Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session KI3: Invited: Space and Astrophysical Plasmas: Reconnection, Acceleration, and Heating |
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Chair: Fatima Ebrahimi Room: Floridian Ballroom CD |
Tuesday, October 22, 2019 3:00PM - 3:30PM |
KI3.00001: Determining the Dominant Acceleration Mechanism during Relativistic Magnetic Reconnection in Large-scale Systems Invited Speaker: Fan Guo While a growing body of research indicates that relativistic magnetic reconnection is a prodigious source of particle acceleration in high-energy astrophysical systems, the dominant acceleration mechanism remains controversial. Using a combination of fully kinetic simulations and theoretical analysis, we demonstrate that Fermi-type acceleration within the large-scale motional electric fields dominates over direct acceleration from non-ideal electric fields within small-scale diffusion regions. This result has profound implications for modeling particle acceleration in large-scale astrophysical problems, since it opens up the possibility of modeling the energetic spectra without resolving microscopic diffusion regions. [Preview Abstract] |
Tuesday, October 22, 2019 3:30PM - 4:00PM |
KI3.00002: Magnetic Reconnection in Relativistic and Semirelativistic Plasmas: Extreme Particle Acceleration and Radiation Invited Speaker: Gregory Werner Magnetic reconnection is a fundamental plasma process that rapidly converts magnetic energy to particle kinetic energy. It has been studied mostly in nonrelativistic electron-ion plasmas relevant to the solar corona, Earth's magnetosphere, and lab plasmas. However, reconnection in collisionless relativistic electron-ion and electron-positron (pair) plasmas may be important in astrophysical systems such as pulsar wind nebulae and black-hole jets, and is, moreover, more amenable to first-principles particle-in-cell (PIC) simulation. A striking consequence of relativistic reconnection is efficient nonthermal particle acceleration (NTPA). With power-law energy distributions extending to high energies, electrons (and positrons) should emit distinctive, observable synchrotron and/or inverse Compton radiation signatures. PIC simulations show that the NTPA power-law slope steepens with decreasing magnetization and guide magnetic field. NTPA also becomes less efficient in the semirelativistic regime where electrons are relativistic but ions are subrelativistic. Even in environments where accelerated particles promptly radiate away most energy gains, self-consistent radiative-PIC simulations show that reconnection still functions efficiently. In this strongly-radiative regime, the beaming of relativistic particles (hence radiation) is enhanced, potentially yielding shorter, brighter flares. While reconnection-driven NTPA has been studied in 2D simulations over a broad range of plasma conditions, a critical question is whether reconnection behaves similarly in 3D. Large 3D PIC simulations show that, in the magnetically-dominated ultrarelativistic pair regime, reconnection and NTPA are similar in 2D and 3D. However, when the magnetic energy is comparable to the plasma energy ($\beta \sim 1$), reconnection slows and is disrupted by 3D effects like the relativistic drift-kink instability; nonetheless, NTPA remains robust even as reconnection dynamics alter significantly. [Preview Abstract] |
Tuesday, October 22, 2019 4:00PM - 4:30PM |
KI3.00003: New Milestones in Comparing Experimental and Simulated Reconnection: Results from TREX and Cylindrical VPIC Invited Speaker: Samuel Greess Magnetic reconnection is studied in the Terrestrial Reconnection Experiment (TREX) under collisionless conditions relevant to the Earth's magnetosphere [1]. The thickness of the reconnection current layer normalized to electron kinetic length scales is one of the features that is most commonly used to identify different sets of reconnection dynamics. Previous studies suggest that experimental current layer widths are larger by a factor of four compared to those observed in kinetic simulations [2]. Contrary to those results, in this talk we will present results from TREX which closely match the current width scaling and geometry seen in both prior 2D laminar kinetic reconnection simulations and new 3D VPIC models that have been developed specifically to reflect the TREX geometry. The results of the newest TREX run, with an adjustable guide field and a pressure anisotropy probe, and their associated VPIC simulation outputs will also be featured. 1. Olson, J. et al. Experimental Demonstration of the Collisionless Plasmoid Instability below the Ion Kinetic Scale during Magnetic Reconnection. Phys. Rev. Lett. (2016). 2. Ji, H. et al. New insights into dissipation in the electron layer during magnetic reconnection. Geophys. Res. Lett. 35, L13106 (2008). [Preview Abstract] |
Tuesday, October 22, 2019 4:30PM - 5:00PM |
KI3.00004: Laboratory experiments to understand the coronal heating problem Invited Speaker: Sayak Bose Coronal holes are regions of the Sun's atmosphere with open magnetic field lines that extend into interplanetary space. These regions are $\approx $ 200 times hotter than the underlying photosphere. Recent observations of damping of Alfv\'{e}n waves in coronal holes suggest that a wave driven process may be responsible for the temperature rise. The mechanism of this wave damping is unknown. We have explored the effectiveness of a longitudinal gradient in Alfv\'{e}n speed in reducing the energy of propagating Alfv\'{e}n waves under conditions scaled to match those in coronal holes. The experiments were conducted in the Large Plasma Device located at the University of California, Los Angeles. Our results show that the energy of the transmitted Alfv\'{e}n wave decreases as the inhomogeneity parameter, $\lambda $/L, increases. Here, $\lambda $ is the wavelength of the Alfv\'{e}n wave and L is the scale length of Alfv\'{e}n speed gradient. For gradients similar to those in coronal holes, the waves are observed to lose a factor of $\approx $ 5 more energy than they do when propagating through a uniform plasma without a gradient. Contrary to theoretical expectations, this reduction in the energy of the transmitted wave is not accompanied by observation of a reflected wave. Nonlinear effects causing reduction in wave energy are ruled out as the amplitude of the initial wave is too small and the wave frequency well below the ion cyclotron frequency. Decrease of Alfv\'{e}n wave energy due to mode coupling is unlikely, as we have not detected any other modes. Since the total energy must be conserved, it is possible that the reduced wave energy is being deposited in the plasma. These results pertaining to coronal holes are presented. [Preview Abstract] |
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