Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session JI01: Astrophysical/Space Plasmas ILive Streamed
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Chair: Maxim Lyutikov, Purdue Univ. Room: Ballroom 100 A |
Tuesday, October 18, 2022 2:00PM - 2:30PM |
JI01.00001: Experimental observation of the standard magnetorotational instability in a modified Taylor-Couette cell Invited Speaker: Yin Wang The standard magnetorotational instability (SMRI) is a promising mechanism for turbulence and rapid accretion in astrophysical disks. It is a magnetohydrodynamic (MHD) instability that destabilizes otherwise hydrodynamically stable disk flow. Unlike other fundamental plasma processes such as Alfvén waves and magnetic reconnections which have been subsequently detected in space and in the laboratory, SMRI remains unconfirmed since its proposal, despite its its astrophysical importance. Its direct detection has been hindered in observations due to its microscopic nature at astronomical distances and stringent requirements in laboratory experiments. In this talk, I will present direct evidence showing that SMRI indeed exists in a novel laboratory setup, where a uniform magnetic field is imposed along the rotation axis of a differentially rotating liquid-metal flow confined radially between two coaxial cylinders and axially by copper endrings. Through in situ measurement of the radial magnetic field Br at different azimuths of the inner cylinder, onset of the axisymmetric SMRI is identified from the nonlinear increase of Br beyond a critical magnetic Reynolds number. The SMRI is found to be accompanied by a nonaxisymmetric MHD instability, which has an exponential growth at its onset and a dominant m=1 mode in the azimuthal direction. Further analysis suggests that the nonaxisymmetric instability is unlikely to be the conventional hydrodynamic instability or the Stewartson-Shercliff layer instability. The experimental results are reproduced by nonlinear three-dimensional numerical simulations, which further show that SMRI causes the velocity and magnetic fields to contribute an outward flux of axial angular momentum in the bulk region, just as it should in an accretion disk. |
Tuesday, October 18, 2022 2:30PM - 3:00PM |
JI01.00002: Violation of the zeroth law of turbulence through the helicity barrier effect Invited Speaker: Romain Meyrand In the standard picture of plasma turbulence, energy injected at large scales cascades conservatively to small scales, eventually thermalizing to heat particles via a variety of mechanisms. This leads to the "zeroth law of turbulence," whereby the macroscopic statistics of the flow and magnetic field are independent of the details of how energy is microscopically dissipated into heat. The helicity barrier is a newly discovered effect that upends this notion for low-beta imbalanced Alfvenic turbulence, as occurs in the solar wind and corona. Because of a conserved generalized helicity invariant, the energy cascade is "stuck" around the ion gyroscale, which causes the turbulent energy to build up in time (if it is continuously forced). The outcome is quite dramatic for the solar wind and for plasma heating in general because it changes the thermodynamics properties of the plasma based on how it is stirred at large scales. The effect explains a number of long-standing puzzles from in-situ solar-wind observations, including the steep "transition range" seen in magnetic spectra, magnetic helicity signatures, and ion distribution functions. It also links two previously well-studied coronal heating mechanisms (turbulence and ion-cyclotron waves), suggesting an important role of the helicity barrier in the coronal heating problem. More generally, it demonstrates how the complex, nonlinear microphysics of collisionless plasmas can have a strong influence on the macroscopic properties of astrophysical processes. |
Tuesday, October 18, 2022 3:00PM - 3:30PM |
JI01.00003: Are There Waves in Magnetized Compressible Turbulence in Space and Astrophysical Plasmas Invited Speaker: Hui Li Turbulence is ubiquitous throughout magnetized plasmas in the Universe. Past research has made great progress on understanding the nature of MHD turbulence, including its anisotropy and its impact on regulating the transport of energetic particles. When the plasma beta is low and turbulent Mach number becomes large, turbulence becomes compressible, characterized by enhanced density fluctuations. Using extensive 3D MHD simulations and the 4D (spatio-temporal) FFT analysis, the nature of compressible MHD turbulence has been examined in detail (Gan, Li, et al. 2022, ApJ, 926:222). Two approaches are used to determine the presence of eigenwave modes – Alfven, Fast and Slow waves, namely the mode decomposition based on spatial variations only and the spatio-temporal 4D FFT analysis of all fluctuations. The latter method enables us to quantify fluctuations that satisfy the dispersion relation of Alfven and compressible modes with finite frequency. Overall, the fraction of fast modes identified via the spatio-temporal 4D FFT approach in total fluctuation power is either tiny with nearly incompressible driving or ∼2% with highly compressible driving. The majority of the turbulence energy resides in the low frequency region with large perpendicular wavenumber. In addition, we will present similar analysis of kinetic simulations of compressible turbulence. The transition from the wave turbulence to strong turbulence in the spatio-temporal domain will also be discussed. These results could have significant implications for understanding the compressible fluctuations in space and astrophysical plasmas, and its impact for heating and accelerating charged particles in such compressible turbulence. |
Tuesday, October 18, 2022 3:30PM - 4:00PM |
JI01.00004: A New Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow Turbulence Invited Speaker: Bindesh Tripathi Straining of magnetic fields by shear flow is widely recognized to generate small-scale magnetohydrodynamic (MHD) turbulence. In astrophysics, where shear flows are ubiquitous, this is at odds with observations of large-scale magnetic fields in stars, galaxies, and beyond. Identified here is a previously unknown mechanism that sequesters magnetic energy at large scales when the flows are unstable. |
Tuesday, October 18, 2022 4:00PM - 4:30PM |
JI01.00005: Enhancement of the non-resonant streaming instability by particle collisions Invited Speaker: Alexis Marret Cosmic rays can power the growth of a seed magnetic field by exciting a non-resonant insta- bility, which can help confine and accelerate high energy particles in supernovae remnants and young stellar jets shocks. This work aims at elucidating the behaviour of the non-resonant cos- mic rays streaming instability in cold and denser environments, such as H II regions and molec- ular clouds, where particle collisions in the background plasma must be taken into account. We investigate theoretically and numerically their impact by including Monte-Carlo Coulomb and neutral collisions in the simulations. We find that in poorly ionized plasmas, where proton- hydrogen collisions dominate, the instability is rapidly suppressed, confirming quantitatively existing multi-fluid linear theory calculations. In contrast, we find that Coulomb collisions in fully ionized plasmas unexpectedly favour the development of the instability by reducing self- generated pressure anisotropies that would otherwise oppose its growth [1]. |
Tuesday, October 18, 2022 4:30PM - 5:00PM |
JI01.00006: Thomas H. Stix Award for Outstanding Early Career Contributions to Plasma Physics Research: Weakly Collisional Heating and Turbulence in Astrophysical Plasmas Invited Speaker: Jonathan Squire Many astrophysical plasmas are weakly collisional, with characteristic macroscopic timescales that rival interparticle collision times. When stirred up by gravitational or magnetic forces, creating turbulence, this leads to rich and interesting plasma physics, with the plasma’s bulk properties – for instance, the relative heating of ions and electrons, or heat and momentum transport – depending on details of the regime and even the stirring mechanism. In turn, changing these bulk properties can completely alter the system’s macroscopic evolution, determining, for example, how the solar wind is accelerated outwards from the sun or the fate of matter falling into a black hole. |
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