Tähtienvälisen aineen magneettiset ilmiöt teorioiden ja havaintojen kannalta

Magnetic fields and turbulent flows pervade the interstellar medium on all scales. The magnetic turbulence that emerges on the large scales cascades towards the small scales where it influences molecular cloud structure, and star formation within the densest and coldest clumps of the clouds. Active star formation results in supernovae, and the supernova-driven turbulence takes part in the galactic dynamo process leading to the inverse cascade of turbulent energy. Such a process is one example of self-organisatory processes in the interstellar medium where order arises from chaos. Supernovae also induce and influence other important processes in the galactic disks, and this thesis examines some of them. Differentially rotating disk systems, such as galaxies, are prone to magnetorotational instability, where weak magnetic fields destabilise the otherwise hydrodynamically stable disk system, and lead to angular momentum transport outwards. However, magnetorotational instability can be quenched or even damped by another source of turbulence such as supernovae. As both magnetorotational instability and supernovae are capable of producing dynamo effects, the galactic large-scale magnetic fields are proposed to arise as an interplay of these two effects. In addition, supernovae are observed to be able to generate and sustain large-scale flows in galaxies through the anisotropic kinetic alpha effect. Thermodynamical effects have a significant influence on the properties of turbulence. Due to baroclinicity, the supernova-driven turbulence is highly vortical in nature. These types of flows produce a narrow, non-Gaussian velocity distribution with extended wings and an exponential magneti field distribution. Such effects should be taken into account when interstellar turbulence is parametrized in the form of initial conditions and forcing functions for the purpose of making smaller scale models of molecular cloud formation. The Gaussianity of the magnetic field fluctuations is a common assumption, for example, when fitting magnetic field models to explain large-scale polarization maps of the interstellar dust, and our results suggest that such assumptions should require more examination. To study these phenomena, a combination of numerical approaches and observational methods are needed. Exploring physics of turbulence requires the tools of high-performance computing and precise, high-order numerical schemes. Because of the rapidly increasing demands of computation, novel approaches have to be investigated. To improve computational efficiency this thesis shows how the sixth-order finite difference method can be accelerated with the help of graphics processing units. The properties of the interstellar medium can be examined best by the emission of atomic/molecular gas and by the emission, absorption and scattering of interstellar dust. Looking at small-scale phenomena, molecular line emission from cold prestellar cores is explored. More large-scale effects are examined with the help of polarized dust emission, by combining radiative transfer calculations with the results of a supernova-driven model of the interstellar medium, including a realistic multiphase structure and dynamo-generated small- and large-scale magnetic fields. This thesis contains seven papers. Of these, three papers examine the processes driving turbulence in differentially rotating disks via numerical modelling, while one paper looks into how graphics processing units can accelerate such calculations. Observationally, two papers study cold cores and early stages of star formation with the help of radio telescopes, and one paper examines how supernova-driven turbulence is reflected in the large-scale emission of diffuse interstellar dust. Keywords: Magnetohydrodynamics, interstellar medium, galactic dynamo, polarization, star formation, GPGPUMagneettikentät ja turbulenttiset virtaukset ovat osallisina tähtienvälisen aineen kaikissa mittakaavoissa. Sellaisen magneettisen turbulenssin vaikutus, joka lähtee liikkeelle suurissa mittakaavoissa, ryöppyää pieniin mittakaavoihin ja siten vaikuttaa sekä molekyylipilvien muodostumiseen että tähtien syntymiseen näiden pilvien tiheissä ja kylmissä ytimissä. Tähtien synty johtaa supernoviin ja supernovien ajama turbulenssi ottaa osaa galaktisen dynamon toimintaan, mikä siirtää energiaa pienestä mittakaavasta suureen. Sellaiset prosessit ovat esimerkki ilmiöistä tähtienvälisessä aineessa, joissa kaoottiset olosuhteet synnyttävät järjestyneitä rakenteita. Supernovat myös vaikuttavat muihin prosesseihin galaksien kiekkojen sisällä, ja tämä väitöskirja tarkastelee joitakin niistä. Differentiaalisesti pyörivät kiekkojärjestelmät, kuten galaksit, voivat muuttua epävakaiksi heikon magneettikentän aiheuttamien jännitysten johdosta. Heikot magneettikentät pyörivissä kiekoissa tekevät muuten virtauksiltaan vakaan kiekon epävakaaksi, mikä johtaa liikemäärämomentin siirtymiseen kiekon ulko-osiin ja turbulenssin kasvuun. Kuitenkin tämä ilmiö saattaa vaimeta tai jopa sammua kokonaan, jos jokin toinen turbulenssin lähde on paikalla - kuten vaikka yllä mainitut supernovat. Koska sekä tämä magnetorotationaalinen epätasapaino että supernovat kykenevät aikaansaamaan dynamoilmiön, syntyvät galaktiset suuren mittakaavan magneettikentät mahdollisesti molempien ilmiöiden vuorovaikutuksen yhdistelmänä. Tämän lisäksi supernovaräjähdysten havaintaan luovan ja ylläpitävän suuren mittakaavan virtauksia epäisotrooppisen kineettisen alfaefektin kautta. Termodynaamisilla efekteillä on merkittävä vaikutus turbulenssin ominaisuuksiin. Väitöskirjassa esitetty tutkimus osoittaa, että supernovien ajama turbulenssi on luonteeltaan voimakkaan pyörteistä, kun lämpöenergian tarkastelu otetaan asianmukaisesti huomioon. Sellaiset virtaukset tuottavat kapean, leveäsiipisen nopeusjakauman ja eksponentiaalisen muotoisen magneettikentän jakauman, mikä olisi hyvä ottaa huomioon, kun tähtienvälistä turbulenssia asetetaan simulaatioiden alkuehdoiksi, pakkovoimafunktioksi molekyylipilvimalleissa, tai käytetään olettamaan magneettikentän fluktuaatioiden jakaumaa havaintoja analysoidessa. Ilmiöiden tutkimiseen vaaditaan yhdistelmä laskennallisia malleja ja erilaisia havaintomenetelmiä. Magnetohydrodynaamisten simulaatioiden lisäksi, laskennan tehostamisen vuoksi, tässä työssä esitetään, kuinka kuudennen kertaluvun erottelumenetelmää voidaan tehostaa grafiikkaprosessorien avulla virtausmekaniikassa. Havaintojen tasolla, pienissä mittakaavoissa tutkitaan kylmien molekyylipilviytimien säteilyä radioteleskoopeilla. Suurissa mittakaavoissa tarkastellaan tähtienvälisien pölyn polarisaatiota yhdistämällä säteilynkuljetussimulaatioita supernovien ajaman, turbulenttisen tähtienvälisen aineen malliin. Mainittuja aiheita käsitellään kokonaisuudessaan seitsemän julkaisun verran. Avainsanat: Magnetohydrodynamiikka, tähtienvälinen aine, galaktinen dynamo, polarisaatio, tähtien synty, GPGP

Exploring the Formation of Resistive Pseudodisks with the GPU Code Astaroth

Pseudodisks are dense structures formed perpendicular to the direction of the magnetic field during the gravitational collapse of a molecular cloud core. Numerical simulations of the formation of pseudodisks are usually computationally expensive with conventional CPU codes. To demonstrate the proof-of-concept of a fast computing method for this numerically costly problem, we explore the GPU-powered MHD code Astaroth, a 6th-order finite difference method with low adjustable finite resistivity implemented with sink particles. The formation of pseudodisks is physically and numerically robust and can be achieved with a simple and clean setup for this newly adopted numerical approach for science verification. The method's potential is illustrated by evidencing the dependence on the initial magnetic field strength of specific physical features accompanying the formation of pseudodisks, e.g. the occurrence of infall shocks and the variable behavior of the mass and magnetic flux accreted on the central object. As a performance test, we measure both weak and strong scaling of our implementation to find most efficient way to use the code on a multi-GPU system. Once suitable physics and problem-specific implementations are realized, the GPU-accelerated code is an efficient option for 3-D magnetized collapse problems.Comment: 29 pages, 1 table, 15 figures, Accepted for publication in the Astrophysical Journa

Scalable communication for high-order stencil computations using CUDA-aware MPI

Modern compute nodes in high-performance computing provide a tremendous level of parallelism and processing power. However, as arithmetic performance has been observed to increase at a faster rate relative to memory and network bandwidths, optimizing data movement has become critical for achieving strong scaling in many communication-heavy applications. This performance gap has been further accentuated with the introduction of graphics processing units, which can provide by multiple factors higher throughput in data-parallel tasks than central processing units. In this work, we explore the computational aspects of iterative stencil loops and implement a generic communication scheme using CUDA-aware MPI, which we use to accelerate magnetohydrodynamics simulations based on high-order finite differences and third-order Runge-Kutta integration. We put particular focus on improving intra-node locality of workloads. In comparison to a theoretical performance model, our implementation exhibits strong scaling from one to $64$ devices at $50\%$--$87\%$ efficiency in sixth-order stencil computations when the problem domain consists of $256^3$--$1024^3$ cells.Comment: 17 pages, 15 figure

Interaction of large- and small-scale dynamos in isotropic turbulent flows from GPU-accelerated simulations

Magnetohydrodynamical (MHD) dynamos emerge in many different astrophysical situations where turbulence is present, but the interaction between large-scale (LSD) and small-scale dynamos (SSD) is not fully understood. We performed a systematic study of turbulent dynamos driven by isotropic forcing in isothermal MHD with magnetic Prandtl number of unity, focusing on the exponential growth stage. Both helical and non-helical forcing was employed to separate the effects of LSD and SSD in a periodic domain. Reynolds numbers (Rm) up to $\approx 250$ were examined and multiple resolutions used for convergence checks. We ran our simulations with the Astaroth code, designed to accelerate 3D stencil computations on graphics processing units (GPUs) and to employ multiple GPUs with peer-to-peer communication. We observed a speedup of $\approx 35$ in single-node performance compared to the widely used multi-CPU MHD solver Pencil Code. We estimated the growth rates both from the averaged magnetic fields and their power spectra. At low Rm, LSD growth dominates, but at high Rm SSD appears to dominate in both helically and non-helically forced cases. Pure SSD growth rates follow a logarithmic scaling as a function of Rm. Probability density functions of the magnetic field from the growth stage exhibit SSD behaviour in helically forced cases even at intermediate Rm. We estimated mean-field turbulence transport coefficients using closures like the second-order correlation approximation (SOCA). They yield growth rates similar to the directly measured ones and provide evidence of $\alpha$ quenching. Our results are consistent with the SSD inhibiting the growth of the LSD at moderate Rm, while the dynamo growth is enhanced at higher Rm.Comment: 22 pages, 23 figures, 2 tables, Accepted for publication in the Astrophysical Journa

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5-6 K. The aim of this study was to determine the kinetic temperature distribution in the low-mass prestellar core Oph D where previous observations suggest a very low central temperature. The densest part of the Oph D core was mapped in the NH3(1,1) and (2,2) inversion lines using the Very Large Array (VLA). The physical quantities were derived from the observed spectra by fitting the hyperfine structure of the lines, and subsequently the temperature distribution of Oph D was calculated using the standard rotational temperature techniques. A physical model of the cores was constructed, and the simulated spectra after radiative transfer calculations with a 3D Monte Carlo code were compared with the observed spectra. Temperature, density, and ammonia abundance of the core model were tuned until a satisfactory match with the observation was obtained. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores. The gas kinetic temperatures, as derived from ammonia towards the centres of the southern and northern core are 7.4 and 8.9 K, respectively. The observed masses of the cores are only 0.2 M_Sun. Their potential collapse could lead to formation of brown dwarfs or low-mass stars.Comment: Accepted for publication in A&A; 10 pages, 9 figure

Quantifying the effect of turbulent magnetic diffusion on the growth rate of the magneto-rotational instability

VK: resolve; Karhunen, J.; ReSoLVEContext. In astrophysics, turbulent diffusion is often used in place of microphysical diffusion to avoid resolving the small scales. However, we expect this approach to break down when time and length scales of the turbulence become comparable with other relevant time and length scales in the system. Turbulent diffusion has previously been applied to the magneto-rotational instability (MRI), but no quantitative comparison of growth rates at different turbulent intensities has been performed. Aims. We investigate to what extent turbulent diffusion can be used to model the effects of small-scale turbulence on the kinematic growth rates of the MRI, and how this depends on angular velocity and magnetic field strength. Methods. We use direct numerical simulations in three-dimensional shearing boxes with periodic boundary conditions in the spanwise direction and additional random plane-wave volume forcing to drive a turbulent flow at a given length scale. We estimate the turbulent diffusivity using a mixing length formula and compare with results obtained with the test-field method. Results. It turns out that the concept of turbulent diffusion is remarkably accurate in describing the effect of turbulence on the growth rate of the MRI. No noticeable breakdown of turbulent diffusion has been found, even when time and length scales of the turbulence become comparable with those imposed by the MRI itself. On the other hand, quenching of turbulent magnetic diffusivity by the magnetic field is found to be absent. Conclusions. Turbulence reduces the growth rate of the MRI in the same way as microphysical magnetic diffusion does.Peer reviewe

Exploring the Formation of Resistive Pseudodisks with the GPU Code Astaroth

Pseudodisks are dense structures formed perpendicular to the direction of the magnetic field during the gravitational collapse of a molecular cloud core. Numerical simulations of the formation of pseudodisks are usually computationally expensive with conventional CPU codes. To demonstrate the proof of concept of a fast computing method for this numerically costly problem, we explore the GPU-powered MHD code Astaroth, a sixth-order finite difference method with low adjustable finite resistivity implemented with sink particles. The formation of pseudodisks is physically and numerically robust and can be achieved with a simple and clean setup for this newly adopted numerical approach for science verification. The method’s potential is illustrated by evidencing the dependence on the initial magnetic field strength of specific physical features accompanying the formation of pseudodisks, e.g., the occurrence of infall shocks and the variable behavior of the mass and magnetic flux accreted on the central object. As a performance test, we measure both weak and strong scaling of our implementation to find the most efficient way to use the code on a multi-GPU system. Once suitable physics and problem-specific implementations are realized, the GPU-accelerated code is an efficient option for 3D magnetized collapse problems