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<p>Planets are believed to form in primordial gas-dust discs surrounding newborn stars. An important breakthrough in our understanding of planetary formation was the discovery of extra-solar planets around sun-like stars, especially the frequent occurrence of giant planets on close orbits (hot Jupiters). The mechanisms involved in the formation of these objects remain uncertain, however the difficulties associated with their formation at their observed orbital radius has awoken an interest in theories for the migration of protoplanetary cores due to gravitational interaction with the disc. There are three fundamental regimes of planet migration. The type I and II migration regimes, driven by the differential Lindblad torques, result mostly in inward migration and concern low- and high-mass planets respectively. Type III migration, driven by the co-orbital gas flow, concerns an intermediate range of planetary masses and does not have a predefined direction.</p><p>In this thesis the orbital evolution of a high-mass, rapidly (type III) migrating planet is investigated using numerical hydrodynamical simulations. For these simulations we used the state-of-the-art hydrodynamics code FLASH. We focus on the physical aspects of type III migration. However, the problem of rapid migration of such massive planets is numerically challenging, and the disc model has to be chosen carefully, using numerical convergence as a discriminator between models (Paper I). We simulate both inward and outward directed migration (Papers II and III) and provide an extensive description of the co-orbital flow responsible for driving the migration, as well as its time evolution. The migration rate due to type III migration is found to be related to the mass of the planet's co-orbital region, making inward and outward directed migration self-decelerating and self-accelerating processes respectively (for a standard disc model). Rapid migration depends strongly on the flow structure in the planet's vicinity, which makes it sensitive to the amount of mass accumulated by the planet as it moves through the disc. This quantity in turn depends on the structure of the accretion region around the planet. The results of the numerical simulations show a good agreement with the analytical formulation of type III migration (Paper IV).</p>

Planets are believed to form in primordial gas-dust discs surrounding newborn stars. An important breakthrough in our understanding of planetary formation was the discovery of extra-solar planets around sun-like stars, especially the frequent occurrence of giant planets on close orbits (hot Jupiters). The mechanisms involved in the formation of these objects remain uncertain, however the difficulties associated with their formation at their observed orbital radius has awoken an interest in theories for the migration of protoplanetary cores due to gravitational interaction with the disc. There are three fundamental regimes of planet migration. The type I and II migration regimes, driven by the differential Lindblad torques, result mostly in inward migration and concern low- and high-mass planets respectively. Type III migration, driven by the co-orbital gas flow, concerns an intermediate range of planetary masses and does not have a predefined direction. In this thesis the orbital evolution of a high-mass, rapidly (type III) migrating planet is investigated using numerical hydrodynamical simulations. For these simulations we used the state-of-the-art hydrodynamics code FLASH. We focus on the physical aspects of type III migration. However, the problem of rapid migration of such massive planets is numerically challenging, and the disc model has to be chosen carefully, using numerical convergence as a discriminator between models (Paper I). We simulate both inward and outward directed migration (Papers II and III) and provide an extensive description of the co-orbital flow responsible for driving the migration, as well as its time evolution. The migration rate due to type III migration is found to be related to the mass of the planet's co-orbital region, making inward and outward directed migration self-decelerating and self-accelerating processes respectively (for a standard disc model). Rapid migration depends strongly on the flow structure in the planet's vicinity, which makes it sensitive to the amount of mass accumulated by the planet as it moves through the disc. This quantity in turn depends on the structure of the accretion region around the planet. The results of the numerical simulations show a good agreement with the analytical formulation of type III migration (Paper IV).

We discuss various topics concerning the propagation, generation, and detec-tionof high-frequency (HF) radio waves in the Earth's ionosphere. With re-gardsto propagation, we derive a full wave Hamiltonian and a polarization evo-lutionequation for electromagnetic waves in a cold, stratified magnetoplasma.With regards to generation, we will be concerned with three experiments con-ducted at the ionosphere- radio wave interaction research facilities at Sura, Rus-siaand Tromsø, Norway. These facilities operate high power HF transmittersthat can inject large amplitude electromagnetic waves into the ionosphere andexcite numerous nonlinear processes. In an experiment conducted at the Surafacility, we were able to measure the full state of polarization of stimulatedelectromagnetic emissions for the first time. It is expected that by using thetechnique developed in this experiment it will be possible to study nonlinearpolarization effects on powerful HF pump waves in magnetoplasmas in the fu-ture.In another experiment conducted at the Sura facility, the pump frequencywas swept automatically allowing rapid, high-resolution measurements of SEEdependence on pump frequency with minimal variations in ionospheric condi-tions.At the Tromsø facility we discovered by chance a highly variable, pumpinduced, HF emission that most probably emanated from pump excited spo-radicE. Regarding detection, we have proposed a set of Stokes parametersgeneralized to three dimension space; and we have used these parameters in aninvention to detect the incoming direction of electromagnetic waves of multiplefrequencies from a single point measurement.

Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 6 uppsatser.

In this thesis we consider perturbations of a perfect fluid on locally rotationally symmetrical (LRS) class II cosmological backgrounds, with a nonvanishing vorticity of the fluid on the perturbed model. The method used is based on the 1+1+2 covariant decomposition of spacetime, motivated by the assumption of anisotropic expansion, followed by a harmonic decomposition of all gauge invariant quantities. All perturbed quantities can be solved for in terms of the time evolution equations of eight harmonic coefficients. This set of eight harmonic coefficients decouple into an even and odd sector, containing five and three variables respectively, where the vorticity is represented as one variable in each sector. We find that the time evolution of the vorticity completely decouples from the other perturbed variables, and can be solved exactly by assuming a linear equation of state. The evolution of the remaining perturbed variables are examined in the geometrical optics approximation, and compared to research looking at the case when the vorticity vanishes on the perturbed model. The results turn out to be the same, except for a source term containing the odd parity of the vorticity in the evolution of the density, which act as a source term for the shear waves. The four remaining variables will represent damped, source free gravitational waves.

The very-high-energy gamma-ray emission of the hard spectrum BL Lac source 1H1914-194 has been studied with Fermi-LAT data covering a nearly ten-year period between August 2008 until March 2018 in the energy range of 300 MeV to 870 GeV. The mean flux has been determined as 8.4 x 10-9±3.5 x 10-10 photon cm-2 s-1. The data processing has been done with the Enrico software using the Fermi Science Tools (v10r0p5) and the Pass 8 version of the data, performing binned analysis in order to handle the long integration time. The lightcurve shows that the source has to be considered as variable in the given time period for a three-month binning. It gives furthermore evidence for at least one quiet and active period lasting slightly over 1.5 years each. Even these shorter periods show a weak variability. The significance of the source has been determined as σ = 57.5 for a one-year period. The spectral analysis of three different time periods have been fitted by PowerLaw2, LogParabola and PLExpCutoff functions resulting in LogParabola being slightly favored in most of the cases. However, the test statistic are not showing enough significance that may lead to an unambiguous preference. The data from the analysis has been implemented in a multiwavelength view of the source, showing that the analysis is in agreement with the data coming from the Fermi catalogs. The overall emission of 1H1914-194 has been modelled with theoretical frameworks based on a one-zone Synchrotron Self Compton (SSC) model providing an acceptable description of the SED.

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The cold dark matter (CDM) paradigm underlying the standard <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CLambda" />CDM model of cosmology is successful on large scales but faces potential problems on small scales partly related to a seeming overproduction of dwarf galaxies. This could be alleviated in exotic dark matter models that suppresses small-scale structure formation. One such attractive model is known as fuzzy dark matter (FDM). FDM positsthat dark matter is composed of ultra-light bosons with masses <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?m_%7B%5Crm%20FDM%7D%20%5Csim%2010%5E%7B-22%7D" /> eV. With such light particle masses, quantum effects become important. More specifically, a pressure-like term appears in the equations of motion that counteracts gravitational collapse on small scales. Because small galaxies form first in CDM, it follows that the early history ot galaxy formation predicted by FDM should be markedly different. One novel way to probe this effect would be to use the 21 cm line of hydrogen which acts as a sensitive probe of the epoch of reionization (EoR) and Cosmic Dawn — when the first galactic sources of X-rays started to reheat theintergalactic medium (IGM). In this thesis, the evolution of the 21 cm signal have been simulated for both CDM and FDM. These simulations indicate that the fluctuationsin the 21 cm signal amenable to future observations are extremely weak (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cll" /> 1 mK) — and probably unobservable — for FDM at high redshifts <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%20%5Csim%2015-16" /> compared to CDM (which tend to yield signals with amplitudes <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cgg" /> 1 mK). This is mainly due to the delayed galaxy formation in FDM resulting in delayed Lyman-<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Calpha" /> coupling of the 21 cm spin temperature to the kinetic temperature of the IGM. A robust prediction from all FDM scenarios explored in this thesis is that any detection of a signal at <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%20%5Csim%2015-16" /> would rule out interesting particle masses for FDM, and would be evidence for CDM-like structure formation. Future work that properly models ionization fluctuations during the EoR could also yield strong predictions at lower redshifts.