Browsing by department "Institutionen för fysik"

Coronal magnetic field governs most of the coronal activities. Despite of its importance in solar atmosphere, there is no accurate method of measuring the coronal magnetic field. The current measurement methods of coronal magnetic fields depend on extrapolation of photospheric magnetic fields. There are different models to study the global structure of coronal magnetic field. The most commonly used models are PFSS model and magnetohydrodynamic (MHD) model. In this thesis, we study the coronal magnetic field condition during major solar energetic particle (SEP) event of 23rd solar cycle by using the PFSS model. We use 114 SEP events observed by the SOHO/ERNE experiment in 1996-2010. In the beginning we identified 43 events that are relatively free from the disturbance caused by interplanetary coronal mass ejections (ICMEs). We examined these SEP events using IDL software developed by Lockheed Martin Solar and Astrophysics Lab (LMSAL). We produced plots of open coronal magnetic field of each events using SolarSoft. We also classified SEP events according to their number of connection points as events with; single connection point, double connection points and multiple connection points. Events with multiple connection points make up almost one third of the total events. These events show that the coronal magnetic connection are typically complicated and neighboring magnetic field lines in the solar wind can be magnetically connected to regions that are well separated in the low corona. We also found that the actual connection longitude (a longitude that takes in to account the coronal magnetic field) is most of the time is closer to the flare site associated with the event than the Parker spiral connection longitude. The Parker spiral longitudes, connection longitudes and flare longitudes are analyzed in detail by histograms. Finally, we chose two example events and analyzed them by using intensity-time profile of particles, plots of from LASCO CME catalog and plots produce by the SolarSoft. Based on our analysis we classified the example events into gradual and hybrid SEP events.

Solar Energetic particle (SEP) events are sudden and temporary increases in the cosmic ray fluxes which are related to solar flares or interplanetary shocks originating from the Sun. Solar energetic particle transport modelling requires a systematic understanding of the properties of the heliosphere. In the current modelling of particle transport in the heliosphere, it is assumed that the interplanetary medium has a steady-state solar wind and that the magnetic field in the heliosphere follows a Parker spiral. The presence of coronal mass ejections (CMEs) or interplanetary coronal mass ejections (ICMEs) in the heliosphere could cause interference with the solar wind and the magnetic field in the heliosphere. In this project we analyse two heliospheric modelling tools, called ENLIL and ENLIL-with-cone models, to see how accurately they could describe the heliosphere in the presence of Coronal mass ejections. To realize this goal we investigated the SEP events of the 23rd solar cycle. At first we investigated 114 SEP events recorded in this cycle for their relationships with CMEs and ICMEs. First, we investigated whether the SEP events could be related to ICME using time-window analysis and the position of the ICME when the SEP event was recorded. Using this process we identified 43 SEP events that are ICME-clean (not related to any ICME according to the two criteria we set). We then modelled the ICME-clean events using ENLIL modelling. We further analysed the ICME-clean events if they have any relation to CMEs. We narrowed our search only to SEP events that have three or less CME that could be associated with them. We then produced a plot for these SEP events to further study the relation between the SEP and the CMEs. We singled out the SEP event that was recorded on May 9, 1999 as a perfect candidate to be further analysed using ENLIL-with-cone model. This event is chosen because it is associated with a fast northward CME that expands into the western hemisphere and could possibly have accelerated the SEP towards Earth. When analysed with ENLIL-with-cone model, we found out that the CME interfered with the magnetic field lines that are directed towards Earth, thus providing a likely origin for the observed SEP event at 1AU. Though the contact between the CME and the Earthward field lines was very brief, it disrupted the Parker spiral structure of the magnetic field lines. From the statistical analysis of the ICMEs and CMEs during the large SEP events of the 23rd solar cycle, we deduced that the two assumptions used in the modelling of heliospheric SEP transport (steady-state solar wind and Parker spiral structure of the magnetic field) could not be made in typical cases. However, more advanced descriptions of the heliospheric field like ENLIL-with-cone could be utilized for modelling instead. From this project we concluded that a future heliospheric modelling tools need to encompass more factors than the two assumptions discussed above.

A new theoretical model for the structure of glasses is presented and used to study the boson peak found in glasses. The model is based on a simple lattice model familiar from crystals, which is disordered using techniques from noncommutative fluid models. First classical crystal models and concepts of lattice vibrations are reviewed, focusing on acoustic and optical waves, the density of vibrational states, heat capacity and the Debye model. Then noncommutative fluid theory and noncommutative geometry are shortly introduced to show the connection to fluids in our model. After these introductions, the glass model is formulated and used to calculate the dispersion relations, the density of vibrational states and the heat capacity. The density of states has a Van Hove singularity at low frequencies, which generates the boson peak seen in experiments. The glass is found to have both acoustic and optical waves, and the acoustic waves are located very close to the frequency of the Van Hove singularity, which hints that the boson peak should be related to acoustic waves.

Hydrografian seuranta Itämerellä on hoidettu perinteisesti tutkimusaluksilla, joiden operoiminen on kallista ja joilla päästään seurantapisteille tekemään luotauksia vain muutaman kerran vuodessa. Ilmatieteen laitos on vuodesta 2011 testannut ja operoinut automaattisia profiloivia Argo-poijuja Selkämerellä. Poijut tarjoavat uudenlaisen menetelmän hydrografian ja syvien virtausten mittaamiseen, mutta Itämeren mataluus ja vähäsuolainen vesi aiheuttavat haasteita niiden operointiin. Tässä tutkielmassa käsitellään Ilmatieteen laitoksen Argo-poijujen viiden ensimmäisen operatiivisen vuoden (2012--2016) profiilimittauksia. Tärkeimpiä tutkimuskysymyksiä ovat, mitä poijujen data kertoo Selkämerestä itsenäisenä datasettinä, missä poijut toimivat paremmin suhteessa perinteiseen seurantaan ja missä eivät, ja miten poijuja kannattaisi käyttää osana olemassa olevaa havaintoverkkoa. Argo-poijujen datan perusteella lasketut keskimääräiset lämpötilan ja suolaisuuden arvot Selkämerellä olivat lähellä kirjallisuudessa esitettyjä, joskin pohjakerroksen suolaisuus oli noin 0,5\,g\,kg$^{-1}$ matalampi. Tämän arveltiin johtuvan osin siitä, etteivät kaikki profiilit ulottuneet pohjaan asti, ja osittain lyhyestä aikasarjasta verrattuna kirjallisuuden klimatologisiin keskiarvoihin. Vuosi 2014 havaittiin poikkeukselliseksi sekä pintalämpötilan että pohjakerroksen suolaisuuden osalta. Korkean suolaisuuden todettiin olevan todennäköisesti merkki varsinaiselta Itämereltä Selkämerelle tulleesta tavallista suuremmasta vesimäärästä. Argo-poijut tuottavat profiileja vesipatsaan hydrografiasta huomattavasti tiheämmin, kuin perinteiset seurantamatkat, joten niiden vahvuus on laivaseurantaan verrattuna lyhytaikaisten muutosten seurannassa. Virtauksia on myös mahdollista arvioida Argo-poijujen ajelehtimisnopeuden perusteella. Selkämeren syvänteelle lasketut virtausnopeudet olivat lähellä kirjallisuudessa esitettyjä (1,4--4,8\,cm\,s$^{-1}$). Virtausnopeudet ovat ennemminkin hydrografiamittausten sivutuote, kun Argo-poijujen käyttötarkoitus, nopeuksien arvioinnissa on paljon virhelähteitä, jotka yhdistettynä matalaan merialueeseen hankaloittavat tulosten tulkintaa. Kaiken kaikkiaan Argo-poijut todettiin Selkämerellä toimiviksi merialueen haaseista huolimatta. Poijut tuottavat profiilidataa tiheydellä, joka on aiemmin ollut saavuttamattomissa laivaseurannan kalleuden takia. Ensimmäisinä operatiivisina vuosina Selkämeren Argo-poijut ovat tuottaneet syvännealueen profiilidatasta jopa 80\,\% (verrattuna HELCOMin seuranta-asemilla tehtyihin mittauksiin). Tulevaisuudessa olisi kiinnostavaa kokeilla poijujen ohjautumista esimerkiksi Ahvenanmerellä, ja tutkia, miten tällä hetkellä (10/2017) Selkämerellä yhtäaikaa mittaavat kolme poijua kuvaavat koko Selkämerta ja olisiko tämä sopiva määrä poijuja yhtäaikaisessa operatiivisessa seurannassa.

Vacuum arc electrical breakdowns cause problems in many appliances operating in high electric field, such as the Compact Linear Collider (CLIC), a proposed next-generation particle accelerator in CERN. The breakdown phenomenon is not well-understood despite decades of research. Diffusive mass transport in metallic surfaces under electric fields is hypothesised to play a role in the events leading to breakdowns. Kinetic Monte Carlo (KMC) is a well established simulation method for studying diffusion. The weakness of KMC is that it requires knowledge of the rates of all processes that can happen during simulation: in the case of diffusion, these are migration events of mobile objects. The rates can be found from migration barriers, which in turn can be calculated using various methods. In this thesis, the parametrisation scheme of an existing atomistic KMC model for studying Cu surface diffusion was improved. In this model, the migration barrier is a function of the local environment of the migrating atom. The barriers in different environments were calculated with the nudged elastic band (NEB) method. It is an accurate way of finding barriers, but too expensive to be used for calculating them all in the improved parametrisation scheme. This problem was treated with a multidisciplinary approach of training an artificial neural network (ANN) to predict the barriers, using a limited dataset calculated with the NEB method. Good prediction performance was achieved for the case of stable migration processes on smooth surfaces, and the predictor function was found to be sufficiently fast to be called during KMC runtime.

This thesis discusses various topics related to the study of strongly coupled quantum field theories at finite density or, equivalently, finite chemical potential. In particular, the focus is on the theory of strong interactions, quantum chromodynamics (QCD). Finite-density QCD is important in the description of numerous physical systems such as neutron stars or heavy-ion collisions, a brief overview of which are given, alongside with the QCD phase diagram as motivational examples. After this, the general construction of a Lagrangian finite-density quantum field theory is described. In contrast with the zero-density setting, a finite-density field theory does not admit a simple description on the lattice, rendering this standard approach to strongly coupled theories impractical due to the so-called sign problem. Various attempts of addressing the sign problem are reviewed, and the so-called Lefschetz thimble approach and the complex Langevin method are discussed in detail. Some mathematical details related to these approaches are elaborated in the appendices. Due to the impracticality of lattice methods, a perturbative description becomes more important at finite density. Perturbative finite-density QCD and methods useful in practical calculations are discussed. Amongst them is a detailed proof of a set of so-called 'cutting rules' that apply to zero-temperature finite-density quantum field theory, an example computation using these rules as well as a discussion on various divergences and their relation to zero-density theory.

Physical and mechanical properties of drill core specimens were determined as a part of investigations into excavation damage in the dedicated study area in ONK-TKU-3620. The main goal of this study was to find indicators of excavation damage in the form of anomalous physical properties linked to increased porosity or lower mechanical strength. Geophysical indicators are desired for their ease, speed and cost-effectiveness. The secondary goal was to find associations between dynamic and static elastic properties, to allow estimation of rock mechanical properties using geophysical measurements. The parameters most sensitive to the presence of (saline) pore space fluid showed depth dependencies. Resistivity showed abnormally low values in the first 0.2 m, and an increase with depth in the first 0.7 m from the study area surface. S-velocity, shear impedance, shear modulus and Young's modulus all showed abnormally low values in the first 0.2 m from the study area surface. In addition to clear depth dependencies, other indicators of excavation damage were found. Specimens in the first 0.7 m from the study area surface showed increased proportion of high (> 0.5 %) porosity values. Combinations of high porosity/shallow depth, low resistivity/shallow depth, high porosity/low resistivity, low IP/shallow depth and low IP/high porosity also seem to separate anomalous specimens. S-velocity, P/S -ratio, Poisson's ratio and all three impedances in respect to depth separated anomalous specimens. Abnormally high S-velocity in respect to other elastic properties also seemed to separate anomalous specimens. On one of the anomalous specimens, the presence of an EDZ feature was confirmed by Posiva geologist. This specimen could be identified based on S-velocity, P/S -ratio, Poisson's ratio and all three impedances. Best indicators for excavation damage based on this study would appear to be resistivity, S-velocity, shear impedance, shear modulus and Young's modulus. Most of the other elastic parameters in conjunction with other parameters could be used to identify anomalous specimens. The results support the use of electrical and seismic methods to identify excavation damage. Estimation of static elastic properties based on dynamic elastic properties does not appear possible based on this study. The views and opinions presented here are those of the author, and do not necessarily reflect the views of Posiva.

The purpose of this study is to research what factors affect a student’s likelihood of successfully reaching his or her goal of becoming a scientist or, more specifically, a physicist. Academic achievement has long been associated with intelligence. This restricted view has not been comprehensive enough and has lacked the study of how noncognitive personality traits relate to success. In this study factors relating to skills and talent in physics, as well as to personality, will be analysed. Their relationship to a physics attitude assessment test will be investigated as well. Three questionnaires were used to gather data for this study: the Grit Survey, the Colorado Learning Attitudes about Science Survey (CLASS) and the Force Concept Inventory (FCI). Grit measures a person’s perseverance of effort and consistency of interest. It is a personality trait that is thought to predict success better than previous personality constructs have. CLASS measures expert-like thinking of students in a physics context. In this study CLASS is used as a predictor of academic achievement. FCI measures a student’s conceptual understanding of the force concept in Newtonian physics. In this study FCI is used as an evaluator of a student’s skills and talent in physics. The sample studied in this thesis consisted of 71 students attending a first-year physics course at the University of Helsinki. 43 of the participants were male and 28 female. Most of the students were students that had decided to major in physics. Results from correlation analysis between grit and CLASS show that no significant relationship was found between these two factors, with a correlation coefficient of only r=0.181 (p=0.131). FCI and CLASS did show a correlation of r=0.312. Correlations between the factors of grit and CLASS were also analysed in this study. Grit did not correlate with CLASS, a predictor of academic achievement. FCI, an evaluator of skill and talent in physics did correlate with CLASS. Grit as a whole does not seem to relate to academic achievement. But its dimension of effort does. Grit effort correlated with CLASS with a correlation coefficient of r=0.241 (p=0.043). The two dimensions of grit seem to measure different things and it is important to be able to analyse them separately.

Magnetic resonance imaging (MRI) provides spatially accurate, three dimensional structural images of the human brain in a non-invasive way. This allows us to study the structure and function of the brain by analysing the shapes and sizes of different brain structures in an MRI image. Morphometric changes in different brain structures are associated with many neurological and psychiatric disorders, for example Alzheimer's disease. Tracking these changes automatically using automated segmentation methods would aid in diagnosing a particular brain disease and follow its progression. In this thesis we present a method for automatic segmentation of MRI brain scans using parametric generative models and Bayesian inference. Our method segments a given MRI scan to 41 different structures including for example hippocampus, thalamus and ventricles. In contrast to the current state-of-the-art methods in whole-brain segmentation, our method does not pose any constraints on the MRI scanning protocol used to acquire the images. Our model is based on two parts: the first part is a labeling model that models the anatomy of the brain and the second part is an imaging model that relates the label images to intensity images. Using these models and Bayesian inference we can find the most probable segmentation of a given MRI scan. We show how to train the labeling model using manual segmentations performed by experts and how to find optimal imaging model parameters using expectation-maximization (EM) optimizer. We compare our automated segmentations against expert segmentations by means of Dice scores and point out places for improvement. We then extend the labeling and imaging models and show, using a database consisting of MRI scans of 30 subjects, that the new models improve the segmentations compared to the original models. Finally we compare our method against the current state-of-the-art segmentation methods. The results show that the new models are an improvement over the old ones, and compare fairly well against other automated segmentation methods. This is encouraging, because there is still room for improvement in our models. The labeling model was trained using only nine expert segmentations, which is quite a small amount, and the automated segmentations should improve as the number of training samples grows. The upside of our method is that it is fast and generalizes straightforwardly to MRI images with varying contrast properties.

Begreppskartor utvecklades ursprungligen på 1980-talet av Joseph Novak och Bob Gowin som ett sätt att strukturera och därmed få djupare förståelse för ny kunskap. Sedan dess har de utvecklats och undersökts som inlärnings-, utbildnings- och bedömningsmetod, inte minsta av forskare som Maria Ruiz-Primo. Den här avhandlingen utgår till stor del från Ruiz-Primos utveckling av begreppskartan som ett verktyg för att bedöma elevers prestationer och undersöker huruvida traditionella prov kunde ersättas med begreppskartsuppgifter inom fysiken. Begreppskartor har en bevisligen positiv effekt vad gäller inlärning, men de används sällan i våra skolor och det är svårt att få elever att anamma en ny och ointuitiv inlärningsmetod. Att använda begreppskartan som en bedömningsmetod i klassen skulle knuffa eleverna mot ett mer begreppsbaserat tänkande och kunde hjälpa dem i sin förståelse av ämnet. Som provuppgift är begreppskartan snabb att göra och rätta men ger läraren goda insikter i elevens förståelse för ett ämne. Ifall man kunde tänka sig att använda begreppskartan som provuppgift åtminstone diagnostiskt skulle det föra med sig en hel drös fördelar. Två klasser undersöktes, en i grundskolan på årskurs sju och en kurs i gymnasiet. Bägge grupper fick lära sig grunderna i att göra begreppskartor på förhand, gymnasisterna hade lite erfarenhet av det också från tidigare. Båda grupperna fick sedan under en kurs skriva ett traditionellt prov samt göra en begreppskarta av materialet de nyligen gått igenom. Begreppskartorna bedömdes med en femstegs-modell utvecklad av Ruiz-Primo och Shavelson och gavs ett vitsord beroende på hur väl de jämfördes med en expertkarta. Resultaten undersöktes skilt för de två grupperna och granskades för svårighetsgrad, korrelation och överensstämning enligt en statistisk metod utvecklad av Bland och Altman 1986. Svårighetsgraden för uppgiften fanns vara lämplig, bägge gruppernas medeltal var något lägre för begreppskartan än för det traditionella provet vilket kan motiveras med att eleverna trots allt har större erfarenhet av traditionella prov än begreppskartor. Korrelationen för grundskolegruppen fanns vara god, 0,825 medanden för gymnasiet var betydligt svagare, 0,412. Bland-Altman metoden gav vidare negativa resultat för gymnasiet med mycket stora kast mellan de enskilda elevernas prestationer i de två uppgifterna. Grundskolegruppen presterade lite mer konsekvent men visade en trend där de verkligt svaga och de verkligt starka eleverna gynnades av begreppskartsuppgiften medan eleverna med medeltal kring 7 gjorde sämre ifrån sig än i det traditionella provet. Korrelationen för grundskolan är så pass stark att det är tänkbart att begreppskartan kunde användas som en bedömningsmetod inom grundskolan. Grundskolans begränsade matematik gör att också en stor del av naturvetenskaperna är fenomen- och begreppsbaserade snarare än baserade på problemlösning. I gymnasiet är det tvärtom, största delen av gymnasiekursen inom fysiken går ut på matematisk uträkning av fenomen, inte på att kunna förklara dem med ord och förstå samband. Som en följd av detta är begreppskartorna mer användbara som en alternativ bedömningsmetod i grundskolan än i gymnasiet.