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  • Nurmela, Arto (Helsingin yliopisto, 2001)
  • Cristina, Jan (Helsingin yliopisto, 2013)
    This thesis studies connections and frames with very little regularity. It developed as a study of the Cartan-Whitney presentations of Heinonen and Sullivan, and the Quasiconformal frames of Heinonen, Pankka and Rajala. The thesis consists of three chapters of which the first is the introduction. The second chapter is devoted to the development of a theory of non-smooth connections on general fibre bundles. The main theorem proved therein, is a holonomy bound for such connections in terms of their non-smooth curvature. This is done using an adapted smooth approximation, and examining the convergence of lifts along this approximation on almost every plane. The third chapter examines quasiconformal (co-)frames, that is vectors of one-forms, whose exterior product bounds their norm point wise almost everywhere, thus mimicking the derivative of a quasiconformal map. A variational problem of minimising the exterior energy in a given conformal class of such frames is studied. The main results of the chapter are existence of minimisers, and a morphism property of quasiconformal maps for such minimisers. This morphism property is then used to derive higher regularity for such a minimiser in certain special cases by applying the theory of p-harmonic maps.
  • Luosto, Panu (Helsingin yliopisto, 2013)
    The normalized maximum likelihood (NML) distribution has an important position in minimum description length based modelling. Given a set of possible models, the corresponding NML distribution enables optimal encoding according to the worst-case criterion. However, many model classes of practical interest do not have an NML distribution. This thesis introduces solutions for a selection of such cases, including for example one-dimensional normal, uniform and exponential model classes with unrestricted parameters. The new code length functions are based on minimal assumptions about the data, because an approach that would be completely free of any assumptions is not possible in these cases. We also use the new techniques in clustering, as well as in density and entropy estimation applications.
  • Parshintsev, Evgeny (Helsingin yliopisto, 2011)
    The Earth s climate is a highly dynamic and complex system in which atmospheric aerosols have been increasingly recognized to play a key role. Aerosol particles affect the climate through a multitude of processes, directly by absorbing and reflecting radiation and indirectly by changing the properties of clouds. Because of the complexity, quantification of the effects of aerosols continues to be a highly uncertain science. Better understanding of the effects of aerosols requires more information on aerosol chemistry. Before the determination of aerosol chemical composition by the various available analytical techniques, aerosol particles must be reliably sampled and prepared. Indeed, sampling is one of the most challenging steps in aerosol studies, since all available sampling techniques harbor drawbacks. In this study, novel methodologies were developed for sampling and determination of the chemical composition of atmospheric aerosols. In the particle-into-liquid sampler (PILS), aerosol particles grow in saturated water vapor with further impaction and dissolution in liquid water. Once in water, the aerosol sample can then be transported and analyzed by various off-line or on-line techniques. In this study, PILS was modified and the sampling procedure was optimized to obtain less altered aerosol samples with good time resolution. A combination of denuders with different coatings was tested to adsorb gas phase compounds before PILS. Mixtures of water with alcohols were introduced to increase the solubility of aerosols. Minimum sampling time required was determined by collecting samples off-line every hour and proceeding with liquid-liquid extraction (LLE) and analysis by gas chromatography-mass spectrometry (GC-MS). The laboriousness of LLE followed by GC-MS analysis next prompted an evaluation of solid-phase extraction (SPE) for the extraction of aldehydes and acids in aerosol samples. These two compound groups are thought to be key for aerosol growth. Octadecylsilica, hydrophilic-lipophilic balance (HLB), and mixed phase anion exchange (MAX) were tested as extraction materials. MAX proved to be efficient for acids, but no tested material offered sufficient adsorption for aldehydes. Thus, PILS samples were extracted only with MAX to guarantee good results for organic acids determined by liquid chromatography-mass spectrometry (HPLC-MS). On-line coupling of SPE with HPLC-MS is relatively easy, and here on-line coupling of PILS with HPLC-MS through the SPE trap produced some interesting data on relevant acids in atmospheric aerosol samples. A completely different approach to aerosol sampling, namely, differential mobility analyzer (DMA)-assisted filter sampling, was employed in this study to provide information about the size dependent chemical composition of aerosols and understanding of the processes driving aerosol growth from nano-size clusters to climatically relevant particles (>40 nm). The DMA was set to sample particles with diameters of 50, 40, and 30 nm and aerosols were collected on teflon or quartz fiber filters. To clarify the gas-phase contribution, zero gas-phase samples were collected by switching off the DMA every other 15 minutes. Gas-phase compounds were adsorbed equally well on both types of filter, and were found to contribute significantly to the total compound mass. Gas-phase adsorption is especially significant during the collection of nanometer-size aerosols and needs always to be taken into account. Other aims of this study were to determine the oxidation products of β-caryophyllene (the major sesquiterpene in boreal forest) in aerosol particles. Since reference compounds are needed for verification of the accuracy of analytical measurements, three oxidation products of β-caryophyllene were synthesized: β-caryophyllene aldehyde, β-nocaryophyllene aldehyde, and β-caryophyllinic acid. All three were identified for the first time in ambient aerosol samples, at relatively high concentrations, and their contribution to the aerosol mass (and probably growth) was concluded to be significant. Methodological and instrumental developments presented in this work enable fuller understanding of the processes behind biogenic aerosol formation and provide new tools for more precise determination of biosphere-atmosphere interactions.
  • Tanskanen, Hanna (Helsingin yliopisto, 2009)
    Noble gases are mostly known as inert monatomic gases due to their limited reactivity with other elements. However, the first predictions of noble-gas compounds were suggested by Kossel in 1916, by von Antropoff in 1924, and by Pauling in 1930. It took many decades until the first noble-gas compound, XePtF6, was synthesized by Neil Bartlett in 1962. This was followed by gradual development of the field and many noble-gas compounds have been prepared. In 1995, a family of noble-gas hydride molecules was discovered at the University of Helsinki. These molecules have the general formula of HNgY, where H is a hydrogen atom, Ng is a noble-gas atom (Ar, Kr, or Xe), and Y is an electronegative fragment. The first molecular species made include HXeI, HXeBr, HXeCl, HKrCl and HXeH. Nowadays the total number of prepared HNgY molecules is 23 including both inorganic and organic compounds. The first and only neutral ground-state argon compound, HArF, was synthetized in 2000. Helium and neon are the only elements in the periodic table that do not form neutral, ground-state molecules. In this Thesis, experimental preparation of eight novel xenon- and krypton-containing organo-noble-gas hydrides made from acetylene (HCCH), diacetylene (HCCCCH) and cyanoacetylene (HCCCN) are presented. These novel species include the first organic krypton compound, HKrCCH, as well as the first noble-gas hydride molecule containing two Xe atoms, HXeCCXeH. Other new compounds are HXeCCH, HXeCC, HXeC4H, HKrC4H, HXeC3N, and HKrC3N. These molecules are prepared in noble-gas matrices (krypton or xenon) using ultraviolet photolysis of the precursor molecule and thermal mobilization of the photogenerated H atoms. The molecules were identified using infrared spectroscopy and ab initio calculations. The formation mechanisms of the organo-noble-gas molecules are studied and discussed in this context. The focus is to evidence experimentally the neutral formation mechanisms of HNgY molecules upon global mobility of H atoms. The formation of HXeCCXeH from another noble-gas compound (HXeCC) is demonstrated and discussed. Interactions with the surrounding matrix and molecular complexes of the HXeCCH molecule are studied. HXeCCH was prepared in argon and krypton solids in addition to a Xe matrix. The weak HXeCCH∙∙∙CO2 complex is prepared and identified. Preparation of the HXeCCH∙∙∙CO2 complex demonstrates an advanced approach to studies of HNgY complexes where the precursor complex (HCCH∙∙∙CO2) is obtained using photolysis of a larger molecule (propiolic acid).
  • Losilla Fernández, Sergio A (Helsingin yliopisto, 2013)
    In this thesis, several numerical methods for electronic structure calculations are presented. The first is a quadrature scheme for the accurate and efficient computation of electrostatic potentials. The quadrature is applied to calculations on real-space grids, and to Coulomb integrals over Gaussian-type orbitals. Second, we introduce a real-space representation for three-dimensional scalar functions encountered in electronic structure calculations. In this representation, each function is partitioned into numerical atom-centred parts (the bubbles), and the remainder is represented on a three-dimensional Cartesian grid. The algorithms to carry out the required operations are discussed, along with benchmarks of their computer implementations. The presented methods are all of a divide-and-conquer nature, breaking the problem into simple pieces which are suitable for execution in emerging massively parallel computer architectures, such as general-purpose graphics processing units.
  • Siljamo, Pilvi (Helsingin yliopisto, 2013)
    Birch pollen is one of the most prevalent allergens in Northern Europe. Nearly every spring, birch pollen is found in Finland before the start of local flowering. Sometimes, its concentrations are so high that they cause symptoms in allergic persons. Such episodes are caused by long-range transport of pollen grains from other regions, in which the flowering started earlier. Prediction of these episodes requires European-scale modelling of pollen distribution. The goal of this study was to develop a European-wide, bio-physical, numerical prediction system for birch flowering, pollen release, and dispersion in the atmosphere. The pollen forecasting system is based on both the atmospheric dispersion modelling system SILAM developed at FMI and numerical weather prediction models. In order to predict birch pollen concentrations, several input datasets and modules for pollen emissions were developed: a European birch habitat map, phenological model parameters for the start of birch flowering in Europe, and a pollen release model. The study also included an evaluation of the features of the input datasets, such as phenological observations in Europe, and the reliability of the new pollen forecasting system. The SILAM pollen forecasting system allows for detailed predictions of pollen distribution in space and time, and enables the simulation of long-range pollen transport episodes, which cannot be predicted from in-situ observations. Moreover, this pollen dispersion model can be helpful in many research applications, such as gene flow studies or in assessing pollen concentrations under future climate conditions. The second goal of this study was to delineate the source areas of long-range transported birch pollen observed in North-Eastern Europe. In general, long-range transported pollen reaches Finland some days, although sometimes even weeks, before birch trees pollinate in Finland. Typically, the long-range transported pollen originates from the south, mainly from the Baltic countries; however, very high pollen counts are observed when pollen grains come from the southeast, from the vast Russian birch stands. In some cases, a smaller proportion of pollen can travel from Sweden, Poland, or even Germany.
  • 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.
  • Niemi, Esa (Helsingin yliopisto, 2015)
    Inverse problems arise, for example, from various imaging applications in medicine and physics. Their inherent property is ill-posedness; even a very small error in the measurement data can lead to a large error in the reconstruction. To overcome this difficulty, regularization is necessary for the inversion. In this work three new computational regularization methods for limited-data inverse problems are introduced and studied. The problems of special interest are stationary and dynamic X-ray tomography (CT) with sparsely sampled X-ray projection data and acoustic inverse scattering with limited-aperture data. In the first article of this thesis we develop a computational reconstruction algorithm for solving stationary sparse-data CT problems. Sparse-data cases arise e.g. from the need to minimize radiation dose in medical imaging. The new reconstruction algorithm is based on total variation regularization, it preserves sharp features of the target and is suitable for large-scale problems such as 3D CT. Its performance is illustrated by numerical results computed from both simulated and real X-ray data. In the second and third articles we introduce an inversion method for dynamic CT application making use of a few fixed X-ray sources and detectors. In this application the attainable temporal resolution is high while the CT data measured at a single time step is extremely sparse. The inversion method is motivated by level set methods and it regularizes the problem in space-time so that certain regularity is required both in spatial and temporal directions. Some of its important theoretical aspects are analyzed, and a computational implementation of the method is tested using both simulated and real X-ray data. The new methodology provides whole new possibilities e.g. for 4D angiographic imaging with high temporal resolution. In the fourth article a numerical implementation of the so-called enclosure method by Masaru Ikehata is introduced and studied using simulated test data. The enclosure method is suitable for limited-aperture obstacle scattering problems, where one uses only one incident wave and measures the far field pattern of the scattered field on some possibly limited aperture. The name of the method comes from the fact that it finds the convex hull of the obstacle, rather than its precise shape. Numerical evidence presented suggests that the method can approximately recover the shape and position of an obstacle from noisy limited-aperture far field data.
  • Tuovinen, Antti-Pekka (Helsingin yliopisto, 2002)
  • Brücken, Jens Erik (Helsingin yliopisto, 2013)
    We have observed exclusive γγ production in proton-antiproton collisions at the Tevatron at sqrt(s) = 1.96 TeV. We use data corresponding to 1.11 ± 0.07 fb^{−1} integrated luminosity taken by the Run II Collider Detector at Fermilab, with a trigger requiring two electromagnetic showers, each with transverse energy ET > 2 GeV, and vetoing on hits in the forward beam shower counters. We select events with two electromagnetic showers, each with transverse energy ET > 2.5 GeV and pseudorapidity |η| < 1.0, with no other particles detected in −7.4 < η < +7.4. The two showers have similar ET and an azimuthal angle separation ∆φ ∼ π; we find 34 events with exactly two matching charged particle tracks, agreeing with expectations for the QED process p pbar → p + e^+ e^− + pbar by two photon exchange; and we find 43 events with no tracks. The latter are candidates for the exclusive process p pbar → p + γγ + pbar by double pomeron exchange. We use the strip and wire chambers at the longitudinal shower maximum position within the calorimeter to measure a possible exclusive background from IP + IP → π^0 π^0 , and conclude that it is consistent with zero and is < 15 events at 95% C.L. The measured cross section is σ_{γγ,excl} (|η| < 1, ET (γ) > 2.5 GeV) = 2.48 +0.40 -0.35 (stat) +0.40 -0.51 (syst) pb and in agreement with the theoretical predictions. This process is closely related to exclusive Higgs boson production pp → p + H + p at the Large Hadron Collider. The observation of the exclusive production of diphotons shows that exclusive Higgs production can happen and could be observed with a proper experimental setup.
  • Seppälä, Annika (Helsingin yliopisto, 2007)
    This work is focused on the effects of energetic particle precipitation of solar or magnetospheric origin on the polar middle atmosphere. The energetic charged particles have access to the atmosphere in the polar areas, where they are guided by the Earth's magnetic field. The particles penetrate down to 20-100 km altitudes (stratosphere and mesosphere) ionising the ambient air. This ionisation leads to production of odd nitrogen (NOx) and odd hydrogen species, which take part in catalytic ozone destruction. NOx has a very long chemical lifetime during polar night conditions. Therefore NOx produced at high altitudes during polar night can be transported to lower stratospheric altitudes. Particular emphasis in this work is in the use of both space and ground based observations: ozone and NO2 measurements from the GOMOS instrument on board the European Space Agency's Envisat-satellite are used together with subionospheric VLF radio wave observations from ground stations. Combining the two observation techniques enabled detection of NOx enhancements throughout the middle atmosphere, including tracking the descent of NOx enhancements of high altitude origin down to the stratosphere. GOMOS observations of the large Solar Proton Events of October-November 2003 showed the progression of the SPE initiated NOx enhancements through the polar winter. In the upper stratosphere, nighttime NO2 increased by an order of magnitude, and the effect was observed to last for several weeks after the SPEs. Ozone decreases up to 60 % from the pre-SPE values were observed in the upper stratosphere nearly a month after the events. Over several weeks the GOMOS observations showed the gradual descent of the NOx enhancements to lower altitudes. Measurements from years 2002-2006 were used to study polar winter NOx increases and their connection to energetic particle precipitation. NOx enhancements were found to occur in a good correlation with both increased high-energy particle precipitation and increased geomagnetic activity. The average wintertime polar NOx was found to have a nearly linear relationship with the average wintertime geomagnetic activity. The results from this thesis work show how important energetic particle precipitation from outside the atmosphere is as a source of NOx in the middle atmosphere, and thus its importance to the chemical balance of the atmosphere.
  • Juusola, Liisa (Helsingin yliopisto, 2009)
    In this thesis, the solar wind-magnetosphere-ionosphere coupling is studied observationally, with the main focus on the ionospheric currents in the auroral region. The thesis consists of five research articles and an introductory part that summarises the most important results reached in the articles and places them in a wider context within the field of space physics. Ionospheric measurements are provided by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network, by the low-orbit CHAllenging Minisatellite Payload (CHAMP) satellite, by the European Incoherent SCATter (EISCAT) radar, and by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Magnetospheric observations, on the other hand, are acquired from the four spacecraft of the Cluster mission, and solar wind observations from the Advanced Composition Explorer (ACE) and Wind spacecraft. Within the framework of this study, a new method for determining the ionospheric currents from low-orbit satellite-based magnetic field data is developed. In contrast to previous techniques, all three current density components can be determined on a matching spatial scale, and the validity of the necessary one-dimensionality approximation, and thus, the quality of the results, can be estimated directly from the data. The new method is applied to derive an empirical model for estimating the Hall-to-Pedersen conductance ratio from ground-based magnetic field data, and to investigate the statistical dependence of the large-scale ionospheric currents on solar wind and geomagnetic parameters. Equations describing the amount of field-aligned current in the auroral region, as well as the location of the auroral electrojets, as a function of these parameters are derived. Moreover, the mesoscale (10-1000 km) ionospheric equivalent currents related to two magnetotail plasma sheet phenomena, bursty bulk flows and flux ropes, are studied. Based on the analysis of 22 events, the typical equivalent current pattern related to bursty bulk flows is established. For the flux ropes, on the other hand, only two conjugate events are found. As the equivalent current patterns during these two events are not similar, it is suggested that the ionospheric signatures of a flux rope depend on the orientation and the length of the structure, but analysis of additional events is required to determine the possible ionospheric connection of flux ropes.
  • Koponen, Ismo (Helsingin yliopisto, 2003)
  • Mönkkönen, Petteri (Helsingin yliopisto, 2005)
  • Riipinen, Ilona (Helsingin yliopisto, 2008)
    Atmospheric aerosol particles have a significant impact on air quality, human health and global climate. The climatic effects of secondary aerosol are currently among the largest uncertainties limiting the scientific understanding of future and past climate changes. To better estimate the climatic importance of secondary aerosol particles, detailed information on atmospheric particle formation mechanisms and the vapours forming the aerosol is required. In this thesis we studied these issues by applying novel instrumentation in a boreal forest to obtain direct information on the very first steps of atmospheric nucleation and particle growth. Additionally, we used detailed laboratory experiments and process modelling to determine condensational growth properties, such as saturation vapour pressures, of dicarboxylic acids, which are organic acids often found in atmospheric samples. Based on our studies, we came to four main conclusions: 1) In the boreal forest region, both sulphurous compounds and organics are needed for secondary particle formation, the previous contributing mainly to particle formation and latter to growth; 2) A persistent pool of molecular clusters, both neutral and charged, is present and participates in atmospheric nucleation processes in boreal forests; 3) Neutral particle formation seems to dominate over ion-mediated mechanisms, at least in the boreal forest boundary layer; 4) The subcooled liquid phase saturation vapour pressures of C3-C9 dicarboxylic acids are of the order of 1e-5 1e-3 Pa at atmospheric temperatures, indicating that a mixed pre-existing particulate phase is required for their condensation in atmospheric conditions. The work presented in this thesis gives tools to better quantify the aerosol source provided by secondary aerosol formation. The results are particularly useful when estimating, for instance, anthropogenic versus biogenic influences and the fractions of secondary aerosol formation explained by neutral or ion-mediated nucleation mechanisms, at least in environments where the average particle formation rates are of the order of some tens of particles per cubic centimeter or lower. However, as the factors driving secondary particle formation are likely to vary depending on the environment, measurements on atmospheric nucleation and particle growth are needed from around the world to be able to better describe the secondary particle formation, and assess its climatic effects on a global scale.
  • Kaila, Risto (Helsingin yliopisto, 2001)