Browsing by department "Fysiikan osasto"

Tämän pro gradu -tutkielman tavoitteena on selvittää onko säteilysumutilanteiden tunnistaminen mahdollista maston eri korkeuksilta saatavien lämpötila- ja kosteushavaintojen avulla ja tarkastella mahdollisten tunnistettujen säteilysumutapausten elinkaarta. Tutkielmassa käytettävä aineisto koostuu Espoon Kivenlahdessa sijaitsevan tv-maston vuosien 2014-2018 havainnoista ja säteilysumutilanteen tunnistamisen onnistumista arvioidaan Espoon Tapiolan ja Nuuksion sekä Helsinki-Vantaan pintasääasemien näkyvyysmittausten perusteella. Tutkimuksessa valitaan maston kahden alimman mittaustason (2m ja 26m) lämpötila- ja kosteushavainnoille suodatuskriteerit, joiden perusteella löydetään 126 mastohavaintojen nojalla säteilysumuksi tulkittavaa tapausta. Näistä tapauksista 38 täyttää alle 1km näkyvyysehdon jollain pintasääasemalla. Koska pintasääasemat sijaitsevat vaihtelevien etäisyyksien päässä mastosta ja pintasääasemien olosuhteissa on paikallista vaihtelua, työssä analysoidaan näkyvyyssuodatettujen tapausten lisäksi myös kaikkien kriteerit täyttävien tapausten elinkaarta. Aineistoa analysoidaan aritmeettisen keskiarvon ja keskihajonnan avulla. Havaintojen nojalla määriteltyjä säteilysumutapauksia edeltävät meteorologiset olosuhteet ovat samansuuntaiset muualla saatujen aiempien tutkimustulosten kanssa. Ennen sumun muodostumista tuulen nopeus heikkenee, lämpötila laskee ja suhteellinen kosteus kasvaa. Tutkimuksen säteilysumutilanteet jaetaan kasvukorkeuden perusteella paksuihin (korkeus ≥ 26m ) sekä ohuisiin (2m ≤ korkeus < 26m) tapauksiin. Paksuja säteilysumutapauksia löytyy kaikkien (näkyvyyssuodatettujen) tapausten joukosta 58 (25) ja ohuita tapauksia 56 (13). Keskimääräinen tuulen suunta on sekä ohuiden että paksujen tapausten osalta etelästä, joten tuulen suunta ei anna suoraa selitystä säteilysumujen kasvukorkeudelle. Ylempien tasojen kosteusolosuhteet sen sijaan antavat viitteitä sumun kasvukorkeuden kehittymisestä suhteellisen kosteuden ollessa keskimäärin 10 prosenttiyksikköä korkeampi paksujen tapausten kuin ohuiden tapausten kohdalla ylemmillä mittaustasoilla. Maston havaintohistorian (1989-2018) kaikki mittaukset eivät ole tutkimuksen kannalta käyttökelpoisia johtuen muun muassa mittauskorkeuksien vaihtumisesta havaintohistorian aikana. Aineiston havaintojen keskihajonta oli siten kautta linjan melko suuri suppeasta otoskoosta johtuen. Vahvempien johtopäätösten tekemiseksi vaadittaisiin enemmän havaintoja. Lisäksi Kivenlahden mastoon asennettavasta näkyvysmittarista olisi suuri apu sumutilanteiden tarkemman tunnistamisen mahdollistamiseksi, sillä toisaiseksi vaihtelevien etäisyyksien päässä sijaitsevat näkyvyysmittarit eivät anna tarkkaa kuvaa Kivenlahden näkyvyysolosuhteista.

We study single scalar field inflation with the standard model Higgs boson as the inflaton. We first review the homogeneous and isotropic description of the universe given by the FLRW model as well as the inflation scenario. Then we study how this scenario can can be achieved by a single scalar field minimally coupled to gravity in the slow-roll approximation. Next we study linear perturbation theory around the FLRW background. Here the perturbations are decoupled into scalar, vector and tensor perturbations which allows to study them separately. The split of physical quantities into perturbations around a background introduces gauge degrees of freedom which we address by reviewing gauge transformation of the scalar and tensor perturbations (the latter which turns out to be gauge-independent). We then use the comoving gauge and define, for the scalar perturbations, the gauge-invariant quantity known as the comoving curvature perturbation. For scalar perturbations the Einstein Field equation yields the Mukhanov-Sasaki equation, which we solve to first order in the slow-roll approximation in terms of the Mukhanov variable. We then quantize this variable using canonical quantization and calculate the power spectrum from vacuum fluctuations. We also carry the same analysis for tensor perturbations. With the power spectra at hand we introduce the spectral parameters and discuss current observations and constraints on such parameters. In Higgs inflation the Standard Model Higgs boson takes the role of the inflaton. Here the Higgs field is also coupled to the Ricci scalar, giving us a non-minimal coupling to gravity. This coupling can be transformed away using a conformal transformation at the expense of a field re-definition. This enables us to use the results reviewed thus far. At tree level we find the inflationary predictions to be in excellent agreement with current observations. However, quantum corrections complicate this picture. We review the tree level unitarity of the model and examine arguments in favour and against it. We also study how quantum corrections can qualitatively change the shape of the potential and the viability of Higgs inflation in each scenario.

Galaxy clusters are the largest tightly gravitationally bound structures in the Universe. They are abundant and uniformly distributed in the sky, which makes them excellent targets for studying the history and properties of our Universe. They can be used to study some of the burning questions in astrophysics and cosmology. For example, how have the large scale structures evolved from billions of years ago to how we see them today, and what are the best values for parameters like matter densities in our cosmological theories. Photometric galaxy cluster surveys have been conducted for decades, but now is the dawn of large scale spectroscopic surveys. Spectroscopy gives more precise redshift values for galaxies and galaxy clusters than photometric observations, which in turn makes other astrophysical parameters like mass estimation of galaxy clusters much more reliable. The SPIDERS galaxy cluster survey is the largest X-ray detected, spectroscopic, visually validated survey conducted to date. It improves the precision of galaxy cluster redshifts by a factor of 10. The precision of redshift has a direct improvement on other distance related parameters calculated by using galaxy clusters. The SPIDERS value added catalogue, which came out of the survey, is a tremendous achievement and will benefit astrophysicists and cosmologist around the world. The catalogue is the result of the work of the SPIDERS team of experts, and it is freely available online. This thesis explains how the SPIDERS survey was conducted; it’s phases, algorithms and the science behind it. I give many examples of the data processing and visual validation of targets, and explain the results and the significance of having such a large and precise data set.

Forskningens mål är att finna de kunskapsdomän som krävs för att lösa fysikuppgifter i studentexamensprovet i fysik på ett framgångsrikt sätt, för att sedan kunna avgöra svårighetsgraden på uppgifterna. Detta gjordes genom att undersöka tidigare forskning i ämnet. Man kom fram till att svårighetsgraden påverkades av de kognitiva färdigheter uppgiften krävde, det fysikområde uppgiften behandlade och de praktiska processer eleven måste göra för att lösa uppgiften. Dessa färdigheter delades in i hierarkiska undernivåer och sammanställdes till en taxonomi för studentexamensuppgifter i fysik. Man undersökte 143 studentexamensuppgifter i fysik från åren 2006 – 2016 och tilldelade dem poäng enligt de undernivåer de representerade i taxonomin för studentexamensuppgifter i fysik. Dessa poäng användes som rådata tillsammans med de medelpoäng elever fått i samma uppgifter. Rådatan användes av ett artificiellt neuronnätverk programmerat i GNU Octave som tränades att förutse medelpoängen för nya uppgifter. Man kunde konstatera att neuronnätverket fungerade bäst för lättare uppgifter där elever fått höga poäng och mindre bra för uppgifter som genererat lägre medelpoäng i studentexamen. Neuronnätet testades och klarade av att beräkna medelpoäng som hade korrelationskoefficienten R^2 = 0,90 med de verkliga medelpoängen. Taxonomin för studentexamensuppgifter i fysik kan användas av provkonstruktörer som vill säkerställa att alla uppgiftstyper representeras i provet och av lärare som vill se till att dessa övas i undervisningen

Active longitudes are areas, where star spot activity is centered in and reappears on a periodic manner. Star spots are cooler areas on the star surface, caused by rising magnetic field lines inhibiting the flow of the convective region. The ways to observe active longitudes is limited, but in some stars the phenomenon has clearly been present, as Lehtinen et al. (2016) has showed. One of the observation methods is to analyse the primary and secondary minima epochs of the star’s light curve relative to its orbital period. Time series analyses are tools to gather these phases from light curves. Here two different methods were used to analyse a RS CVn binary member IM Pegasi. Continuous Period Search (CPS) (Lehtinen et al. 2011) defines an adaptive,single periodic model to a moving window of observations, allowing the light curve to contain sudden changes. Discrete Chi-square Method (DCM) (Jetsu 2020)) applies a multiperiod, polynomial-trended model to fit to the data with constant parameters, assuming all changes in the light curve are part of periodic changes. Using these two methods the light curve of IM Pegasi is studied in order to determine if there could be active longitudes present. Four data segments were chosen to be further analysed with DCM based on the CPS results. One of the segments showed a flip-flop effect in the CPS phase results, which was showed to be apparent based on the successful DCM performance. Two segments, which had rather steady phase trend in the CPS results, performed well with the DCM analysis. The fourth segment, which showed strong migrating of the secondary minima phase in CPS analysis, had problems performing with DCM as a whole segment. The primary periodicity is detected in both CPS and DCM withing good limits of agreement. The DCM dual-periodic model results in all four segments indicate of an additional, more fragile irregular structure in the star, like separated dynamo waves.

The solar dynamo is a highly complex system where the small-scale turbulence in the convection zone of the Sun gives rise to large-scale magnetic fields. The mean-field theory and direct numerical simulations (DNSs) are widely used to describe these dynamo processes. In mean-field theory the small-scale and large-scale fields are separated into a fluctuating field and a mean field with their own characteristic length scales. The evolution of the mean fields is governed by the contributions of fluctuating fields and these contributions are often written as so-called turbulent transport coefficients (TTCs). Thus obtaining accurate values for these coefficients is important for the evaluation of mean-field models. Recently, test-field method (TFM) has been used to obtain TTCs from DNSs. The aim of this work is to use mean-field dynamo simulations to validate a test-field module (TFMod), which is an implementation of TFM. Both the TFMod and a mean-field module (MFMod), which is used to calculate the mean-field simulations, are written as modules in Pencil Code, which is a versatile magnetohydrodynamics simulation software. The validation is done by comparing magnetic fields generated using DNS with magnetic fields generated using a mean-field simulation that uses TTCs obtained using the TFMod. In addition the TTCs measured by the TFMod are compared against expected values that are calculated using the second-order correlation approximation (SOCA). The model chosen for this validation is an α 2 -dynamo driven by helical forcing as described by Mitra et al. (2010). The accuracy of the MFMod is confirmed by replicating mean-field models by Krause and Rädler (1980), Steenbeck and Krause (1969) and Jouve et al. (2008). The measured TTCs replicate various expected properties with the exception of the β-tensor, which does not appear to be isotropic. This finding was shown by Viviani et al. (2019) to be an artefact from the original TFM tensor decomposition. Using all TTCs the MFMod is able to replicate various features of the DNS such as the growth rate, oscillation period and the field configurations of the magnetic fields. The validation gives credence to the usage of the TFMod for the measurement of TTCs from DNSs, but it also shows that mean-field simulations such as the one described here are important for testing the validity of the TTCs produced by the TFM. It also suggests that utilizing the combination of the TFM and mean- field simulations in conjunction with DNS on more complicated dynamos could be an area of interest for future research.

A series of x-ray microtomography (micro-CT) measurements was performed on a set of rabbit femur bone samples containing artificial scaffolds of bioactive glass BAG-S53P4, implanted into an intentionally induced defect, i.e. a gap, in the femur. The scaffolds, some additionally enveloped in PLGA, were supportive structures composed of small granules of bioactive glass, intended to enhance, stimulate and guide the healing and regeneration of bone. The 34 samples were harvested from the rabbits at three different stages of healing and bone regeneration: 2 weeks, 4 weeks and 8 weeks. In addition to 27 samples that contained scaffolds of BAG-S53P4 or BAG-S53P4-PLGA, which had been implanted into the femur of a rabbit, 3 scaffolds of BAG-S53P4(-PLGA) that were not implanted and 7 control samples containing inert PMMA-implants were also included in the measurements for comparison. During the healing process the bioactive glass granules are gradually dissolved into the surrounding bodily fluids and a thin reaction layer composed of silica gel forms onto the surfaces of the granules. Subsequently an additional surface layer composed of HCA, a material that closely resembles natural hydroxyapatite, is formed onto the granules. As the healing process to regenerate the bone in the gap progresses, a complex three-dimensional network of newly formed trabecular bone grows in between the granules, attaching onto the surface layers and eventually enveloping the gradually dissolving granules entirely. Ultimately, the scaffold is intended to degrade completely, and a structure of regenerated, remodeled cortical bone is expected to be formed into the volume of the initial defect. As the thicknesses of both the surface layers of the granules and the individual trabeculae of the newly formed bone are in the micrometre range, x-ray microtomography was employed to evaluate and assess the complex three-dimensional structure, consisting of trabecular bone intertwined with granules at varying stages of dissolution. By evaluating the rate of formation of these structures at three different stages, i.e. time points, of regeneration, valuable information on the effectiveness of the bioactive glass BAG-S53P4(-PLGA) for the regeneration of defected bone can be obtained. The measurements were performed at University of Helsinki’s Laboratory of Microtomography using its Nanotom-apparatus with 80kV voltage, 150microA current and a voxel size of 15micrometres. 1000 projection images per sample were used in 37 reconstructions utilizing the FBP-algorithm. Subsequent image processing to analyze and compare the samples was conducted using ImageJ. A procedure to reduce image artefacts – due to metal parts in the samples – was developed, utilizing Gaussian filtering, as well as a preliminary image segmentation scheme, utilizing Morphological filtering, to automatically separate the bone from the granules and their surface layers.