Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session TO5: Solar Wind, Turbulence, Particle Motion |
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Chair: Bart Van Compernolle, University of California, Los Angeles Room: 200 |
Thursday, November 19, 2015 9:30AM - 9:42AM |
TO5.00001: Application of Self-Similar Kinetic Theory to the Solar Wind: Data and Simulations Konstantinos Horaites, Stanislav Boldyrev, Sergei Krasheninnikov, Chadi Salem, Stuart Bale, Marc Pulupa If the temperature Knudsen number $\gamma(x) \sim T(dT/dx)/n$ in a plasma is constant throughout the system, the collisional kinetic equation for electrons admits self-similar solutions. These solutions have the novel property that the ``shape'' of the electron velocity distribution function (eVDF) does not vary in space. Such a theory should be applicable to the solar wind in the inner heliosphere, where the density and temperature are observed to vary as power laws with heliocentric distance r such that $\gamma(r) \sim$constant. We present results of numerical simulations, where we find the steady-state eVDF for various $\gamma$. We then compare our predictions with observations from the Helios satellite. Our theory successfully produces a strahl population, which we interpret to be comprised of thermal runaway electrons that originated from the corona. For the large (collisionless) Knudsen numbers that are typically observed in the solar wind, this population contributes significantly to the total electron heat flux. [Preview Abstract] |
Thursday, November 19, 2015 9:42AM - 9:54AM |
TO5.00002: Log-normal Intermittency in Alfvenic Turbulence Vladimir Zhdankin, Stanislav Boldyrev, Christopher Chen Random cascade models provide an attractive phenomenological framework for describing intermittency in turbulence. These models generally predict the distribution of local coarse-grained energy dissipation rates, from which the scaling of structure function exponents is inferred after assuming the refined similarity hypothesis. However, it remains unclear to what extent random cascade models can be applied to describe intermittency in large-scale turbulent plasmas. To address this, we investigate the statistics of the coarse-grained energy dissipation rate in numerical simulations of magnetohydrodynamic turbulence and in measurements of the solar wind, using magnetic field gradients as a surrogate variable in the latter case. We present evidence that the resulting distributions and their moments are described remarkably well as a log-normal random cascade. We find that the intermittency parameter matches in the two systems when a weak guide field is used for the simulations, while a strong guide field makes the dynamics more intermittent. We discuss possible implications for other measured quantities in the solar wind, including the statistics of magnetic discontinuities. [Preview Abstract] |
Thursday, November 19, 2015 9:54AM - 10:06AM |
TO5.00003: Role of shell distribution instabilities in formation of parallel and perpendicular spectra of the solar wind turbulence Vitaly Galinsky, Valentin Shevchenko Alfven waves propagating along the local background magnetic field pitch angle scatter particles into shell-like distribution. Results of a study of dispersive Alfven modes propagating outward from the Sun in streaming inhomogeneous plasma for the inner heliosphere ($\leq$ 1AU) region show that an interplay of macro scale and resonant wave-particle instabilities of shell-like distribution combined with nonlinear wave-wave interaction of shear and kinetic Alfven branches is responsible for formation of both parallel and perpendicular spectra of the solar wind turbulence. The study does not use any of the available nonlinear models of imbalanced incompressible MHD turbulence but nevertheless correctly reproduces several peculiarities of the solar wind turbulence spectra that currently generate significant difficulties for the solar wind MHD turbulence models. Our approach correctly explains the fine structure of parallel fluctuations, quantitatively describes the prevalence of a turbulent energy in perpendicular ($k_\perp$) scales over energy contained in propagating parallel ($k_\parallel$) to the local magnetic field perturbations. The radial dependence of the turbulent spectral break is also predicted rather well. [Preview Abstract] |
Thursday, November 19, 2015 10:06AM - 10:18AM |
TO5.00004: Firehose, Mirror, and Magnetorotational Instabilities in a Collisionless Shearing Plasma Matthew Kunz, Alexander Schekochihin, James Stone, Scott Melville, Eliot Quataert Describing the large-scale behavior of weakly collisional magnetized plasmas, such as the solar wind, black-hole accretion flows, or the intracluster medium of galaxy clusters, necessitates a detailed understanding of the kinetic-scale physics governing the dynamics of magnetic fields and the transport of momentum and heat. This physics is complicated by the fact that such plasmas are expected to exhibit particle distribution functions with unequal thermal pressures in the directions parallel and perpendicular to the local magnetic field. This pressure anisotropy can trigger fast Larmor-scale instabilities -- namely, firehose and mirror -- which solar-wind observations suggest to be effective at regulating the pressure anisotropy to marginally stable levels. Results from weakly nonlinear theory and hybrid-kinetic particle-in-cell simulations that address how marginal stability is achieved and maintained in a plasma whose pressure anisotropy is driven by a shearing magnetic field are presented. Fluctuation spectra and effective collisionality are highlighted. These results are placed in the context of our ongoing studies of magnetorotational turbulence in collisionless astrophysical accretion disks, in which microscale plasma instabilities regulate angular-momentum transport. [Preview Abstract] |
Thursday, November 19, 2015 10:18AM - 10:30AM |
TO5.00005: Vlasov Simulations of Ionospheric Turbulence near the Upper Hybrid Layer Amir Najmi, Bengt Eliasson, Xi Shao, Gennady Milikh, Surja Sharma, Konstantinos Papadopoulos High-frequency, ordinary (O) mode electromagnetic waves incident on a magnetized plasma near the upper hybrid resonance can excite magnetic field aligned density striations, associated with both turbulence and electron heating. We have used Vlasov simulations, which combine low noise and high resolution of all areas of phase space, in one spatial and two velocity dimensions to study the induced turbulence in the presence of striations near the upper hybrid resonance, where the O-mode pump is mode converted to large amplitude upper hybrid oscillations trapped in a striation. By taking moments of the resulting electron and ion distribution functions, we were able to correlate the evolution of stationary electron and ion oscillations with the onset of turbulence, and the heating of electrons in the striation with large amplitude, short wavelength electron Bernstein waves. These Bernstein waves excite stochastic electron heating when the normalized gradients of their electric field exceed the electron gyroradius, breaking the drift approximation, and causing particle orbits in phase space to diverge exponentially, rapidly increasing the electron temperature by several thousand Kelvin. These results are relevant to ongoing high-latitude heating experiments. [Preview Abstract] |
Thursday, November 19, 2015 10:30AM - 10:42AM |
TO5.00006: Energy transfers in large-scale and small-scale dynamos Mahendra Verma, Ravi Samtaney, Rohit Kumar We present the energy transfers, mainly energy fluxes and shell-to-shell energy transfers in small-scale dynamo (SSD) and large-scale dynamo (LSD) using numerical simulations of MHD turbulence for Pm = 20 (SSD) and for Pm = 0.2 on $1024^3$ grid. For SSD, we demonstrate that the magnetic energy growth is caused by nonlocal energy transfers from the large-scale or forcing-scale velocity field to small-scale magnetic field. The peak of these energy transfers move towards lower wavenumbers as dynamo evolves, which is the reason for the growth of the magnetic fields at the large scales. The energy transfers U2U (velocity to velocity) and B2B (magnetic to magnetic) are forward and local. For LSD, we show that the magnetic energy growth takes place via energy transfers from large-scale velocity field to large-scale magnetic field. We observe forward U2U and B2B energy flux, similar to SSD.\footnote{R. Kumar, M. K. Verma, and R. Samtaney, EPL, {\bf 104}, 54001 (2013); J. Turbulence, {\bf 16}, 1114, (2015).} [Preview Abstract] |
Thursday, November 19, 2015 10:42AM - 10:54AM |
TO5.00007: Nonlinear growth of electron holes in cross-field wakes Ian Hutchinson, C.B. Haakonsen, C. Zhou Cross-field plasma flow past an obstacle is key to the physics underlying Mach-probes, space-craft charging, and the wakes of non-magnetic bodies: the solar-wind wake of the moon is a typical example. We report associated new nonlinear instability mechanisms. Ions are accelerated along the $B$-field into the wake, forming two beams, but they are not initially unstable to ion two-stream instabilities. Electron Langmuir waves become unstable much earlier because of an electron velocity-distribution distortion called the ``dimple''. The magnetic field, perpendicular to the flow, defines the 1-D direction of particle dynamics. In high-fidelity PIC simulations at realistic mass ratio, small electron holes --- non-linearly self-binding electron density deficits --- are spawned by the dimple in $f_e(v)$ near the phase-space separatrix. Most holes accelerate rapidly out of the wake, along $B$. However, some remain at very low speed, and grow until they are large enough to disrupt the two ion-streams, well before the ions are themselves linearly unstable. This non-linear hole growth is caused by the same mechanism that causes the dimple: cross-field drift from a lower to a higher density. Related mechanisms cause plasma near magnetized Langmuir probes to be unsteady. [Preview Abstract] |
Thursday, November 19, 2015 10:54AM - 11:06AM |
TO5.00008: CPIC: A Parallel Particle-In-Cell Code for Studying Spacecraft Charging Collin Meierbachtol, Gian Luca Delzanno, David Moulton, Louis Vernon CPIC is a three-dimensional electrostatic particle-in-cell code designed for use with curvilinear meshes [1]. One of its primary objectives is to aid in studying spacecraft charging in the magnetosphere. CPIC maintains near-optimal computational performance and scaling thanks to a mapped logical mesh field solver [2], and a hybrid physical-logical space particle mover (avoiding the need to track particles). CPIC is written for parallel execution, utilizing a combination of both OpenMP threading and MPI distributed memory. New capabilities are being actively developed and added to CPIC, including the ability to handle multi-block curvilinear mesh structures. Verification results comparing CPIC to analytic test problems will be provided. Particular emphasis will be placed on the charging and shielding of a sphere-in-plasma system. Simulated charging results of representative spacecraft geometries will also be presented. Finally, its performance capabilities will be demonstrated through parallel scaling data. \\[4pt] [1] G.L. Delzanno, et al., ``CPIC: A Curvilinear Particle-In-Cell Code for Plasma-Material Interaction Studies,'' IEEE Trans. Plas. Sci., 41 (12), 3577 (2013).\\[0pt] [2] J.E. Dendy, ``Black Box Multigrid,'' J. Comp. Phys., 48, 366 (1982). [Preview Abstract] |
Thursday, November 19, 2015 11:06AM - 11:18AM |
TO5.00009: A First-Principle Kinetic Theory of Meteor Plasma Formation Yakov Dimant, Meers Oppenheim Every second millions of tiny meteoroids hit the Earth from space, vast majority too small to observe visually. However, radars detect the plasma they generate and use the collected data to characterize the incoming meteoroids and the atmosphere in which they disintegrate. This diagnostics requires a detailed quantitative understanding of formation of the meteor plasma. Fast-descending meteoroids become detectable to radars after they heat due to collisions with atmospheric molecules sufficiently and start ablating. The ablated material then collides into atmospheric molecules and forms plasma around the meteoroid. Reflection of radar pulses from this plasma produces a localized signal called a head echo. Using first principles, we have developed a consistent collisional kinetic theory of the near-meteoroid plasma. This theory shows that the meteoroid plasma develops over a length-scale close to the ion mean free path with a non-Maxwellian velocity distribution. The spatial distribution of the plasma density shows significant deviations from a Gaussian law usually employed in head-echo modeling. This analytical model will serve as a basis for more accurate quantitative interpretation of the head echo radar measurements. [Preview Abstract] |
Thursday, November 19, 2015 11:18AM - 11:30AM |
TO5.00010: Hybrid Particle Code Simulations of Mars: The Role Ionospheric Escape in Explaining Water Loss from Mars Stephen Brecht, Stephen Ledvina The results of our latest hybrid particle simulations using the HALFSHEL code are discussed. The presentation will address assorted processes that produce differing ion escape rates from Mars. The simulations investigate the role of the neutral atmosphere (Univ. of Michigan's MTGCM) in its dynamic form (neutral winds and co-rotation) in the calculation of the ionospheric loss from Mars. In addition, the effect of crustal magnetic field orientation in ion escape from Mars will be discussed. Further, the presentation addresses reasons for these differences and details of the interaction around the crustal magnetic fields. Finally, these results and others will be compared to fits to data. The estimated loss rates from a variety of missions and times were fit to the solar EUV flux. Our results will be compared to this fit. [Preview Abstract] |
Thursday, November 19, 2015 11:30AM - 11:42AM |
TO5.00011: A study of full particle orbit effects in stochastic magnetic fields Shun Ogawa, Benjamin Cambon, Xavier Leoncini, Diego del-Castillo Negrete, Michel Vittot, Guilhem Dif-Pradalier, Xavier Garbet Full orbit effects of charged particle motion in a stochastic magnetic field are investigated. Particles move following the Lorentz force in a prescribed static magnetic field with no electric field in a cylinder with periodic boundary condition. The magnetic field model consists of the perturbation of equilibrium fields with monotonic and reversed shear q-profiles. Unlike the gyrokinetic theory, the adiabatic invariance of the magnetic momentum is not assumed, and the full Hamiltonian equations of motion are numerically integrated by using a symplectic method. Contrary to the simpler case of magnetic field line tracing, the dynamical properties of full orbit is not easily straightforward. To address this issue, we propose a method to construct reduced Poincar\'e plots from the full particle trajectory in three-dimensional space. This diagnostic is used to clarify the nontrivial relationship between the integrability and stochasticity of field lines and particle orbits. A problem of particular interest is the study of finite Larmor radius effects on the stochasticity and the topology of orbits. [Preview Abstract] |
Thursday, November 19, 2015 11:42AM - 11:54AM |
TO5.00012: Surfatron Acceleration of Charged Particles in the Presence of Electromagnetic Fluctuations Dmitri Vainchtein, Africa Ruiz Mora In the present talk we discuss the properties of the surfatron acceleration of charged particles in the presence of random high-frequency fluctuations of the background magnetic field. We show that fluctuations significantly affect both capture into resonance, by forcing particles to escape from the surfatron resonance and thus altering the resulting energy spectrum of particles. Using the Probability distribution function approach, we compute the final energy distribution of particles as a function of the strength of the background magnetic field, the properties of the wave and the statistical properties of the fluctuations. [Preview Abstract] |
Thursday, November 19, 2015 11:54AM - 12:06PM |
TO5.00013: Fractional diffusion by Levy stochastic motion of charged particles in the presence of a magnetic field Sara Moradi, Diego Del-Castillo Negrete The motion of charged particles in the presence of alpha-stable Levy noise in a constant external magnetic field and linear friction is studied via Monte Carlo numerical simulations. The Levy noise is introduced to model the effect of non-local transport due to fractional diffusion in velocity space. The statistical properties of the velocity moments and energy for various values of the Levy index $\alpha $ are investigated. Of particular interest is the study of the resulting non-Maxwellian particle distribution functions and their dependence on alpha, the magnetic field amplitude, and the friction. We also explore the role of asymmetric Levy noise, the interplay of regular and fractional diffusion, and compute the statistical moments of displacements. [Preview Abstract] |
Thursday, November 19, 2015 12:06PM - 12:18PM |
TO5.00014: Langevin model of crossover in multiscale fluctuations: Substorm time-scales in Earth's magnetosphere A. Surjalal Sharma, Venkat Anurag Setty Multiscale fluctuations are usually characterized by a power law with a scaling exponent but many systems require more than one exponent and thus exhibit crossover behavior. The scaling exponents, such as Hurst exponents, represent the nature of correlation in the system and the crossover shows the presence of more than one type of correlation. An accurate characterization of the crossover behavior is thus needed for a better understanding of the inherent correlations in the system. A multi-step process is developed for accurate computation of the crossover behavior. First the detrended fluctuation analysis is used to remove the trends in the data and the scaling exponents are computed. The crossover point is then computed by a Hyperbolic regression technique, with no prior assumptions. The time series data of the magnetic field variations in the Earth's magnetosphere is analyzed with these techniques and yields a crossover behavior with a time scale of $\sim$ 4 hrs. A Langevin model of the magnetospheric dynamics yields an excellent fit to the crossover in the scaling exponents and thus provide a good model of magnetospheric dynamics. [Preview Abstract] |
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