven by dawn and night ULF powers. The AE index is found to correlate with ULF power during all of the events, which indicates that even the non-geoeffective event produces some kind of substorm activity. Solar wind speed is found to correlate well with the ULF power during SIRs. It is important to take into account the MLT dependence of ULF waves since their generation mechanisms are different at different parts of the magnetosphere. In addition, we found that the ULF power in the four MLT sectors can behave differently at the same moment.
  • Al-Adulrazzaq, Aula (2023)
    Dark matter direct detection experiments still have found no evidence of the dark matter WIMPs. The search has therefore been expanded for lighter dark matter candidates. Light dark matter is nearly invisible to current detectors through the elastic nuclear recoils. This thesis is meant to provide understanding on the inelastic atomic scatterings, which are one good way to detect dark matter particles with mχ ∼ GeV. In this thesis we consider spin-independent scatterings. Inelastic scatterings are based on the fact that in an atom, electrons do not follow the motion of the recoil nucleus immediately, but instead it takes time. This results in a small probability of observable ionization or excitation of the atom. This is known as the Migdal effect. We will first study the theoretical framework of dark matter-nucleus scatterings, showing how to get the event rate and how it is factorized into the astrophysical, the particle physics and the target response part. Then we will move to the inelastic processes, Migdal and Bremsstrahlung effects, deriving their event rates. In the first, we try to detect ionized electrons. The latter one, the Bremsstrahlung, is a similar process to the Migdal, but there we try to detect photons emitted from the de-excitations of atoms excited in the inelastic recoils. We will also look into the Migdal in semiconductors. Because of the smaller gap for electron excitations in crystals, we find that the rate for the Migdal effect is much higher in semiconductors than in atomic targets, thus allowing the search for even lighter dark matter particles. The rate can be expressed in terms of the energy loss function of the target material.
  • Mozejko, Arik (2023)
    Dark matter (DM) is introduced and explored in a holistic perspective. Topics include observational evidence, various DM properties, potential candidates, and the tenets of indirect versus direct DM detection. Then an emphasis is placed on understanding the cryogenic detection of weakly interacting massive particles, with explicit connection to phonon-based detection of DM. The importance of improving methods of DM direct detection are emphasised, with specifically the usage of molecular dynamics simulations as an avenue of studying defect creation in cryogenic detector materials. Previous investigations into this area are reviewed and expanded upon through novel experimentation into how defect properties vary when changing thermal motion of the crystal lattice. This experimentation is conducted via the usage of molecular dynamics simulations on sapphire (Al2O3) as a DM direct detection material, and it is found that while atomic velocity does not impact the overall emergent defect structure, it does have an impact on the energy lost in these defects. Changing the temperature of the lattice produces the expected results, generating greater variance in both defect band structure as well as average energy loss.
  • Gibson, Natalie (2023)
    The search for a profound connection between gravity and quantum mechanics has been a longstanding goal in theoretical physics. One such connection is known as the holographic principle, which suggests that the dynamics within a given region of spacetime can be fully described on its boundary surface. This concept led to the realization that string theory provides a lower-dimensional description that encapsulates essential aspects of spacetime. While the "AdS/CFT correspondence" exemplifies the success of this holographic theory, it was discovered soon after that the Universe has a positive cosmological constant, Λ. This immediately sparked interest in a potential correspondence centered around de Sitter (dS) space, which is also characterized by a positive cosmological constant. This thesis comprehensively explores the de Sitter/Conformal Field Theory (dS/CFT) correspondence from various perspectives, along with the unique challenges posed by the distinct nature of dS space. The original dS/CFT duality proposes that a two-dimensional Conformal Field Theory resides on the boundary of three-dimensional asymptotic dS space. However, the definition and interpretation of physical observables within the dS/CFT framework remain open questions. Therefore, the discussions in this thesis not only cover the original dS/CFT conjecture, but also extend into more recent advancements in the field. These advancements include a higher-spin dS/CFT duality, the relationship between string theory and dS space, and the intriguing proposal of an "elliptical" dS space. While the dS/CFT correspondence is still far from being well-defined, there have been extensive efforts devoted to shedding light on its intricate framework and exploring its potential applications. As the Universe may be evolving towards an approximately de Sitter phase, understanding the dS/CFT correspondence offers a unique opportunity for gaining fresh insights into the link between gravity and quantum field theory.
  • Häkkinen, Jenni (2024)
    Gravitational waves from cosmological phase transitions are a promising probe of the early universe. Many theories beyond the Standard Model predict the early universe to have undergone a cosmological first-order phase transition at the electroweak scale. This transition would have produced gravitational waves potentially detectable with the future space-based detector Laser Interferometer Space Antenna (LISA). We study the gravitational wave power spectrum generated by sound waves, which are a dominant source of gravitational waves from first-order phase transitions. We compare two methods for calculating the sound wave power spectrum: a simulation-motivated broken power-law fit of the shape of the spectrum, and a wider theoretical framework called the Sound Shell Model, which includes hydrodynamic calculations of the phase transition. We present an implementation of the Sound Shell Model into the PTPlot tool, which is currently based on the broken power-law fit. With PTPlot, we calculate the signal-to-noise ratios of LISA for the sound wave power spectrum of each method. The signal-to-noise ratio allows us to estimate the detectability of gravitational wave signals with LISA. We analyse how the detectability of certain particle physics models changes between the two different methods. Our results show that the Sound Shell Model has a potentially significant impact on the signal-to-noise ratio predictions, but it does not uniformly improve or worsen the detectability of the gravitational wave signals compared to the broken power law. The code implementation is overall successful and lays the foundation for an updated release of PTPlot and future work within this topic.
  • Takala, Saara (2024)
    Ultra-low frequency (ULF) waves in the Pc4-Pc5, 2 – 25 mHz range have been observed to accelerate trapped 1 – 10 MeV electrons in the Earth’s radiation belts. This acceleration can lead to particle losses and injections that occur on timescales comparable to the particle drift periods. Current models rely on diffusion equations written in terms of Fokker-Planck equations and are not suitable for describing fast temporal variations in the distribution function. This thesis is a study of fast transport of equatorially trapped electrons in the radiation belts. We look at solutions for the time evolution of the linear part of the perturbed distribution function using both analytical and numerical methods. Based on this work we build a simple model of fast transport in the radiation belts using a spectral PDE framework called Dedalus. The resulting program is a computationally inexpensive, simple approach to modelling drift-periodic signatures on fast timescales. In this study we investigate the behavior of the distribution function in three systems: a simple system without a wave term, and systems with a single non-resonant and resonant ULF wave. The wave solutions are evaluated with magnetic field perturbations of different magnitudes. The Earth’s magnetic field is modelled with the Mead field. The numerical solution of the perturbed differential equation is studied for relativistic equatorially trapped electrons. Phase-mixing is found to happen regardless of field fluctuations or resonance. The non-resonant wave solution shows time-delayed, spatially localized structures in the equatorial plane forming in the presence of large magnetic field fluctuations. These transients are also seen in the analytical solution and provide a new theoretical explanation for the ubiquitous observation of drift echoes in the inner and outer radiation belts of the Earth (Li et al., 2024).
  • Kostamo, Iida (2023)
    In this thesis, 16 gas-free galaxy merger simulations were run in order to determine the dominant effect that causes the formation of cores, i.e. regions of ”missing light”, in the centers of massive early-type galaxies. The simulations were run on the Mahti supercomputer at the Finnish IT Centre for Science (CSC), using the simulation codes GADGET-3 and KETJU. The merging of galaxies will eventually lead to the merging of their central supermassive black holes (SMBHs). The evolution of a SMBH binary can be divided into three phases: the dynamical friction phase, the three-body interaction phase and gravitational wave (GW) emission phase. After the GW emission phase, the merged SMBH may receive a recoil velocity. To study the effects of these three phases on the formation of the core, three kinds of simulations were run. These include three GADGET runs and thirteen KETJU runs that can be divided into two groups, since eight of the runs had GW recoils enabled. When using only GADGET, the three-body interactions are not modeled due to the softening of gravity. Using KETJU allows for modeling the later evolution of the SMBH binary, including the three-body interaction phase, the GW emission phase and the resulting GW recoil. The initial conditions for the progenitor galaxies were motivated by the galaxy NGC 1600. The same stellar and dark matter profiles were used for each simulated galaxy. Two different SMBH masses were used. One KETJU merger was also run without SMBHs. For the runs with spinning SMBHs, the spin directions and magnitudes were chosen in such a way that different magnitudes of recoil velocity would be achieved. The size of the core can be determined from the brightness profile of the galaxy, assuming a constant mass-to-light ratio. The commonly used core-Sérsic profile was fit to the surface mass density profiles of the merger remnant galaxies. The best-fit parameter values were then used to estimate the sizes of the cores, such as the core radii. The amount of missing mass in the centers of the galaxies, i.e. the mass deficits, were also computed based on the core-Sérsic fits. We found that the mass deficit correlates positively with the mass of the SMBH for all the KETJU runs. Including GW recoils in the simulations was found to increase the mass deficits by roughly the equivalent of one SMBH mass compared to the KETJU runs without the recoils. The formation of cores was the weakest for the GADGET runs, since the cores were created only by the large-scale dynamics. No core was formed in the run without SMBHs, as expected. Thus, we can conclude that SMBHs are essential for the formation of cores in massive early type galaxies, and the largest cores are formed when GW recoils are included in the model.
  • Grön, Julia (2023)
    Mars is a rocky planet in the Solar System, fairly similar to the Earth. It is known for its red color and the theory that there has been liquid water and possibly life in Mars at some point in its history. Mars has been a target of study since space exploration began, with the first fly-by mission occuring in 1965. In the past, Martian climate could have supported a functioning hydrological system and Mars even could have had an ocean. This system would have been similar to the one happening in Antarctica due to cold weather. The Martian atmosphere is mostly composed of carbon dioxide, much thinner and colder compared to the Earth's atmosphere. Methane is a main point of interest due to it being a possible biosignature, a sign of life. Another interesting feature about the atmosphere are the dust devils that contibute to the climate by lifting dust into the air. The Martian soil is composed of rocks and dust containing toxic perchlorates. To colonize Mars, several requirements need to be met: the transportation and funding, designing the settlement efficiently with everything needed and determining how many people are necessary. The risk assessment to both people and equipment needs to be made and taken into account.
  • Garcia Sturba, Sebastian (2023)
    Quantum field theory is often presented without clearly defined mathematical structures, especially in the case of field operators. We discuss axiomatic quantum field theory, where quantum fields and states are defined rigorously using distribution theory, alongside their assumed properties in the form of the Wightman axioms. We present the two key results that come from this construction, namely CPT symmetry and the spin-statistics connection. We then consider the construction of quantum fields in curved spacetime so as to discuss their behaviour in regions of large curvature, such as near black holes. This requires us to redefine fields and states in terms of *-algebras. We then present the GNS reconstruction theorem which allows us to get back the original definitions of these objects in Minkowski spacetime.
  • Luttikhuis, Thijs (2022)
    One of the most noticeable effects of solar–terrestrial physics is the aurora which regularly appears in the polar regions. This polar light is the result of the excitation of atmospheric species by charged particles originating from the solar wind and magnetosphere that enter the Earth’s atmosphere, which are called precipitating particles. We present the first results on auroral proton precipitation into the ionosphere using a global 3-dimensional simulation of near-Earth space plasma with the Vlasiator hybrid-Vlasov model, driven with a southward interplanetary magnetic field and steady solar wind parameters. The hybrid-Vlasov approach describes ions through their velocity distribution function in phase space (3-dimensional ordinary space and 3-dimensional velocity space), while electrons are represented by a massless charge-neutralizing fluid. Vlasiator is a global model describing the whole region of near-Earth space including the Earth’s magnetosphere (whole dayside and part of the magnetotail), the magnetosheath, as well as the foreshock region and some solar wind. The precipitating proton differential number fluxes for this run are determined from the proton phase-space density contained within the bounce loss-cone, which is set at a constant angle of 10 degrees everywhere. To determine the precipitation of particles at ionospheric altitudes (in this case a height of 110 km above the Earth’s surface), we trace magnetic field lines from the ionosphere to the inner boundary of the Vlasiator domain using the Tsyganenko model. With this, we obtain a magnetic local time–geomagnetic latitude map of differential number flux of precipitating protons in 9 energy bins between 0.5 and 50 keV. From the differential number flux, proton integral energy fluxes and mean energies can be obtained. The integral energy fluxes in the Vlasiator run are then compared to data of the Precipitation Electron/Proton Spectrometer (SSJ) instrument of the Defense Meteorological Satellite Program (DMSP) for several satellite overpasses during events with similar solar wind conditions as in the Vlasiator run. The SSJ instrument bins proton energies between 0.03 and 30 keV. Typical values of the total integral energy flux are between 5 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the cusp and between 1 · 10^6 and 3 · 10^7 keV cm−2 s−1 sr−1 in the evening sector for both Vlasiator and DMSP, although DMSP fluxes can locally be up to an order of magnitude higher. Additionally, global precipitation patterns in Vlasiator are compared to Ovation Prime, which is an empirical model based on data from DMSP which can be used to forecast precipitation of auroral electrons and protons. Although Ovation Prime shows a much wider cusp region compared to Vlasiator, both show similar maximum integral energy fluxes around 1 to 2 · 10^7 keV cm−2 s−1 sr−1 in the cusp region, and between 3 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the nightside oval.
  • Kukkola, Antti (2023)
    A stream of charged particles known as the solar wind constantly flows with supersonic speed in our solar system. As the supersonic solar wind encounters Earth's magnetic field, a bow shock forms where the solar wind is compressed, heated and slowed down. Not all ions of the solar wind pass through the shock but rather a portion are reflected back upstream. What happens to the reflected ions depends on the magnetic field geometry of the shock. In the case where the angle between the upstream magnetic field and the shock normal vector is small, the reflected ions follow the magnetic field lines upstream and form a foreshock region. In this case the shock is called quasi-parallel. In the case of a quasi-perpendicular shock, where the angle is large, the reflected ions gyrate back to the shock, accelerated by the convection electric field. Upon returning to the shock, the ions have more energy and either pass through the shock or are reflected again, repeating the process. Ion reflection is important for accelerating ions in shocks. In this work we study the properties and ion reflection of the quasi-perpendicular bow shock in Vlasiator simulations. Vlasiator is a plasma simulation which models the interaction between solar wind and the Earth's magnetic field. The code simulates the dynamics of plasma using a hybrid-Vlasov model, where ions are represented as velocity distribution functions (VDF) and electrons as magnetohydrodynamic fluid. Two Vlasiator runs are used in this work. The ion reflection is studied by analysing VDFs at various points in the quasi-perpendicular shock. The analysis is performed with reflections in multiple different frames. A virtual spacecraft is placed in the simulation to study shock properties and ion dynamics, such as the shock potential and ion reflection efficiency. These are compared to spacecraft observations and other simulations to test how well Vlasiator models the quasi-perpendicular bow shock. We find that the ion reflection follows a model for specular reflection well in all tested frames, especially in the plane perpendicular to the magnetic field. In addition, the study was extended to model second specular reflections which were also observed. We conclude that the ions in Vlasiator simulations are nearly specularly reflected. The properties of the quasi-perpendicular bow shock are found to be in quantitative agreement with spacecraft observations. Ion reflection efficiency is found to match observations well. Shock potential investigations revealed that spacecraft observations may have large uncertainties compared to the real shock potential.
  • Rajala, Aatu (2024)
    Color confinement, the inability for free quarks to exist at normal temperatures and densities, is one of the most important properties of Quantum Chromodynamics, the quantum field theory (QFT) of the strong interaction. A simple representation of confinement can be obtained by considering a static (i.e. infinitely massive) quark-antiquark pair. The potential of the pair contains a term linear with respect to the separation. Therefore, separating the pair would require infinite energy meaning that the quarks are confined. The static potential can be related to the expectation value of a QFT operator called the Wilson loop. In the non-perturbative large coupling regime lattice field theory can be applied to estimate expectation values of observables. The idea of lattice field theory is to replace continuous spacetime with a lattice, where the fields are defined on the sites and links. The discretization of spacetime allows evaluating path integrals numerically using Monte Carlo methods. An alternate way of computing the Wilson loop expectation value is provided by holographic duality. It is an equivalence between a QFT in four-dimensional flat spacetime and a higher-dimensional theory of gravity in curved spacetime. The duality allows evaluation of hard, non-perturbative QFT calculations with easy classical computations on the gravity side. From the lattice data of the static potential, we can construct a holographic model that could produce those results in a process called bulk reconstruction. The constructed holographic model can then be used to compute other QFT quantities such as entanglement entropy. These quantities allow us to study confinement among other things. In this thesis, lattice field theory measurements of the static potential at different temperatures are presented. Then using a machine learning method, the corresponding holographic metrics are constructed from the lattice results. The lattice simulations have been done as a learning exercise and for the bulk reconstruction method, this thesis is a proof of concept, where no further computations are done using the constructed metrics. The lattice results are in good agreement with previous ones and the bulk reconstruction method seems to work as intended. In future works the method should be applied to a bigger dataset and other quantities should be computed with the constructed metrics.
  • Pänkäläinen, Lauri (2024)
    The Sun's corona is constantly releasing a stream of charged particles known as the solar wind. When the solar wind interacts with Earth's magnetic field, it drags the field lines forming a magnetic tail behind Earth. This magnetotail hosts a process called magnetic reconnection, which converts magnetic energy into plasma heating, plasma kinetic energy and particle acceleration. Reconnection occurs at magnetic structures called X-lines. Magnetic reconnection is thought to be responsible for creating bursty bulk flows (BBFs), short-lived plasma velocity increases in the magnetotail's central plasma sheet region. In satellite measurements, BBFs are seen as minute-timescale velocity increases along Earth-Sun direction. Closely related to BBFs are dipolarization fronts (DFs), sudden increases in $B_z$, the magnetic field component aligned with Earth’s magnetic dipole axis. Despite their short timescales, both phenomena greatly affect the energy distribution and flux transport within the magnetotail. The three-dimensional nature of BBFs and DFs is studied using Vlasiator, a simulation code utilizing a hybrid-Vlasov approach where ions are modelled using distribution functions and electrons are treated as a charge-neutralizing fluid. DFs are identified using a magnetic field derivative threshold $|dB_z/dt|<0.35$ nT/s. BBFs are defined based on a velocity threshold, and they are studied on a case-by-case basis. DFs moving away from Earth are found at magnetic islands that form between multiple X-lines, while DFs moving earthward are mostly seen in finger-like structures of high earthward velocity. BBFs matching satellite observations are also seen at the same structures. Events registered as BBFs in simulated satellite measurements also originate from other sources, including magnetic reconnection at multiple X-lines, movement of reconnecting X-lines and vertical movement of a current sheet within the central plasma sheet. Most DFs are accompanied by velocity increases, but BBFs often result in only small magnetic field enhancements. The results imply that there are multiple different types of dipolarization fronts, and similar satellite measurements of bursty bulk flows can arise from different physical phenomena. The certainty of the results is reduced by low magnetic field variations in the simulation compared to spacecraft measurements and a small DF dataset. The findings may still help with interpreting satellite observations in the magnetotail.
  • Raivio, Riku (2023)
    In high energy physics the microscopic nature of our universe is studied. A common experimental way is to collide particles such as protons with almost the speed of light and study the fragments that fly away from the interaction point. The results are compared to existing theories and help to modify or create knowledge of our universe. However, our senses are not capable of directly measuring those tiny and fast fragments from the collisions. Therefore, we need dedicated devices called particle detectors. Several types of radiation detectors are known to us. Most of those utilize the fact that particles in such experiments are ionizing when traversing matter. The detectors we are interested in contain gas as the main interacting material for the ionizing radiation, called gaseous detectors. One of these is the Gas Electron Multiplier (GEM), which uses electric fields to make electrons drift through the holes of one or several foils producing a signal that can be detected. Gain and ion backflow are two properties that can be used to evaluate the effectiveness of the detector. The effect of the hole diameters of the foil on the gain and on the ion backflow is insufficiently known, however. In this thesis such measurements have been performed. For this purpose a GEM detector operating in proportional region was constructed. The detector contained a special foil with four quadrants in such a way that the diameters of the holes were different in each quadrant. The functionality of the detector was verified by measuring the leakage current, by using a multichannel analyzer, and by calculating the sum of all currents. The detector constructed passed all the tests. The diameters of the holes in the foil quadrants were estimated by calculating the amount of pixels from the foil image taken. The outer hole diameters were around 50 µm and inner hole diameters 40 µm in the quadrant with smallest holes. The corresponding diameters in the quadrant with largest holes were around 70 µm and 66 µm. The measurements were started by searching the proper values for the drift and the induction fields and for the GEM voltage range. A drift field of 2 kV/cm, an induction field of 7 kV/cm, and a voltage range from 400 V to 500 V were chosen. The results from the actual measurements were similar on both sides of the foil. The effective gain increased steadily along with the voltage being around 10-fold at 500 V compared to that at 400 V . The ion backflow, on the other hand, stayed constant or even slightly decreased. The results measured from the four quadrants differed clearly from each other. In the quadrant with the smallest holes the effective gain was about twice as high as in the quadrant with the largest holes. Respectively the ion backflow was about 30 % higher in the quadrant with small holes than in the quadrant with large holes.