@article{VafinRafighiPohletal.2018, author = {Vafin, Sergei and Rafighi, Iman and Pohl, Martin and Niemiec, Jacek}, title = {The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {857}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aab552}, pages = {12}, year = {2018}, abstract = {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.}, language = {en} } @article{KrtičkaFeldmeier2021, author = {Krtička, Jiř{\´i} and Feldmeier, Achim}, title = {Stochastic light variations in hot stars from wind instability}, series = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, volume = {648}, journal = {Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/202040148}, pages = {9}, year = {2021}, abstract = {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.}, language = {en} } @article{VafinLazarFichtneretal.2018, author = {Vafin, Sergei and Lazar, M. and Fichtner, H. and Schlickeiser, R. and Drillisch, M.}, title = {Solar wind temperature anisotropy constraints from streaming instabilities}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {613}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731852}, pages = {5}, year = {2018}, abstract = {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.}, language = {en} } @article{KliemToeroekTitovetal.2014, author = {Kliem, Bernhard and Toeroek, Tibor and Titov, Viacheslav S. and Lionello, Roberto and Linker, Jon A. and Liu, Rui and Liu, Chang and Wang, Haimin}, title = {Slow rise and partial eruption of a double-decker filament. II. A double flux rope model}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {792}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/792/2/107}, pages = {10}, year = {2014}, abstract = {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.}, language = {en} } @article{VafinDekaPohletal.2019, author = {Vafin, Sergei and Deka, Pranab Jyoti and Pohl, Martin and Bohdan, A.}, title = {Revisit of Nonlinear Landau Damping for Electrostatic Instability Driven by Blazar-induced Pair Beams}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {873}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/ab017b}, pages = {12}, year = {2019}, abstract = {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.}, language = {en} } @article{NiemiecPohlBretetal.2012, author = {Niemiec, Jacek and Pohl, Martin and Bret, Antoine and Wieland, Volkmar}, title = {Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {759}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/759/1/73}, pages = {20}, year = {2012}, abstract = {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.}, language = {en} } @article{RuedigerSchultz2020, author = {R{\"u}diger, G{\"u}nther and Schultz, Manfred}, title = {Large-scale dynamo action of magnetized Taylor-Couette flows}, series = {Monthly notices of the Royal Astronomical Society}, volume = {493}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/staa293}, pages = {1249 -- 1260}, year = {2020}, abstract = {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.}, language = {en} } @article{BohdanNiemiecPohletal.2019, author = {Bohdan, Artem and Niemiec, Jacek and Pohl, Martin and Matsumoto, Yosuke and Amano, Takanobu and Hoshino, Masahiro}, title = {Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {878}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/ab1b6d}, pages = {11}, year = {2019}, abstract = {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.}, language = {en} } @article{VafinRiazantsevaYoon2017, author = {Vafin, Sergei and Riazantseva, M. and Yoon, P. H.}, title = {Kinetic Features in the Ion Flux Spectrum}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {850}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aa9519}, pages = {7}, year = {2017}, abstract = {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.}, language = {en} } @article{SeissSpahn2011, author = {Seiss, Martin and Spahn, Frank}, title = {Hydrodynamics of saturn's dense rings}, series = {Mathematical modelling of natural phenomena}, volume = {6}, journal = {Mathematical modelling of natural phenomena}, number = {4}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0973-5348}, doi = {10.1051/mmnp/20116409}, pages = {191 -- 218}, year = {2011}, abstract = {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.}, language = {en} }