Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session DI3: Space and Astrophysical Plasmas; Laboratory Astrophysics |
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Chair: Steve Spangler, University of Iowa Room: OCC Oregon Ballroom 204 |
Monday, November 5, 2018 3:00PM - 3:30PM |
DI3.00001: MMS Observations of Electron Magnetic Reconnection without Ion Coupling in the Turbulent Magnetosheath Invited Speaker: Tai Phan In the standard model of magnetic reconnection, the process occurs in a minuscule electron-scale diffusion region. On larger scales, the ions couple to the newly-reconnected field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed. Much of the magnetic energy is converted into ion jetting and heating in spatially extended ion exhausts. In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to play a major role in the dissipation of turbulent energy at kinetic scales. In this talk, I will describe MMS observations of reconnection which involves only electrons in the turbulent magnetosheath region downstream of Earth’s quasi-parallel shocks. These observations reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling. |
Monday, November 5, 2018 3:30PM - 4:00PM |
DI3.00002: Quantifying heating by magnetic pumping through in situ spacecraft observations Invited Speaker: Emily R Lichko Superthermal electrons and ions in power-law tails are observed throughout the universe in a variety of astrophysical systems, but how these particles are energized is an open question. It is well known that plasma can be heated by waves, but most theories of particle energization are based on wave-particle resonances which are only effective at particle velocities near the phase velocity of the wave, v ~ ω/k. Starting from the drift kinetic equation, we have derived a magnetic pumping model, similar to the magnetic pumping well known in fusion research, where particles are heated by the largest scale turbulent fluctuations. We have shown that this is a complementary heating mechanism to the turbulent cascade, effective up to v ≤ ω/k, which results in power-law distributions like those observed in the solar wind [1]. However, compressional Alfvenic turbulence has the ability to magnetically trap superthermal particles. Magnetic trapping renders magnetic pumping an effective Fermi heating process for particles with v >> ω/k, and produces superthermal power-law distributions. To test this, we used satellite observations of the strong, compressional magnetic fluctuations near the Earth's bow shock from the Magnetospheric Multiscale (MMS) mission and found strong agreement with our model. Given the ubiquity of such fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated. [1] E. Lichko, J. Egedal, W. Daughton, and J. Kasper. Astrophys. J. Lett. 2, 850 (2017) |
Monday, November 5, 2018 4:00PM - 4:30PM |
DI3.00003: Creation of a Hall-MHD Parker Spiral in the Lab Invited Speaker: Ethan E Peterson When magnetically-dominated plasma co-rotating with the Sun expands outward it transitions to flow-dominated as the magnetic field strength drops. At that point it can stretch, bend, and break the dipolar fieldlines of the Sun - carrying a spiraling magnetic field out into the solar system known as the Parker Spiral. The evolution of plasma through this transitional region where MA ≈ 1 (known as the Alfvén surface) has never been accessible to any in-situ satellite measurements until now (Parker Solar Probe will be the first) but is readily accessible with novel laboratory experiments on the Big Red Plasma Ball (BRB) at the Wisconsin Plasma Physics Lab. The experiment uses a SmCo permanent magnet and J x B stirring to produce a rapidly rotating magnetosphere that can give rise to the formation of an Alfvén critical point and the Parker Spiral. NIMROD MHD simulations performed with experimental parameters and current injection confirm the production of a rotating magnetosphere, radial wind, Alfvén critical point, and Parker Spiral as well as axisymmetric plasmoids in the magnetospheric current sheet. 2D maps of magnetic data taken from a three-axis hall sensor array during the experiment show clear formation of an Alfvén surface, a Syrovatskii layer, and a magnetic Parker Spiral. The Parker spiral current sheet region is quite dynamic with coherent magnetic and density fluctuations that are consistent with plasmoid ejection. Experimental magnetic data agrees very well with NIMROD MHD simulations but significant discrepancies arise between the simulated ion flow and experimental measurements that implicate the important role of two-fluid effects in this Hall-dominated experimental Parker Spiral. |
Monday, November 5, 2018 4:30PM - 5:00PM |
DI3.00004: Efficient non-thermal particle acceleration mediated by the current-driven kink instability in jets Invited Speaker: E. Paulo Alves Astrophysical jets shine across the entire electromagnetic spectrum and are among the most powerful particle accelerators in the universe. Yet, the dominant mechanisms underlying their particle acceleration are not well understood. Global magnetohydrodynamic simulations suggest that the development of the current-driven kink instability (KI) can play an important role in the dissipation of the jet’s internal magnetic field near recollimation regions, but it remains unclear if such process could lead to efficient non-thermal particle acceleration. We have performed large-scale 3D particle-in-cell simulations to investigate the self-consistent particle acceleration associated with the development of the KI in conditions relevant to magnetized relativistic jets. We find that the development of the KI mediates the efficient dissipation of the magnetic field into high-energy particles. Interestingly, we find that efficient acceleration is achieved via a new acceleration mechanism, that is distinct from the commonly invoked shock and magnetic reconnection mechanisms. Non-thermal particles are accelerated by the combination of a coherent large-scale inductive electric field, that develops throughout the unstable region during the nonlinear stage of the KI, and efficient scattering in the highly tangled magnetic fields. This results in the development of a power-law energy tail that contains 50% of the initial magnetic energy and is robust for a large range of initial conditions and system sizes. In the context of the bright knots in AGN jets, such as HST-1 and Knot A in M87, we show that this mechanism can account for the spectrum of synchrotron radiating particles, and offers a viable means for accelerating ultra-high energy cosmic rays. |
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