In the paper attached to this thesis, Paper I, we have calculated the flux of neutrinos that emanate from cosmic ray collisions in the solar atmosphere. These neutrinos are created in the cascades that follow the primary collision and can travel from their production point to a detector on Earth, interacting with the solar material and oscillating on the way. The motivation is both a better understanding of the cosmic ray interactions in the solar environment but also the fact that this neutrino flux presents an almost irreducible background for the searches for neutrinos from annihilations between dark matter particles in the Sun’s core. This interesting connection between neutrinos and dark matter make use of the Sun as a laboratory to investigate new models of particle physics. If dark matter consists of weakly interacting massive particles (WIMPs), the Sun will sweep up some of these WIMPs when it moves through the halo of dark matter that our galaxy lies in. These WIMPs will become gravitationally bound to the Sun and over time accumulate in the Sun’s core. In most models WIMPs can annihilate to Standard Model particles when encountering each other. The only particle that can make it out of the Sun without being absorbed is the neutrino. The buildup of WIMPs in the solar interior can therefore lead to a detectable flux of neutrinos. Neutrino telescopes therefore search for an excess of neutrinos from the Sun. To be able to ensure that a detected flux is in fact coming from dark matter annihilations one must properly account for all other sources of neutrinos. At higher energies these are primarily neutrinos created in energetic collisions between cosmic rays and particles in the Earth’s atmosphere, but also the solar atmospheric neutrinos. The latter will be tougher to disentangle from a WIMP signal since they also come from the Sun. We calculate in Paper I the creation of the neutrinos in the solar atmosphere and propagate these neutrinos to a detector on Earth, including oscillations and interactions in the Sun and vacuum oscillations between the Sun and the Earth. We find that the expected flux is small but potentially detectable by current neutrino telescopes, although further studies are needed to fully ascertain the possibility of discovery as well as how to properly disentangle this from a potential WIMP-induced neutrino signal.

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.

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 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.

Supernova remnants (SNRs) are the end products of supernova (SN) explosions. These explosions happen when certain massive stars face the ends of their evolutionary cycle and create shock waves propagating in the surrounding medium by ejecting part of the star’s material. An SNR is the region defined by these powerful shocks. Due to the nature of SNRs spreading stellar material to their surroundings, they play a major part in the chemical evolution of the universe.  Some SNRs are observed to contain a pulsar (PSR), a spinning neutron star emitting electromagnetic radiation through its magnetic poles, and a pulsar-wind nebula (PWN), where relativistic particles and magnetic fields escaping the pulsar interact with the surroundings. This is the case for SNR 0540-69.3 (SNR0540), the so called twin of one of the most studied astronomical objects in the sky, the Crab Nebula. The attached paper (Paper I) is based on VLT observations of the central regions of SNR 0540 with the instruments MUSE and X-Shooter. The MUSE observations provide a possibility to study the optical spatial variations of SNR 0540 for the first time utilising spectroscopy, and are in general one of the few such studies of PWNe in the optical. On the other hand, earlier works focusing on the shape of the SNR 0540 continuum spectrum in the optical wavelengths have mostly utilised narrow band photometry, which has yielded conflicting results. The X-shooter observations of SNR 0540 providing the first near-infrared (NIR) spectrum with a good coverage in the optical can be used to tackle these problems.  In Paper I, the continuum emission is modelled by power-law models to constrain the underlying conditions in the PSR and PWN in SNR 0540. An important parameter in these models is called the spectral index (α), which determines the slope of the spectrum. We find significant spatial variations in the spectral index that reveal a torus-jet structure around the PSR, confirming earlier results. Surprisingly, we also find that the spectral index decreases (from α ~ 1.7 to α ~ 0.5) toward the outer parts of the PWN and is the largest for the PSR (α1 ~ 1 in the low and α2 ~ 2 in the high frequencies), in contrast to theoretical expectations for the basic scenario of synchrotron cooling. Additionally, two spectral indices seem to be required to characterise both the PSR’s and PWN’s optical(-NIR) spectra.  Future observations in the optical but also in the infrared and X-rays would help understanding the complex conditions in the central regions of SNRs. Most importantly, optical observations of other SNRs would shed light on whether SNR 0540 is a special case. The unexpected spectral index variations in SNR 0540 highlight the need for further theoretical work to better understand the origin of the optical synchrotron emission in PSRs and PWNe. / Supernovarester (SNR) är slutprodukterna av supernovaexplosioner (SN). Dessa explosioner inträffar när vissa massiva stjärnor möter ändarna av sin evolutionära cykel och skapar chockvågor som fortplantar sig i det omgivande mediet genom att stöta ut en del av stjärnans material. En SNR är regionen som definieras av dessa kraftfulla stötar. På grund av naturen hos SNR som sprider stjärnmaterial till sin omgivning spelar de en stor roll i universums kemiska utveckling.  Vissa SNR:er har observerats innehålla en pulsar (PSR), en snurrande neutronstjärna som sänder ut elektromagnetisk strålning genom sina magnetiska poler, och en pulsar-vindnebulosa (PWN), där relativistiska partiklar och magnetfält som flyr pulsarn interagerar med omgivningen. Detta är fallet för SNR 0540-69.3 (SNR 0540), den så kallade tvillingen till ett av de mest studerade astronomiska objekten på himlen, Krabbanebulosan. Bifogade papper (Paper I) är baserad på VLT-observationer av de centrala regionerna i SNR 0540 med instrumenten MUSE och X-Shooter. MUSE-observationerna ger en möjlighet att studera de optiska rumsliga variationerna av SNR 0540 för första gången med hjälp av spektroskopi, och är i allmänhet en av få sådana studier av PWNe i optiken. A andra sidan har tidigare arbeten med fokus på formen av SNR 0540-kontinuumspektrumet i de optiska våglängderna mestadels använt smalbandsfotometri, vilket har gett motstridiga resultat. X-shooter-observationerna av SNR 0540 som ger det första nära-infraröda (NIR) spektrumet med en bra täckning i optiken kan användas för att ta itu med dessa problem.  I Paper I modelleras synkrotronemissionen av potenslagsmodeller för att begränsa de underliggande förhållandena i PSR och PWN i SNR 0540. En viktig parameter i dessa modeller kallas spektralindex (α), som bestämmer spektrumets lutning. Vi hittar betydande rumsliga variationer i det spektrala indexet som avslöjar en torus-jet-struktur runt PSR, vilket bekraftar tidigare resultat. Överraskande nog finner vi också att spektralindexet minskar (från α ~ 1.7 till α ~ 0.5) mot de yttre delarna av PWN och är störst för PSR (α1 ~ 1 i de låga och α2 ~ 2 i de höga frekvenserna), i motsats till teoretiska förväntningar på grundscenariot med synkrotronkylning. Dessutom verkar två spektrala index krävas för att karakterisera både PSR:s och PWN:s optiska(-NIR) spektra.  Framtida observationer inom det optiska, men också i det infraröda och röntgenstrålar skulle hjälpa till att förstå de komplexa förhållandena i de centrala regionerna av SNR. Det viktigaste är att optiska observationer av andra SNR:er skulle belysa om SNR 0540 är ett specialfall. De oväntade spektrala indexvariationerna i SNR 0540 belyser behovet av ytterligare teoretiskt arbete för att bättre förstå ursprunget till den optiska synkrotronemissionen i PSR och PWNe. / <p>QC 2023-05-23</p>