Skip to main content
Login | Suomeksi | På svenska | In English

Browsing by study line "De astrofysikaliska vetenskaperna"

Sort by: Order: Results:

  • Leppälä, Ari (2023)
    The thesis presents results for simulating an orbit determination process with least-squares fitting (LSF) of synthetic observations for non-cooperative satellites. Five satellites with different masses, sizes, and orbital inclinations are orbiting in Low Earth Orbit (LEO). Three different inclinations (53°, 85°, and 98°) are simulated using Orekit (Orbit Extrapolation Kit), an open-source astro- dynamical software library. Satellites are observed with four hypothetical radars. The thesis is a feasibility study. It addresses four main research questions: How reliable are predictions of the satellite position for 24 h, 48 h, 72 h, and 96 h after the first detection when using the observations from four consecutive overpasses and the LSF-estimated orbit? How accurately the Keplerian or- bital elements be modelled with LSF? How does the satellite’s mass affect the results’ reliability, and can the satellite’s drag, reflection, and absorption coefficients be estimated with Orekit? The simulation utilizes the Orekit software library in a Python environment, and the simulation incorporates several perturbing forces: Earth’s gravitational potential model EGM2008, an atmo- sphere model with space weather data NRLMSISE-00, tidal forces, and point-like masses of the Sun and the Moon. Synthetic observations were simulated by assuming Gaussian distribution for the uncertainties of the measurements. The initial estimate of the orbit is done with Gooding’s method, and the Levenberg–Marquardt algorithm is used for the LSF. The propagation of the actual satellite for 96 h was compared with the 96-h propagation of estimated orbital elements and with a satellite with estimated size and mass. Simulations were conducted with overestimated and underestimated initial masses. Each combination of a satellite, an observing radar, and an orbit was repeated 100 times. The results for reliability were promising. The likelihood of the sky-plane projected error of the position staying under the 3.0-km for up to 3 days was found to be 90% for two of the simulated radars, but only in cases of orbits having an inclination of 53°. Two other orbits had less successful results, but simulations revealed a potential increase in reliability by adjusting the observational strategy. There was a small bias in the results hinting that underestimating the satellite’s mass could result in less accurate orbit estimates than overestimating it. The satellite’s drag, absorption, and reflection coefficients could not be obtained due to the short observational time intervals simulated.
  • Hietala, Hilppa (2020)
    The aim of this thesis is to explore applications of machine learning to the study of asteroid spectra, and as such, its research question can be summarized as: How can asteroid spectra be analyzed using machine learning? The question is explored through evaluation of the obtained solutions to two tasks: the optimal locations of spectrophotometric filters for asteroid classification success and the formation of an asteroid taxonomy through unsupervised clustering. First, background theory for asteroids and particularly spectroscopy of asteroids is presented. Next, the theory of machine learning is briefly discussed, including a focus on the method utilized to solve the first task: neural networks. The first task is executed by developing an optimization algorithm that has access to a neural network that can determine the classification success rate of data samples that would be obtained using spectrophotometric filters at specific locations within the possible wavelength range. The second task, on the other hand, is evaluated through determining the optimal number of clusters for the given dataset and then developing taxonomies with the clustering algorithm k-means. The obtained results for the first task involving the optimal locations of filters for spectrophotometry seem reliable, and correlate relatively well with well-known mineralogical features on asteroid surfaces. The taxonomic systems developed by the unsupervised clustering also succeeded rather well, as many of the formed clusters seem to be meaningful and follow the trends in other asteroid taxonomies. Therefore, it seems that based on the two investigated tasks, machine learning can be applied well to asteroid spectroscopy. For future studies, larger datasets would be required for improving the overall reliability of the results.
  • Vuori, Mikko (2023)
    A method for deriving the complex refractive index of a mm-sized single particle in a specific wavelength using laboratory measurements is presented. Laboratory measurements were done using the 4π scatterometer, which measures Mueller matrix elements of a particle suspended in air using acoustic levitation as a function of scattering angle. To obtain the complex refractive index of the particle, measurements were compared to simulations from a newly developed SIRIS4 Fixed Orientation (SIRIS4 FO) geometric optics simulation. The 4π scatterometer is a unique instrument which measures Mueller matrix elements from a particle using linear polarizers and a detector rotating about the particle on a rotational stage. The scatterometer uses an acoustic levitator as a sample holder which provides nondestructive measurements and full orientation control of the sample. To compare the measurement results to simulations, SIRIS4 single-particle geometric optics code was modified to handle particles in a fixed orientation. The original code is able to calculate the Mueller matrix elements for a given 3D model, but averages the results over the orientation of the particle. The modified SIRIS4 FO calculates the Mueller matrix elements over the full solid angle as functions of the two scattering angles, which give the direction of observation of the scattered wave compared to the direction of the incident wave. A 3D model of the shape of the measured particle was constructed using X-ray microtomography, and was translated to SIRIS4 FO. The complex refractive index was obtained with a nonlinear least squares analysis by minimizing the sum of squared residuals between the measurements and simulations with varying refractive index values. Finally, confidence regions were constrained for the results, by estimating the computed residuals between simulations and measurements as the random errors in the nonlinear model.
  • Lehtinen, Simo (2021)
    The solar corona constantly emits a flow of charged particles, called the solar wind, into interplanetary space. This flow is diverted around the Earth by the magnetic pressure of the Earth’s own geomagnetic field, shielding the Earth from the effect of this particle radiation. On occasion the Sun ejects a large amount of plasma outwards from the corona in an event called a Coronal Mass Ejection (CME). Such events can drive discontinuities in the solar wind plasma, called interplanetary shocks. Shocks can affect the Earth’s magnetosphere, compressing it inwards and generating electromagnetic waves inside it. In this thesis we will cover a study of the ultra-low frequency (ULF) wave response in the magnetosphere to CME-driven shocks. Geomagnetic pulsations are ultra-low frequency plasma waves in the magnetosphere, observable from ground-based magnetometers. The compression of the magnetosphere by interplanetary shocks generates geomagnetic pulsations in the Pc4 and Pc5 frequency ranges (2 - 22 mHz). These waves play an important role in magnetospheric dynamics and the acceleration and depletion of high energy electrons in the radiation belts. We consider 39 interplanetary shock events driven by CMEs, and analyse ground-based magnetometer data from stations located near local noon at the time of the shock arrival. Solar wind measurements are used to categorise interplanetary shocks based on their Mach number and the dynamic pressure differential as main indicators of shock strength. The importance of these parameters in determining the strength of the wave response in the geomagnetic field is then studied using wavelet analysis and superposed epoch analysis. Stronger shocks are found to result in larger increases in wave activity, especially in the Pc4 range. Ground stations at higher latitudes observe higher wavepower, but there is an interesting anomaly in the Pc4 range at stations magnetically connected to regions near the plasmapause, which show an enhanced wavepower response. We quantify the decay time of the wave activity and find that it is around 20 hours for Pc5 waves and 7 hours for Pc4 waves.
  • Suni, Jonas (2021)
    Magnetosheath jets are a class of structures in the Earth's magnetosheath usually defined by an enhancement of the dynamic pressure of the plasma. Magnetosheath jets have been observed by several different spacecraft over the past few decades, but their origin and formation mechanism have remained unclear. The aim of this thesis is to use data from a global simulation to investigate the origin of magnetosheath jets. We defined two different kinds of structures, magnetosheath jets and foreshock compressive structures (FCS), and collected a database of individual jets and FCSs from 4 Vlasiator global hybrid-Vlasov simulation runs, all of which simulate only the ecliptic plane. We then conducted a statistical analysis of the properties of jets and FCSs, and their occurrence rates as a function of the definition of the FCS criterion. Jets were separated into two categories: jets that form in contact with FCSs (FCS-jets), and those that do not (non-FCS-jets). We found that up to 75% of magnetosheath jets form in association with an FCS impacting the Earth's bow shock. We also found that FCS-jets penetrate deeper into the magnetosheath than non-FCS-jets. Finally, we found no conclusive explanation for the formation of non-FCS-jets. The properties of both jets and FCSs agree qualitatively and to some extent quantitatively with spacecraft observations and other simulations in the literature. The formation of jets from FCSs impacting the bow shock is similar to the proposed theory that jets are linked to Short Large-Amplitude Magnetic Structures (SLAMS). In the future, we will study magnetosheath jets and FCSs in polar plane simulation runs as well, and ultimately in full 3D simulation runs. If made possible by new simulations, the effects of electron kinetic effects on jets and FCSs will also be studied. Comparison studies with spacecraft observations of jet formation from FCSs will also be conducted, if and when such observations are found and become available.
  • Kostamo, Iida (2023)
    In this thesis, 16 gas-free galaxy merger simulations were run in order to determine the dominant effect that causes the formation of cores, i.e. regions of ”missing light”, in the centers of massive early-type galaxies. The simulations were run on the Mahti supercomputer at the Finnish IT Centre for Science (CSC), using the simulation codes GADGET-3 and KETJU. The merging of galaxies will eventually lead to the merging of their central supermassive black holes (SMBHs). The evolution of a SMBH binary can be divided into three phases: the dynamical friction phase, the three-body interaction phase and gravitational wave (GW) emission phase. After the GW emission phase, the merged SMBH may receive a recoil velocity. To study the effects of these three phases on the formation of the core, three kinds of simulations were run. These include three GADGET runs and thirteen KETJU runs that can be divided into two groups, since eight of the runs had GW recoils enabled. When using only GADGET, the three-body interactions are not modeled due to the softening of gravity. Using KETJU allows for modeling the later evolution of the SMBH binary, including the three-body interaction phase, the GW emission phase and the resulting GW recoil. The initial conditions for the progenitor galaxies were motivated by the galaxy NGC 1600. The same stellar and dark matter profiles were used for each simulated galaxy. Two different SMBH masses were used. One KETJU merger was also run without SMBHs. For the runs with spinning SMBHs, the spin directions and magnitudes were chosen in such a way that different magnitudes of recoil velocity would be achieved. The size of the core can be determined from the brightness profile of the galaxy, assuming a constant mass-to-light ratio. The commonly used core-Sérsic profile was fit to the surface mass density profiles of the merger remnant galaxies. The best-fit parameter values were then used to estimate the sizes of the cores, such as the core radii. The amount of missing mass in the centers of the galaxies, i.e. the mass deficits, were also computed based on the core-Sérsic fits. We found that the mass deficit correlates positively with the mass of the SMBH for all the KETJU runs. Including GW recoils in the simulations was found to increase the mass deficits by roughly the equivalent of one SMBH mass compared to the KETJU runs without the recoils. The formation of cores was the weakest for the GADGET runs, since the cores were created only by the large-scale dynamics. No core was formed in the run without SMBHs, as expected. Thus, we can conclude that SMBHs are essential for the formation of cores in massive early type galaxies, and the largest cores are formed when GW recoils are included in the model.
  • Grön, Julia (2023)
    Mars is a rocky planet in the Solar System, fairly similar to the Earth. It is known for its red color and the theory that there has been liquid water and possibly life in Mars at some point in its history. Mars has been a target of study since space exploration began, with the first fly-by mission occuring in 1965. In the past, Martian climate could have supported a functioning hydrological system and Mars even could have had an ocean. This system would have been similar to the one happening in Antarctica due to cold weather. The Martian atmosphere is mostly composed of carbon dioxide, much thinner and colder compared to the Earth's atmosphere. Methane is a main point of interest due to it being a possible biosignature, a sign of life. Another interesting feature about the atmosphere are the dust devils that contibute to the climate by lifting dust into the air. The Martian soil is composed of rocks and dust containing toxic perchlorates. To colonize Mars, several requirements need to be met: the transportation and funding, designing the settlement efficiently with everything needed and determining how many people are necessary. The risk assessment to both people and equipment needs to be made and taken into account.
  • Luttikhuis, Thijs (2022)
    One of the most noticeable effects of solar–terrestrial physics is the aurora which regularly appears in the polar regions. This polar light is the result of the excitation of atmospheric species by charged particles originating from the solar wind and magnetosphere that enter the Earth’s atmosphere, which are called precipitating particles. We present the first results on auroral proton precipitation into the ionosphere using a global 3-dimensional simulation of near-Earth space plasma with the Vlasiator hybrid-Vlasov model, driven with a southward interplanetary magnetic field and steady solar wind parameters. The hybrid-Vlasov approach describes ions through their velocity distribution function in phase space (3-dimensional ordinary space and 3-dimensional velocity space), while electrons are represented by a massless charge-neutralizing fluid. Vlasiator is a global model describing the whole region of near-Earth space including the Earth’s magnetosphere (whole dayside and part of the magnetotail), the magnetosheath, as well as the foreshock region and some solar wind. The precipitating proton differential number fluxes for this run are determined from the proton phase-space density contained within the bounce loss-cone, which is set at a constant angle of 10 degrees everywhere. To determine the precipitation of particles at ionospheric altitudes (in this case a height of 110 km above the Earth’s surface), we trace magnetic field lines from the ionosphere to the inner boundary of the Vlasiator domain using the Tsyganenko model. With this, we obtain a magnetic local time–geomagnetic latitude map of differential number flux of precipitating protons in 9 energy bins between 0.5 and 50 keV. From the differential number flux, proton integral energy fluxes and mean energies can be obtained. The integral energy fluxes in the Vlasiator run are then compared to data of the Precipitation Electron/Proton Spectrometer (SSJ) instrument of the Defense Meteorological Satellite Program (DMSP) for several satellite overpasses during events with similar solar wind conditions as in the Vlasiator run. The SSJ instrument bins proton energies between 0.03 and 30 keV. Typical values of the total integral energy flux are between 5 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the cusp and between 1 · 10^6 and 3 · 10^7 keV cm−2 s−1 sr−1 in the evening sector for both Vlasiator and DMSP, although DMSP fluxes can locally be up to an order of magnitude higher. Additionally, global precipitation patterns in Vlasiator are compared to Ovation Prime, which is an empirical model based on data from DMSP which can be used to forecast precipitation of auroral electrons and protons. Although Ovation Prime shows a much wider cusp region compared to Vlasiator, both show similar maximum integral energy fluxes around 1 to 2 · 10^7 keV cm−2 s−1 sr−1 in the cusp region, and between 3 · 10^6 and 5 · 10^7 keV cm−2 s−1 sr−1 in the nightside oval.
  • Björn, Vesa (2023)
    Mercury is the smallest planet in the Solar System. The planet has a tenuous atmosphere, which means that it can be modeled as an atmosphereless object. Such Solar System bodies are covered in loose material called regolith, which affects how the object scatters light from the Sun. Photometry is a type of measurement that records the intensity of scattered light as a function of the viewing geometry, which is defined using angles from the surface towards the Sun and towards the observing instrument. Mercury was studied in 2011–2015 by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission of the United States’ National Aeronautics and Space Admin- istration, NASA. The present thesis uses spectrophotometric data, i.e., brightness as a function of wavelength, from the spacecraft’s Mercury Dual Imaging System (MDIS) instrument. Two theo- retical models, the Lommel–Seeliger (LS) and particulate medium (PM) models, are fitted to the observed reflectance using the least-squares method. The PM model is the more complicated of the two, and it includes a shadowing correction that de- pends on three model parameters. The parameters describe the properties of a particulate medium, i.e., regolith. The most important of the parameters is the packing density of the regolith, which is defined as the ratio of the volume of the particles to the total volume. The other two parameters describe the medium’s surface roughness in horizontal and vertical directions. The PM model is fitted to the observed reflectance for various different combinations of the model parameters. Ini- tially, only discrete and predetermined parameter values are used, but the parameter values are extended to arbitrary values using interpolation. Trilinear interpolation is utilized using several methods, followed by Markov chain Monte Carlo (MCMC) sampling for the final results. Most of the methods agree with one another, and fall within the uncertainties of the best solution, which allows to form an argumented conclusion about the best parameter values of the PM model. The best parameter values correspond to a densely-packed regolith with horizontally smooth surface and large height variations. The results of the present study can be used in the BepiColombo mission to Mercury, which is planned to begin its science operations in early 2026.
  • Minkkinen, Tiina (2023)
    Gravitational waves predicted by the theory of general relativity are providing us with an opportunity to study cosmological processes well beyond the horizon for electromagnetic observations. One such process is a first-order phase transition as the universe cools down, which could generate gravitational waves observable in the millihertz range today. The upcoming Laser Interferometer Space Antenna (LISA) is a space-based gravitational wave observatory with peak sensitivity in the millihertz range, so it has the potential to observe a sufficiently strong phase transition signal. The LISA Consortium have developed the LISA simulation pipeline for the purposes of creating mock data, which can then be used to test data analysis methods in preparation for the real LISA data analysis. Our aim is to first test the simulation pipeline by injecting a phase transition signal, with added instrument noises and galactic binary confusion noise, which we expect to be present in the real observations as well. Second, we will attempt to recover the injected signal to see whether it is detectable based on the deviance information criterion (DIC). We will do this for 25 different parameter combinations in order to chart the detectability of signals from different phase transition scenarios. Our results show better detectability for phase transition signals with higher amplitudes and frequencies centered around the mHz range, which is where the expected peak sensitivity of LISA lies. The confusion noise appears to be less of a distraction to our observations than the instrument noises, which set limits in the extremes of the LISA frequency range.
  • Benke, Petra (2021)
    Active galactic nuclei (AGN) are one of the most powerful sources of the luminous Universe. Radio-loud AGN exhibit prominent relativistic outflows known as jets, whose synchrotron radiation can be detected in the radio domain. The launching, evolution and variable nature of these sources is still not fully understood. We study 3C 84, because its proximity, brightness and the intermittent nature of its jet makes it a good target to investigate these open questions of the AGN phenomena. 3C 84 (optical counterpart: NGC 1275) is a Fanaroff-Riley type I radio galaxy, located in the Perseus cluster at z = 0.0176. Due to its close proximity, 3C 84 has been a favourable target for observations throughout the entire electromagnetic spectrum, especially for ones in the radio domain. Its most recent activity started 2003, when a new component emerged from the core in the form of a restarted parsec-scale jet. This provided a rare opportunity to study the formation and evolution of a jet (see Nagai et al. 2010, 2014, 2017 and Suzuki et al. 2012). The highest resolution results were obtained by Giovannini et al. (2018), who imaged the source with the Global VLBI Network together with the Space Radio Telescope, RadioAstron. This enabled them to capture the limb-brightened structure of the restarted jet and measure its collimation profile from ~350 gravitational radii. In this work I present the 22 GHz RadioAstron observations carried out 3 years later, in a similar configuration, but with a significantly different sampling of the space baselines than the ones presented in Giovannini et al. (2018). The calibration was carried out in the Astronomical Image Processing System (AIPS), whereas imaging was done in Difmap (Shepherd 1997). The aim of this thesis work was to obtain a high-resolution image of the source, measure the collimation profile of the restarted jet, and compare the results with those of Giovannini et al. (2018) and verify the observed source structures and measured jet properties, if possible. Comparing the images of the two epochs (angular resolution of the 2016 observations is 0.217x0.072 mas at Pa=-49.6°), they both show a similar structure, with the radio core, a diffuse emission region (C2), and the hotspot (C3) at the end of the restarted jet. Edge-brightening is confirmed in the jet and the counter-jet. However, the jet has advanced ~1 mas, corresponding to the velocity of 0.55c. C3 has moved from the center of the feature to the jet head, indicating an interaction between the jet and the clumpy external medium (Kino et al. , 2018 and Nagai et al., 2017). The base of the jet has also changed between the observation, approximately by ~20°. In the light that in the 1990s the jet pointed towards C2, then swinged westwards when the jet emerged (Suzuki et al., 2012 and Giovannini et al., 2018), and on the 2016 image has moved towards its initial position. This suggest a precessing jet, observed and modeled by Dominik et al. (2021) and Britzen et al. (2019). Measuring the brightness temperature of the core and the hotspot shows a signifacant drop of 70% and 50% since the 2013 measurements, respectively, due to emission of jet material and the expansion of the jet. Jet width measurements between 1200 and 19000 gravitational radii reveal a less cylindrical collimation profile, with r ~ z0.31 – where z is the de-projected distance from the core and r is the width of the jet. The evolution of the restarted jet’s profile from quasi-cylindrical (Giovannini et al. 2018) to less cylindrical implies that the cocoon surrounding the jet (Savolainen, 2018) cannot confine the jet material as it moves further from the core. The measured collimation profile corresponds to a slowly decreasing density, and more steeply decreasing pressure gradient in the external medium. Since the closest jet width measurement is only at 1200 gravitational radii from the core (here the jet width is 750 gravitational radii), it cannot confirm the wide jet base measured by Giovannini et al. (2018) at 350 gravitational radii. Based on this result, we arrive at the same conclusion as Giovannini et al. (2018), that the jet is either launched from the accretion disk, or it is ergosphere-launched, but undergoes a quick lateral expansion below 1000 gravitational radii.
  • Normo, Sanna (2023)
    Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the solar corona. Fast CMEs can drive shock waves which are capable of accelerating charged particles to high energies. These accelerated particles emit electromagnetic radiation, including radio emission. Studying radio emission associated with CME-driven shocks offers a way to remotely investigate shock-accelerated electrons as well the shock itself. Solar radio bursts are transient events where the radio emission of the Sun rises above the background level. A classical division based on their appearance in a dynamic spectrum divides solar radio bursts into five categories: types I-V. Of these five different types, type II and type IV radio bursts are most commonly associated with CMEs. Occasionally, type II radio bursts exhibit a bursty fine structure known as herringbones. These are regarded as signatures of individual electron beams accelerated by CME-driven shocks. This thesis studies the radio emission associated with a CME that erupted on 1 September 2014. White-light imaging of the CME revealed a prominent shock wave. Simultaneously, the dynamic spectrum exhibited spike-like radio emission resembling herringbones. The aim of the study presented in this thesis is to find the source location of this radio emission relative to a three dimensional reconstruction of the shock. The source location of the radio emission can be used to conclude the likely origin of the electrons responsible for it. Additionally, in situ electron flux measurements are investigated in an attempt to connect the remote and in situ detections of energetic electrons. Using interferometric radio observations of the Sun and reconstructing the CME shock in three dimension revealed the location of the radio emission to be at the flank of the CME-driven shock. Such location suggests that the spike-like radio emission observed in the dynamic spectrum originates from shock-accelerated electrons. The location of the radio emission at the flanks of the CME shock was also used to get an estimation of the lateral expansion of the CME. Although the in situ electron flux measurements detected high-energy electrons, their inferred release time at the Sun did not coincide with observed radio emission.
  • Suortti, Joonas (2020)
    Core galaxies are bright elliptical galaxies that contain a shallow central surface brightness profile. They are expected to form in mergers of massive gas-poor elliptical galaxies that contain supermas- sive black holes (SMBHs) in their respective centres. During the merger process, these black holes form a coalescing binary, which causes the ejection of stars from the centre of the galaxy merger in complex three-body interactions, resulting in the creation of a low-luminosity core. I have studied whether core galaxies can form according to the formation model described above. I analysed the results of seven galaxy merger simulations done using KETJU, a simulation code specifically made for studying the dynamics of supermassive black holes in galaxies. KETJU is a regularised tree-code, combining both the GADGET-3 tree-code and an AR-CHAIN integrator. This allows for the simultaneous simulation of both general galactic dynamics and accurate particle motion near black holes, respectively. All seven simulations consisted of a merger of two identical galaxies. Six of the simulations had galaxies with equal mass central SMBHs, where the mass of the black holes changed from one simulation to another, and ranged from 8.5 × 10 8 M to 8.5 × 10 9 M . For the sake of comparison, the galaxies in the seventh simulation did not contain SMBHs. The other properties of the merged galaxies were determined in such a way, that the resulting merger remnants would be as similar as possible to the well studied core galaxy NGC 1600. Naturally, these properties were identical across all of the simulation runs. By calculating the surface brightness profiles of the merger remnants in the simulation results, I found out that only simulations that contained SMBHs produced remnants with cores. Furthermore, I identified a clear positive correlation between the size of the core and the mass of the coalescing binary SMBH. Both of these results corroborate the theory, that the cores are formed by interacting SMBH binaries. This interpretation of the results was further enforced by the fact that, according to their velocity anisotropy profiles, stellar orbits near the centre of the remnants were tangentially dominated, implying that stellar particles on more radial orbits had been ejected from the system. I also generated 2D maps of the stellar line-of-sight velocity distributions in the simulated merger remnants. These maps showed kinematic properties similar to observed core galaxies, such as "kinematically distinct cores". Finally, I compared both photometric and kinematic properties of the simulated merger remnant containing the largest SMBH binary to the observed properties of NGC 1600. I found that the simulation and the observations agree well with each other. Since the properties of the simulated merger remnants follow theoretical expectations and is in general good agreement with the obser- vations, I conclude that the formation of the cores in bright elliptical galaxies is likely caused by coalescing binary black holes in dry mergers of elliptical galaxies.
  • Ruohotie, Julia (2022)
    Small-scale flux ropes (SFRs) are structures with helical magnetic field and they are frequently observed in the solar wind. In addition to the solar wind, SFRs can also be found within larger structures, like interplanetary coronal mass ejections (ICMEs) and their sheath regions that form between the shock and the leading edge of the ICME if the ICME propagates fast enough. ICME-driven sheaths are composed of shocked and compressed solar wind plasma. SFRs can be swept from the upstream solar wind into ICME sheaths when the upstream wind is deflected and compressed into ICME sheaths. Alternatively, SFRs can be formed within ICME sheaths through number of processes. This thesis includes the first comprehensive study of the occurrence of SFRs specifically in ICME-driven sheath regions. SFRs are identified from spacecraft data in both ICME sheaths and the upstream solar wind using the wavelet analysis method. This method calculates normalized reduced magnetic helicity, normalized cross-helicity, and normalized residual energy and uses them to identify SFRs and Alfvén waves. The method is applied to 55 ICME-driven sheath regions observed by Wind spacecraft. The occurrence of SFRs is studied in three different frequency ranges between 10−2 − 10−4 Hz. SFRs are found to be common structures in ICME-driven sheaths and they are more common in ICME sheaths than in the upstream solar wind. This suggest that SFRs are at least to some extent generated within ICME sheaths. The occurrence of SFRs behaves differently in different frequency ranges. The occurrence of SFRs is relatively constant at high frequencies (smallest scale) while the occurrence of low-frequency (largest scale) SFRs increases towards the leading edge of the ICME. This suggests that high- and low-frequency SFRs are generated by different processes. The occurrence of Alfvén waves was found to be somewhat similar in the upstream solar wind and ICME sheaths. However, there was an increase in the occurrence of Alfvén waves near the shock. This indicates that SFRs and Alfvén waves are generated by different processes and shock related processes might be important in the generation of Alfvén waves while the processes near the leading edge of the ICME are important in the generation of larger scale SFRs.
  • Haris-Kiss, Andras Kristof (2022)
    Over the last thirty years more than five thousand exoplanets have been discovered around a wide variety of stellar objects. Most exoplanets have been discovered using the transit method, which relies on observing the periodic brightness changes of stars as their planet transits in front of them. The discovery efficiency of these planets has been strongly enhanced with the advent of space telescopes dedicated to the discovery of planets using the transit method. Planetary signals in the photometric data of active stars can be challenging to find, as the surface features of the stars combined with their rotation might produce signals which are orders of magnitude stronger than those caused by the planetary transit. The question of what statistical methods should be applied to account for the innate variability of stars in order to identify the transits of exoplanets in the lightcurves of active stars is being investigated in this thesis. I test a number of statistical methods in order to combat stellar activity and to identify planetary transit signals. The rotation period of the star is investigated using the Lomb-Scargle and likelihood ratio periodograms. Starspot induced variability is approximated with a number of sinusoids, with periods based on the star's rotation period. Additional stellar activity is filtered out using autoregressive and moving average models. Model fittings are performed with least squares fitting, and using samples generated by the Adaptive Metropolis algorithm. After the lightcurve has been detrended for stellar activity, the likelihoods of planetary transit signals are assessed with a box-fitting algorithm. Models are compared with the Bayesian and Akaike information criteria. Planetary characteristics are then estimated by modeling the shape of the transit lightcurve. These methods are tested and performed on the lightcurve of HD~110082, a highly active young star with one confirmed planetary companion, based on the observations of the TESS space telescope. I find that stellar activity is sufficiently filtered out with a model containing four sinusoid signals. The signal corresponding to the planet is confirmed by the box fitting algorithm, agreeing with results available in scientific literature.
  • Tarvus, Vertti (2020)
    The magnetic field of Earth interacts with the supersonic solar wind that emanates from the outer part of the Sun’s atmosphere. The interaction results in the formation of Earth’s magnetosphere with a bow shock and a foreshock upstream of it. Together, they form a complex system that hosts a large number of different phenomena, ranging from aurorae visible with the naked eye from Earth’s surface to magnetic waves and transient structures only observable by spacecraft with in-situ measurements. In addition to spacecraft measurements, numerical simulations performed with computers have become increasingly important in space research with the constantly growing amount of available computing power. The topic of this thesis, two types of transient structures found upstream of the bow shock in the foreshock, cavitons and spontaneous hot flow anomalies (SHFAs), are examples of phenomena that have been discovered and studied with the combination of numerical simulations and spacecraft observations. These transient types are related, as cavitons can evolve into SHFAs. In this thesis, cavitons and SHFAs are studied with the global hybrid-Vlasov simulation Vlasiator. The transients are studied statistically in a global simulation for the first time, granting the largest statistical sample up to date. The approach taken in this study is to track individual transients in time, for which purpose a tracking algorithm was developed as a part of this thesis. With this method, the first detailed investigation of the evolution of cavitons and SHFAs is conducted. The statistical results obtained in this work indicate that cavitons and SHFAs form in a uniform region near the bow shock. There is a distinct distance to the shock within which cavitons can become SHFAs, and it is found that SHFAs can either form independently, or evolve from cavitons. The properties of the transients are found to have some dependence on the transients’ location relative to the bow shock. The propagation velocity of the transients is measured, and is found to agree with prior spacecraft observations.
  • Keitaanranta, Atte (2022)
    Results from cosmological zoom-in simulations focusing on the formation and evolution of massive early-type galaxies starting from redshift z=50 until the present time are presented in this thesis. In addition, the dynamics of supermassive black hole (SMBH) binaries found in a subset of the galaxies as a result of galaxy mergers are studied. The first 12 zoom-in simulations were run with GADGET-3, an N-body hydrodynamical tree code on the Puhti supercomputer at the Finnish IT Centre for Science (CSC). The zoom-in regions were chosen from a low resolution large-volume simulation, run with the University of Helsinki computing cluster Kale. The SMBH binary dynamics were simulated with the regularized integrator code KETJU. In total three simulations using KETJU were run on the CSC supercomputer Mahti. The GADGET-3 simulations included both dark matter only runs and runs containing also baryons. The simulations were run at two different resolutions. No significant differences between the two resolutions were found for the dark matter only runs, whereas the runs including a baryonic component showed large differences. The medium resolution runs had too low particle numbers and too large particle masses to correctly resolve star formation and feedback, leading to rotation curves that missed their central peaks. The results from the high resolution simulations agreed with earlier published results. The rotation curves peak in the central regions of the galaxies with the curves becoming almost constant at large radii. The star formation rates peak in the redshift range z~2-3 and at smaller redshifts star formation can momentarily increase due to the occuring of galaxy mergers. At the present day, all of the studied galaxies include mainly old stellar populations, resulting in red colours for all the galaxies. Still, the galaxy formation efficiency parameter of each galaxy is somewhat higher than what is seen in the observations. Finally, all three SMBH binaries for which the dynamics were studied using KETJU led to a coalescence of the two black holes. The orbital decay of SMBH binaries occur in three phases: dynamical friction, three-body interactions and gravitational wave emission. The merger times strongly depend on the eccentricity of the binaries with the semimajor axis of the binary with the highest eccentricity decreasing the fastest. This is expected from theory, with the evolution from subparsec scales to coalescence agreeing very well with the theoretical evolution taking into account the post-Newtonian correction term 2.5PN, which is the lowest order post-Newtonian term responsible for gravitational wave emission.
  • Kosowski, Jacob (2023)
    We investigated the connection between the 3D physical properties of stellar clusters and their measured counterparts from their 2D observed images; primarily focusing on the relationship be- tween the 3D half-mass radius (Rh3D) and the effective radius (Rheff) (also known as the 2D half-light radius) of stellar clusters. We generated an ensemble of 3D models of stellar clusters using the McLuster code. This ensemble is made up of subgroups consisting of different stellar counts, half-mass radius, concentration, maximum mass of the initial mass function, and degree of mass segregation. Each subgroup covered a broad range of their respective property in order to provide a comprehensive overview of the Rh3D to Rheff relationship as a function of these variables. Then, utilizing myosotis, we created synthetic observations of these models and investigated how the Rh3D of the cluster could be inferred from the measured Rheff of the synthetic photometric map. Our analysis reveals that for systems where all stars are of equal mass, independent of their size, the half-mass radius is equal to Rh3D ≈ 1/α Rheff where α ∼ 0.76. We show that the value of α can be inferred by a geometric relationship. We also find that this relationship holds for systems with varying values of concentration. For unsegregated systems of unequal stellar masses, we observe that the value of α oscillates around 0.76, with the amplitude of the oscillations increasing as the maximum mass of the system increases. As Rh3D by construction does not change, the only parameter to cause this variation in α is the Rheff . When we looked at mass segregated systems, we found that the value of Rheff (and similarly α) decreases generally monotonically as a function of the degree of the segregation. The presence of stars of unequal mass is the dominant factor that determines the measurements of Rheff , beyond the geometric effects of projection. The prevalence of this factor is attributed to the non-linear relationship between mass and luminosity that results in a few tens of massive stars greatly influencing the overall luminosity of the cluster, and therefore, its effective radius.
  • Rawlings, Alexander (2021)
    This thesis presents the results from seventeen collisionless merger simulations of massive early-type galaxies in an effort to understand the coalescence of supermassive black holes (SMBHs) in the context of the Final Parsec Problem. A review of the properties of massive early-type galaxies and their SMBHs is presented alongside a discussion on SMBH binary coalescence to motivate the initial conditions used in the simulations. The effects of varying SMBH mass and stellar density profiles in the progenitor initial conditions on SMBH coalescence was investigated. Differing mass resolutions between the stellar particles and the SMBHs for each physical realisation were also tested. The simulations were performed on the supercomputers Puhti and Mahti at CSC, the Finnish IT Centre for Science. SMBH coalescence was found to only occur in mergers involving SMBH binaries of equal mass, with the most rapid coalescence observed in galaxies with a steep density profile. In particular, the eccentricity of the SMBH binary was observed to be crucial for coalescence: all simulations that coalesced displayed an orbital eccentricity in excess of e=0.7 for the majority of the time for which the binary was bound. Simulations of higher mass resolution were found to have an increased number of stellar particles able to positively interact with the SMBH binary to remove orbital energy and angular momentum, driving the binary to coalescence. The gravitational wave emission from an equal mass SMBH binary in the final stages before merging was calculated to be within the detection limits required for measurement by pulsar timing arrays. Mergers between galaxies of unequal mass SMBHs were unable to undergo coalescence irrespective of mass resolution or progenitor density profile, despite the binary in some of these simulations displaying a high orbital eccentricity. It was determined that the stellar particles interacting with the SMBH binary were unable to remove the required orbital energy and angular momentum to bring the SMBHs to within the separation required for efficient gravitational wave emission. A trend between increasing mass resolution and increasing number of stellar particles able to remove energy from the SMBH binary was observed across all the simulation suites. This observation is of paramount importance, as three-body interactions are essential in removing orbital energy and angular momentum from the SMBH binary, thus overcoming the Final Parsec Problem. As such, it is concluded that the Final Parsec Problem is a numerical artefact arising from insufficient mass resolution between the stellar particles and the SMBHs rather than a physical phenomenon.
  • Koikkalainen, Venla (2023)
    The aim of this study is to inspect fluctuations in the solar wind magnetic field for four different types of solar wind time series. The events considered are fast and slow solar wind, along with magnetic clouds and sheath regions, which are found in coronal mass ejections (CMEs). Time series measurements of these processes are analysed using methods from Information Theory and Complex Network Analysis. The techniques that are used here are the Fisher-Shannon information plane, the Jensen-Shannon complexity-entropy plane, and Horizontal Visibility Graph Analysis. Statistical and information theory measures as well as network analysis have recently been applied to studying time series in an attempt to determine their internal structure. There is promising research into these methods quantifying data as either chaotic, stochastic, or periodic. Knowing whether a process has e.g. a deterministic origin could shed light on the creation of said process. Applying these methods to solar wind, more information could be found about its formation at the Sun. In general, the solar wind data analysed in this thesis was found to be stochastic, which agrees with previous studies. In addition, when analysing magnetic field magnitude B, magnetic clouds appear to have more internal structure in the time series signal than the other types of solar wind data tested. The results obtained here are promising in terms of finding differences in structure within solar wind, and could be investigated further with the use of more solar wind data.