Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session QI2: Turbulence/Waves |
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Chair: Mike Brown, Swarthmore College Room: 102ABC |
Wednesday, October 25, 2017 3:00PM - 3:30PM |
QI2.00001: Magnetorotational Turbulence and Dynamo in a Collisionless Plasma Invited Speaker: Matthew Kunz Low-luminosity black-hole accretion flows are collisionless. A kinetic approach is thus necessary to understand the transport of heat and angular momentum, the acceleration of particles, and the growth and structure of the magnetic field in these systems. I present results from the first 6D kinetic simulation of magnetorotational turbulence and dynamo, which was performed using the hybrid-kinetic particle-in-cell code Pegasus. Special attention will be paid to the transport of angular momentum by the anisotropic-pressure stress, as well as to the ion-Larmor-scale kinetic instabilities (firehose, mirror, ion-cyclotron) that regulate it. The latter endow the plasma with an effective viscosity that is biased with respect to the magnetic-field direction and spatiotemporally variable. Energy spectra suggest an Alfv\'{e}n-wave cascade at large scales and a kinetic-Alfv\'{e}n-wave cascade at small scales, with strong small-scale density fluctuations and weak nonaxisymmetric density waves. Ions undergo nonthermal particle acceleration, their distribution accurately described by a $\kappa$ distribution. Dedicated nonlinear studies of firehose and mirror instabilities in a shearing plasma will also be presented as a complement to the study of the magnetorotational instability. The profits, perils, and price of using a kinetic approach are discussed. [Preview Abstract] |
Wednesday, October 25, 2017 3:30PM - 4:00PM |
QI2.00002: Intermittency, Anisotropy and the onset of reconnection in strong Alfv\'enic turbulence Invited Speaker: Alfred Mallet On length scales larger than the ion gyroradius, the turbulence in a plasma with a strong mean magnetic field may be modelled using the equations of reduced magnetohydrodynamics, which describe the evolution of Alfv$\'e$nic fluctuations propagating up and down the magnetic field. This (strongly nonlinear) turbulence is (i) ``critically balanced" - anisotropic with respect to the direction of the local mean magnetic field, (ii) ``aligned" - vector fluctuations in the fields in the perpendicular plane point in the same direction to within a small angle, and the structures are highly sheet-like, (iii) highly intermittent - the shape of the probability distributions of many (but not all!) turbulent quantities depends on scale in a non-trivial way. I will discuss the work we have performed connecting these phenomena, and the resulting statistical model for the Alfv$\'e$nic turbulence we have developed. Finally, I will discuss recent results concerning the onset of reconnection at the small scales of Alfv$\'e$nic turbulence, and the subsequent disruption of the sheet-like turbulent structures at these scales. This dramatically affects the turbulent cascade at small scales, and may provide a resolution to recent disagreements as to the value of asymptotic spectral index of the turbulence. [Preview Abstract] |
Wednesday, October 25, 2017 4:00PM - 4:30PM |
QI2.00003: Kinetic Alfvén Wave Turbulence: New Insight from Gyrokinetics and beyond Invited Speaker: Daniel Told One of the most eminent unsolved problems in space physics is the nature of the turbulent energy dissipation at the smallest spatial scales, which is thought to explain the localized plasma heating observed in the solar wind [1]. This work focuses on new results obtained from gyrokinetic simulations of Kinetic Alfvén Wave turbulence, a major ingredient of solar wind turbulence [2]. For conditions similar to the solar wind at 1 AU, previous work [3,4] showed that electron Landau damping can become important even on ion spatial scales and is responsible for about 70% of the turbulent heating, underscoring the importance of retaining electron kinetic physics. In addition, studies of linear wave physics in various kinetic models [5,6] indicate that this dominance of electron damping may be enhanced even more in conditions of plasma beta < 1, which is characteristic of the solar wind closer to the sun. Making use of multi-scale nonlinear simulations, we shed light on how such findings carry over to nonlinear simulations for different plasma beta values. We focus in particular on characterizing the kinetic mechanisms that catalyze heating, their dependence on plasma parameters, and their relative importance to each particle species. [1] R. Bruno, V. Carbone, Living Rev. Sol. Phys. 10: 2 (2013) [2] C. H. K. Chen, S. Boldyrev, Q. Xia et al., Phys. Rev. Lett. 110, 225002 (2013) [3] D. Told, F. Jenko, J. M. TenBarge et al., Phys. Rev. Lett. 115, 025003 (2015) [4] A. Bañón Navarro, B. Teaca, D. Told et al., Phys. Rev. Lett. 117, 245101 (2016) [5] G. G. Howes, Mon. Not. R. Astron. Soc. Lett. 409 (1): L104-L108 (2010) [6] D. Told, J. Cookmeyer, F. Muller, et al., New J. Phys. 18, 065011, 2016 [Preview Abstract] |
Wednesday, October 25, 2017 4:30PM - 5:00PM |
QI2.00004: First Satellite Measurement of the ULF Wave Growth Rate in the Ion Foreshock Invited Speaker: Seth Dorfman Waves generated by accelerated particles are important throughout our heliosphere. These particles often gain their energy at shocks via Fermi acceleration. At the Earth's bow shock, this mechanism accelerates ion beams back into the solar wind; the beams can then generate ultra low frequency (ULF) waves via an ion-ion right hand resonant instability. These waves influence the shock structure and particle acceleration, lead to coherent structures in the magnetosheath, and are ideal for non-linear interaction studies relevant to turbulence.\newline \newline We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock [1]. This is made possible by employing the two ARTEMIS spacecraft orbiting the moon at $\sim 60$ Earth radii from Earth to characterize crescent-shaped reflected ion beams and relatively monochromatic ULF waves. The event to be presented features spacecraft separation of $\sim 2.5$ Earth radii ($0.9\pm0.1$ wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. By contrast, most prior ULF wave observations use spacecraft with insufficient separation to see wave growth and are so close to Earth (within $\sim 30$ Earth radii) that waves convected from different events interfere.\newline \newline Using ARTEMIS data, the ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results not only advance our understanding of the foreshock, but will also inform future nonlinear studies related to turbulence and dissipation in the heliosphere.\newline \newline [1] S. Dorfman, H. Hietala, P. Astfalk, and V. Angelopoulos, Geophys. Res. Let. 44 (2017). [Preview Abstract] |
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