Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session BO06: Astrophysical Turbulence and DynamosOn Demand
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Chair: Archie Bott, Princeton University Room: Rooms 310-311 |
Monday, November 8, 2021 9:30AM - 9:42AM |
BO06.00001: Is the Landau Theory Important for Stellar Dynamics? Amitava Bhattacharjee, Chung-Sang Ng From considerations of the Vlasov equation, Lau and Binney [1] have recently advocated the use of Case-Van Kampen eigenmodes in studies of collisonless stellar systems. Invoking the properties of these modes as rigorous eigenmodes of the collisionless system, they have made the surprising but interesting suggestion that the Landau approach, which does not produce eigenmodes of the system, is not useful for the study of such systems. However, in recent years the limit of zero collisions of a very weakly collisional plasma has been a subject of interest in the plasma physics community, and has led to new insights on the relative importance of Van-Kampen modes and Landau solutions. Using the Lenard-Bernstein collision operator, it has been shown that in the limit of zero collisions, the Case-Van Kampen continuous spectrum is eliminated and the discrete Landau solutions become true eigemodes of the weakly collisional system [2]. We apply the theory to the Jeans instability of stellar systems and demonstrate that in the limit of zero collisions of the collisional problem, large N-body simulations of stellar and galactic dynamics must reckon with Landau solutions as true eigenmodes of such systems. |
Monday, November 8, 2021 9:42AM - 9:54AM |
BO06.00002: Collisions of relativistic Alfvén waves as sources of weak turbulence, current sheets, and efficient mode conversion Jens F Mahlmann, Bart Ripperda, Alexander Chernoglazov, Jason M TenBarge, Elias R Most, James L Juno, Yajie Yuan, Alexander A Philippov, Amitava Bhattacharjee Alfvén waves are the primary building blocks of turbulence in magnetized plasmas. We extend the validity of this statement to relativistic astrophysical plasmas beyond the heliosphere. Such environments, commonly found in black hole accretion disks coronae and neutron star magnetospheres, are highly magnetized and relativistic. Starting from counter-propagating Alfvén waves, we unveil nonlinear dynamics of Alfvén waves by drawing on theory and three-dimensional numerical simulations in both relativistic magnetohydrodynamics (MHD) and force-free MHD. |
Monday, November 8, 2021 9:54AM - 10:06AM |
BO06.00003: Large Scale Magnetic Field Growth and Stability in Hall-MHD Simulations of Quasi-Keplerian Flows Matthew C Pharr, Fatima Ebrahimi, Eric Blackman Understanding the self-generation of large-scale magnetic fields in plasmas and their relation to magnetorotational instability and momentum transport is an outstanding problem in plasma astrophysics. One hypothesis is that large-scale dynamo magnetic fields could cause the magnetorotational instability to enter the stable regime and therefore suppress turbulence in the plasma. Here, we examine this behavior in quasi-keplerian cylindrical flows. |
Monday, November 8, 2021 10:06AM - 10:18AM |
BO06.00004: Kinetic Turbulence in Pressure-Anisotropic, High-Beta Plasmas Lev Arzamasskiy, Matthew W Kunz, Jonathan Squire, Eliot Quataert, Alexander A Schekochihin Many space and astrophysical plasmas, such as the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium, are hot and dilute, which makes them weakly collisional or collisionless, with plasma beta of order unity or larger. Kinetic processes, occurring on scales much smaller than what could realistically be observed from Earth, can dramatically influence the emission from these systems by shaping particles' distribution functions through the dissipation of the cascade and by introducing magnetic-field-biased deviations from local thermodynamic equilibrium, which could make the plasma unstable to a number of kinetic microinstabilities. These instabilities introduce an effective collisionality into otherwise collisionless plasma and impact the dynamics of turbulent fluctuations. In this talk, the results from hybrid-kinetic simulations are used to explore the interplay between kinetic micro-instabilities and the turbulent cascade. In particular, we obtain the effective collisionality and viscous scale in collisionless high-beta turbulence, quantify the non-local energy transfer mediated by the micro-instabilities, and determine the partition of energy between particle species. |
Monday, November 8, 2021 10:18AM - 10:30AM |
BO06.00005: Exploration of Magnetic-Field Generation via Biermann Battery Using the FLASH Code to Model Experiments Performed at UCLA’s Phoenix Laboratory Marissa B Adams, Scott Feister, Jessica J Pilgram, Carmen G Constantin, Christoph Niemann, Pierre-Alexandre Gourdain, Petros Tzeferacos Magnetic fields are omnipresent in our universe and a key astrophysical process behind their origin is the Biermann battery mechanism.[1] This mechanism generates magnetic fields caused by misaligned density and temperature gradients, also encountered in terrestrial laser-driven plasma experiments.[2] Therefore, the latter are ideal for validating the theory and simulation tools used to model magnetic-field generation in astrophysical and laboratory environments. Recent high-repetition-rate laser experiments performed by the HEDP Group at UCLA[3] on the PEENING laser are furnishing large data sets of Bierman battery magnetic-field measurements in expanding plasma plumes via B-dot probes, centimeters away from the laser–target interaction. In this talk we present numerical simulations that model these experiments, using the multiphysics radiation-magnetohydrodynamics code FLASH. The simulations allow us to explore a variety of questions regarding the plasma properties of the expanding plasma plumes and the strength and spatial distribution of the Biermann battery magnetic fields. [1] L. Biermann and A. Schlüter, Z. Naturforschg. 5a, 65 (1950). [2] M. G. Haines, Plasma Phys. Control. Fusion 28, 1705 (1986). [3] J. J. Pilgram et al., this conference. |
Monday, November 8, 2021 10:30AM - 10:42AM |
BO06.00006: Radiative Turbulence in Magnetically Dominated Plasmas: Particle Acceleration and Cooling Luca Comisso, Lorenzo Sironi Magnetized turbulence is often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic particle-in-cell simulations that include self-consistently the radiation reaction force, we investigate the acceleration and cooling of particles in turbulent plasmas in the strong cooling regime. This regime is relevant to a variety of high-energy astrophysical phenomena such as the prompt emission of gamma-ray bursts and the gamma-ray flares from the Crab Nebula. We show that reconnecting current sheets, which develop self-consistently in the turbulent plasma, inject particles with a hard power-law distribution and low pitch angle due to the acceleration powered by magnetic-field-aligned electric fields. Particles cool down by increasing their pitch angle, which affects the cooled particle distribution. Due to the low pitch angle of the accelerated particles, significant synchrotron radiation is emitted above the synchrotron burnoff limit, as is required to explain the gamma-ray emission in Crab flares. Synchrotron radiation from the accelerated particles give rise to a synchrotron energy flux with a power-law range $\nu F_\nu \propto \nu^s$ with $s \sim 1$, up to the peak frequency $\nu_{\rm{peak}}$ consistent to the observed prompt emission of gamma-ray bursts. The presented results have also implications for understanding the generation of nonthermal particles in other high-energy astrophysical sources. |
Monday, November 8, 2021 10:42AM - 10:54AM |
BO06.00007: Kinetic Simulations of Imbalanced Turbulence in a Relativistically-hot Plasma Amelia Hankla, Vladimir V Zhdankin, Gregory R Werner, Dmitri A Uzdensky, Mitchell C Begelman Turbulent high-energy astrophysical systems often feature asymmetric energy injection or driving: for instance, nonlinear interactions between Alfvén waves propagating from an accretion disk into its corona. Such systems---relativistic analogs of the solar wind---are "imbalanced": the energy fluxes parallel and anti-parallel to the large-scale magnetic field are unequal and the plasma possesses net cross-helicity. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In the present study, we investigate externally-driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using three-dimensional particle-in-cell simulations. We find that a turbulent cascade forms for every value of imbalance covered by the simulations and that injected Poynting flux efficiently converts into net momentum of the plasma, with implications for the launching of a disk wind. Surprisingly, particle acceleration remains efficient even for very imbalanced turbulence. These results characterize properties of imbalanced turbulence in a collisionless plasma and have ramifications for black hole accretion disk coronae, winds, and jets. |
Monday, November 8, 2021 10:54AM - 11:06AM |
BO06.00008: Reconnection-controlled decay of magnetohydrodynamic turbulence and the role of invariants David Hosking, Alexander A Schekochihin We present a new theoretical picture of magnetically dominated, decaying turbulence in the absence of a mean magnetic field. With direct numerical simulations, we demonstrate that the rate of turbulent decay is governed by the reconnection of magnetic structures, and not necessarily by ideal dynamics, as has previously been assumed. We obtain predictions for the magnetic-energy decay laws by proposing that turbulence decays on reconnection timescales, while respecting the conservation of certain integral invariants representing topological constraints satisfied by the reconnecting magnetic field. As is well known, the magnetic helicity is such an invariant for initially helical field configurations, but does not constrain non-helical decay, where the volume-averaged magnetic-helicity density vanishes. For such a decay, we propose a new integral invariant, analogous to the Loitsyansky and Saffman invariants of hydrodynamic turbulence, that expresses the conservation of the random [scaling as volume^(1/2)] magnetic helicity contained in any sufficiently large volume. We verify that this invariant is indeed well-conserved in our numerical simulations. Our treatment leads to novel predictions for the magnetic-energy decay laws, and to a natural explanation of the 'inverse-transfer' phenomenon reported by previous numerical studies. |
Monday, November 8, 2021 11:06AM - 11:18AM |
BO06.00009: Collisionless magnetorotational turbulence in electron-ion plasmas Fabio Bacchini, Dmitri A Uzdensky, Gregory R Werner, Mitchell C Begelman, Vladimir V Zhdankin The magnetorotational instability (MRI) is a fundamental process occurring in astrophysical accretion disks. The MRI promotes accretion by driving turbulence on macroscopic scales. In the process, magnetic reconnection and other collective plasma phenomena can accelerate particles to high (possibly nonthermal) energies. The resulting plasma emission may be measurable with observational campaigns. |
Monday, November 8, 2021 11:18AM - 11:30AM |
BO06.00010: Scaling of Turbulent Viscosity and Resistivity: Extracting a Scale-dependent Turbulent Magnetic Prandtl Number Xin Bian, Jessica K Shang, Eric Blackman, Gilbert Collins, Hussein Aluie Turbulent viscosity μt and resistivity ηt are perhaps the simplest models for turbulent transport of angular momentum and magnetic fields, respectively. The associated turbulent magnetic Prandtl number Prt = μt/ηt has been well recognized to determine the final magnetic configuration of accretion disks. Here, we present an approach to determining these "effective transport" coefficients acting at different length-scales using coarse-graining and recent results on decoupled kinetic and magnetic energy cascades. By analyzing the kinetic and magnetic energy cascades from a suite of high-resolution simulations, we show that our definitions of μt, ηt, and Prt have power-law scalings in the "decoupled range." We observe that Prt ≈1 to 2 at the smallest inertial-inductive scales, increasing to ≈5 at the largest scales. However, based on physical considerations, our analysis suggests that Prt has to become scale-independent and of order unity in the decoupled range at sufficiently high Reynolds numbers (or grid-resolution), and that the power-law scaling exponents of velocity and magnetic spectra become equal. In addition to implications to astrophysical systems, the scale-dependent turbulent transport coefficients offer a guide for large eddy simulation modeling. |
Monday, November 8, 2021 11:30AM - 11:42AM |
BO06.00011: Effective Collision Operator for Heat-Flux-Generated Whistler Turbulence Evan L Yerger, Matthew W Kunz, Anatoly Spitkovsky High-beta plasmas can be highly magnetized at the largest astrophysical scales, e.g., in the intracluster medium (ICM) of galaxy clusters. If the plasma is furthermore weakly collisional, the transport of momentum and heat is highly anisotropic with respect to the magnetic field direction. In thermally stratified plasmas at sufficiently high beta, the parallel heat flux can be large enough to trigger a kinetic whistler instability, which back-reacts on the transport by deforming the field lines on electron-Larmor scales. In this work, we use the particle-in-cell code Tristan-MP to calculate the steady-state heat flux through a stratified, high-beta, collisionless, magnetized plasma. By tracking a quarter million particles and simulating across a range of beta and temperature gradient length scales, we calculate the effective collision operator for heat-flux-driven whistler turbulence and use it to solve the Chapman-Enskog-Braginskii problem. We compare the resulting transport equations with existing models and discuss their implications for magneto-thermal convection and the structure of the ICM. |
Monday, November 8, 2021 11:42AM - 11:54AM |
BO06.00012: Cyclical Generation of Magnetic Field Structures with Significant Electron Temperature and Density Gradients Bamandas Basu, Bruno Coppi The generation of magnetic field structures over macroscopic scale distances is an issue of well recognized importance in astrophysics and for laboratory experiments. A relevant process is identified [1] involving a plasma, imbedded in a pre-existing sheared magnetic structure, with a significant gradient of the longitudinal (to the stationary component of the magnetic field) electron temperature. A periodic emergence of a reconnected magnetic structure is the basic feature of this process that is likened to an "alternator". The oscillations amplitude can be amplified in the presence of a density gradient (aligned with the electron temperature gradient) and a non-negligible particle diffusion. Regimes where the longitudinal electron thermal conductivities are large relative to the associated transverse conductivities are considered, a two-fluid description being adopted as a start. |
Monday, November 8, 2021 11:54AM - 12:06PM |
BO06.00013: Stretching, mixing, and tearing: Magnetic self-organization in high-resolution simulations of the turbulent dynamo in Pm>1 plasma Alisa Galishnikova, Matthew W Kunz, Alexander A Schekochihin Turbulence in a conducting plasma can amplify seed magnetic fields in what is known as the turbulent, or small-scale, dynamo. The associated growth rate and emergent magnetic-field geometry depends sensitively on the material properties of the plasma, in particular the magnetic Prandtl number Pm. For Pm>1, the amplified magnetic field is gradually arranged into a folded structure, with direction reversals at the resistive scale and field lines curved at the larger scale of the flow. As the magnetic energy grows to come into approximate equipartition with the fluid motions, this folded structure is thought to persist. Using analytical theory and high-resolution MHD simulations with the Athena++ code, it is shown that these magnetic folds become unstable to tearing during the nonlinear stage of the dynamo for magnetic Reynolds numbers Rm≳104. An Rm- and Pm-dependent tearing scale, at and below which fold disruption occurs, is theoretically predicted and found to match well the characteristic field-reversal scale measured in the simulations. This disruption increases the amount of viscous dissipation in this tearing-limited dynamo. In the saturated state, the spectral peak of the magnetic energy appears to be independent of the resistive scale, belying the customary “small-scale” moniker. |
Monday, November 8, 2021 12:06PM - 12:18PM |
BO06.00014: Spontaneous magnetization of collisionless plasmas through the action of a shear flow Muni Zhou, Vladimir V Zhdankin, Matthew W Kunz, Nuno F Loureiro, Dmitri A Uzdensky The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming pervasive cosmic magnetic fields. Both the mechanisms of seed-field production and the nature of plasma dynamos have been studied independently. However, in the weakly collisional intergalactic/intracluster medium, the turbulent motions of dynamo may also give rise to seed fields and thus magnetize the plasma non-inductively. The strength and morphology of these seed fields have not been studied self-consistently. We study in a fully-kinetic framework the generation of seed fields through the Weibel instability in an initially unmagnetized plasma driven by large-scale shear force. We develop an analytical model that describes the development of thermal pressure anisotropy via phase-mixing, the ensuing exponential growth of magnetic fields in the linear Weibel phase, and its saturation due to the particle magnetization. The predicted scaling dependencies of the saturated seed fields on key parameters (e.g., scale separation between the forcing scale and electron skin depth, the forcing amplitude) are confirmed by our 3D and 2D particle-in-cell simulations. This work demonstrates the spontaneous magnetization of a collisonless plasma through large-scale motions as simple as a shear flow. |
Monday, November 8, 2021 12:18PM - 12:30PM |
BO06.00015: Energy diffusion and advection coefficients in kinetic simulations of relativistic plasma turbulence Kai W Wong, Dmitri A Uzdensky, Vladimir V Zhdankin, Mitchell C Begelman, Gregory R Werner Turbulent magnetised plasmas, which are ubiquitous in high-energy astrophysical systems, feature extended broadband nonthermal emission spectra implying nonthermal particle energy distributions. The underlying turbulent nonthermal particle acceleration processes have traditionally been modeled with a Fokker-Planck momentum-space diffusion-advection equation. We analyse the energy histories of large numbers of particles in kinetic simulations of driven pair plasma turbulence with varying initial magnetisation and system size. For each simulation, we test the energy-diffusion assumption of the Fokker-Planck particle acceleration framework, and then measure the energy diffusion and advection coefficients D and A as a function of particle energy ε. In the nonthermal energy range, we find the diffusion coefficient scaling is consistent with ε2σ3/2 where σ is the instantaneous magnetisation. We also measure the evolution of the power-law index of the particle energy distribution and find that it is well-described by an exponential in time. We then construct a model connecting the Fokker-Planck coefficients and the observed power-law evolution, predicting that A ∽ ε log(ε/ε*), which is consistent with the measurements. These results enhance our understanding of turbulent nonthermal particle acceleration. |
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