Matemaattis-luonnontieteellinen tiedekunta

 

Recent Submissions

  • Heinonen, Esko (Helsingin yliopisto, 2017)
    This dissertation consists of four research articles whose unifying theme is the existence and non-existence of continuous entire non-constant solutions for nonlinear differential operators on Riemannian manifolds. The existence of such solutions depends heavily on the geometry of the manifold and, in the case of complete and simply connected Riemannian manifolds, we prove the existence under assumptions on the sectional curvature. In the first and fourth article we study the existence of minimal graphic functions by solving the asymptotic Dirichlet problem. Here the idea is to compactify the Cartan-Hadamard manifold by adding an asymptotic boundary and equipping the resulting space with the cone topology. Then one can solve the asymptotic Dirichlet problem i.e. prove the existence of entire solutions with prescribed continuous boundary values on the asymptotic boundary. In the fourth article we prove also a non-existence result by showing that asymptotically non-negative sectional curvature implies uniform gradient estimate for minimal graphic functions with at most linear growth. The second article deals with the existence of A-harmonic functions and the third article with the existence of f-minimal graphs. In the case of A-harmonic functions, we improve an earlier result of A. Vähäkangas by relaxing the assumption on the curvature upper bound. Here, again, we solve the asymptotic Dirichlet problem in order to get the existence result. We solve the asymptotic Dirichlet problem also for the f-minimal equation, but differing from the other papers, here we consider also the existence in the case of bounded domains.
  • Liu, Jinxiu (Helsingin yliopisto, 2017)
    Land cover is critical information to various land management and scientific applications, including biogeochemical and climate modeling. In addition, fire is an essential factor in shaping of vegetation structures, as well as for the functioning of savanna ecosystems. Remote sensing has long been an important and effective means of mapping and monitoring land cover and burned area over large areas in a consistent and robust way. Owing to the free and open Landsat archive and the increasing availability of high spatial resolution imagery, seasonal features from the temporal domain and the use of texture features from the spatial domain create new opportunities for land cover characterization and burned area detection. This thesis examined the application of satellite image time series and texture information in land cover characterization and burned area detection. First, the utility of seasonal features derived from Landsat time series (LTS) in improving accuracies of land cover classification and attribute prediction in a savanna area in southern Burkina Faso was studied. Then, the temporal profiles from LTS were explored for mapping burned areas over a 16 year period, and MODIS burned area product was used for comparison. Finally, the application of texture features derived from high spatial resolution data in land cover classification and attribute predictions was investigated in a savanna area of Burkina Faso and an urban fringe area in Beijing. According to the results, firstly, seasonal features from LTS based on all available imagery during one year as input led to a significant increase in land cover classification accuracy in comparison to the dry and wet season single date imagery. The harmonic model used for time series modeling provided a robust method for extracting seasonal features, and the influence of burned pixels on seasonal features could be considered simultaneously. Secondly, the annual burned area mapping based on a harmonic model and breakpoint identification with LTS was capable of detecting small and patchy burn scars with higher accuracy than MODIS burned area product. The approach demonstrated the potential of LTS for improving burned area detection in savannas, and was robust against data gaps caused by clouds and Landsat 7 missing lines. Thirdly, predictive models of tree crown cover (CC) using RapidEye and LTS imagery achieved similar accuracy, indicating the importance of texture and seasonal features from RapidEye and LTS imagery, respectively. Predictions of aboveground carbon and tree species richness, which were strongly correlated with CC, were promising using RapidEye and LTS imagery. Finally, the optimized window size texture classification improved classification accuracy in comparison to the classifications with single window size texture features and multiple window size texture features in an urban fringe area in Beijing, indicating the importance of multiscale texture information. Keywords: Landsat time series, texture, land cover classification, burned area, savanna, tree crown cover
  • Zhang , Yurui (Helsingin yliopisto, 2017)
    As the latest epoch of the Earth’s history, the Holocene is commonly defined as the last 11.7 ka BP (hereafter referred to as ka) and represents a new phase, encompassing the time span of human civilization. The last deglaciation lasted well into the Holocene, implying that the early Holocene was characterized by a large-scale reorganization with transitions in various components of the climate system. Studying the Holocene can provide insights into how the climate system functions, apart from the theoretical contributions to climate history itself. We first conducted sets of simulations with different combinations of climate forcings for 11.5 ka and for the entire Holocene to investigate the response of the climate–ocean system to the main climate forcings. In particular, two possible freshwater flux (FWF) scenarios were further tested considering the relatively large uncertainty in reconstructed ice-sheet melting. Moreover, we compared four Holocene simulations performed with the LOVECLIM, CCSM3, FAMOUS and HadCM3 models by identifying the regions where the multi-model simulations are consistent and where they are not, and analysing the reasons at the two levels (of the models’ variables and of the model principles and physics) where mismatches were found. After this, these multi-model simulations were systematically compared with data-based reconstructions in five regions of the Northern Hemisphere (NH) extratropics, namely Fennoscandia, Greenland, North Canada, Alaska and high-latitude Siberia. Potential uncertainty sources were also analysed in both model simulations and proxy data, and the most probable climate histories were identified with the aid of additional evidence when available. Additionally, the contribution of climate change, together with forest fires and human population size, to the variation in Holocene vegetation cover in Fennoscandia was assessed by employing the variation partitioning method. With effects of climate forcings, including variations in orbital-scale insolation (ORB), melting of the ice sheets and changes in greenhouse gas (GHG) concentrations, the climate shows spatial heterogeneity both at 11.5 ka and over the course of the Holocene. At 11.5 ka, the positive summer ORB forcing overwhelms the minor negative GHG anomaly and causes a higher summer temperatures of 2–4 °C in the extratropical continents than at 0 ka. The ice-sheet forcings primarily induce climatic cooling, and the underlying mechanisms include enhanced surface albedo over ice sheets, anomalous atmospheric circulation, reduced the Atlantic Meridional Overturning Circulation (AMOC) and relevant feedbacks. In particular, the most distinct feature is a thermally contrasting pattern over North America, with simulated temperatures being around 2 °C higher than those at 0 ka for Alaska, whereas over most of Canada, temperatures are more than 3 °C lower. The geographical variability of simulated temperatures is also reflected in Holocene temperature evolution, especially during the early Holocene, as constant Holocene cooling in Alaska contrasts with strong early-Holocene warming (warming rate over 1 °C kyr-1) in northern Canada. The early-Holocene climate is sensitive to the FWF forcings and a brief comparison with proxy records suggests that our updated FWF (FWF-v2, with a larger FWF release from the Greenland ice sheet and a faster FWF from the Fennoscandian Ice sheet (FIS)) represents a more realistic Holocene temperature scenario regarding the early-Holocene warming and Holocene temperature maximum (HTM). Comparison of multiple simulations suggests that the multi-model differences are spatially heterogeneous, despite overall consistent temperatures in the NH extratropics as a whole. On the one hand, reasonably consistent temperature trends (a temporal pattern with the early-Holocene warming, following a warm period and a gradual decrease toward 0 ka) are found over the regions where the climate is strongly influenced by the ice sheets, including Greenland, N Canada, N Europe and central-West Siberia. On the other hand, large inter-model variation exists in the regions over which the ice sheet effects on the climate were relatively weak via indirect influences, such as in Alaska, the Arctic, and E Siberia. In these three regions, the signals of multi-model simulations during the early Holocene are incompatible, especially in winter, when both positive and negative early-Holocene anomalies are suggested by different models. These divergent temperatures can be attributed to inconsistent responses of model variables. Southerly winds, surface albedo and sea ice can result in divergent temperature trends across models in Alaska, Siberia and the Arctic. Further comparisons reveal that divergent responses in these climate variables across the models can be partially caused by model differences (e.g. different model physics and resolution). For instance, the newly adopted formulation of the turbulent transfer coefficient in CCSM3 causes an overestimated albedo over Siberia at 0 ka, which leads to a stronger early-Holocene warmth than in other models. Moreover, the relatively simplified sea ice representation in FAMOUS probably leads to overestimated sea ice cover in the Arctic Ocean. The coarse vertical resolution in LOVECLIM might also introduce strong responses in atmospheric circulation over Alaska. From the perspective of climate features, the transient feature of the early-Holocene climate driven by the retreating ice sheets also influences the inter-model comparisons, as this transient feature induces a large degree of uncertainty into the FWF forcing. Comparisons of multiple model results with compiled proxy data at the sub-continental scale of NH high latitudes (i.e. Fennoscandia, Greenland, north Canada, Alaska and Siberia) reveal regionally-dependent consistencies in Holocene temperatures. In Fennoscandia, simulations and pollen data suggest a summer warming of 2 °C by 8 ka, although this is less expressed in chironomid data. In Canada, an early-Holocene warming of 4 °C in summer is suggested by both the simulations and pollen results. In Greenland, the magnitude of early-Holocene warming of annual mean ranges from 6 °C in simulations to 8 °C in δ18O-based temperatures. By contrast, simulated and reconstructed summer temperatures are mismatched in Alaska. Pollen data suggest 4 °C early-Holocene warming, while the simulations indicate 2 °C Holocene cooling, and chironomid data show a stable trend. Meanwhile, a high frequency of Alaskan peatland initiation before 9 ka can either reflect a high temperature, high soil moisture content or large seasonality. In high-latitude Siberia, simulations and proxy data depict high Holocene temperatures, although these signals are noisy owing to a large spread in the simulations and to a difference between pollen and chironomid results. On the whole, these comparisons of multi-model simulations with proxy reconstructions further confirm the Holocene climate evolution patterns in Fennoscandia, Greenland and North Canada. This implies that the Holocene temperatures in these regions have been relatively well established, with a reasonable representation of Holocene climate in the multiple simulations and a plausible explanation for the underlying mechanisms. However, the Holocene climate history and underlying mechanisms in the regions of Siberia and Alaska remain inconclusive. Variation partitioning revealed that climate was the main driver of vegetation dynamics in Fennoscandia during the Holocene as a whole and before the onset of farming. Forest fires and population size had relatively small contributions to vegetation change. However, the size of the human population became a more important driver of variation in vegetation composition than climate during the agricultural period, which can be estimated to have begun at 7–6 ka in Sweden and 4–3 ka in Finland. There is a clear region-dependent pattern of change caused by the human population: the impact of human activities on vegetation dynamics was notably higher in south Sweden and southwest Finland, where land use was more intensive, in comparison with central Sweden and southeast Finland. This thesis investigates the climate responses to the main forcings during the Holocene through various approaches, which has potential implications for the interactions between ice sheets and the climate, the Holocene climate history and current global change. The atmosphere-ocean system was sensitive to the FWF forcing during the early Holocene, implying that existing uncertainties in reconstructions of ice-sheet dynamics can be constrained by applying different freshwater scenarios via a comparison with proxy data. The Holocene climate history in most of the Northern Hemisphere extratropics is relatively well established, especially in regions that were strongly influenced by ice sheets. The implications of our investigation (on the transient early-Holocene) for the current global change are twofold. First, regional heterogeneity of the climate responses implies that regional differences should be taken into account when adapting to the current global change. Second, apart from the different scenarios of GHG forcing, inter-model comparison would be a good option to reduce model-dependency in estimation of the future climate.
  • Gilmore, Clifford (Helsingin yliopisto, 2017)
    Linear dynamics has been a rapidly evolving area of research since the late 1980s. Its central notion is known as hypercyclicity and many natural continuous linear operators turn out to be hypercyclic. This study is concerned with the hypercyclic and frequently hypercyclic properties of particular classes of operators. The first article of this thesis investigates the dynamics of commutator maps acting on separable Banach ideals of operators. Several necessary conditions are established which identify large classes of non-hypercyclic commutator maps. The main result proves that the commutator map induced by scalar multiples of the backward shift fails to be hypercyclic. In the second article concrete examples of hypercyclic generalised derivations acting on separable Banach ideals of operators are analysed and some necessary conditions for their hypercyclicity are identified. Conditions are also established under which the general class of elementary operators are never hypercyclic on certain Banach algebras. Notably, some curious hypercyclic behaviour is uncovered in relation to the remarkable Banach space constructed by Argyros and Haydon. The third article of this thesis solves a problem, originally posed by Blasco, Bonilla and Grosse-Erdmann (2010), on the minimal growth rates of harmonic functions that are frequently hypercyclic for the partial differentiation operators. This is done by explicitly constructing the frequently hypercyclic harmonic function.
  • Myllys, Minna (Helsingin yliopisto, 2017)
    The solar wind is a continuous plasma flow from the Sun into the interplanetary space. It consist of large number of charged particles that carry the solar magnetic field with it. When the solar wind reaches the Earth, it interacts with the terrestrial magnetic field and creates a magnetosphere around our planet. At times, significant plasma and energy transfer occurs from the solar wind into the magnetosphere causing strong disturbances to the Earth’s inner magnetic field. The solar wind is often considered to be a single fluid that has macroscopic measurable parameters like velocity, density, pressure and magnetic field. These parameters have a different role in controlling the solar wind-magnetosphere coupling and thus, the relative and absolute variations of the parameters affects to the magnetospheric response. The motivation for this thesis stems from the need to improve our understanding on the solar wind-magnetosphere coupling, and thus, ultimately the space weather forecasting ability. The thesis consists of four peer-reviewed scientific publications and introduction. The main objectives were to study the coupling efficiency of different large-scale solar wind structures and how the efficiency varies with the geomagnetic latitude. This thesis also studies how the solar wind parameters control the plasma convection in the high-latitude magnetosphere. All four publications are statistical studies that combine in-situ solar wind measurements combined with the ground-based magnetometer data. This thesis gives significant new insight on how different solar wind parameters affect the magnetospheric response. The published articles suggest that the strongest geomagnetic disturbances are caused by the solar wind structures with the combination of the high geoeffective electric field, and high solar wind velocity and dynamic pressure. Such conditions are found generally from sheath regions of coronal mass ejections. The published articles also show new observational features of well-known phenomenon, called the polar cap potential saturation, that decreases coupling between the solar wind and magnetosphere
  • Saksala, Teemu (Helsingin yliopisto, 2017)
    This thesis focuses on geometric inverse problems. By this, we mean that the mathematical framework is the Riemannian geometry and the objects of interest are smooth Riemannian manifolds with or without boundaries. Electric impedance tomography, sonography, and seismic imaging are examples of geometric inverse problems that have been studied extensively. In inverse problems, one tries to obtain more information about the object of interest by doing indirect measurements and combining this additional information with some type of a priori information. The a priori information together with the measurements are called “the data”. We want to show that Riemannian manifolds with the same data have also some other geometric properties in common. For instance, two Riemannian manifolds admit the same data if and only if they are Riemannian isometric. In this thesis, we focus, on the uniqueness questions of the geometric inverse problems. In the first article, we study an inverse problem related to the obtaining information about the deep structures of the Earth from the travel time differences of seismic waves produced by earthquakes. We show that, under certain assumptions about the measurement area, the travel time difference functions determine the Riemannian manifold up to an isometry. In the second article, we show that, if in an open set of Euclidean space we have been given a wave that is produced by a single realization of a white noise source, then we can determine the Riemannian metric tensor, provided that the metric tensor is non-trapping and coincides with the Euclidean metric outside some compact set. In addition we also show that, if the solution mapping of the Riemannian wave equation with interior source is given in some open set, then we can determine the Riemannian structure up to an isometry. In the third article, we study an inverse problem of a reconstruction of a compact Riemannian manifold with a smooth boundary from the scattering data of internal sources. This data consists of the exit directions of geodesics that emanate from the interior points of the manifold. We show, that under certain generic assumptions on the metric, one can reconstruct an isometric copy of the manifold from such scattering data measured on the boundary. In the fourth article, we consider a generalization of the first article.
  • Riuttanen, Laura (Aerosolitutkimusseura ry, 2017)
    Climate change and air quality are the big challenges of our time. In this work, air pollution transport to Finland, air pollution properties, their direct radiative effect on climate as well as their effect on upper tropospheric humidity are studied by using satellite data of aerosol optical properties, precipitation, clouds and upper tropospheric humidity, in situ measurement data of gases and aerosols at three measurement stations, combined with flight measurements, air mass trajectories, synoptic maps and meteorological reanalysis data. South-eastern transport routes with continental origin were found on average to bring the highest concentrations of anthropogenic gaseous and particulate pollution to southern Finland. The air masses coming from the north-east brought air that was clean of anthropogenic pollution but included high concentrations of nucleation and Aitken mode particles. Air mass trajectory statistical methods were found to be useful tools to study air pollution transport, but they were not able to reproduce emission source maps. The most severe air pollution cases in the history of air pollution measurements in Finland were produced by the biomass burning events in Eastern Europe in 2006 and 2010. The smoke was transported hundreds and even thousands of kilometres and the smoke particles were observed to grow in size during transportation. Particulate air pollution in southern Finland scattered solar radiation and had a cooling effect on climate. High absorption coefficients were measured in continental air pollution from Eastern Europe as well as in biomass burning smoke, but also these air pollutants were on average more scattering than absorbing. Based on satellite observations we were able to show the first observational evidence, with a strong emphasis on causality, that aerosols increase upper tropospheric humidity and have thus a so-far overlooked warming effect on climate. The magnitude of the observed increase is so big that the mechanism potentially has relevance for the global climate.
  • Ikonen, Jussi (Helsingin yliopisto, 2017)
    The geological disposal of spent nuclear fuel may represent a good solution after a thorough risk assessment. In Finland and Sweden, the host rock for geological disposal is crystalline rock and the repository is based on the Swedish KBS-3 multi-barrier design. When radionuclides from spent nuclear fuel are potentially released into the bedrock, they are then transported by advection along water-conducting fractures. The retardation of radionuclides can occur via molecular diffusion into stagnant pore water and/or via chemical retardation onto mineral surfaces in the rock matrix. When assessing the risk of spent nuclear fuel deposition, it is important to understand the transport behaviour of radionuclides in the bedrock, that is, at the rock–groundwater interface. Most parameters, such as the diffusion coefficient, the distribution coefficient and the rock porosity, associated with radionuclide migration in the rock are obtained from laboratory scale experiments. In order to apply results from laboratory experiments to the full-scale nuclear fuel deposition, in-situ experiments are performed. For example, the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) have been conducting extensive in-situ experiments at the Grimsel Test Site (GTS). During the first in-situ test, tritiated water (HTO), 22Na, 134Cs and 131I, as well as non-radioactive isotopes 127I and 23Na, were circulated in a borehole interval isolated by packers 70 cm apart from each other. The second ongoing Long Term Diffusion (LTD) experiment primarily uses the same radionuclides as well as the non-radioactive element selenium. This thesis presents the laboratory analysis of HTO and iodine from an in-situ diffusion experiment and supporting laboratory studies aimed at determining the sorption and diffusion of caesium and selenium on Grimsel granodiorite (GG). Caesium sorption was studied through batch sorption experiments using crushed rock, while selenium diffusion and sorption relied on batch and block scale experiments using Kuru grey granite (KGG) and GG rock blocks. HTO and iodine diffusion was modelled using the time domain diffusion (TDD) method among in in-situ rock blocks as well as selenium in a laboratory using rock blocks. The outleaching method proved successful for analysing non-sorbing radionuclides from the connected pore network of the GG. TDD modelling of the results lead to an apparent diffusion coefficient of 3 × 10-10 m2/s for both HTO and iodine. No significant difference between the in-situ and laboratory diffusion coefficient was detected. Caesium sorption stood at 0.107 ± 0.003 m3/kg on GG at a 10−8 M Cs concentration. Sorption was highest on biotite at Kd = 0.304 ± 0.005 m3/kg, explaining the in-situ diffusion results of caesium which followed the biotite veins in GG. The sorption of selenium was significantly overestimated when the determination was conducted on crushed rock using the batch sorption method compared to studies conducted on intact rock. The outleaching method proved successful in the analysis of non-sorbing radionuclides, while flexible TDD modelling proved to be a quite useful tool in handling measured data from both block scale experiments and in-situ experiments of weakly and non-sorbing tracers.
  • Nyrhinen, Hannu (Helsingin yliopisto, 2017)
    For a hundred years Einstein's general relativity (GR) has persisted as the standard model of gravity. To date, no observations conflict it. Moreover, GR has predicted effects that have been later confirmed, such as the deflection of light, the redshift of light in gravitational field, and gravitational waves. Modifications to GR have been studied since the early days of relativistic gravity. In this thesis, we present three projects on different modified gravity models. These include Palatini f(R), bimetric variational principle, and scalar-tensor theories. In Palatini f(R) theories, a function of the Ricci scalar, f(R), acts as the gravitational Lagrangian. The connection is independent of the metric. In GR, the function f(R) = R, and the Einstein equations follow regardless of whether or not the connection is independent. We show that for a system of compact objects, the difference between Palatini f(R) and GR is the scaling of masses. However, without complementary measurement of masses, such systems are observationally indistinguishable. In bimetric variational principle, the independent spacetime connection is constructed of a tensor apart from the physical metric. The physical metric and this new tensor then give the field equations. We study Einstein-Hilbert and f(R) actions of the Ricci scalar of the connection. We use ADM formalism to show that without further constraints the resulting Hamiltonian contains a dynamical variable without a lower bound. This leads to decaying to lower energy states by radiating energy ad infinitum. Hence, these theories are physically unviable. We also study scalar-tensor theories with disformally coupled matter. In these theories matter couples to the scalar field and its derivatives. We focus on a system of disformally coupled matter surrounding a black hole. In Brans-Dicke-type theories such systems suffer from the so called spontaneous scalarisation. In it, the scalar field develops stable scalar hair around the black hole. This conflicts the no-hair theorem according to which the only observable properties of a black hole are its mass, angular momentum, and electric charge. We show that disformal coupling can further destabilise the system. We find a range of disformal coupling strengths where the coupling enhances the scalarisation. Outside this range, the effect of the disformal coupling is stabilising.
  • Ren, Wei (Helsingin yliopisto, 2017)
    Nanowires (NWs) and nanotubes (NTs) are considered to be of great importance for future nanotechnology applications, due to the roles of dimensionality and small system size. Potential applications of NWs and NTs range from field-effect transistors to biological applications. However, the one-dimensional (1D) nanostructures and most of their applications are still in an early stage of technical development. There are several issues that need to be addressed before they are ripe for industrial applications. Irradiation has been widely used in semiconductor industry to modify the properties of materials since the 1950s. Irradiation in 1D nanomaterials has been studied to tailor the mechanical, electronic, optical and even magnetic properties in a controlled manner, to improve the functionality of the devices based on the 1D nanomaterials. This thesis focuses on the structural and mechanical modifications of the 1D nanomaterials under energetic ion irradiation, as well as the formation mechanisms of the composites of the functional one-dimensional nanomaterials to improve their usage. In the first part, we studied the defect production of GaN NWs under Ar irradiation. The difference of defect production between NWs and the bulk counterpart was studied. The effect of the large surface-area-to-volume ratio was found to play an importance role in defect production in NWs. The irradiation energy of the maximum damage production in the NWs has been obtained. In the second part of the thesis, we studied the formation of the composite nanomaterials of diamond-like-carbon (DLC) and carbon nanotubes (CNTs). We used the classical MD method to simulate the deposition process of carbon atoms on the CNT systems to provide the atomic insights into structural changes. The results show that high-sp$^3$-content DLC can be formed provided the deposition conditions allow for sidewards pressure to form from a substrate close beneath the CNTs.
  • Bukonte, Laura (Helsingin yliopisto, 2017)
    Defects always exists in a crystal lattice at temperatures above absolute zero. Our knowledge of defect concentration and mobility is crucial, due to their profound influence on the material properties. It has been shown that the presence of light impurities may enhance vacancy formation in many metals and metal alloys. The main reason for this phenomenon, often referred to as the superabundant vacancy formation, is the lowering of the vacancy formation energy due to the impurity trapping. In this thesis a theoretical thermodynamics model has been developed to study the equilibrium vacancy concentrations as a function of impurity concentration and temperature. The diffusion of monovacancies and hydrogen in tungsten is studied due to its relevance to fusion research. The molecular dynamics method has been used to simulate the diffusion of hydrogen and monovacancies in tungsten. The commonly accepted and so far used H diffusion migration barrier is revised and a new analysis method to determine diffusion coefficients that accounts for the random oscillation of atoms around the equilibrium position is presented. The results show the presence of multiple nearest neighbour jumps of monovacancy above 2/3 of the melting temperature of tungsten, that partly explains the upward curvature of Arrhenius diagram of self-diffusion experiments in tungsten. For the first time, the W monovacancy diffusion prefactor is calculated, and found to be unexpectedly high, resulting in a monovacancy diffusion attempt frequency of about 2-3 orders of magnitude higher than the values commonly used. A comparative study between the molecular dynamics and a Monte Carlo method – binary collision approximation has been carried out by simulating the single ion impacts on silicon and tungsten surfaces. The results from both methods are compared and found to be in a good agreement for crystalline structures. However, large discrepancies between the two methods arise for materials that are amorphous or become amorphized during ion irradiation.
  • Suomi, Irene (Helsingin yliopisto, 2017)
    Wind gusts, which are short duration (typically 3 s) wind speed maxima, are representative of the extreme wind conditions. They are very important for human activity, because the strongest gusts associated with storms are the most significant single cause of natural hazards. The impact of wind gusts on different structures depends on the characteristics of each structure. For example for wind energy, it is important to know both the probability of extreme maximum gusts in time scales of decades for the design of power plants and in the shorter term to support wind turbine operations. For wind gust forecasting it is essential to have reliable wind gust observations. Traditionally, observations have only been available from weather stations where the wind is usually measured at a reference height of 10 m. For wind energy, information is needed at greater heights, as the hub heights of the largest turbines extend even above 150 m. The main aim of this work has been to investigate wind gusts across the entire atmospheric boundary layer based on observations from tall meteorological masts as well as applying new measurement methods developed in this dissertation. The novel methods are based on turbulence measurements taken on board a research aircraft and by a Doppler lidar. The research aircraft can fly long distances in a short time, so the measured wind speeds do not represent wind speed variation in time but they are a function of flight distance. The new method developed in this dissertation to compare temporal and spatial scales allows the measurement of wind gusts from a research aircraft. Then, observations can be obtained from places where traditional weather stations or meteorological masts cannot be deployed. Applying the new method, the observed wind gusts from the marine Arctic matched well with those observed at a meteorological mast in the Baltic Sea, although also differences were observed between these environments. Doppler lidar provides radial wind speed measurements along a laser beam transmitted by the instrument. When data from at least three lines of sight are combined, the three-dimensional wind vector can be derived. However, the measurements from multiple lines of sight take several seconds, and the different beams represent different measurement volumes. For these reasons, the measured wind speed maxima from the Doppler lidar used in this work were higher than the corresponding wind gusts from the nearby meteorological mast. In this dissertation, we developed a new theoretical method that significantly reduced this positive bias. Wind gust measurements are usually prone to measurement errors, called outliers. The use of a spike removal algorithm typically applied in traditional turbulence measurements, resulted in significantly improved Doppler lidar data quality. The method performed even better than the traditional data quality assurance methods based on carrier-to-noise ratio, by removing the unrealistic outliers present in the time series. Based on the above wind gust measurements, it was found that in the lowest part of the atmospheric boundary layer the ratio of the wind gust speed and the mean wind speed, called the gust factor, decreases strongly with measurement height. The higher the aerodynamic roughness of the surface, the greater is the change. Moreover, the static stability of the atmosphere affects the gust factor: the decrease of the gust factor with height is clearly smaller in unstable than in stable conditions. The gust parameterizations used in numerical weather prediction models were originally designed for the reference measurement height of 10 m. A new parameterization was developed that takes into account not only the effects of surface roughness and atmospheric stability but also the height above the surface. Based on meteorological mast and research aircraft measurements, the new parametrization yielded better results than previous methods.
  • Ponto, Heli (Helsingin yliopisto, 2017)
    This study, positioned in the fields of humanistic geography and young people’s geographies, deals with young people’s place experiences in the city. I consider such experiences subjective, and study place from the perspective of personal relations and experienced insideness and outsideness. I also understand place as an intersubjective experience comprising social encounters. My research contributes to the literature on mobility, specifically in examining place from the perspective of daily mobility and personal networks. The participants were young people in upper-secondary education in the Helsinki metropolitan area, Finland. The research material consists of go-along interviews, photographs, GPS recordings and independent assignments, gathered in accordance with participatory methods. Earlier research findings indicate that adults define and restrict young people’s places in urban space, and that their experiences of insideness and outsideness are strongly elated to other meaningful social encounters. Daily mobility also tends to be perceived as a bodily and routine practice, whereas mobility opportunities are connected to young people’s experienced independence. According to the participants, socio-spatial tensions that typically arise between adults and young people no longer influenced the construction of their place experiences. The meanings of familiar childhood places, used for activities such as hanging out, changed as these young people were more commonly encountered as ‘almost adults’ in different places. Childhood places still evoked strong memories, but were no longer at the centre of daily life. They still had a significant bridge-building role in reconciling memories of familiar childhood places with contradictory experiences of new adulthood places. Thus, I claim that young people are experientially living between their childhood and adulthood places, actively constructing new personal places in their everyday environments in the process of growing up. The young people emphasised the importance of friends in fostering feelings of insideness with place. Groups of friends had their private ‘our’ places, but also more open places in which ‘they’ were welcome. Meetings with ‘them’ were easier if friends were present. Social encounters sometimes evoked strong feelings of outsideness despite the presence of friends, however, especially if such feelings were shared. Nevertheless, those who had the skills and resources to handle experiences related to encounters with different people in their daily places seemed to retain their feelings of insideness. My findings reveal that the construction of (inter)subjective meanings of places is tightly intertwined with daily mobility. The young people described how daily moving structured the experiential and bodily connections between them and their places, supporting their way of living and being in the city and enabling them to practise new adulthood. They had plenty of freedom related to daily, bodily mobility, and were restricted by a poor public-transport network, and the lack of a driving licence and vehicle, rather than parental strictures. Daily places form webs of meaningful places and experienced (dis)connections between places and people, in which (inter)subjective meaning-making is intermingled with daily mobilities. Given that experienced disconnections appear to arouse feelings of outsideness, young people should learn to recognize their experiential ties with places and handle the breakages. To do this they need places that support their everyday agency in terms of actively influencing their personal connections with places and promoting feelings of insideness. Keywords: young people, place experiences, place, everyday life, urban space
  • Lehtonen, Ilari (Helsingin yliopisto, 2017)
    The aim of this work was to study the climate change impact on two specific abiotic risks affecting forests in Finland: fires and heavy snow loads. Approximately 1000 forest fires occur annually in Finland, but thanks to effective fire suppression, the average size of fires is only about 0.5 ha. Occasionally, heavy snow loading causes forest damage, which reduces stand quality in boreal forests experiencing cold winters. In Finnish forests, snow damage occurs most commonly in the eastern and northern parts of the country. The basic tools used in this work to evaluate the climate change impact were climate models. In addition, observational weather data and fire statistics were used. In evaluating the forest fire risk, the Canadian Fire Weather Index (FWI) system was used. Snow load amounts were estimated mainly by applying a snow load model developed at the Finnish Meteorological Institute (FMI). The results indicate that forest fire risk will most likely increase in the future due to increasing temperature and enhanced evaporation. However, there is large uncertainty regarding the rate of change, which originates from the differences between climate model responses to the same radiative forcing. Moreover, an increase in forest fire risk will at the same time increase the risk of conflagrations. Crown snow loads were projected to become heavier in northern Finland and in the regions of Kainuu and North Karelia next to the Russian border. In southern and western Finland the risk of snow damage is expected to decrease. The largest decrease in the risk is projected to occur in coastal areas. In the areas expected to experience increased risk of snow damage, conditions favouring both heavy wet snow loading and rime accretion were predicted to become more common. The results of this work can be utilized when considering climatically-driven risks in forest management.
  • Hauptmann, Andreas (Helsingin yliopisto, 2017)
    Electrical impedance tomography (EIT) is a rather new approach to medical imaging that is motivated by using electricity to determine the inside of a body. The clear advantage lies in the usage of harmless electric currents, in contrast to the ionizing radiation of X-rays, whereas the mathematical problem is inherently more challenging. In EIT we seek to reconstruct an image of the inner organs by determining their conductivity, i.e. how well electricity is conducted. As a medical imaging modality it is most promising in pulmonary and cardiac imaging, due to considerably different conductivity values in the air filled lungs (low conductive) and the blood filled heart (high conductive). EIT is in principle capable of monitoring the respiratory process, detecting pathologies in the lungs, and monitoring the heart activity. The main focus of this work is on the partial-boundary problem in EIT, that means one has only access to a certain part of the boundary and data can only be collected there. In a hospital setting these situations can arise when monitoring a critical or unconscious patient and hence one can only access the front of the torso (ventral position). Furthermore, practical complications can arise due to faulty, dislocated, or dispatched electrodes and hence leading to incomplete data. The methods presented in this thesis are capable of dealing with such incomplete data. Following the tradition of mathematical research we are also interested in quantifying the error incomplete data introduces to the reconstruction. In a short summary, this thesis investigates how to improve EIT reconstructions from partial-boundary data by utilizing concepts from an ideal mathematical setting as well as how to apply these methods to real electrode models and measurement data.