TY - GEN A1 - Makwana, Kirit D. A1 - Yan, Huirong T1 - Properties of magnetohydrodynamic modes in compressively driven plasma turbulence T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfven Mach number (M-A), plasma beta, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high M-A. The slow-mode properties are similar to the Alfven mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M-A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1225 KW - mhd turbulence KW - star formation KW - simulations KW - Anisotropy KW - diffusion Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-531607 SN - 1866-8372 VL - 10 IS - 3 PB - American Physical Society (APS) CY - College Park ER - TY - GEN A1 - Norell, Jesper A1 - Jay, Raphael Martin A1 - Hantschmann, Markus A1 - Eckert, Sebastian A1 - Guo, Meiyuan A1 - Gaffney, Kelly J. A1 - Wernet, Philippe A1 - Lundberg, Marcus A1 - Föhlisch, Alexander A1 - Odelius, Michael T1 - Fingerprints of electronic, spin and structural dynamics from resonant inelastic soft X-ray scattering in transient photo-chemical species T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L-3-edge RIXS in the ferricyanide complex Fe(CN)(6)(3-), we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 779 KW - charge-transfer KW - relaxation dynamics KW - absorption-spectra KW - energy-conversion KW - basis-sets KW - ab-initio KW - complexes KW - photoelectron KW - spectroscopy KW - simulations Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-437493 SN - 1866-8372 IS - 779 SP - 7243 EP - 7253 ER - TY - GEN A1 - Tillner, Elena A1 - Langer, Maria A1 - Kempka, Thomas A1 - Kühn, Michael T1 - Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Injection of fluids into deep saline aquifers causes a pore pressure increase in the storage formation, and thus displacement of resident brine. Via hydraulically conductive faults, brine may migrate upwards into shallower aquifers and lead to unwanted salinisation of potable groundwater resources. In the present study, we investigated different scenarios for a potential storage site in the Northeast German Basin using a three-dimensional (3-D) regional-scale model that includes four major fault zones. The focus was on assessing the impact of fault length and the effect of a secondary reservoir above the storage formation, as well as model boundary conditions and initial salinity distribution on the potential salinisation of shallow groundwater resources. We employed numerical simulations of brine injection as a representative fluid. Our simulation results demonstrate that the lateral model boundary settings and the effective fault damage zone volume have the greatest influence on pressure build-up and development within the reservoir, and thus intensity and duration of fluid flow through the faults. Higher vertical pressure gradients for short fault segments or a small effective fault damage zone volume result in the highest salinisation potential due to a larger vertical fault height affected by fluid displacement. Consequently, it has a strong impact on the degree of shallow aquifer salinisation, whether a gradient in salinity exists or the saltwater-freshwater interface lies below the fluid displacement depth in the faults. A small effective fault damage zone volume or low fault permeability further extend the duration of fluid flow, which can persist for several tens to hundreds of years, if the reservoir is laterally confined. Laterally open reservoir boundaries, large effective fault damage zone volumes and intermediate reservoirs significantly reduce vertical brine migration and the potential of freshwater salinisation because the origin depth of displaced brine is located only a few decametres below the shallow aquifer in maximum. The present study demonstrates that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinisation of shallower aquifers strongly depends on initial salinity distribution, location of hydraulically conductive faults and their effective damage zone volumes as well as geological boundary conditions. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 548 KW - geological CO2 storage KW - brine migration KW - fluid-flow KW - pressure management KW - dynamic flow KW - permeability KW - sequestration KW - simulations KW - injection KW - leakage Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-411854 SN - 1866-8372 IS - 548 ER - TY - GEN A1 - Ehlert, Christopher A1 - Holzweber, Markus A1 - Lippitz, Andreas A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - A detailed assignment of NEXAFS resonances of imidazolium based ionic liquids N2 - In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([CnC1im]+[NTf2]- and [C4C1im]+[I]-). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Furthermore, a detailed assignment of resonance features to excitation centers (carbon or nitrogen atoms) leads to a consistent interpretation of the spectra. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 315 KW - ray absorption-spectroscopy KW - fine-structure KW - spectra KW - simulations KW - molecules KW - dynamics KW - graphene KW - surface KW - salts Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-394417 SP - 8654 EP - 8661 ER - TY - GEN A1 - Sunyer, M. A. A1 - Hundecha, Y. A1 - Lawrence, D. A1 - Madsen, H. A1 - Willems, Patrick A1 - Martinkova, M. A1 - Vormoor, Klaus Josef A1 - Bürger, Gerd A1 - Hanel, Martin A1 - Kriaučiūnienė, J. A1 - Loukas, A. A1 - Osuch, M. A1 - Yücel, I. T1 - Inter-comparison of statistical downscaling methods for projection of extreme precipitation in Europe T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Information on extreme precipitation for future climate is needed to assess the changes in the frequency and intensity of flooding. The primary source of information in climate change impact studies is climate model projections. However, due to the coarse resolution and biases of these models, they cannot be directly used in hydrological models. Hence, statistical downscaling is necessary to address climate change impacts at the catchment scale. This study compares eight statistical downscaling methods (SDMs) often used in climate change impact studies. Four methods are based on change factors (CFs), three are bias correction (BC) methods, and one is a perfect prognosis method. The eight methods are used to downscale precipitation output from 15 regional climate models (RCMs) from the ENSEMBLES project for 11 catchments in Europe. The overall results point to an increase in extreme precipitation in most catchments in both winter and summer. For individual catchments, the downscaled time series tend to agree on the direction of the change but differ in the magnitude. Differences between the SDMs vary between the catchments and depend on the season analysed. Similarly, general conclusions cannot be drawn regarding the differences between CFs and BC methods. The performance of the BC methods during the control period also depends on the catchment, but in most cases they represent an improvement compared to RCM outputs. Analysis of the variance in the ensemble of RCMs and SDMs indicates that at least 30% and up to approximately half of the total variance is derived from the SDMs. This study illustrates the large variability in the expected changes in extreme precipitation and highlights the need for considering an ensemble of both SDMs and climate models. Recommendations are provided for the selection of the most suitable SDMs to include in the analysis. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 512 KW - climate-change impacts KW - model output KW - assessing uncertainties KW - multimodel ensemble KW - bias correction KW - simulations KW - scenarios KW - variability KW - basin KW - UK Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-408920 SN - 1866-8372 IS - 512 ER - TY - GEN A1 - Wirth, Jonas A1 - Neumann, Rainer A1 - Antonietti, Markus A1 - Saalfrank, Peter T1 - Adsorption and photocatalytic splitting of water on graphitic carbon nitride BT - a combined first principles and semiempirical study N2 - Graphitic carbon nitride, g-C₃N₄, is a promising organic photo-catalyst for a variety of redox reactions. In order to improve its efficiency in a systematic manner, however, a fundamental understanding of the microscopic interaction between catalyst, reactants and products is crucial. Here we present a systematic study of water adsorption on g-C₃N₄ by means of density functional theory and the density functional based tight-binding method as a prerequisite for understanding photocatalytic water splitting. We then analyze this prototypical redox reaction on the basis of a thermodynamic model providing an estimate of the overpotential for both water oxidation and H⁺ reduction. While the latter is found to occur readily upon irradiation with visible light, we derive a prohibitive overpotential of 1.56 eV for the water oxidation half reaction, comparing well with the experimental finding that in contrast to H₂ production O₂ evolution is only possible in the presence of oxidation cocatalysts. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - paper 172 KW - augmented-wave method KW - hydrogen KW - initio molecular-dynamics KW - oxidation KW - photooxidation KW - reduction KW - simulations KW - tight-binding KW - transition KW - visible-light Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-74391 SP - 15917 EP - 15926 ER - TY - THES A1 - Sorce, Jenny T1 - From Spitzer mid-infrared observations and measurements of peculiar velocities to constrained simulations of the local universe T1 - Von Spitzer-Beobachtungen im mittleren Infrarotbereich und Messungen der Pekuliargeschwindigkeiten zu Constrained Simulationen des Lokalen Universums N2 - Galaxies are observational probes to study the Large Scale Structure. Their gravitational motions are tracers of the total matter density and therefore of the Large Scale Structure. Besides, studies of structure formation and galaxy evolution rely on numerical cosmological simulations. Still, only one universe observable from a given position, in time and space, is available for comparisons with simulations. The related cosmic variance affects our ability to interpret the results. Simulations constrained by observational data are a perfect remedy to this problem. Achieving such simulations requires the projects Cosmic flows and CLUES. Cosmic flows builds catalogs of accurate distance measurements to map deviations from the expansion. These measures are mainly obtained with the galaxy luminosity-rotation rate correlation. We present the calibration of that relation in the mid-infrared with observational data from Spitzer Space Telescope. Resulting accurate distance estimates will be included in the third catalog of the project. In the meantime, two catalogs up to 30 and 150 Mpc/h have been released. We report improvements and applications of the CLUES' method on these two catalogs. The technique is based on the constrained realization algorithm. The cosmic displacement field is computed with the Zel'dovich approximation. This latter is then reversed to relocate reconstructed three-dimensional constraints to their precursors' positions in the initial field. The size of the second catalog (8000 galaxies within 150 Mpc/h) highlighted the importance of minimizing the observational biases. By carrying out tests on mock catalogs, built from cosmological simulations, a method to minimize observational bias can be derived. Finally, for the first time, cosmological simulations are constrained solely by peculiar velocities. The process is successful as resulting simulations resemble the Local Universe. The major attractors and voids are simulated at positions approaching observational positions by a few megaparsecs, thus reaching the limit imposed by the linear theory. N2 - Die Verteilung der Galaxien liefert wertvolle Erkenntnisse über die großräumigen Strukturen im Universum. Ihre durch Gravitation verursachte Bewegung ist ein direkter Tracer für die Dichteverteilung der gesamten Materie. Die Strukturentstehung und die Entwicklung von Galaxien wird mithilfe von numerischen Simulationen untersucht. Es gibt jedoch nur ein einziges beobachtbares Universum, welches mit der Theorie und den Ergebnissen unterschiedlicher Simulationen verglichen werden muß. Die kosmische Varianz erschwert es, das lokale Universum mit Simulationen zu reproduzieren. Simulationen, deren Anfangsbedingungen durch Beobachtungsdaten eingegrenzt sind (Constrained Simulations'') stellen eine geeignete Lösung dieses Problems dar. Die Durchführung solcher Simulationen ist das Ziel der Projekte Cosmicflows und CLUES. Im Cosmicflows-Projekt werden genaue Entfernungsmessungen von Galaxien erstellt, welche die Abweichung von der allgemeinen Hubble-Expansion abbilden. Diese Messungen werden hauptsächlich aus der Korrelation zwischen Leuchtkraft und Rotationsgeschwindigkeit von Spiralgalaxien gewonnen. In dieser Arbeit wird die Kalibrierung dieser Beziehung im mittleren Infrarot mithilfe von Daten vom Spitzer Space Telescope vorgestellt. Diese neuen Entfernungsbestimmungen werden im dritten Katalog des Cosmicflows-Projekts enthalten sein. Bisher wurden zwei Kataloge veröffentlicht, mit Entfernungen bis zu 30 beziehungsweise 150 Mpc/h. In dieser Arbeit wird die CLUES-Methode auf diese zwei Kataloge angewendet und Verbesserungen werden vorgestellt und diskutiert. Zunächst wird das kosmische Verschiebungsfeld mithilfe der Zeldovich-Näherung bestimmt. In umgekehrter Richtung kann man damit die aus heutigen Beobachtungsdaten rekonstruierten dreidimensionalen Constraints an ihren Ursprungsort im frühen Universum zurückzuversetzen. Durch den großen Datenumfang des cosmicflows-2 Katalogs (8000 Galaxien bis zu einer Entfernung von 150 Mpc/h) ist es besonders wichtig, den Einfluss verschiedener Beobachtungsfehler zu minimieren. Eine für das lokale Universum angepasste Korrekturmethode lässt sich durch die Untersuchung von Mock-Katalogen finden, welche aus kosmologischen Simulationen gewonnen werden. Schließlich stellt diese Arbeit erstmals kosmologische Simulationen vor, die ausschließlich durch Pekuliargeschwindigkeiten eingegrenzt sind. Der Erfolg dieser Methode wird dadurch bestätigt, dass die dadurch erzeugten Simulationen dem beobachteten lokalen Universum sehr ähnlich sind. Die relevanten Attraktoren und Voids liegen in den Simulationen an Positionen, welche bis auf wenige Megaparsec mit den beobachteten Positionen übereinstimmen. Die Simulationen erreichen damit die durch die lineare Theorie gegebene Genauigkeitsgrenze. KW - Beobachtungen KW - Entfernungen KW - Pekuliargeschwindigkeiten KW - Simulationen KW - observations KW - distances KW - peculiar velocities KW - simulations Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-72486 ER -