TY - JOUR A1 - Vafin, Sergei A1 - Schlickeiser, R. A1 - Yoon, P. H. T1 - AMPLIFICATION OF COLLECTIVE MAGNETIC FLUCTUATIONS IN MAGNETIZED BI-MAXWELLIAN PLASMAS FOR PARALLEL WAVE VECTORS. I. ELECTRON-PROTON PLASMA JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The general electromagnetic fluctuation theory is a powerful tool to analyze the magnetic fluctuation spectrum of a plasma. Recent works utilizing this theory for a magnetized non-relativistic isotropic Maxwellian electron-proton plasma have demonstrated that the equilibrium ratio of vertical bar delta B vertical bar/B-0 can be as high as 10(-12). This value results from the balance between spontaneous emission of fluctuations and their damping, and it is considerably smaller than the observed value vertical bar delta B vertical bar/B-0 in the solar wind at 1 au, where 10(-3) less than or similar to vertical bar delta B vertical bar/B-0 less than or similar to 10(-1). In the present manuscript, we consider an anisotropic bi-Maxwellian distribution function to investigate the effect of plasma instabilities on the magnetic field fluctuations. We demonstrate that these instabilities strongly amplify the magnetic field fluctuations and provide a sufficient mechanism to explain the observed value of vertical bar delta B vertical bar/B-0 in the solar wind at 1 au. KW - instabilities KW - magnetic fields KW - solar wind KW - turbulence KW - waves Y1 - 2016 U6 - https://doi.org/10.3847/0004-637X/829/1/41 SN - 0004-637X SN - 1538-4357 VL - 829 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Aldoretta, E. J. A1 - St-Louis, N. A1 - Richardson, N. D. A1 - Moffat, Anthony F. J. A1 - Eversberg, T. A1 - Hill, G. M. A1 - Shenar, Tomer A1 - Artigau, E. A1 - Gauza, B. A1 - Knapen, J. H. A1 - Kubat, Jiří A1 - Kubatova, Brankica A1 - Maltais-Tariant, R. A1 - Munoz, M. A1 - Pablo, H. A1 - Ramiaramanantsoa, T. A1 - Richard-Laferriere, A. A1 - Sablowski, D. P. A1 - Simon-Diaz, S. A1 - St-Jean, L. A1 - Bolduan, F. A1 - Dias, F. M. A1 - Dubreuil, P. A1 - Fuchs, D. A1 - Garrel, T. A1 - Grutzeck, G. A1 - Hunger, T. A1 - Kuesters, D. A1 - Langenbrink, M. A1 - Leadbeater, R. A1 - Li, D. A1 - Lopez, A. A1 - Mauclaire, B. A1 - Moldenhawer, T. A1 - Potter, M. A1 - dos Santos, E. M. A1 - Schanne, L. A1 - Schmidt, J. A1 - Sieske, H. A1 - Strachan, J. A1 - Stinner, E. A1 - Stinner, P. A1 - Stober, B. A1 - Strandbaek, K. A1 - Syder, T. A1 - Verilhac, D. A1 - Waldschlaeger, U. A1 - Weiss, D. A1 - Wendt, A. T1 - An extensive spectroscopic time series of three Wolf-Rayet stars - I. The lifetime of large-scale structures in the wind of WR 134 JF - Monthly notices of the Royal Astronomical Society N2 - During the summer of 2013, a 4-month spectroscopic campaign took place to observe the variabilities in three Wolf-Rayet stars. The spectroscopic data have been analysed for WR 134 (WN6b), to better understand its behaviour and long-term periodicity, which we interpret as arising from corotating interaction regions (CIRs) in the wind. By analysing the variability of the He ii lambda 5411 emission line, the previously identified period was refined to P = 2.255 +/- 0.008 (s.d.) d. The coherency time of the variability, which we associate with the lifetime of the CIRs in the wind, was deduced to be 40 +/- 6 d, or similar to 18 cycles, by cross-correlating the variability patterns as a function of time. When comparing the phased observational grey-scale difference images with theoretical grey-scales previously calculated from models including CIRs in an optically thin stellar wind, we find that two CIRs were likely present. A separation in longitude of Delta I center dot a parts per thousand integral 90A degrees was determined between the two CIRs and we suggest that the different maximum velocities that they reach indicate that they emerge from different latitudes. We have also been able to detect observational signatures of the CIRs in other spectral lines (C iv lambda lambda 5802,5812 and He i lambda 5876). Furthermore, a DAC was found to be present simultaneously with the CIR signatures detected in the He i lambda 5876 emission line which is consistent with the proposed geometry of the large-scale structures in the wind. Small-scale structures also show a presence in the wind, simultaneously with the larger scale structures, showing that they do in fact co-exist. KW - instabilities KW - methods: data analysis KW - techniques: spectroscopic KW - stars: individual: WR 134 KW - stars: massive KW - stars: Wolf-Rayet Y1 - 2016 U6 - https://doi.org/10.1093/mnras/stw1188 SN - 0035-8711 SN - 1365-2966 VL - 460 SP - 3407 EP - 3417 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Kretschmar, Peter T1 - Clumped stellar winds in supergiant high-mass X-ray binaries T2 - Postprint der universität Potsdam : Mathematisch Naturwissenschaftliche Reihe N2 - The clumping of massive star winds is an established paradigm, which is confirmed by multiple lines of evidence and is supported by stellar wind theory. We use the results from time-dependent hydrodynamical models of the instability in the line-driven wind of a massive supergiant star to derive the time-dependent accretion rate on to a compact object in the Bondi-Hoyle-Lyttleton approximation. The strong density and velocity fluctuations in the wind result in strong variability of the synthetic X-ray light curves. Photoionization of inhomogeneous winds is different from the photoinization of smooth winds. The degree of ionization is affected by the wind clumping. The wind clumping must also be taken into account when comparing the observed and model spectra of the photoionized stellar wind. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 573 KW - accretion KW - instabilities KW - stars: mass loss KW - X-rays: binaries Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413916 SN - 1866-8372 IS - 573 SP - 287 EP - 288 ER - TY - JOUR A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Kretschmar, Peter T1 - Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization JF - Monthly notices of the Royal Astronomical Society N2 - The clumping of massive star winds is an established paradigm, which is confirmed by multiple lines of evidence and is supported by stellar wind theory. The purpose of this paper is to bridge the gap between detailed models of inhomogeneous stellar winds in single stars and the phenomenological description of donor winds in supergiant high-mass X-ray binaries (HMXBs). We use the results from time-dependent hydrodynamical models of the instability in the line-driven wind of a massive supergiant star to derive the time-dependent accretion rate on to a compact object in the BondiHoyleLyttleton approximation. The strong density and velocity fluctuations in the wind result in strong variability of the synthetic X-ray light curves. The model predicts a large-scale X-ray variability, up to eight orders of magnitude, on relatively short time-scales. The apparent lack of evidence for such strong variability in the observed HMXBs indicates that the details of the accretion process act to reduce the variability resulting from the stellar wind velocity and density jumps. KW - accretion, accretion discs KW - instabilities KW - stars: neutron KW - X-rays: binaries KW - X-rays: stars Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-2966.2012.20507.x SN - 0035-8711 VL - 421 IS - 4 SP - 2820 EP - 2831 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Stroman, Thomas A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Bret, Antoine T1 - Could cosmic rays affect instabilities in the Transition layer of nonrealativistic collisionless shocks? JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - There is an observational correlation between astrophysical shocks and nonthermal particle distributions extending to high energies. As a first step toward investigating the possible feedback of these particles on the shock at the microscopic level, we perform particle-in-cell (PIC) simulations of a simplified environment consisting of uniform, interpenetrating plasmas, both with and without an additional population of cosmic rays. We vary the relative density of the counterstreaming plasmas, the strength of a homogeneous parallel magnetic field, and the energy density in cosmic rays. We compare the early development of the unstable spectrum for selected configurations without cosmic rays to the growth rates predicted from linear theory, for assurance that the system is well represented by the PIC technique. Within the parameter space explored, we do not detect an unambiguous signature of any cosmic-ray-induced effects on the microscopic instabilities that govern the formation of a shock. We demonstrate that an overly coarse distribution of energetic particles can artificially alter the statistical noise that produces the perturbative seeds of instabilities, and that such effects can be mitigated by increasing the density of computational particles. KW - cosmic rays KW - instabilities KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/746/1/24 SN - 0004-637X VL - 746 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Vafin, Sergei A1 - Riazantseva, Maria A1 - Pohl, Martin T1 - Coulomb collisions as a candidate for temperature anisotropy constraints in the solar wind JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - Many solar wind observations at 1 au indicate that the proton (as well as electron) temperature anisotropy is limited. The data distribution in the (A(a), beta(a),(parallel to))-plane have a rhombic-shaped form around beta(a),(parallel to) similar to 1. The boundaries of the temperature anisotropy at beta(a),(parallel to) > 1 can be well explained by the threshold conditions of the mirror (whistler) and oblique proton (electron) firehose instabilities in a bi-Maxwellian plasma, whereas the physical mechanism of the similar restriction at beta(a),(parallel to) < 1 is still under debate. One possible option is Coulomb collisions, which we revisit in the current work. We derive the relaxation rate nu(A)(aa) of the temperature anisotropy in a bi-Maxwellian plasma that we then study analytically and by observed proton data from WIND. We found that nu(A)(pp) increases toward small beta(p),(parallel to) < 1. We matched the data distribution in the (A(p), beta(p),(parallel to))-plane with the constant contour nu(A)(pp) = 2.8 . 10(-6) s(-1), corresponding to the minimum value for collisions to play a role. This contour fits rather well the left boundary of the rhombic-shaped data distribution in the (A(p), beta(p),(parallel to))-plane. Thus, Coulomb collisions are an interesting candidate for explaining the limitations of the temperature anisotropy in the solar wind with small beta(a),(parallel to) < 1 at 1 au. KW - instabilities KW - plasmas KW - scattering KW - solar wind KW - Sun: heliosphere Y1 - 2019 U6 - https://doi.org/10.3847/2041-8213/aafb11 SN - 2041-8205 SN - 2041-8213 VL - 871 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Kobzar, Oleh A1 - Pohl, Martin T1 - Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit the extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta beta p =5.10(-4) and 0.5 beta p =), we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane, while keeping it at 90 degrees to the shock normal. Ion reflection off of the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all of the cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field, the fraction of suprathermal electrons is much higher than for other configurations, because only in this case are the Buneman modes fully captured by the 2D simulation grid. Shocks in plasma with moderate bp provide more efficient pre-acceleration. The relevance of our results to the physics of fully 3D systems is discussed. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa872a SN - 0004-637X SN - 1538-4357 VL - 847 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Hassanin, Alshaimaa A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helical kink instability in the confined solar eruption on 2002 May 27 JF - Astronomische Nachrichten = Astronomical notes KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2016 U6 - https://doi.org/10.1002/asna.201612446 SN - 0004-6337 SN - 1521-3994 VL - 337 SP - 1082 EP - 1089 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helicity shedding by flux rope ejection JF - Astronomy and astrophysics : an international weekly journal N2 - We quantitatively address the conjecture that magnetic helicity must be shed from the Sun by eruptions launching coronal mass ejections in order to limit its accumulation in each hemisphere. By varying the ratio of guide and strapping field and the flux rope twist in a parametric simulation study of flux rope ejection from approximately marginally stable force-free equilibria, different ratios of self- and mutual helicity are set and the onset of the torus or helical kink instability is obtained. The helicity shed is found to vary over a broad range from a minor to a major part of the initial helicity, with self helicity being largely or completely shed and mutual helicity, which makes up the larger part of the initial helicity, being shed only partly. Torus-unstable configurations with subcritical twist and without a guide field shed up to about two-thirds of the initial helicity, while a highly twisted, kink-unstable configuration sheds only about one-quarter. The parametric study also yields stable force-free flux rope equilibria up to a total flux-normalized helicity of 0.25, with a ratio of self- to total helicity of 0.32 and a ratio of flux rope to external poloidal flux of 0.94. These results numerically demonstrate the conjecture of helicity shedding by coronal mass ejections and provide a first account of its parametric dependence. Both self- and mutual helicity are shed significantly; this reduces the total initial helicity by a fraction of ∼0.4--0.65 for typical source region parameters. KW - instabilities KW - magnetic fields KW - magnetohydrodynamics (MHD) KW - Sun KW - corona KW - coronal mass ejections (CMEs) KW - flares Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202142422 SN - 0004-6361 SN - 1432-0746 VL - 659 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Seiss, Martin A1 - Spahn, Frank T1 - Hydrodynamics of saturn's dense rings JF - Mathematical modelling of natural phenomena N2 - The space missions Voyager and Cassini together with earthbound observations revealed a wealth of structures in Saturn's rings. There are, for example, waves being excited at ring positions which are in orbital resonance with Saturn's moons. Other structures can be assigned to embedded moons like empty gaps, moon induced wakes or S-shaped propeller features. Furthermore, irregular radial structures are observed in the range from 10 meters until kilometers. Here some of these structures will be discussed in the frame of hydrodynamical modeling of Saturn's dense rings. For this purpose we will characterize the physical properties of the ring particle ensemble by mean field quantities and point to the special behavior of the transport coefficients. We show that unperturbed rings can become unstable and how diffusion acts in the rings. Additionally, the alternative streamline formalism is introduced to describe perturbed regions of dense rings with applications to the wake damping and the dispersion relation of the density waves. KW - granular gas KW - instabilities KW - hydrodynamics KW - planetary rings Y1 - 2011 U6 - https://doi.org/10.1051/mmnp/20116409 SN - 0973-5348 SN - 1760-6101 VL - 6 IS - 4 SP - 191 EP - 218 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Seiß, Martin A1 - Spahn, Frank T1 - Hydrodynamics of Saturn’s dense rings T2 - Postprints der Universität Potsdam : Postprint Mathematisch Naturwissenschaftliche Reihe N2 - The space missions Voyager and Cassini together with earthbound observations re-vealed a wealth of structures in Saturn’s rings. There are, for example, waves being excited at ring positions which are in orbital resonance with Saturn’s moons. Other structures can be assigned to embedded moons like empty gaps, moon induced wakes or S-shaped propeller features. Further-more, irregular radial structures are observed in the range from 10 meters until kilometers. Here some of these structures will be discussed in the frame of hydrodynamical modeling of Saturn’s dense rings. For this purpose we will characterize the physical properties of the ring particle ensemble by mean field quantities and point to the special behavior of the transport coefficients. We show that unperturbed rings can become unstable and how diffusion acts in the rings. Additionally, the alternative streamline formalism is introduced to describe perturbed regions of dense rings with applications to the wake damping and the dispersion relation of the density waves. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 574 KW - granular gas KW - instabilities KW - hydrodynamics KW - planetary rings Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413139 SP - 191 EP - 218 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 - TY - JOUR A1 - Bohdan, Artem A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Matsumoto, Yosuke A1 - Amano, Takanobu A1 - Hoshino, Masahiro T1 - Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants BT - I. Electron Shock-surfing Acceleration JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Electron injection at high Mach number nonrelativistic perpendicular shocks is studied here for parameters that are applicable to young SNR shocks. Using high-resolution large-scale two-dimensional fully kinetic particle-in-cell simulations and tracing individual particles, we in detail analyze the shock-surfing acceleration (SSA) of electrons at the leading edge of the shock foot. The central question is to what degree the process can be captured in 2D3V simulations. We find that the energy gain in SSA always arises from the electrostatic field of a Buneman wave. Electron energization is more efficient in the out-of-plane orientation of the large-scale magnetic field because both the phase speed and the amplitude of the waves are higher than for the in-plane scenario. Also, a larger number of electrons is trapped by the waves compared to the in-plane configuration. We conclude that significant modifications of the simulation parameters are needed to reach the same level of SSA efficiency as in simulations with out-of-plane magnetic field or 3D simulations. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab1b6d SN - 0004-637X SN - 1538-4357 VL - 878 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Rüdiger, Günther A1 - Schultz, Manfred T1 - Large-scale dynamo action of magnetized Taylor-Couette flows JF - Monthly notices of the Royal Astronomical Society N2 - A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler-Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining a effect and eddy diffusivity. If both degenerated modes with m = +/- 1 are excited with the same power then the global a effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite alpha effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an alpha(2) dynamo. Moreover, an alpha Omega dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm similar or equal to 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, eta(T) similar or equal to eta. By basically lower or larger eddy diffusivities the dynamo instability is suppressed. KW - dynamo KW - instabilities KW - MHD KW - magnetic fields Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa293 SN - 0035-8711 SN - 1365-2966 VL - 493 IS - 1 SP - 1249 EP - 1260 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Driel-Gesztelyi, L. van A1 - Baker, Daniel N. A1 - Török, Tibor A1 - Pariat, Etienne A1 - Green, L. M. A1 - Williams, D. R. A1 - Carlyle, J. A1 - Valori, G. A1 - Démoulin, Pascal A1 - Matthews, S. A. A1 - Kliem, Bernhard A1 - Malherbe, J.-M. T1 - Magnetic reconnection driven by filament eruption in the 7 June 2011 event T2 - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe N2 - During an unusually massive filament eruption on 7 June 2011, SDO/AIA imaged for the first time significant EUV emission around a magnetic reconnection region in the solar corona. The reconnection occurred between magnetic fields of the laterally expanding CME and a neighbouring active region. A pre-existing quasi-separatrix layer was activated in the process. This scenario is supported by data-constrained numerical simulations of the eruption. Observations show that dense cool filament plasma was re-directed and heated in situ, producing coronal-temperature emission around the reconnection region. These results provide the first direct observational evidence, supported by MHD simulations and magnetic modelling, that a large-scale re-configuration of the coronal magnetic field takes place during solar eruptions via the process of magnetic reconnection. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 608 KW - MHD KW - instabilities KW - Sun: activity KW - magnetic fields KW - coronal mass ejections (CMEs) KW - filaments KW - methods: numerical KW - data analysis Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-415671 IS - 608 SP - 502 EP - 503 ER - TY - JOUR A1 - Niemiec, Jacek A1 - Pohl, Martin A1 - Bret, Antoine A1 - Wieland, Volkmar T1 - Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present results of 2D3V particle-in-cell simulations of nonrelativistic plasma collisions with absent or parallel large-scale magnetic field for parameters applicable to the conditions at young supernova remnants. We study the collision of plasma slabs of different density, leading to two different shocks and a contact discontinuity. Electron dynamics play an important role in the development of the system. While nonrelativistic shocks in both unmagnetized and magnetized plasmas can be mediated by Weibel-type instabilities, the efficiency of shock-formation processes is higher when a large-scale magnetic field is present. The electron distributions downstream of the forward and reverse shocks are generally isotropic, whereas that is not always the case for the ions. We do not see any significant evidence of pre-acceleration, neither in the electron population nor in the ion distribution. KW - acceleration of particles KW - instabilities KW - ISM: supernova remnants KW - methods: numerical KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/759/1/73 SN - 0004-637X SN - 1538-4357 VL - 759 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Alawashra, Mahmoud T1 - Plasma instabilities of TeV pair beams induced by blazars T1 - Plasma Instabilitäten von TeV-Paar Strahlen durch Blazare induziert N2 - Relativistic pair beams produced in the cosmic voids by TeV gamma rays from blazars are expected to produce a detectable GeV-scale cascade emission missing in the observations. The suppression of this secondary cascade implies either the deflection of the pair beam by intergalactic magnetic fields (IGMFs) or an energy loss of the beam due to the electrostatic beam-plasma instability. IGMF of femto-Gauss strength is sufficient to significantly deflect the pair beams reducing the flux of secondary cascade below the observational limits. A similar flux reduction may result in the absence of the IGMF from the beam energy loss by the instability before the inverse Compton cooling. This dissertation consists of two studies about the instability role in the evolution of blazar-induced beams. Firstly, we investigated the effect of sub-fG level IGMF on the beam energy loss by the instability. Considering IGMF with correlation lengths smaller than a few kpc, we found that such fields increase the transverse momentum of the pair beam particles, dramatically reducing the linear growth rate of the electrostatic instability and hence the energy-loss rate of the pair beam. Our results show that the IGMF eliminates beam plasma instability as an effective energy-loss agent at a field strength three orders of magnitude below that needed to suppress the secondary cascade emission by magnetic deflection. For intermediate-strength IGMF, we do not know a viable process to explain the observed absence of GeV-scale cascade emission and hence can be excluded. Secondly, we probed how the beam-plasma instability feeds back on the beam, using a realistic two-dimensional beam distribution. We found that the instability broadens the beam opening angles significantly without any significant energy loss, thus confirming a recent feedback study on a simplified one-dimensional beam distribution. However, narrowing diffusion feedback of the beam particles with Lorentz factors less than 1e6 might become relevant even though initially it is negligible. Finally, when considering the continuous creation of TeV pairs, we found that the beam distribution and the wave spectrum reach a new quasi-steady state, in which the scattering of beam particles persists and the beam opening angle may increase by a factor of hundreds. This new intrinsic scattering of the cascade can result in time delays of around ten years, thus potentially mimicking the IGMF deflection. Understanding the implications on the GeV cascade emission requires accounting for inverse Compton cooling and simulating the beam-plasma system at different points in the IGM. N2 - Relativistische Teilchenstrahlen, erzeugt in den Weiten des Weltraums durch TeV- Gammastrahlen von Blazaren, sollen eine Art von Emission im GeV-Bereich erzeugen. Diese Emission wurde jedoch bisher nicht beobachtet. Der Grund für diese fehlende Emission könnte eine von zwei Ursachen sein: Entweder werden die Teilchenstrahlen von den Magnetfeldern im Weltraum (den sogenannten intergalaktischen Magnetfeldern oder IGMFs) umgeleitet, oder die Strahlen verlieren ihre Energie aufgrund einer Art von Instabilität namens Strahlungs-Plasma-Instabilität. Wenn die IGMFs extrem schwach sind (im Femto-Gauss-Bereich gemessen), können sie dennoch eine große Wirkung auf die Teilchenstrahlen haben, indem sie diese von ihrem Kurs abbringen und die Menge der fehlenden Emission verringern. Andererseits kann die Strahlungs-Plasma-Instabilität die Energieverluste der Strahlen verursachen, wenn es keine IGMFs gibt. Diese Forschung besteht aus zwei Studien. In der ersten Studie haben Wissenschaftler erforscht, wie schwache IGMFs den Energieverlust der Strahlen aufgrund von Instabilität beeinflussen. Sie stellten fest, dass diese schwachen Felder den Impuls der Teilchenstrahlen erheblich verändern können, was den Energieverlust aufgrund der Instabilität erheblich verlangsamt. Dies bedeutet, dass selbst extrem schwache IGMFs die Strahlungs Plasma-Instabilität unwirksam machen können, wenn es darum geht, Energieverluste zu verursachen. In der zweiten Studie haben sie untersucht, wie die Strahlungs-Plasma-Instabilität die Teilchenstrahlen beeinflusst. Sie entdeckten, dass die Instabilität den Winkel der Strahlen erweitert, ohne signifikante Energieverluste zu verursachen. Im Laufe der Zeit könnten jedoch Partikel mit niedrigeren Energien anfangen, Energie zu verlieren. Wenn man die kontinuierliche Erzeugung von hochenergetischen Teilchen berücksichtigt, stellten sie fest, dass die Verteilung der Strahlen und das Wellenspektrum schließlich einen stabilen Zustand erreichen, in dem die Partikel weiterhin gestreut werden und der Strahlenwinkel sich erheblich vergrößern kann. Diese intrinsische Streuung der Emission kann Zeitspannen verursachen, die es so aussehen lassen, als ob die IGMFs die Emission umlenken. Um die Auswirkungen auf die fehlende GeV-Emission vollständig zu verstehen, müssen Wissenschaftler Faktoren wie inverse-Compton-Kühlung berücksichtigen und die Wechselwirkung zwischen den Teilchenstrahlen und dem umgebenden Plasma an verschiedenen Stellen im Weltraum simulieren. KW - gamma rays: general KW - instabilities KW - blazar KW - relativistic processes KW - waves KW - Blazar KW - Gammastrahlen: allgemein KW - Instabilitäten KW - relativistische Prozesse KW - Wellen Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-630131 ER - TY - JOUR A1 - Vafin, Sergei A1 - Deka, Pranab Jyoti A1 - Pohl, Martin A1 - Bohdan, Artem T1 - Revisit of Nonlinear Landau Damping for Electrostatic Instability Driven by Blazar-induced Pair Beams JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We revisit the effect of nonlinear Landau (NL) damping on the electrostatic instability of blazar-induced pair beams, using a realistic pair-beam distribution. We employ a simplified 2D model in k-space to study the evolution of the electric-field spectrum and to calculate the relaxation time of the beam. We demonstrate that the 2D model is an adequate representation of the 3D physics. We find that nonlinear Landau damping, once it operates efficiently, transports essentially the entire wave energy to small wave numbers where wave driving is weak or absent. The relaxation time also strongly depends on the intergalactic medium temperature, T-IGM, and for T-IGM << 10 eV, and in the absence of any other damping mechanism, the relaxation time of the pair beam is longer than the inverse Compton (IC) scattering time. The weak late-time beam energy losses arise from the accumulation of wave energy at small k, that nonlinearly drains the wave energy at the resonant k of the pair-beam instability. Any other dissipation process operating at small k would reduce that wave-energy drain and hence lead to stronger pair-beam energy losses. As an example, collisions reduce the relaxation time by an order of magnitude, although their rate is very small. Other nonlinear processes, such as the modulation instability, could provide additional damping of the nonresonant waves and dramatically reduce the relaxation time of the pair beam. An accurate description of the spectral evolution of the electrostatic waves is crucial for calculating the relaxation time of the pair beam. KW - gamma rays: general KW - instabilities KW - magnetic fields KW - relativistic processes KW - waves Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab017b SN - 0004-637X SN - 1538-4357 VL - 873 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kliem, Bernhard A1 - Toeroek, Tibor A1 - Titov, Viacheslav S. A1 - Lionello, Roberto A1 - Linker, Jon A. A1 - Liu, Rui A1 - Liu, Chang A1 - Wang, Haimin T1 - Slow rise and partial eruption of a double-decker filament. II. A double flux rope model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically becoming unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I. It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium. KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/792/2/107 SN - 0004-637X SN - 1538-4357 VL - 792 IS - 2 PB - IOP Publ. Ltd. CY - Bristol 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 - Krtička, Jiří A1 - Feldmeier, Achim T1 - Stochastic light variations in hot stars from wind instability BT - finding photometric signatures and testing against the TESS data JF - Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO) N2 - Context Line-driven wind instability is expected to cause small-scale wind inhomogeneities, X-ray emission, and wind line profile variability. The instability can already develop around the sonic point if it is initiated close to the photosphere due to stochastic turbulent motions. In such cases, it may leave its imprint on the light curve as a result of wind blanketing. Aims We study the photometric signatures of the line-driven wind instability. Methods We used line-driven wind instability simulations to determine the wind variability close to the star. We applied two types of boundary perturbations: a sinusoidal one that enables us to study in detail the development of the instability and a stochastic one given by a Langevin process that provides a more realistic boundary perturbation. We estimated the photometric variability from the resulting mass-flux variations. The variability was simulated assuming that the wind consists of a large number of independent conical wind sectors. We compared the simulated light curves with TESS light curves of OB stars that show stochastic variability. Results We find two typical signatures of line-driven wind instability in photometric data: a knee in the power spectrum of magnitude fluctuations, which appears due to engulfment of small-scale structure by larger structures, and a negative skewness of the distribution of fluctuations, which is the result of spatial dominance of rarefied regions. These features endure even when combining the light curves from independent wind sectors. Conclusions The stochastic photometric variability of OB stars bears certain signatures of the line-driven wind instability. The distribution function of observed photometric data shows negative skewness and the power spectra of a fraction of light curves exhibit a knee. This can be explained as a result of the line-driven wind instability triggered by stochastic base perturbations. KW - stars: winds KW - outflows KW - stars: mass-loss KW - stars: early-type KW - hydrodynamics KW - instabilities KW - stars: variables: general Y1 - 2021 U6 - https://doi.org/10.1051/0004-6361/202040148 SN - 1432-0746 VL - 648 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Vafin, Sergei A1 - Rafighi, Iman A1 - Pohl, Martin A1 - Niemiec, Jacek T1 - The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - This work revisits the electrostatic instability for blazar-induced pair beams propagating through the intergalactic medium (IGM) using linear analysis and PIC simulations. We study the impact of the realistic distribution function of pairs resulting from the interaction of high-energy gamma-rays with the extragalactic background light. We present analytical and numerical calculations of the linear growth rate of the instability for the arbitrary orientation of wave vectors. Our results explicitly demonstrate that the finite angular spread of the beam dramatically affects the growth rate of the waves, leading to the fastest growth for wave vectors quasi-parallel to the beam direction and a growth rate at oblique directions that is only a factor of 2-4 smaller compared to the maximum. To study the nonlinear beam relaxation, we performed PIC simulations that take into account a realistic wide-energy distribution of beam particles. The parameters of the simulated beam-plasma system provide an adequate physical picture that can be extrapolated to realistic blazar-induced pairs. In our simulations, the beam looses only 1% of its energy, and we analytically estimate that the beam would lose its total energy over about 100 simulation times. An analytical scaling is then used to extrapolate the parameters of realistic blazar-induced pair beams. We find that they can dissipate their energy slightly faster by the electrostatic instability than through inverse-Compton scattering. The uncertainties arising from, e.g., details of the primary gamma-ray spectrum are too large to make firm statements for individual blazars, and an analysis based on their specific properties is required. KW - gamma rays: general KW - instabilities KW - magnetic fields KW - relativistic processes KW - waves Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab552 SN - 0004-637X SN - 1538-4357 VL - 857 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER -