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  • Määttä, Jussi (Helsingin yliopisto, 2016)
    Model selection is the task of selecting from a collection of alternative explanations (often probabilistic models) the one that is best suited for a given data set. This thesis studies model selection methods for two domains, linear regression and phylogenetic reconstruction, focusing particularly on situations where the amount of data available is either small or very large. In linear regression, the thesis concentrates on sequential methods for selecting a subset of the variables present in the data. A major result presented in the thesis is a proof that the Sequentially Normalized Least Squares (SNLS) method is consistent, that is, if the correct answer (i.e., the so-called true model) exists, then the method will find it with probability that approaches one as the amount of data increases. The thesis also introduces a new sequential model selection method that is an intermediate form between SNLS and the Predictive Least Squares (PLS) method. In addition, the thesis shows how these methods may be used to enhance a novel algorithm for removing noise from images. For phylogenetic reconstruction, that is, the task of inferring ancestral relations from genetic data, the thesis concentrates on the Maximum Parsimony (MP) approach that tries to find the phylogeny (family tree) which minimizes the number of evolutionary changes required. The thesis provides values for various numerical indicators that can be used to assess how much confidence may be put in the phylogeny reconstructed by MP in various situations where the amount of data is small. These values were obtained by large-scale simulations and they highlight the fact that the vast number of possible phylogenies necessitates a sufficiently large data set. The thesis also extends the so-called skewness test, which is closely related to MP and can be used to reject the hypothesis that a data set is random, possibly indicating the presence of phylogenetic structure.
  • Kupiainen-Määttä, Oona (Helsingin yliopisto, 2016)
    A large fraction of atmospheric aerosol particles are formed from condensable vapors in the air. This particle formation process has been observed to correlate in many locations with the sulfuric acid concentration, but the very first steps of cluster formation have remained beyond the reach of experimental investigation until recently. Charged clusters can now be detected and characterized starting from the smallest sizes and even neutral clusters consisting of only a few molecules can be detected, although their composition cannot be fully characterized. However, measuring the concentrations of different cluster types does not tell the full story of how the clusters were formed, and detailed simulations are needed in order to get a full understanding of the cluster formation pathways. Cluster formation is described by a set of nonlinear differential equations that cannot be solved analytically in any realistic situation. The best way to understand the complex behavior of cluster populations is by cluster kinetics simulations. The focus of this Thesis is on developing tools for simulating cluster formation, and using the simulation results to improve the detailed understanding of atmospheric aerosol particle formation. As sulfuric acid has been identified as the main driving force of cluster formation in many locations, it is also the main compound in the simulations of this Thesis. It cannot explain the observed atmospheric particle formation rates alone, and other possible participating species considered in this Thesis are ammonia, dimethylamine and water. In the first two papers of the Thesis, theoretical values are used for the collision and evaporation rates, and simulated cluster concentrations and formation rates are compared to experimental observations. The simulation results agree well with experimental findings from two very different studies. The third and fourth paper asses existing methods for interpreting cluster measurements and point out details that should be taken into account: the effect of dipole moments on chemical ionization of neutral molecules and clusters, and the conditions for the widely used nucleation theorem to be valid. The last paper introduces a new method for extracting cluster evaporation rates from measured cluster distributions.
  • Alves Antunes Soares, Joana Soares (2016)
    Atmospheric aerosols are subject to extensive research, due to their effect on air quality, human health and ecosystems, and hold a pivotal role in the Earth s climate. The first focus of this study is to improve the modelling of aerosol emissions and its dispersion in the atmosphere in both spatial and temporal scales and secondly, to integrate the dispersion modelling with population activity data that leads to exposure metrics. The mathematical models used in this study are fully or partially developed by the Finnish Meteorological Institute: a global-to -mesoscale chemical transport model, SILAM; a local-scale point/line-source dispersion model, UDM/CAR-FMI; and a human exposure and intake fraction assessment model, EXPAND. One of the outcomes of this work was the refinement of the emissions modelling for global-to-mesoscale dispersion model. Firstly, a new parameterisation for bubble-mediated sea salt emissions has been developed by combining and re-assessing widely used formulations and datasets. This parameterisation takes into account the effects of wind speed and seawater salinity and temperature, and can be applicable to particles with dry diameters raging between 0.01 and 10 µm. The parameterization is valid for low-to-moderate wind speed, seawater salinity ranging between 0 and 33 and seawater temperature ranging between -2 and 25 °C. Secondly, the near-real time fire estimation system, IS4FIRES, based on Fire Radiative Power (FRP) data from MODIS, was refined to reduce the overestimation of particulate matter (PM) emissions by including more vegetation types, improving the diurnal variation, removing highly-energetic sources and recalibrating the emission factors. Applying dynamic emission modelling brought more insight to the spatial distribution of these emissions, their contribution to the atmospheric budget, and possible impact on air quality and climate. The modelling shows that sea salt aerosol (SSA) can be transported far over land and contribute up to 6 µg m-3 to PM10 (at annual level), and indicate that the Mediterranean has sharp gradients of concentrations, becoming an interesting area to analyse for climate considerations. For fire, the simulations show the importance of meteorology and vegetation type for the intensity of the emissions. The simulations also show that MODIS FRP is accounting for highly energetic sources as a wildland fire, bringing up to an 80% overestimation in AOD, close to the misattributed sources. The second outcome is related to urban-scale modelling. The emissions for Helsinki Metropolitan Area (HMA) were revised to bring up-to-date the emissions for traffic and energy sectors in use for urban-scale modelling. The EXPAND model was revised to combine concentrations and activity data in order to compute parameters such as population exposure or intake fraction. EXPAND includes improvements of the associated urban emission and dispersion modelling system, time use of population, and infiltration coefficients from outdoor to indoor air. This refinement showed that PM2.5 in HMA is mainly originated from long-range transport, with the largest local contributors being vehicular emissions and shipping (at harbours and its vicinity). At annual level, the population living mostly indoors (home and work) is mainly exposed to PM2.5 with an acutely increased exposure while commuting.
  • Tapiola, Olli (Helsingin yliopisto, 2016)
    Different dyadic techniques are an inseparable part of modern-day harmonic analysis both in the Euclidean space and in metric spaces. In this dissertation, we improve adjacent and random dyadic techniques in metric spaces and apply these and previously known techniques for questions related to metric, Euclidean and vector-valued analysis. The dissertation consists of an introductory part and four research articles. In the first article, we present a general randomization procedure for dyadic systems in metric spaces which can be used for constructing both random and adjacent dyadic systems. As an application of the new random systems, we improve the continuity properties of metric wavelets of P. Auscher and T. Hytönen by exploiting the improved ``smallness of boundary'' property of our random cubes. In the third article, we prove some additional properties for our adjacent dyadic systems to prove a decomposition result for dyadic systems in metric spaces. With its help, we give an alternative proof for the quantitative bound of the Lp norm of shift operators acting on vector-valued functions in metric spaces. In the second article, we explore certain properties of the Muckenhoupt weight classes, the class of Reverse Hölder weights and their weakened versions in spaces of homogeneous type. In the Euclidean setting, the Muckenhoupt weight classes have numerous different equivalent definitions but in spaces of homogeneous type some of those equivalences break down. We show that although certain definitions are no longer equivalent in this context, their weakened versions still define the same weight classes. We also show that every weak Reverse Hölder weight has a self-improving property. In the literature, these types of weak weights appear especially in the theory of partial differential equations. In the fourth article, we prove quantitative weighted bounds for so called rough homogeneous singular integrals by combining older techniques with a quantitative version of M. Lacey's recent extension of the A2 theorem. The proof of this extension is based on a domination technique which provides a way to dominate Calderón-Zygmund operators pointwise with the help of a finite number of simple sparse operators associated with adjacent dyadic systems.
  • Mod, Heidi (Helsingin yliopisto, 2016)
    The effects of co-occurring species, namely biotic interactions, govern performance and assemblages of species along with abiotic factors. They can emerge as positive or negative, with the outcome and magnitude of their impact depending on species and environmental conditions. However, no general conception of the role of biotic interactions in functioning of ecosystems exists. Implementing correlative spatial modelling approaches, combined with extensive data on species and environmental factors, would complement the understanding of biotic interactions and biodiversity. Moreover, the modelling frameworks themselves, conventionally based on abiotic predictors only, could benefit from incorporating biotic interactions and their context-dependency. In this thesis, I study the influence of biotic interactions in ecosystems and examine whether their effects vary among species and environmental gradients (sensu stress gradient hypothesis = SGH), and consequently, across landscapes. Species traits are hypothesized to govern the species-specific outcomes, while the SGH postulates that the frequency of positive interactions is higher under harsh environmental conditions, whereas negative interactions dominate at benign and productive sites. The study applies correlative spatial models utilizing both regression models and machine-learning methods, and fine-scale (1 m2) data on vascular plant, bryophyte and lichen communities from Northern Finland and Norway (69°N, 21°E). In addition to conventional distribution models of individual species (SDM), also species richness, traits and fitness are modelled to capture the community-level impacts of biotic interactions. The underlying methodology is to incorporate biotic predictors into the abiotic-only models and to examine the impacts of biotic interactions and their dependency on species traits and environmental conditions. Cover values of the dominant species of the study area are used as proxies for the intensity of their impact on other species. The results show, firstly, that plant plant interactions consistently and significantly affect species performance and richness patterns. Secondly, the results make evident that the impacts of biotic interactions vary between species, and, more importantly, that the guild, geographic range and traits of species can indicate the outcome and magnitude of the impact. For instance, vascular plant species, particularly competitive ones, respond mainly negatively to the dominant species, whereas lichens tend to show more positive responses. Thirdly, as proposed, the manifestation of biotic interactions also varies across environmental gradients. Support for the SGH is found as the effect of the dominant species is more negative under ameliorate conditions for most species and guilds. Finally, simulations of species richness, where the cover of the dominant species is modified, demonstrate that the biotic interactions exhibit a strong control over landscape-level biodiversity patterns. These simulations also show that even a moderate increase in the cover of the dominant species can lead to drastic changes in biodiversity patterns. Overall, all analyses consistently demonstrate that taking into account biotic interactions improves the explanatory power and predicting accuracy of the models. There are global demands to understand species-environment relationships to enable predictions of biodiversity changes with regard to a warming climate or altered land-use. However, uncertainties in such estimates exist, especially due to the precarious influence of biotic interactions. This thesis complements the understanding of biotic interactions in ecosystems by demonstrating their fundamental, yet species-specific and context-dependent, role in shaping species assemblages and performance across landscapes. From an applied point of view, our study highlights the importance of recognizing biotic interactions in future forecasts of biodiversity patterns.
  • Koivisto, Juhani (Helsingin yliopisto, 2016)
    The dissertation Amenability of metric measure spaces and fixed point properties of groups consists of three articles revolving around amenability and property (T) in different contexts, and a summary. In the first article, (non-)amenability of hyperbolic metric spaces is considered. In it, we prove that a uniformly coarsely proper hyperbolic cone of a connected bounded metric space containing at least two points is non-amenable. In particular, this implies that any uniformly coarsely proper visual Gromov hyperbolic space with connected boundary containing at least two points is non-amenable. In the second article, the degree of amenability of metric measure spaces is considered in general. Here, we prove a homological characterisation of global weighted Sobolev inequalities for quasiconvex uniform metric measure spaces that support a local weak (1,1)-Poincaré inequality using methods from large scale algebraic topology. Returning to the topic of the first article, we show that a quasiconvex visual Gromov hyperbolic uniform metric measure space that supports a local weak (1,1)-Poincaré inequality with a connected boundary containing at least two points satisfies a global Sobolev inequality. In the third article, fixed point conditions for uniformly bounded group actions on Hilbert spaces are considered. In the article, we establish a spectral condition for the vanishing of the first cohomology group of the complex of square integrable cochains twisted by a uniformly bounded representation of an automorphism group of a 2-dimensional simplicial complex. In particular, if the automorphism group acts properly discontinuously and cocompactly on the complex this implies that every affine action of the automorphism group on the Hilbert space with linear part given by the representation has a fixed point. In the summary, the results of the articles are further explained and placed in a larger context: mathematically as well as historically.
  • Aurela, Minna (Helsingin yliopisto, 2016)
    Atmospheric aerosols have a significant effect on people and the environment. They cause adverse health effects especially for cardiorespiratory patients. Atmospheric aerosols also affect the Earth s climate directly by scattering and absorbing solar radiation and indirectly by modifying amounts and properties of clouds. Reliable characterisation and quantification of airborne particles and their sources are essential for developing control strategies for atmospheric pollution and decreasing the uncertainties of estimating particles effects on climate change. Aerosols can be divided into fine and coarse particles based on their size. This thesis concentrated on fine particles, which are either directly emitted into the atmosphere (primary particles) or formed in the atmosphere through gas-to-particle conversion (secondary particles). The main sources of atmospheric fine particles are natural and anthropogenic combustion, industry and secondary aerosol formation (biogenic and anthropogenic). The overall objective of this thesis was to investigate the chemical composition of fine particles with different temporal and seasonal time-scales. For this purpose, a variety of different sampling techniques, off-line analytical methods and on-line instruments were used to characterise the main chemical species of fine particles in simultaneous and independent campaigns at several sites mainly in southern Finland but also in South Africa. More specific objectives were to investigate the dominant sources of particulate organic matter in the Helsinki area and chemically characterise the fine particles originating from biomass burning. Determining the effect of air mass origin on the chemical composition and concentration of fine particles was also one of the specific aims. In this thesis, it was found that the chemical composition of fine particles had strong spatial and temporal variation, although on average the mass concentrations of fine particles were quite similar between different sites. The main components in fine particulate matter in southern Finland was particulate organic matter (POM), followed by sulphate, whereas sulphate had the highest contribution to particulate mass in South Africa. Source apportionment analysis of POM revealed clear primary sources from traffic and from biomass burning. However, the secondary organic aerosol had the largest contribution of POM, even though the campaigns were conducted in specific environments such as residential areas where biomass combustion is commonly used or traffic environments. Occasionally, the contribution of biomass burning organic aerosol increased substantially as ambient air temperature decreased. Additionally, simultaneous measurements pointed out a high contribution of common regional or long-range transported sources over large areas of southern Finland. Similarly, the air mass passing over either clean or polluted areas showed a significant effect on the mass concentrations in Finland and South Africa.
  • Granberg, Fredric (Helsingin yliopisto, 2016)
    The need for energy and electricity in the modern society places large interest in producing power in more efficient and environmentally friendly ways. Most of the new concepts also need new or improved materials. One key component of the power plants is the structural materials. Both current and future power plants are and will be built from some kinds of steel, which has shown very good properties. Alloying and choice of manufacturing technique have been shown to change the properties of steel and even better materials have been obtained. Conventionally, these improvements have been studied by conducting experiments on the possible materials, but recently computer simulations have started to be an essential tool in the search for better materials. The mechanical properties of materials are known to be determined by the movement of dislocations. If the dislocation movement is hindered, the material will become harder, but also more brittle. Previous computational studies have investigated the effect of voids, bubbles and other dislocation structures on the movement of dislocations. However, the effect of non-coherent obstacles, man-made or irradiation induced, have received much less attention. To address this lack of data, we have investigated the effect of carbides, to represent non-coherent obstacles, on the dislocation movement. In this study we investigated different carbides and compared them with other defect structures, voids and fixed atoms. The effect of size and temperature were also determined. We obtained two parameters for each obstacle, the needed unpinning stress and the unpinning mechanism, that can be used in other simulation methods to be able to investigate larger systems. We found that the nanostructure of the obstacle can drastically change the needed unpinning stress. We also found that the obstacles with same unpinning mechanism showed similar unpinning stresses, but to obtain the subtle differences we need to consider the nanostructure. A comparison of spherical and rod shaped obstacles showed that the surface curvature will be an important factor in the unpinning event. Recently, a new class of metal alloys have been found, where the concentrations of the alloying elements are in large or equal fractions. We wanted to investigate the possibility to use these equiatomic multicomponent alloys in environments where radiation is present. These alloys have been shown to have very promising properties, but there were no data on the irradiation response of these alloys. We studied two equiatomic multicomponent alloys, two-element and three-element alloys, and compared them with the single element specimen. We found a major reduction of defect build-up in the alloys compared to the single element sample, in agreement with experiments. We found one of the key mechanisms of this reduction to be the reduced dislocation mobility in the alloys.
  • Luisto, Rami (Helsingin yliopisto, 2016)
    In this dissertation we study the theory of quasiregular- and BLD-mappings. The unifying theme of the dissertation is how results and techniques used in the study of one of these classes gives rise to ideas in the theory of the other. The first paper in the thesis concerns the theory of quasiregular mappings, which are non-homeomorphic generalizations of quasiconformal mappings; heuristically quasiregular mappings can be defined as mappings that map infinitesimal balls to infinitesimal ellipsoids with a bound on the eccentricity of the ellipsoid. The first article is especially focused on quasiregularly elliptic manifolds, i.e. those closed Riemannian manifolds for which there exists a non-constant quasiregular mapping from the euclidean space of same dimension to the manifold. The classes of quasiregularly elliptic 2- and 3-manifolds are classified up to a quasiconformal homeomorphism. In higher dimensions it is still a widely open question which closed manifolds are quasiregularly elliptic, but even though a complete classification of quasiregularly elliptic manifolds is hard, there are results restricting the structure of quasiregulary elliptic manifolds. One of the basic results is the Varopoulos theorem that gives an upper bound to the growth rate of the fundamental group of a quasiregularly elliptic manifold. The main result of the first paper is that if the growth rate of the fundamental group of a quasiregularly elliptic manifold is as large as possible, then the manifold is in fact a finite quotient of a torus. Mappings of bounded length distorion, BLD-mappings for short, were first defined by Martio and Väisälä in 1988 in euclidean domains as a subclass of quasiregular mappings. Even though the original defintion of BLD-mappings was analytical, Martio and Väisälä showed that there exists also a definition with a more metric flavor. Indeed, BLD-mappings can be defined as those open, continuous and discrete mappings that preserve the length of all paths up to a uniform multiplicative constant. This definition lends itself to all (path-)metric spaces, and in the latter two articles of the thesis we study which classical results of euclidean BLD- or quasiregular mappings can be proven in the setting of locally compact and complete path-metric spaces.
  • Gross, Oskar (Helsingin yliopisto, 2016)
    In order to analyse natural language and gain a better understanding of documents, a common approach is to produce a language model which creates a structured representation of language which could then be used further for analysis or generation. This thesis will focus on a fairly simple language model which looks at word associations which appear together in the same sentence. We will revisit a classic idea of analysing word co-occurrences statistically and propose a simple parameter-free method for extracting common word associations, i.e. associations between words that are often used in the same context (e.g., Batman and Robin). Additionally we propose a method for extracting associations which are specific to a document or a set of documents. The idea behind the method is to take into account the common word associations and highlight such word associations which co-occur in the document unexpectedly often. We will empirically show that these models can be used in practice at least for three tasks: generation of creative combinations of related words, document summarization, and creating poetry. First the common word association language model is used for solving tests of creativity -- the Remote Associates test. Then observations of the properties of the model are used further to generate creative combinations of words -- sets of words which are mutually not related, but do share a common related concept. Document summarization is a task where a system has to produce a short summary of the text with a limited number of words. In this thesis, we will propose a method which will utilise the document-specific associations and basic graph algorithms to produce summaries which give competitive performance on various languages. Also, the document-specific associations are used in order to produce poetry which is related to a certain document or a set of documents. The idea is to use documents as inspiration for generating poems which could potentially be used as commentary to news stories. Empirical results indicate that both, the common and the document-specific associations, can be used effectively for different applications. This provides us with a simple language model which could be used for different languages.
  • Soultanis, Elefterios (Helsingin yliopisto, 2016)
    This dissertation studies classical questions in the field of geometric analysis in the context of metric spaces. The dissertation is comprised of three research articles. The first is on the connection of quasiconformal maps and the quasihyperbolic metric. The remaining two concern notions of homotopy classes of Sobolev type maps between metric spaces, comparison with the manifold case, and the existence of minimizers of a p-energy in these homotopy classes. The unifying theme of all three articles is analysis on metric spaces. That is, all three papers deal with questions concerning maps between metric spaces. The particular type of metric spaces involved is generally referred to as PI-spaces. PI-spaces satisfy conditions allowing one to extend a large part of classical first order calculus, such as the theory of Sobolev maps and, á posteriori, differentiability of Lipschitz functions.
  • Rautiainen, Sari (Helsingin yliopisto, 2016)
    Selective catalytic transformations are essential for the sustainable production of chemicals using renewable biomass feedstocks. In particular, catalytic oxidations utilizing oxygen or hydrogen peroxide reduce the use of toxic chemicals providing environmentally benign routes to replace petroleum-derived chemicals. Recently, catalysts based on gold nanoparticles have gained a lot of interest due to their excellent performance, stability and reusability in selective oxidation under mild conditions, and especially in oxidation of biomass-derived substrates. In this thesis, selective and environmentally friendly methods for oxidation of alcohols, aldehydes and carbohydrates were developed using gold nanoparticles supported on metal oxides. The work expands the substrate scope of gold catalysts and, on the other hand, elucidates the effect of reaction conditions on the oxidation. The effect of reaction media on the chemoselectivity of gold catalysis was studied using benzyl alcohol as a model compound. As a result, we introduced a new one for all concept for oxidation of alcohol into a variety of products. In a further study, vanillic acid was produced by oxidation of vanillin, an aromatic aldehyde available from biomass lignin. Oxidation of carbohydrates with gold catalysts was studied both in alkaline and base-free conditions. Uronic acids, available from hemicelluloses and pectin, were selectively oxidized into sugar diacids under very mild conditions. Base-free oxidation of glucose to gluconic acid was studied using microwave irradiation and hydrogen peroxide as green oxidant. In both studies, significantly shortened reaction times and very high activities compared to previous reports were achieved.
  • Ruotsalainen, Kari (Helsingin yliopisto, 2016)
    Inelastic X-ray scattering spectroscopies form a versatile family of experimental techniques that are capable of probing the ground state properties as well as the single-particle and collective excitations of condensed-matter systems. Electronic excitations are of fundamental importance in determining the optical and transport properties of solids and also take part in the screening of the Coulomb interaction, thus contributing to ground-state properties. Transition-metal and rare-earth compounds exhibit a large variety of physical phenomena such as metal-insulator transitions, colossal magnetoresistance, unconventional magnetic ground states etc. These compounds are of fundamental and applied interest due to challenges in their theoretical description and their potential use in various devices ranging from transistors to infrared detectors. This thesis presents four applications of inelastic X-ray scattering methods in transition-metal and rare-earth oxides as well as transition-metal dichalcogenides. First, we demonstrate that valence excitations in 55-nm-thick LaAlO3 /SrTiO3 heterostructures can be measured. The data is analyzed utilizing free-ion multiplet and first-principles calculations. Second, it is demonstrated that the electron momentum-density difference across the metal-insulator phase transition in VO2 is observable using the Compton-scattering spectroscopy, which probes the electronic ground state. The experimental results are compared against first- principles calculations. Third, a new experimental resonant X-ray emission method that utilizes a X-ray standing wave to excite the resonant scattering process is demonstrated using a Gd3Ga5O13 single crystal. The method is shown to be very sensitive to quadrupole excitations. Its potential in achieving atomic-site sensitivity and electronic-state symmetry selectivity are discussed. Last, a joint experimental and computational study on the high- energy plasmon excitations in the transition-metal dichalcogenides Cu0.2NbS2 and NbSe2 is presented. The primary significance of the first three studies lies in in demonstrations of new types of experiments, with several interesting possible applications in solid-state physics and materials science. The last study contributes to the discussion on the optical properties of transition-metal dichalcogenides.
  • Lehtinen, Jyri (Helsingin yliopisto, 2016)
    The outer convective envelopes of late-type stars are believed to support turbulent dynamos that are able to generate strong and dynamic magnetic fields. When these magnetic fields penetrate the stellar surface, they give rise to directly observable activity phenomena, such as dark spots and line emission from the chromospheric layers. Observing and analyzing these activity indicators allows us to characterize the behaviour of the dynamos on a wide range of different stars as well as to put the behaviour of the Sun into a wider context. This thesis presents a study of the activity of a sample of 21 young solar-type stars that can be seen as analogues of the Sun during the first few hundred million years of its existence. The aim is to characterize their activity behaviour on different time scales from months to decades as well as to derive estimates for the magnitude of their surface differential rotation. The results of the analysis provide important observational constraints for the dynamo theory. The study is primarily based on the time series analysis of up to three decades of photometric monitoring of the sample stars. The light curves of these stars display quasiperiodic variations that are induced by changing patterns of dark starspots on the stellar surface and the rotation of the star. The time series analysis results are supplemented by spectroscopic observations that are used to determine the chromospheric activity levels of the stars. The time series analysis is performed using the Continuous Period Search and Carrier Fit methods along with a selection of additional complementary tools. The development of the Continuous Period Search method forms a part of this thesis. This method is fully characterized and its performance is tested on noisy low amplitude light curve data. The photometric analysis reveals new results from both activity cycles and the longitudinal distribution of the spot activity on the stars. Furthermore, it confirms previously published results concerning these and the strength of the surface differential rotation using a new sample of stars. The results show that activity cycles of different lengths are common on active solar-type stars and that their lengths fall on a set of distinct branches. When the longitudinal distribution of the spot activity is compared to the activity level of the stars, the results show a clear domain shift between stars that have axisymmetric spot distributions and stars where the spot activity shows stable longitudinal concentration. Such concentration of activity, known as active longitudes, only appears on the faster rotating more active stars. This suggests a transition between axisymmetric and non-axisymmetric dynamo modes as a function of the activity level of a star. The active longitudes also show a tendency to follow prograde propagation with respect to the stellar rotation. This may be interpreted either as a signature of radial differential rotation or as an azimuthal dynamo wave.
  • Ahonen, Teppo (Helsingin yliopisto, 2016)
    Measuring similarity in music data is a problem with various potential applications. In recent years, the task known as cover song identification has gained widespread attention. In cover song identification, the purpose is to determine whether a piece of music is a different rendition of a previous version of the composition. The task is quite trivial for a human listener, but highly challenging for a computer. This research approaches the problem from an information theoretic starting point. Assuming that cover versions share musical information with the original performance, we strive to measure the degree of this common information as the amount of computational resources needed to turn one version into another. Using a similarity measure known as normalized compression distance, we approximate the non-computable Kolmogorov complexity as the length of an object when compressed using a real-world data compression algorithm. If two pieces of music share musical information, we should be able to compress one using a model learned from the other. In order to use compression-based similarity measuring, the meaningful musical information needs to be extracted from the raw audio signal data. The most commonly used representation for this task is known as chromagram: a sequence of real-valued vectors describing the temporal tonal content of the piece of music. Measuring the similarity between two chromagrams effectively with a data compression algorithm requires further processing to extract relevant features and find a more suitable discrete representation for them. Here, the challenge is to process the data without losing the distinguishing characteristics of the music. In this research, we study the difficult nature of cover song identification and search for an effective compression-based system for the task. Harmonic and melodic features, different representations for them, commonly used data compression algorithms, and several other variables of the problem are addressed thoroughly. The research seeks to shed light on how different choices in the scheme attribute to the performance of the system. Additional attention is paid to combining different features, with several combination strategies studied. Extensive empirical evaluation of the identification system has been performed, using large sets of real-world music data. Evaluations show that the compression-based similarity measuring performs relatively well but fails to achieve the accuracy of the existing solution that measures similarity by using common subsequences. The best compression-based results are obtained by a combination of distances based on two harmonic representations obtained from chromagrams using hidden Markov model chord estimation, and an octave-folded version of the extracted salient melody representation. The most distinct reason for the shortcoming of the compression performance is the scarce amount of data available for a single piece of music. This was partially overcome by internal data duplication. As a whole, the process is solid and provides a practical foundation for an information theoretic approach for cover song identification.
  • Zhu, Cheng (Helsingin yliopisto, 2016)
    Noble-gas chemistry was started in 1962 when Neil Bartlett reported the first noble-gas compound, xenon hexafluoroplatinate. This discovery was followed by successful synthesis of many other noble-gas compounds. Matrix-isolation infrared spectroscopy has been an important tool in noble-gas chemistry. In 1995, noble-gas hydrides HNgY (Ng is a noble-gas atom and Y is an electronegative fragment) were characterized by using this method. To date, a substantial number of noble-gas hydrides have been identified including the first neutral ground-state Ar compound, HArF. A few noble-gas compounds of the YNgY’ type (Y and Y’ are electronegative fragments) have also been prepared in matrices. This thesis consists of two parts. In the first part, two new HNgY compounds (HKrCCCl and HXeCCCl) and three new YNgY’ compounds (FKrCN, FXeCN, and FXeNC) are described. These molecules are prepared in noble-gas matrices (Kr and Xe) by ultraviolet photolysis of the precursors and subsequent annealing and characterized by infrared spectroscopy. The assignments are supported by ab initio calculations at the MP2(full) and CCSD(T) levels of theory. The precursors HCCCl and FCN are synthesized using a microwave discharge of HClC=CCl2/Ng and (FCN)3/Ng gaseous mixtures, respectively. The infrared spectrum of CCCl radical is also reported. In the second part, the complexation effect on HXeI and the matrix effect on HXeI and HXeH are studied. Two new complexes (HXeI···HCl and HXeI···HCCH) are prepared in a Xe matrix and assigned based on the ab initio calculations at the MP2(full) and CCSD(T) levels of theory. Similarly to the previously studied systems, the H–Xe stretching mode of HXeI in these complexes show substantial blue shifts. The decomposition of the HXeI complexes (including known HXeI···HBr and HXeI···HI) by broadband IR radiation of the spectrometer slows down as the H–Xe stretching frequency increases. HXeI and HXeH are studied in Ar, Kr, and Xe matrices. The order of the H–Xe stretching frequency of HXeH in these matrices (ν(Xe) < ν(Kr) < ν(Ar)) is the same as that of previously studied HXeCl, HXeBr, and HXeCCH. For HXeI, the frequency order is diffe rent: ν(Xe) < ν(Ar) < ν(Kr), which is a remarkable observation. The experimental results are successfully simulated using the hybrid quantum-classical calculations.
  • Vitikainen, Anne-Mari (Helsingin yliopisto, 2016)
    Individually navigated transcranial magnetic stimulation (nTMS) has been used to locate and map the primary motor cortical areas since the inception of the technique. Recently, it has been added to the pre-surgical routine for epilepsy and brain tumor patients. The accuracy of the mappings in healthy volunteers and brain tumor patients and their feasibility in the pre-surgical evaluation of brain tumor patients have been established. The originating causes for epilepsy are variable and affect the functional localizations in relation to conventional anatomy. A reliable and versatile pre-surgical method for the localization of the functional cortical areas is essential for pre-surgical risk-benefit assessments and it is important to the success of surgical treatments. In this thesis, I describe an nTMS mapping protocol suitable for clinical use and evaluate the accuracy of the motor cortical mappings by comparing the results with the results of direct electrical stimulation of the primary motor cortex. The accuracy, 11 ± 4 mm for the hand and 16 ± 7 mm for the arm muscle groups, is sufficiently good for pre-surgical evaluation in patients with severe epilepsy. With this patient group, the nTMS technique enables the mapping of the abnormally excitable tissue, which has an impact on the interpretation and reliability of the mappings as well. In addition to the mapping of the motor cortical areas, the cortical areas related to speech are of key interest in neurosurgery. The speech-related cortical areas are commonly localized noninvasively with functional magnetic resonance imaging techniques. The dominant hemisphere for language functions can be discriminated with the invasive Wada test in the pre- surgical evaluation of epilepsy patients. Recently, nTMS protocols have been introduced for localization of speech-related cortical areas. The analysis of the nTMS elicited modifications in the language task performance have commonly been analyzed manually from video recordings and the methods for the reliable determination of the nTMS elicited speech-response latencies, their categorization and analysis, have been sparse. In the last part of this dissertation, I developed a semi-automated script for the speech-response latency difference calculation based on the accelerometer signal of the speech-response elicited vibrations of the larynx. The developed script was individually optimized for speech-response detection. According to the presented results the method is capable of determining the speech-response latencies with a sensitivity of 96% and a specificity of 71%, against the manual review from the video and visual observations from the accelerometer signals. Based on the results presented in this thesis, nTMS is a reliable method for the mapping of the functional cortical areas pre-surgically in patients with severe epilepsy. It also enables the mapping of abnormally excitable brain areas.
  • Hatz, Richard (Helsingin yliopisto, 2016)
    Intermolecular interactions are ubiquitous, and their intricate network plays a decisive role in most of the phenomena encountered in our everyday lives. The focus of this thesis is on the London dispersion forces, a component present in all interactions between atoms and molecules, and often the most important one at long intermolecular distances. The quantum-mechanical origin of these forces can be traced to the correlated fluctuations of the molecular charge distributions, which however render the dispersion interactions challenging to calculate accurately, due to the high-level electronic structure methods required. The aim of the research presented in this thesis is to investigate the dispersion interactions, and to develop a viable method for modeling them. The systems studied in the accompanying research articles mainly encompass small clusters of coinage (Cu, Ag, and Au) and volatile (Zn, Cd, and Hg) metals. The long-range forces present in these clusters are calculated by means of highly correlated electronic structure methods, and the interaction potentials are used to develop a simple but effective model, capable of accurately describing the dispersion interactions in a variety of systems. Some original theoretical considerations are also elaborated. A novel formula is derived for the tensor describing all intermolecular interactions, and it is applied to investigate the long-range interaction potential of coinage metal hydrogen clusters. The method developed to account for the dispersion energy is a pair-potential model, where the total intermolecular London forces are calculated by means of atomic dispersion coefficients describing the magnitude and orientation dependence of the interaction. The coefficients are calculated based on small model systems, and they are used to compute the dispersion energy in larger clusters at no additional cost. Encouraging results are also obtained for the computed orientation averaged interaction potentials. All things considered, the publications included in this thesis indicate that the methods proposed and implemented to analyze the studied systems are capable of accurately modeling the non-covalent forces in a straightforward fashion.
  • Peltola, Timo Hannu Tapani (Helsingin yliopisto, 2016)
    The position sensitive silicon particle detectors are widely used in the tracking systems of High Energy Physics experiments such as the CMS at LHC, the world's largest particle accelerator at CERN. The foreseen upgrade of the LHC to its high luminosity (HL) phase, will enable the use of maximal physics potential of the facility. However, after 10 years of operation the expected fluence will result in a radiation environment that is beyond the capacity of the present tracking system design. The required upgrade of the all-silicon central trackers will include higher granularity and radiation hard sensors that can tolerate the increased occupancy and the higher radiation levels. To address this, extensive measurement and simulation studies have been performed to investigate different designs and silicon materials. The work in this thesis has been carried out within the CMS Tracker Upgrade Project and the multi-experiment RD50 Collaboration. Simulations serve a vital role in device structure optimization and predicting the electric fields and trapping in the silicon sensors. The main objective of device simulations is by using professional software to develop an approach to both model and predict the performance of the irradiated silicon detectors. In the course of this thesis, an effective non-uniform defect model is developed using the Sentaurus TCAD simulation framework. The model reproduces both the observed bulk and surface properties and can predict the performance of strip detectors up to HL-LHC fluences. When applied to measurements of the position dependence of Charge Collection Efficiency, the model can provide a means for the parametrization of oxide charge accumulation at the detector s SiO2/Si interface as a function of irradiation dose. TCAD simulations are also applied for a comparative study of a thin p-on-p pixel sensor and a more conventional p-on-n pixel sensor. The simulations are used to provide an explanation to the measured charge collection behavior and for a detailed investigation of the electrical properties of the two sensor types. Finally, the scope of TCAD simulations is extended to GaAs, a compound semiconductor material. By implementing the observed deep donor defect level to the simulation, the resulting electrical properties are in close agreement with the measurements of an epitaxial GaAs radiation detector. Also, the transferred electron effect observed in the transient current measurements is reproduced by the simulation. The combined results of this thesis demonstrate the versatility and power of the TCAD simulations of semiconductor detectors as a tool to bridge the gap from observation to parametrization.
  • Klein, Robert (Helsingin yliopisto, 2016)
    This dissertation presents paleomagnetic results from Baltica for three significant time intervals during the evolution of Precambrian supercontinents: i) 1.87 Ga the onset of the supercontinent Nuna; ii) 1.58 1.57 Ga maximum compaction of Nuna; and iii) ca. 0.6 0.57 Ga the break-up between Baltica and Laurentia which marked the final break-up of the supercontinent Rodinia. Paleogeographic reconstructions at 1.25 Ga and 1.05 Ga which marks the transition from Nuna to Rodinia - is briefly discussed based on existing paleomagnetic results; no new data were produced for these two reconstructions. New paleomagnetic results were obtained from samples from the 1.87 Ga Keuruu diabase dyke swarm (Central Finland), 1.58-1.57 Ga Åland intrusives (Åland archipelago, SW Finland) and ca. 0.6-0.57 Ga Hailuoto sedimentary formation (Western Finland). In addition to these, new paleomagnetic results were obtained from the Satakunta sandstone formation (SW Finland), providing a new paleomagnetic age estimate of ca. 1.63 1.45 Ga for the sandstone deposition, older than previously thought (ca. 1.4-1.3 Ga). The 1.87 Ga Keuruu diabase dykes yield a dual polarity intermediate NNW downward or SSE upward characteristic remanent magnetization (ChRM), positioning Baltica at low latitudes in a way that would proceed into the 1.8-1.2 Ga NENA (North Europe North America) configuration. The age of the Keuruu dykes are defined by a U-Pb (zircon) age of 1870 ± 9 Ma and a Pb-Pb (zircon) age of 1869 ± 7 Ma. The primary nature of the Keuruu ChRM is supported by positive baked contact tests. The Åland intrusives and Satakunta sandstone both yield dual polarity shallow (upward and downward) NNE or SSW characteristic remanent magnetizations (ChRM), positioning Baltica on equatorial paleolatitudes and supporting the NENA configuration between Baltica and Laurentia during the Mesoproterozoic. Well determined U-Pb (zircon) ages narrows the occurrence of intrusion to 1570 1580 Ma. The primary nature of the Åland intrusives ChRM is supported by positive baked contact tests. The Hailuoto sedimentary samples were obtained from a 272 m deep fully oriented drill core. The depostional age of the Hailuoto sediments is poorly constrained at 570-600 Ma, based on fossil records. The Hailuoto pole adds to the scattered Ediacaran paleomagnetic data of Baltica and indicates large distances between other late Neoproterozoic and early Cambrian paleomagnetic poles, implying rapid movement of Baltica from high latitudes at 615 Ma, over the polar region, to low latitudes at 550 Ma. In all these cases a secondary component with shallow to intermediate downward inclinations and northeasterly declinations were observed. This secondary component is commonly observed in the Fennoscandian Shield and has previously been interpreted as a result of oxidizing fluids during the Phanerozoic. In the case of Keuruu and Åland, meteorite impact (from Keurusselkä and Lumparn, respectively) cannot be ruled out as a possible cause of the secondary component. This component is mostly observed at low coercivities/blocking temperatures, but sometimes also at higher coercivities/blocking temperatures, causing asymmetry in the dual polarity results of Keuruu diabase dykes and Åland intrusives. In summary, the work presented in this dissertation provides new paleomagnetic results for Baltica, adding two new key poles to the Paleoproterozoic-Mesoproterozoic apparent polar wander path (APWP) of Baltica. Furthermore, the results provide a better understanding of the movement of Baltica during the enigmatic Ediacaran period. Paleogeographic reconstructions are presented at 1.87 Ga, 1.57 Ga, 1.25 Ga, and 1.05 Ga. The 1.87 Ga reconstruction do not support the NENA or SAMBA configurations, but indicates the onset of Nuna. The 1.57 Ga and 1.25 Ga reconstructions indicate similarities in the relative positions between Baltica, Laurentia, Amazonia, and Siberia during this period. The transition between Nuna and Rodinia is marked by a rotation between Laurentia, and Baltica and Amazonia (and possibly West Africa), forming the Grenville collisional belt.