Den här licenciatuppsatsen bygger på två vetenskapliga artiklar, varav den första är skriven som ensamförfattare och den andra är skriven tillsammans med Dr. Giacomo Monari. De är båda på temat mörk materia i Vintergatan. Den första artikeln handlar om mörk materia som fångas i solen. Förutsatt att mörk materia består av partiklar som interagerar via den svaga kraften, med en massa av storleksordningen 10--1000 GeV, så kan sådana partiklar kollidera med atomkärnor i solens inre, förlora rörelseenergi och bli gravitationellt bundna. Väl infångad så kommer en mörk materia--partikel att fortsätta kollidera och förlora rörelseenergi tills den har uppnått termisk jämvikt med solens kärna. Givet att infångade och termaliserade mörk materia--partiklar har blivit tillräckligt många till antalet så kan dessa partiklar annihiliera och producera standard modell--partiklar. Neutriner som produceras i en sådan process skulle kunna detekteras i ett neutrinoteleskop på jorden, vilket vore ett sätt att indirekt detektera mörk materia. I artikeln har jag utforskat denna termaliseringsprocess och den tid det tar för en infångad partikel att uppnå termisk jämvikt. Jag har funnit att termaliseringstiden är kort jämfört med solens ålder och kan försummas, utom i vissa finjusterade specialfall. I den andra artikeln har vi utfört en dynamisk mätning av massdensiteten i solens närområde. Astrometri från rymdteleskopet Gaia ger information om stjärnors vertikala hastighetsfördelning och hur deras antaltäthet avtar med avstånd från galaxskivan. Genom att anta jämvikt så kan man relatera dessa två fördelning till varandra genom gravitationspotentialen de rör sig genom, vilket i sin tur ger galaxskivans massfördelning. Först och främst så har vi gjort framsteg i fråga om statistisk modellering; för första gången har mätfel på alla enskilda stjärnor tagits i beaktning. Vi har funnit en massdensitet som stämmer överens med de flesta tidigare mätningar och har även kunnat dra slutsatser om solens position och hastighet i förhållande till galaxskivan.

In the last few years, there has been significant progress in the development of a new model for explaining magnetic flux concentrations, by invoking the negative effective magnetic pressure instability (NEMPI) in a highly stratified turbulent plasma. According to this model, the suppression of the turbulent pressure by a large-scale magnetic field leads to a negative contribution of turbulence to the effective magnetic pressure (the sum of non-turbulent and turbulent contributions). For large magnetic Reynolds numbers the negative turbulence contribution is large enough, so that the effective magnetic pressure is negative, which causes a large-scale instability (NEMPI). One of the potential applications of NEMPI is to explain the formation of active regions on the solar surface. On the other hand, the solar dynamo is known to be responsible for generating large-scale magnetic field in the Sun. Therefore, one step toward developing a more realistic model is to study a system where NEMPI is excited from a dynamo-generated magnetic field. In this context, the excitation of NEMPI in spherical geometry was studied here from a mean- field dynamo that generates the background magnetic field. Previous studies have shown that for NEMPI to work, the background field can neither be too weak nor too strong. To satisfy this condition for the dynamo-generated magnetic field, we adopt an “alpha squared dynamo” with an α effect proportional to the cosine of latitude and taking into account alpha quenching. We performed these mean-field simulations (MFS) using the Pencil Code. The results show that dynamo and NEMPI can work at the same time such that they become a coupled system. This coupled system has then been studied separately in more detail in plane geometry where we used both mean-field simulations and direct numerical simulations (DNS). Losada et al. (2013) showed that rotation suppresses NEMPI. However, we now find that for higher Coriolis numbers, the growth rate increase again. This implies that there is another source that provides the excitation of an instability. This mechanism acts at the same time as NEMPI or even after NEMPI was suppressed. One possibility is that for higher Coriolis numbers, an α2 dynamo is activated and causes the observed growth rate. In other words, for large values of the Coriolis numbers we again deal with the coupled system of NEMPI and mean-field dynamo. Both, MFS and DNS confirm this assumption. Using the test-field method, we also calculated the dynamo coefficients for such a system which again gave results consistent with previous studies. There was a small difference though, which is interpreted as being due to the larger scale separation that we have used in our simulations. Another important finding related to NEMPI was the result of Brandenburg et al. (2013), that in the presence of a vertical magnetic field NEMPI results in magnetic flux concentrations of equipartition field strength. This leads to the formation of a magnetic spot. This finding stimulated us to investigate properties of NEMPI for imposed vertical fields in more detail. We used MFS and DNS together with implicit large eddy simulations (ILES) to confirm that an initially uniform weak vertical magnetic field will lead to a circular magnetic spot of equipartition field strength if the plasma is highly stratified and scale separation is large enough. We determined the parameter ranges for NEMPI for a vertical imposed field. Our results show that, as we change the magnitude of the vertical imposed field, the growth rate and geometry of the flux concentrations is unchanged, but their position changes. In particular, by increasing the imposed field strength, the magnetic concentration forms deeper down in the domain.

No description available.

In these pages we will go through the topic of astrophysical magnetic fields, focusing on galactic fields, their observation and the theories that have been developed for a proper understanding of the these physical phenomena.We review the main work in the study of galactic magnetic fields, often seeing how it is important to deal with problems of general validity in order to be able to point out the right elements needed for a correct interpretation of specific situations. We also aim to summarize some of the conflicts that arise using different theoretical approaches to be proficient in future choices of our research guidelines.This thesis consists in an introductory text and three papers dealing with some specific topics that are introduced in the first three chapters.In the first chapter we will talk about the state of the art of the observations of galactic fields. We review current techniques and observations.In the second chapter we describe the current theories that best describe the generation of magnetic fields. We also mention here two of the three works presented in this licentiate thesis. We will then deal with the possibility to have a proper measure of the $\alpha$ effect in numerical simulationsof dynamo action.Then we consider a particular aspect of magnetic helicity, that is, its connection with the topology of the magnetic field in a given system.In the third chapter we focus on theories related to galactic fields and their validity.We also present our work on the generation of vorticity in the interstellar medium as well as a study ofturbulent diffusivity in a system presenting spherical expansion waves.

Meteors are light events appearing in the sky, looking like strips of light. A meteor occurs when a space object, a meteoroid, enters the earth’s atmosphere at high velocity and starts to glow. Meteor triangulation is an important part in astronomical research, by mapping streams of material in our solar system the knowledge about the solar system is extended. By making measurements with meteor cameras and calculate the meteors trajectories with triangulation methods, the meteors origin in space can be derived. Previous research has shown that by including time measurements, and not only use triangulation techniques with spatial measurements, the trajectory for a meteor can be calculated more accurately. This project seeks to explore if including the time information from the Swedish meteor data would create more precise solutions for the meteor’s paths through the atmosphere. This was done by investigating how the time variable can be implemented and running the Swedish 2020 Perseid data through the algorithm that implements the time variable. The accuracy of the methods is determined by how exact the Perseids radiant position is calculated and also by looking at the computed velocities for the meteor events. The results from this project show that the time variable should be considered in the Swedish meteor triangulation algorithm. With inclusion of the time variable, the radiant position of the Perseids can be more closely determined and the velocities for the meteors are in the expected range for Perseid meteors. But it turns out that the timestamps between the meteor cameras are not synchronized, which means that time offsets between the stations are required. / Meteorer är ljusfenomen som kan ses på natthimlen, de ser ut som ljusstreck och kallas även stjärnfall. En meteor inträffar när ett objekt från rymden, en meteoroid, kommer in i jordatmosfären med hög hastighet och börjar glöda. Meteortriangulering är en viktig del i astronomisk forskning, genom att kartlägga strömmar av material i solsystemet kan kunskapen om vårt solsystem breddas. Meteorers ursprung kan beräknas genom att göra mätningar med meteorkameror, för att sedan med trianguleringsmetoder beräkna dess banor genom atmosfären.  Tidigare forskning har visat att man kan få mer noggranna bestämningar för meteorspår genom att inkludera tidmätningar i beräkningarna, i Sverige används just nu endast trianguleringsmetoder som bygger på spatiella mätningar. Målet med det här projektet är att undersöka om inkludering av tidmätningar från den svenska meteordatan kan ge mer exakta lösningar för meteorspår. Detta undersöktes genom att kolla på hur tidsvariabeln kan integreras i den svenska trianguleringsalgoritmen och sedan testköra svensk perseiddata från år 2020. Precisionen i metoderna avgörs genom att kolla på hur exakt perseidernas radiantposition kan bestämmas och genom att kolla på meteorernas beräknade hastigheter i atmosfären. Resultaten från det här projektet visar att tidsvariabeln borde tas i beaktning i den svenska trianguleringsalgoritmen. Genom att inkludera tidsvariabeln kan radiantpositionerna bestämmas mer exakt och hastigheterna är inom det förväntade spannet för perseidmeteorer. Men det visade sig att tidmätningarna mellan de olika kamerorna är osynkroniserade, så tidsoffsets måste inkluderas i algoritmen.

By including dissipative fluxes in the description, this thesis extends previous results regarding first order perturbations on homogeneous and hypersurface orthogonal locally rotationally symmetric (LRS) class II cosmologies using the 1 + 1 + 2 covariant split of spacetime. Whereas previous works consider perturbations of perfect fluid type, perturbations pertaining to heat flux and fluid viscosity are here studied with the aim to ascertain their effect on the evolution of the fluid vorticity. The studied perturbations include scalar, vector, and tensor modes, and are harmonically decomposed to yield a system of ordinary differential equations. These equations, originating from the Bianchi identities, the Ricci identities for certain preferred vector fields, and the thermodynamic Eckart theory, then decouple into two independent systems. These separately closed systems, with four and eight remaining variables respectively, describe the evolution of perturbations pertaining to the Weyl tensor and the fluid shear, vorticity, heat flow, energy density, and number density. From the final system of equations it is seen that the inclusion of heat flux and fluid viscosity has the possibility to yield mechanisms for generating vorticity, even if this vorticity vanishes initially. This is in contrast to the case of barotropic perfect fluids, for which it can be shown that vorticity perturbations cannot be generated. The validity of the results presented here can be questioned, as the Eckart theory, which violates causality, is employed to describe the detailed thermodynamic properties of the fluid. However, on time scales much larger than the relaxation times of the fluid, it should still provide a decent description of the dissipative phenomena, provided that certain couplings between the dissipative fluxes can be neglected.

In this Licentiate thesis I review the properties of the solar atmosphere and the diagnostic value ofdifferent spectral lines in the visible and ultraviolet (UV) along with the millimeter (mm) continua in theelectromagnetic spectrum of the Sun.While the solar atmosphere has been routinely observed in high-resolution from ground-based opti-cal telescopes such as the Swedish Solar Telescope (SST), and more recently in the UV from space tele-scopes such as the Interface Region Imaging Spectrograph (IRIS), radio observations lag behind despitetheir great usefulness. This is likely to change thanks to the Atacama Large Millimeter Array (ALMA)that only started observing the Sun in 2016 with a few limitations, but the first results are promising.ALMA observations probe the solar chromosphere at different heights by tuning into slightly differentfrequencies at potentially milliarcsecond scales if the full array is able to operate with the longest base-lines. This new spectral window onto the Sun is expected to advance various fields of research suchas wave propagation and oscillations in the chromosphere, thermal structure of filaments/prominences,triggering of flares and microflares, and more generally chromospheric and coronal heating, because themm-intensities can be modelled by simply assuming local-thermodynamic equilibrium.In da Silva Santos et al. (2018) we find that coordinated observations from SST, IRIS and ALMA willpermit us to estimate with greater accuracy the full thermodynamical state of the plasma as a functionof optical depth based on experiments with a snapshot of a three-dimensional magnetohydrodynamicsimulation of the Sun’s atmosphere. Particularly, the mm-continuum improves the accuracy of inferredtemperatures in the chromosphere. Here we expand on the Why and How this can be done. The goal isto better constrain the temperature stratification in the solar atmosphere in order to understand chromo-spheric heating.

Solar plage has been the topic of many studies since its initial description in the mid 19th century, but as of yet it has not been understood to the point where we can reproduce all aspects of these active regions in quasi-realistic numerical models. To a large extent, this is caused by an incomplete understanding of the magnetic structure that drives the activity in these areas. Detailed measurements have been done of the magnetic field configuration of plage in the photosphere since the late 20th century, but only a handful of papers have managed to make any measurements at all in the higher situated chromosphere, despite the fact that the magnetic field vector of plage is important in understanding chromospheric magnetic fields in general, as well as the heating processes of the higher atmosphere. In Pietrow et al. (2020) we add to these measurements by introducing what is to our knowledge the first full Stokes inversion of chromospheric plage, which allowed us to estimate the magnetic field vector at an optical depth of logτ = -3.5. The obtained value is |B| = 440 ± 90 G in the plage with an inclination of 10° ± 16° with respect to the local vertical. Our reported magnetic field strength matches with a recent study by Morosin et al. (2020), but is higher by a factor of two or more compared to previous studies that measured the field using other methods. Additionally we measure an average magnetic field strength of |B| = 300 ± 50 G in a fibrillar region close to the plage. In this thesis we explore the difficulties of measuring this magnetic field vector. Since plage exists in a complex environment, we will begin with a general description of the structure and properties of the solar atmosphere and the layers from which it is composed, as well as review the types of active regions that can be found in the solar atmosphere. Our focus then narrows to the chromosphere, the diagnostic properties of spectral lines that are sensitive to this layer (mainly the \cair line), plage regions, and plage chromospheric magnetic fields. Additionally, we touch upon the theory of radiative transfer and how physical characteristics of the atmosphere can be inferred from polarised light. We also give attention to the observing process with the Swedish 1-m Solar Telescope (SST) and the workings of the reduction pipeline and post-reduction methods as well as the process spectropolarimetric inversions. Finally, once we have understood why and how this project has been done, we summarize our findings and compare them to current literature.

No description available.

Developing efficient and accurate numerical methods to simulate dynamics of physical systems has been an everlasting challenge in computational physics. Physics-Informed Neural Networks (PINNs) are neural networks that encode laws of physics into their structure. Utilizing auto-differentiation, they can efficiently solve partial differential equations (PDEs) by minimizing the loss function at certain points within the domain of interest. The remarkable efficiency exhibited by these networks when solving PDEs positions them as ideal solvers for simulating complex systems.  In this pioneering work, we take a first step towards simulating galaxy dynamics using PINNs by solving the gravitational Poisson equation. We initially substantiate the capacity of PINNs to solve the gravitational Poisson equation for the simple Hernquist (Hernquist, 1990) radial density profile, and for the parametric Dehnen (Dehnen, 1993) radial density profile. Following this, we extended our study to encompass a more complex axisymmetric density profile describing a Thick Exponentiel Disk.  The capacity of PINNs to generate comparatively accurate results has been validated with an average error of 1.71% and 3.75% respectively for the spherically symmetric Hernquist and Dehnen models. While for the axisymmetric thick exponentiel disk model the PINN demonstrated an average relative error of 0.36% with a maximum error of just 0.99% after fine-tuning the PINN’s hyperparameters. Although this model typically relies on the two coordinates R and z along with the ratio η of the model’s scale lengths, the PINN is here trained using a fixed, predetermined value of η.  Drawing upon the outcomes of the grid search implemented for the thick exponen- tial disk model, we provide a succinct examination of how the hyperparameters of the PINN impact the relative error. Given the limited quantity of datapoints, we refrain from formulating definitive conclusions, yet we do exhibit certain discernible patterns. Specifically, we demonstrate that the hyperbolic tangent (tanh) activation function con- sistently outperforms other activation functions in the context of our model. Addition- ally, it appears that augmenting the depth of the network offers superior error reduction in comparison to increasing its width, reinforcing the importance of architectural con- siderations in the optimization of Physics-Informed Neural Networks  Our results show clear advantages of PINNs over regular solvers in terms of efficiency. Despite the success of the two-parameter PINN for the thick exponential disk, further work is required to confirm its extension to three dimensions. This pioneering research offers a promising foundation for further developments in the field, and demonstrates the genuine practical utility of PINNs for simulating complex systems such as galaxies.