TY - JOUR A1 - Warren, Donald C. A1 - Ellison, Donald C. A1 - Barkov, Maxim V. A1 - Nagataki, Shigehiro T1 - Nonlinear Particle Acceleration and Thermal Particles in GRB Afterglows JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The standard model for GRB afterglow emission treats the accelerated electron population as a simple power law, N(E) proportional to E-p for p greater than or similar to 2. However, in standard Fermi shock acceleration, a substantial fraction of the swept-up particles do not enter the acceleration process at all. Additionally, if acceleration is efficient, then the nonlinear back-reaction of accelerated particles on the shock structure modifies the shape of the nonthermal tail of the particle spectra. Both of these modifications to the standard synchrotron afterglow impact the luminosity, spectra, and temporal variation of the afterglow. To examine the effects of including thermal particles and nonlinear particle acceleration on afterglow emission, we follow a hydrodynamical model for an afterglow jet and simulate acceleration at numerous points during the evolution. When thermal particles are included, we find that the electron population is at no time well fitted by a single power law, though the highest-energy electrons are; if the acceleration is efficient, then the power-law region is even smaller. Our model predicts hard-soft-hard spectral evolution at X-ray energies, as well as an uncoupled X-ray and optical light curve. Additionally, we show that including emission from thermal particles has drastic effects (increases by factors of 100 and 30, respectively) on the observed flux at optical and GeV energies. This enhancement of GeV emission makes afterglow detections by future gamma-ray observatories, such as CTA, very likely. KW - acceleration of particles KW - cosmic rays KW - gamma-ray burst: general KW - shock waves KW - turbulence Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa56c3 SN - 0004-637X SN - 1538-4357 VL - 835 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Polarization of submillimetre lines from interstellar medium JF - Monthly notices of the Royal Astronomical Society N2 - Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. KW - polarization KW - turbulence KW - H II regions KW - ISM: magnetic fields KW - photodissociation region (PDR) KW - submillimetre: ISM Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3164 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 2 SP - 2415 EP - 2420 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Lebiga, O. A1 - Santos-Lima, Reinaldo A1 - Yan, Huirong T1 - Kinetic-MHD simulations of gyroresonance instability driven by CR pressure anisotropy JF - Monthly notices of the Royal Astronomical Society N2 - The transport of cosmic rays (CRs) is crucial for the understanding of almost all high-energy phenomena. Both pre-existing large-scale magnetohydrodynamic (MHD) turbulence and locally generated turbulence through plasma instabilities are important for the CR propagation in astrophysical media. The potential role of the resonant instability triggered by CR pressure anisotropy to regulate the parallel spatial diffusion of low-energy CRs (less than or similar to 100 GeV) in the interstellar and intracluster medium of galaxies has been shown in previous theoretical works. This work aims to study the gyroresonance instability via direct numerical simulations, in order to access quantitatively the wave-particle scattering rates. For this, we employ a 1D PIC-MHD code to follow the growth and saturation of the gyroresonance instability. We extract from the simulations the pitch-angle diffusion coefficient D-mu mu produced by the instability during the linear and saturation phases, and a very good agreement (within a factor of 3) is found with the values predicted by the quasi-linear theory (QLT). Our results support the applicability of the QLT for modelling the scattering of low-energy CRs by the gyroresonance instability in the complex interplay between this instability and the large-scale MHD turbulence. KW - MHD KW - plasmas KW - turbulence KW - cosmic rays Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty309 SN - 0035-8711 SN - 1365-2966 VL - 476 IS - 2 SP - 2779 EP - 2791 PB - Oxford Univ. Press CY - Oxford ER - TY - THES A1 - Zhang, Heshou T1 - Magnetic fields in the universe BT - Diagnostics, turbulent properties, and their implications N2 - The galactic interstellar medium is magnetized and turbulent. The magnetic field and turbulence play important roles in many astrophysical mechanisms, including cosmic ray transport, star formation, etc. Therefore, measurements of magnetic field and turbulence information are crucial for the proper interpretation of astronomical observations. Nonetheless, the magnetic field observation is quite challenging, especially, there is not universal magnetic tracer for diffuse medium. Moreover, the modelling of turbulence can be oversimplified due to the lack of observational tools to diagnose the plasma properties of the turbulence in the galactic interstellar medium. The studies presented in this thesis have addressed these challenges by bridging the theoretical studies of magnetic field and turbulence with numerical simulations and observations. The following research are presented in this thesis. The first observational evidence of the novel magnetic tracer, ground state alignment (GSA), is discovered, revealing the three-dimensional magnetic field as well as 2 orders of magnitude higher precision comparing to previous observational study in the stellar atmosphere of the post-AGB 89 Herculis. Moreover, the application of GSA in the sub-millimeter fine-structure lines is comprehensively studied for different elements and with magnetohydrodynamic simulations. Furthermore, the influence of GSA effect on the spectroscopy is analyzed and it is found that measurable variation will be produced on the spectral line intensity and the line ratio without accounting for the optical pumping process or magnetic field. Additionally, a novel method to measure plasma modes in the interstellar medium, Signatures from Polarization Analysis (SPA), is proposed and applied to real observations. Magneto-sonic modes are discovered in different types of interstellar medium. An explanation is provided for the long-standing mystery, the origin of γ-ray enhanced emission “Cygnus Cocoon”, based on the comparison between the outcome of SPA and multi-waveband observational data. These novel methods have strong potentials for broader observational applications and will play crucial roles in future multi-wavelength astronomy. N2 - Das interstellare Medium ist magnetisiert und turbulent. Das Magnetfeld und die Turbulenz spielen eine wichtige Rolle in vielen astrophysikalischen Prozessen, unter anderem beim Transport kosmischer Strahlung, bei der Entstehung von Sternen usw. Daher sind Messungen des Magnetfelds und der Turbulenz entscheidend bei der korrekten Interpretation astronomischer Beobachtungen. Dennoch ist die Messung des astronomischen Magnetfeldes eine große Herausforderung, besonders da es keinen universellen magnetischen Tracer für diffuse Medien gibt. Außerdem kann es aufgrund des Mangels an geeigneten Beobachtungswerkzeugen für die Plasmaeigenschaften der galaktischen Turbulenz zu übermäßigen Vereinfachungen bei der Turbulenzmodellierung kommen. Die in dieser Arbeit vorgestellten Forschungsergebnisse beschäftigen sich mit diesen Herausforderungen; die Theorien des Magnetfeldes und der Turbulenz werden durch numerische Simulationen und Beobachtungen anwendbar gemacht. Die erste Anwendung eines neuen magnetischen Markers, Ground State Alignment (GSA, deutsch „Grundzustandsausrichtung“), wird vorgestellt. Dabei wird das dreidimensionale Magnetfeld in der Analyse von Beobachtungsdaten aus der stellaren Atmosphäre des post-AGB 89 Herculis um zwei Größenordnungen genauer bestimmt als bei den bisherigen Methoden. Zusätzlich wird die Anwendung von GSA bei der Analyse von sub-millimeter Feinstrukturlinien ausführlich für verschiedene Elemente mit Hilfe von magnetohydrodynamischen Simulationen getestet. Auch der Einfluss von GSA-Effekten auf spektroskopische Analysen wird untersucht; dabei stellt sich heraus, dass messbare Variationen in der Linienbreite und im Verhältnis der Linien verursacht werden, ohne dass optische Pumpprozesse oder das Magnetfeld berücksichtigt werden. Des Weiteren wird die Signatures from Polarization Analysis (SPA, deutsch „Signatur-aus-Polarisationsanalyse“) als neue Messmethode für Plasmamoden im galaktischen Medium vorgestellt und auf reale Beobachtungen angewandt. In verschiedenen galaktischen Medien werden magneto-akustische Moden gefunden. Durch den Vergleich von Ergebnissen der SPA mit Beobachtungsdaten aus verschiedenen elektromagnetischen Frequenzbereichen kann die bisher ungeklärte Herkunft der gammastrahlenverstärkten Emission „Cygnus Cocoon“ erklärt werden. Diese neuen Methoden haben das Potential für eine breitere Anwendung bei der Analyse von Beobachtungen und werden in der Zukunft eine wichtige Rolle in der Multiwellenlängen-Astronomie spielen. KW - magnetic field KW - turbulence KW - polarization KW - interstellar medium KW - cosmic ray propagation Y1 - 2021 ER - TY - JOUR A1 - Vafin, Sergei A1 - Lazar, M. A1 - Fichtner, H. A1 - Schlickeiser, R. A1 - Drillisch, M. T1 - Solar wind temperature anisotropy constraints from streaming instabilities JF - Astronomy and astrophysics : an international weekly journal N2 - Due to the relatively low rate of particle-particle collisions in the solar wind, kinetic instabilities (e.g., the mirror and firehose) play an important role in regulating large deviations from temperature isotropy. These instabilities operate in the high beta(parallel to) > 1 plasmas, and cannot explain the other limits of the temperature anisotropy reported by observations in the low beta beta(parallel to) < 1 regimes. However, the instability conditions are drastically modified in the presence of streaming (or counterstreaming) components, which are ubiquitous in space plasmas. These effects have been analyzed for the solar wind conditions in a large interval of heliospheric distances, 0.3-2.5 AU. It was found that proton counter-streams are much more crucial for plasma stability than electron ones. Moreover, new instability thresholds can potentially explain all observed bounds on the temperature anisotropy, and also the level of differential streaming in the solar wind. KW - solar wind KW - instabilities KW - waves KW - turbulence Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731852 SN - 1432-0746 VL - 613 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Brose, Robert A1 - Sushch, Iuri A1 - Pohl, Martin A1 - Luken, K. J. A1 - Filipovic, M. D. A1 - Lin, R. T1 - Nonthermal emission from the reverse shock of the youngest galactic supernova remnant G1.9+0.3 JF - Astronomy and astrophysics : an international weekly journal N2 - Context. The youngest Galactic supernova remnant G1.9+0.3 is an interesting target for next-generation gamma-ray observatories. So far, the remnant is only detected in the radio and the X-ray bands, but its young age of approximate to 100 yr and inferred shock speed of approximate to 14 000 km s(-1) could make it an efficient particle accelerator. Aims. We aim to model the observed radio and X-ray spectra together with the morphology of the remnant. At the same time, we aim to estimate the gamma-ray flux from the source and evaluate the prospects of its detection with future gamma-ray experiments. Methods. We performed spherical symmetric 1D simulations with the RATPaC code, in which we simultaneously solved the transport equation for cosmic rays, the transport equation for magnetic turbulence, and the hydro-dynamical equations for the gas flow. Separately computed distributions of the particles accelerated at the forward and the reverse shock were then used to calculate the spectra of synchrotron, inverse Compton, and pion-decay radiation from the source. Results. The emission from G1.9+0.3 can be self-consistently explained within the test-particle limit. We find that the X-ray flux is dominated by emission from the forward shock while most of the radio emission originates near the reverse shock, which makes G1.9+0.3 the first remnant with nonthermal radiation detected from the reverse shock. The flux of very-high-energy gamma-ray emission from G1.9+0.3 is expected to be close to the sensitivity threshold of the Cherenkov Telescope Array. The limited time available to grow large-scale turbulence limits the maximum energy of particles to values below 100 TeV, hence G1.9+0.3 is not a PeVatron. KW - acceleration of particles KW - turbulence KW - ISM: supernova remnants KW - gamma rays: ISM Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834430 SN - 1432-0746 VL - 627 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Rüdiger, Günther A1 - Küker, Manfred A1 - Kapyla, P. J. A1 - Strassmeier, Klaus G. T1 - Antisolar differential rotation of slowly rotating cool stars JF - Astronomy and astrophysics : an international weekly journal N2 - Rotating stellar convection transports angular momentum towards the equator, generating the characteristic equatorial acceleration of the solar rotation while the radial flux of angular momentum is always inwards. New numerical box simulations for the meridional cross-correlation < u(theta)u(phi)>, however, reveal the angular momentum transport towards the poles for slow rotation and towards the equator for fast rotation. The explanation is that for slow rotation a negative radial gradient of the angular velocity always appears, which in combination with a so-far neglected rotation-induced off-diagonal eddy viscosity term nu(perpendicular to) provides "antisolar rotation" laws with a decelerated equator Similarly, the simulations provided positive values for the rotation-induced correlation < u(r)u(theta)>, which is relevant for the resulting latitudinal temperature profiles (cool or warm poles) for slow rotation and negative values for fast rotation. Observations of the differential rotation of slowly rotating stars will therefore lead to a better understanding of the actual stress-strain relation, the heat transport, and the underlying model of the rotating convection. KW - stars: solar-type KW - convection KW - stars: rotation KW - turbulence Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935280 SN - 1432-0746 VL - 630 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Pratt, Jane A1 - Busse, Angela A1 - Mueller, W-C A1 - Watkins, Nikolas W. A1 - Chapman, Sandra C. T1 - Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection JF - New journal of physics : the open-access journal for physics N2 - We investigate the utility of the convex hull of many Lagrangian tracers to analyze transport properties of turbulent flows with different anisotropy. In direct numerical simulations of statistically homogeneous and stationary Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD Boussinesq convection a comparison with Lagrangian pair dispersion shows that convex hull statistics capture the asymptotic dispersive behavior of a large group of passive tracer particles. Moreover, convex hull analysis provides additional information on the sub-ensemble of tracers that on average disperse most efficiently in the form of extreme value statistics and flow anisotropy via the geometric properties of the convex hulls. We use the convex hull surface geometry to examine the anisotropy that occurs in turbulent convection. Applying extreme value theory, we show that the maximal square extensions of convex hull vertices are well described by a classic extreme value distribution, the Gumbel distribution. During turbulent convection, intermittent convective plumes grow and accelerate the dispersion of Lagrangian tracers. Convex hull analysis yields information that supplements standard Lagrangian analysis of coherent turbulent structures and their influence on the global statistics of the flow. KW - turbulence KW - magnetohydrodynamics KW - Lagrangian statistics KW - magnetoconvection KW - turbulent transport Y1 - 2017 U6 - https://doi.org/10.1088/1367-2630/aa6fe8 SN - 1367-2630 VL - 19 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kobzar, Oleh A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Bohdan, Artem T1 - Spatio-temporal evolution of the non-resonant instability in shock precursors of young supernova remnants JF - Monthly notices of the Royal Astronomical Society N2 - A non-resonant cosmic ray (CR) current-driven instability may operate in the shock precursors of young supernova remnants and be responsible for magnetic-field amplification, plasma heating and turbulence. Earlier simulations demonstrated magnetic-field amplification, and in kinetic studies a reduction of the relative drift between CRs and thermal plasma was observed as backreaction. However, all published simulations used periodic boundary conditions, which do not account for mass conservation in decelerating flows and only allow the temporal development to be studied. Here we report results of fully kinetic particle-in-cell simulations with open boundaries that permit inflow of plasma on one side of the simulation box and outflow at the other end, hence allowing an investigation of both the temporal and the spatial development of the instability. Magnetic-field amplification proceeds as in studies with periodic boundaries and, observed here for the first time, the reduction of relative drifts causes the formation of a shock-like compression structure at which a fraction of the plasma ions are reflected. Turbulent electric field generated by the non-resonant instability inelastically scatters CRs, modifying and anisotropizing their energy distribution. Spatial CR scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads to significant non-adiabatic heating of the background plasma maintaining bulk equipartition between ions and electrons. The highest temperatures are reached at sites of large-amplitude electrostatic fields. Ion spectra show supra-thermal tails resulting from stochastic scattering in the turbulent electric field. Together, these modifications in the plasma flow will affect the properties of the shock and particle acceleration there. KW - acceleration of particles KW - shock waves KW - turbulence KW - methods: numerical KW - cosmic rays KW - ISM: supernova remnants Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx1201 SN - 0035-8711 SN - 1365-2966 VL - 469 SP - 4985 EP - 4998 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Vafin, Sergei A1 - Riazantseva, M. A1 - Yoon, P. H. T1 - Kinetic Features in the Ion Flux Spectrum JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - An interesting feature of solar wind fluctuations is the occasional presence of a well-pronounced peak near the spectral knee. These peaks are well investigated in the context of magnetic field fluctuations in the magnetosheath and they are typically related to kinetic plasma instabilities. Recently, similar peaks were observed in the spectrum of ion flux fluctuations of the solar wind and magnetosheath. In this paper, we propose a simple analytical model to describe such peaks in the ion flux spectrum based on the linear theory of plasma fluctuations. We compare our predictions with a sample observation in the solar wind. For the given observation, the peak requires similar to 10 minutes to grow up to the observed level that agrees with the quasi-linear relaxation time. Moreover, our model well reproduces the form of the measured peak in the ion flux spectrum. The observed lifetime of the peak is about 50 minutes, which is relatively close to the nonlinear Landau damping time of 30-40 minutes. Overall, our model proposes a plausible scenario explaining the observation. KW - instabilities KW - solar wind KW - turbulence KW - waves Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa9519 SN - 0004-637X SN - 1538-4357 VL - 850 PB - IOP Publ. Ltd. CY - Bristol ER -