The focus of this thesis is to investigate magnetic reconnection in the solar chromosphere by usinghigh spatial resolution data including polarimetric signals. Magnetic reconnection occurs whenmagnetic field lines of different directions come in contact and change the configuration of the fieldlines. When this happens in the solar atmosphere, magnetic energy is transformed into thermalenergy (heating) and kinetic energy. This is observed as brightenings and fast plasma motions. The data was taken with the Swedish 1-m Solar Telescope (SST) on La Palma (Spain) in 2016 andincludes the spectral lines CaIIK, CaII8542 Å , FeI6301/6302 Å and Hα. The observations ofCaII8542 Å and FeI6301/6302 Å also include polarimetric signals. The analysis is done by inferringthe atmospheric conditions in which the events are evolving with the inversion code STiC using thefirst three lines mentioned. Before the inversion, several methods were used to improve the datasuch as denoising using a convolution neural network. The improvements were done to enhancethe polarimetric signals of Stokes Q and U. These signals are usually hard to detect since thepolarization signal is smaller by a factor 10^3-10^4 than the observed intensity. In this thesis, two reconnection events were studied in detail. The first event was identified as anEllerman bomb type event. We detect a compact, smaller than 12, enhancement in the temperatureand bidirectional flow of plasma where two magnetic concentrations of opposite polarities came incontact. The second event is identified as a light-bridge jet type event where the vertical magneticfield of a pore is interacting with the horizontal magnetic field of a light bridge. It is characterized bystrong fan-shaped jets, high-velocity plasma flows, and an elongated temperature enhancement.

The formation of magnetic flux concentrations in the Sun is still a matter of debate. One observable manifestations of such concentrations is sunspots. A mechanism able to spontaneously form magnetic flux concentrations in strongly stratified hydromagnetic turbulence and in the presence of a weak magnetic field is the negative effective magnetic pressure instability (NEMPI). This instability is caused by the local suppression of the turbulence by the magnetic field. Due to the complexity of the system, and in order to understand the fundamental physics behind the instability, the study started by considering simplified conditions. In this thesis we aim to move towards the complexity of the Sun. Here we want to know whether the instability can develop under rotation and in the case of a polytropic stratification instead of the simpler isothermal stratification. We perform different kinds of simulations, namely direct numerical simulations (DNS)and mean field simulations (MFS) of strongly stratified turbulence in the presence of weak magnetic fields. We then study separately the effects of rotation and the change in stratification. It is found that slow rotation can suppress the instability. For Coriolis numbers larger than $0.1$ the MFS no longer result in growth, whereas the DNS start first with adecreaseof the growth rate of the instability % with the speed-up of rotation is alleviated and then, for $\Co > 0.06$, an increase owing to the fact that rotation leads to  the onset of the dynamo instability, which couples with NEMPI in a combined system. In fact, the suppression implies a constraint on the depth where the instability can operate in the Sun. Since rotation is very weak in the uppermost layers of the Sun, the formation of the flux concentration through this instability might be a shallow phenomenon. The same constraint is found when we study the effects of polytropic stratificationon NEMPI. In this case, the instability also develops, but it is much more concentrated in the upper parts of the simulation domain than in the isothermal case. In contrast to the isothermal case, where the density scale height is constant inthe computational domain, polytropic layers decrease their stratification deeper down, so it becomes harder for NEMPI to operate. With these studies we confirm that NEMPI can form magnetic flux concentrations even in the presence of weak rotation and for polytropic stratification. When applied to the Sun, the effects of rotation and the change of stratification constrain the depth where NEMPI can develop to the uppermost layers, where the rotational influence is weak and the stratification is strong enough.

No description available.

Helically shaped magnetic field structuresknown as coronal mass ejections (CMEs) are closely related to so-called eruptive flares. On the one hand, these events are broadly believed to be due tothe buoyant rise of magnetic flux tubes from the bottom of the convection zone to the photosphere where they form structures such as sunspots. On the other hand, models of eruptive flares and CMEs have no connection to the convection zone and the magnetic field generated bydynamo action. It is well known that a dynamo can produce helical structures and twisted magnetic fields as observed in the Sun. In this work we ask, how a dynamo-generated magnetic field appears above the surface without buoyancy force and how this field evolves inthe outer atmosphere of the Sun. We apply a new approach of a two layer model, where the lower one represents the convection zone and the upper one the solar corona. The two layers are included in one single simulation domain. In the lower layer, we use a helical forcing function added to the momentum equation to create a turbulent dynamo. Due to dynamo action, a large-scale field is formed. As a first step we use a Cartesian cube. We solve the equations of the so-called force-free model in the upper layer to create nearly force-free coronal magnetic fields. In a second step we use a spherical wedge, which extends radially from 0.7 to 2 solar radii. We include density stratification due to gravity in anisothermal domain. The wedge includes both hemispheres of the Sun and we apply a helicalforcing with different signs in each hemisphere. As a result, a large-scale field is generated by a turbulent dynamo acting underneath the surface. Due to the latitudinal variation of the helicity produced by the helical forcing, the dynamo is oscillating in the spherical wedge. Twisted magnetic fields emerge above the surface and form arch-like structures with strong current sheets. Plasmoids and CME-like structures are ejected recurrently into the outerlayers. In the spherical simulations we find that the magnetic helicity changes sign in the exterior, which is in agreement with recent analysis of the solar wind data.

No description available.

Thesis (Ph.D.)--Boston University / This dissertation confronts contemporary physical cosmology with Karl Popper's standards of scientific method and theory construction. To the degree to which there are differences, an attempt is made to criticize the major cosmological models in the light of Popper's analysis and, in turn, to explore revisions necessitated in this analysis by the unique problems of cosmology. As background, the major facets of Popper's work are presented in detail: his falsifiability criterion for demarcating scientific theories from metaphysics, his hypothetico-deductive method, and his rejection of induction. Then the origins of general relativity and its competitors are analyzed both as explanatory background to modern cosmology and so as to reveal the history of certain problems pertinent to Popper's scheme: for instance, the use of arguments from simplicity, the ideas of the utility of analogy and models, and the relation of theory to reality. Finally, the great variety of evolutionary, fundamentalistic, and steady-state models available for study is explored in detail as to presupposition and methodology so that their distinctives are revealed and a basis for comparison with Popper's suggestions provided. [TRUNCATED]

The advancing resolution of modern spectrometers uncovers increasingly detailed spectral features in astrophysical observations which can be attributed to properties of the energy structure of the corresponding atomic systems such as fine, hyperfine and Zeeman splitting. At the same time, increasing computational power enables us to include these quantum mechanical interactions on different levels of sophistication in our theoretical atomic structure calculations. This project aims at investigating the hyperfine and Zeeman splitting in five spectral lines of singly-ionized Europium which are relevant for astrophysical studies. To that end, we perform ab initio atomic structure calculations with the latest versions of the code packages Grasp2018 and Rhyze in which we treat the two interactions with a) first-order perturbation theory and b) a generalised full-matrix (`all-order') approach. For both sets of atomic data, we synthesize stellar Stokes I and V spectra for a model atmosphere and stellar magnetic field regime typical for magnetic, chemically peculiar Ap stars. We confirm the overall importance of accounting for hyperfine and Zeeman interactions in the atomic data and find significant differences between the two approaches in the synthesized spectra for the Zeeman interaction. The established computational machinery represents a systematic and largely generalised approach to synthesize spectra of magnetic stars from purely ab initio atomic calculations, including hyperfine and Zeeman interaction simultaneously for any atomic species in the periodic table.

The plan of this paper is to present some observations and then in subsequent chapters to examine the two basic cosmological ideas in the light of these observations. The first of the observations involves an estimate of the angular momentum in a given volume of space and the ratio of this momentum to the mass or number of nucleons in the given volume. We therefore wish to obtain a rough estimate of the angular momentum in that part of the universe which man may call his immediate surroundings, i.e., the solar system and galaxy.

The goal of this work is to detect Zeeman split lines and measure magnetic field strength in a sample of Ap stars. This work aims to analyse spectra of 454 stars and find stars that show spectral lines resolved into their magnetically split components. The work included writing and applying a computer code to visualize astronomical spectra and estimate the mean surface magnetic field strength by fitting resolved Zeeman split lines. The data represent spectroscopic measurements obtained from November 2018 to March 2020 with the HERMES spectrograph. This study investigated high-resolution spectra of 454 Ap stars and identified 31 stars showing resolved Zeeman split lines. The majority of these stars were already known to have resolved magnetically split lines, but this paper reports the discovery of 12 new stars having this property. This paper presents 67 measurements of the mean magnetic field modulus of 31 Ap stars with resolved magnetically split lines.

The majority of stars in the Milky Way are M dwarfs whichmake up 75 % of stars in the vicinity of the sun. As the magneticfield of stars can significantly affect the interiors and theatmospheres of the exoplanets they host within the stellar system,characterising this magnetic field is of major importance.In this work, we selected a sample of 23 exoplanet hostM dwarfs and analysed the observations made by the highresolutionspectropolarimeters ESPaDOnS and NARVAL availablein PolarBase for these stars. In particular, we used theLSD Stokes I and V spectra to measure the projection alongthe line of sight of the average magnetic field over the stellarsurface. 60% of stars had a likely magnetic signatures with adetection greater than 2. By taking the maximum value foreach star over the different observations, the median magneticfield strength measured was 55 G and only a few stars exceeded100 G in strength.