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In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.

Solar flares and eruptions are manifestations of violent releases of magnetic energy from the solar atmosphere. They are powered by magnetic reconnection, a mechanism in which magnetic field lines change their connectivities to reach a lower-energetic state. Theoretical predictions regarding the generalised three-dimensional magnetic reconnection are imposed by the standard flare model in 3D. In this work we present the results of five peer-reviewed publications in which we focused on different predicted aspects of magnetic reconnection in 3D. We analyse evolution and morphology of seven eruptive flares, primarily using observations of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. In the first publication, (Lörinčík et al., 2019a), we interpreted variations of velocities of slipping flare kernels using the mapping norm of field line connectivity simulated via the model. In Lörinčík et al. (2019b) we showed that the observed conversion of filament strands to flare loops is a signature of the 'ar-rf' reconnection geometry between erupting flux rope and overlying coronal arcades. In another observation (Dudík, Lörinčík et al. (2019)), all constituents of this geometry were successfully identified together with the constituents of the 'rr-rf' geometry between two...

This project aims to investigate the orientation of plasma jet fronts in Earth's magnetotail using multi-spacecraft measurement data. The orientations are estimated by applying minimum variance analysis (MVA) and multi-spacecraft timing analysis for finding normal vectors to the jet fronts as they pass over the spacecraft. An agreement between the two analysis methods is found when applied to a data set of fronts. The obtained results are compared to measurements of the ion bulk velocities of the fronts, where a discrepancy is found. Limitations of the analysis are addressed and alternative approaches are presented. / I detta projekt undersöks riktningen hos fronter till plasma-jetstrålar i jordens magnetsvans genom analysering av mätdata från en samling rymdfarkoster. Riktningarna uppskattas genom att applicera minimum variance analysis (MVA) samt multi-spacecraft timing för att hitta normalvektorer till fronterna som passerar rymdfarkosterna. De två metoderna uppnår liknande resultat när de tillämpas på en uppsättning fronter. De erhållna normalvektorerna jämförs även med riktningen av uppmätta jonhastigheter från rymdfarkosterna där en tydlig skillnad förekommer. Begränsningar av analysmetoden påpekas och förslag på alternativa tillvägagångssätt läggs fram.

<p>Active Galactic Nuclei (AGNs) are accreting supermassive black holes at the center of galaxies, known for rich spectral features and multi-time scale variability in their electromagnetic emission. The origin of the variability in AGN light curves can be either intrinsic, meaning related processes that take place inside the AGN system, or extrinsic, i.e., from the propagation of light towards Earth. In this dissertation, I present my work focusing on AGN variability. The first two works focus on the variability of blazars, a subclass of AGN with their relativistic jets beaming towards the observer. The first work combines 3D relativistic magnetohydrodynamics (RMHD) simulations with radiation transfer and shows the kink instability within the blazar jet can cause quasi-periodic radiation signatures within a typical period of time scales from weeks to months. The second work combines 2D Particle-in-Cell (PIC) simulations with radiation transfer and shows that isolated and merging plasmoids due to magnetic reconnection in a blazar environment could produce rich radiation and polarization signatures. The last work explores an extrinsic origin for AGN variability: a scenario in which interstellar medium (ISM) within our galaxy can refract light coming from AGNs. It suggests that plasma structures in ISM with an axisymmetric geometry can account for extreme scattering events (ESEs) in AGN observations. Future research directions include studies of the kink instability in jets that propagate in different environments and simulations of magnetic reconnection in 3D which may reveal additional particle acceleration mechanisms, which may play important role in the resulting radiation and polarization signatures. </p>

In this thesis examples of translationally invariant one-dimensional (1D) Vlasov-Maxwell (VM) equilibria are investigated. The 1D VM equilibrium equations are equivalent to the motion of a pseudoparticle in a conservative pseudopotential, with the pseudopotential being proportional to one of the diagonal components of the plasma pressure tensor. A necessary condition on the pseudopotential (plasma pressure) to allow for force-free 1D VM equilibria is formulated. It is shown that linear force-free 1D VM solutions correspond to the case where the pseudopotential is an attractive central potential. The pseudopotential for the force-free Harris sheet is found and a Fourier transform method is used to find the corresponding distribution function. The solution is extended to include a family of equilibria that describe the transition between the Harris sheet and the force-free Harris sheet. These equilibria are used in 2.5D particle-in-cell simulations of magnetic reconnection. The structure of the diffusion region is compared for simulations starting from anti-parallel magnetic field configurations with different strengths of guide field and self-consistent linear and non-linear force-free magnetic fields. It is shown that gradients of off-diagonal components of the electron pressure tensor are the dominant terms that give rise to the reconnection electric field. The typical scale length of the electron pressure tensor components in the weak guide field case is of the order of the electron bounce widths in a field reversal. In the strong guide field case the scale length reduces to the electron Larmor radius in the guide magnetic field.

<p>Magnetized planets, such as Earth, are strongly influenced by the solar wind. The Sun is very dynamic, releasing varying amounts of energy, resulting in a fluctuating energy and momentum exchange between the solar wind and planetary magnetospheres. The efficiency of this coupling is thought to be controlled by magnetic reconnection occurring at the boundary between solar wind and planetary magnetic fields. One of the main tasks in space physics research is to increase the understanding of this coupling between the Sun and other solar system bodies. Perhaps the most important aspect regards the transfer of energy from the solar wind to the terrestrial magnetosphere as this is the main source for driving plasma processes in the magnetosphere-ionosphere system. This may also have a direct practical influence on our life here on Earth as it is responsible for Space Weather effects. In this thesis I investigate both the global scale of the varying solar-terrestrial coupling and local phenomena in more detail. I use mainly the European Space Agency Cluster mission which provide unprecedented three-dimensional observations via its formation of four identical spacecraft. The Cluster data are complimented with observations from a broad range of instruments both onboard spacecraft and from groundbased magnetometers and radars.</p><p>A period of very strong solar driving in late October 2003 is investigated. We show that some of the strongest substorms in the history of magnetic recordings were triggered by pressure pulses impacting a quasi-stable magnetosphere. We make for the first time direct estimates of the local energy flow into the magnetotail using Cluster measurements. Observational estimates suggest a good energy balance between the magnetosphere-ionosphere system while empirical proxies seem to suffer from over/under estimations during such extreme conditions.</p><p>Another period of extreme interplanetary conditions give rise to accelerated flows along the magnetopause which could account for an enhanced energy coupling between the solar wind and the magnetosphere. We discuss whether such conditions could explain the simultaneous observation of a large auroral spiral across the polar cap.</p><p>Contrary to extreme conditions the energy conversion across the dayside magnetopause has been estimated during an extended period of steady interplanetary conditions. A new method to determine the rate at which reconnection occurs is described that utilizes the magnitude of the local energy conversion from Cluster. The observations show a varying reconnection rate which support the previous interpretation that reconnection is continuous but its rate is modulated.</p><p>Finally, we compare local energy estimates from Cluster with a global magnetohydrodynamic simulation. The results show that the observations are reliably reproduced by the model and may be used to validate and scale global magnetohydrodynamic models.</p>

Cette thèse s’intéresse à la modélisation magnétohydrodynamique des écoulements de fluides conducteurs d’électricité multi-échelles en mettant l’emphase sur deux applications particulières de la physique solaire: la modélisation des mécanismes des variations de l’irradiance via la simulation de la dynamo globale et la reconnexion magnétique. Les variations de l’irradiance sur les périodes des jours, des mois et du cycle solaire de 11 ans sont très bien expliquées par le passage des régions actives à la surface du Soleil. Cependant, l’origine ultime des variations se déroulant sur les périodes décadales et multi-décadales demeure un sujet controversé. En particulier, une certaine école de pensée affirme qu’une partie de ces variations à long-terme doit provenir d’une modulation de la structure thermodynamique globale de l’étoile, et que les seuls effets de surface sont incapables d’expliquer la totalité des fluctuations. Nous présentons une simulation globale de la convection solaire produisant un cycle magnétique similaire en plusieurs aspects à celui du Soleil, dans laquelle le flux thermique convectif varie en phase avec l’ ́energie magnétique. La corrélation positive entre le flux convectif et l’énergie magnétique supporte donc l’idée qu’une modulation de la structure thermodynamique puisse contribuer aux variations à long-terme de l’irradiance. Nous analysons cette simulation dans le but d’identifier le mécanisme physique responsable de la corrélation en question et pour prédire de potentiels effets observationnels résultant de la modulation structurelle. La reconnexion magnétique est au coeur du mécanisme de plusieurs phénomènes de la physique solaire dont les éruptions et les éjections de masse, et pourrait expliquer les températures extrêmes caractérisant la couronne. Une correction aux trajectoires du schéma semi-Lagrangien classique est présentée, qui est basée sur la solution à une équation aux dérivées partielles nonlinéaire du second ordre: l’équation de Monge-Ampère. Celle-ci prévient l’intersection des trajectoires et assure la stabilité numérique des simulations de reconnexion magnétique pour un cas de magnéto-fluide relaxant vers un état d’équilibre. / This thesis concentrates on magnetohydrodynamical modeling of multiscale conducting fluids with emphasis on two particular applications of solar physics: the modeling of solar irradiance mechanisms via the numerical simulation of the global dynamo and of magnetic reconnection. Irradiance variations on the time scales of days, months, and of the 11 yr solar cycle are very well described by changes in the surface coverage by active regions. However, the ultimate origin of the long-term decadal and multi-decadal variations is still a matter of debate. In particular, one school of thought argues that a global modulation of the solar thermodynamic structure by magnetic activity is required to account for part of the long-term variations, in addition to pure surface effects. We hereby present a global simulation of solar convection producing solar-like magnetic cycles, in which the convective heat flux varies in phase with magnetic energy. We analyze the simulation to uncover the physical mechanism causing the positive correlation and to predict potential observational signatures resulting from the flux modulation. Magnetic reconnection is central to many solar physics phenomena including flares and coronal mass ejections, and could also provide an explanation for the extreme temperatures (T ∼ 106K) that charaterize the coronna. A trajectory correction to the classical semi-Lagrangian scheme is presented, which is based on the solution to a second-order nonlinear partial differential equation: the Monge-Amp`ere equation. Using the correction prevents the intersection of fluid trajectories and assures the physical realizability of magnetic reconnection simulations for the case of a magneto- fluid relaxing toward an equilibrium state.

The aim with this review was to investigate whether a reconnection of an oxbow lake can contribute to higher biodiversity. However, oxbow lakes can be divided in three categories: Lentic- (connected with both ends to the river bed), semi-lentic- (connected with one end) and lotic oxbow lakes (Isolated from the riverbed). Aquatic organisms such as fish, aquatic invertebrates, amphibians and macrophytes has been studied. The result showed that hydrological connectivity determines both biodiversity and water quality in oxbow lakes. Lotic oxbow lakes consisted low biodiversity and it’s dominated by amphibians. Semi-lentic oxbow lakes contributes with highest biodiversity of macrophytes, fish and aquatic invertebrates. Lentic oxbow lakes consisted less biodiversity and was dominated by fish. In conclusion, reconnection of an oxbow lake can be used as a restoration project to improve biodiversity.

The Magnetospheric Multiscale mission led by NASA has been designed to study the micro-physics of Magnetic Reconnection in Earth's magnetosphere by using four identical spacecrafts with instruments with high temporal and spatial resolutions. Among these instruments are the Dual Ion Spectrometers (DIS) engineered to measure the 3D distribution of ion flux in space. The detector assembly of the DIS consists of Micro-Channel Plates (MCP) mounted in Chevron configuration. Centre d'Etude Spatiale des Rayonnements (CESR), Toulouse is responsible for the provision and testing of all fifty MCP pairs for this mission. The goal of the work was to participate in the testing and characterization of the first prototype of the MCPs. It was achieved by understanding the working and characteristics of the MCPs in general and getting familiar with the detector assembly of the DIS i.e. the MCP pair and the detector circuit board in particular. To perform the testing, it was necessary to understand the testing system as well. These topics are described in this report along with the testing procedure and the data analysis. The testing procedure was developed eventually after facing several problems during the testing. MCP pair characteristics like pulse height distributions, gain, resistance and the MCP operating voltages for the mission were determined on analyzing the data. Crosstalk was found in the circuit board of the detector assembly and has also been discussed. / Validerat; 20101217 (root)