Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session GI01: Astro/Space I: Space and Astrophysical Plasma PhenomenaInvited Live
|
Hide Abstracts |
Chair: Bill Amatucci, Naval Research Laboratory Room: Ballroom B |
Tuesday, November 9, 2021 9:30AM - 10:00AM |
GI01.00001: Turbulence, waves, and thermodynamics in expanding, collisionless plasma Invited Speaker: Archie Bott The plasma composing many different astrophysical systems of interest – from the solar wind to the intracluster medium of galaxy clusters – is often weakly collisional or collisionless, with the Larmor radii of the constituent particles being many orders of magnitude below their Coulomb mean free paths. This feature results in a complex interplay between a plasma's macrophysical evolution (e.g., due to expansion, compression, or large-scale shear) and its microphysical response (e.g., triggering of kinetic instabilities). In this talk, the results of several hybrid-kinetic simulations that elucidate this phenomenon will be presented. We show how the nonlinear dynamics of strong Alfvénic turbulence in a collisionless plasma efficiently adapts to changes in fundamental wave physics that are induced by the effect of macroscopic expansion on microscopic particle motions. This adaptation holds irrespective of a qualitative transformation to the plasma’s thermodynamics caused by pressure-anisotropy-driven kinetic instabilities. We also demonstrate that different rates of expansion can lead to qualitatively distinct thermodynamic states, with dramatic ramifications for both macroscopic wave and turbulent dynamics. Our results may help to disentangle the signatures of kinetic instabilities and strong Alfvénic turbulence in key observables in the near-Earth solar wind, such as magnetic power spectra and ion velocity distribution functions. |
Tuesday, November 9, 2021 10:00AM - 10:30AM |
GI01.00002: Structure and Dynamics of a Compressed Current Sheet in the Earth's Magnetotail Invited Speaker: Ami M DuBois When a current sheet with a reversed magnetic field configuration is compressed to kinetic scales during geomagnetically active periods, in situ observations show evidence of intense lower hybrid wave activity, magnetic reconnection, and substorm onset. The waves are often attributed to the density gradient driven lower hybrid drift instability. However, studies have shown that velocity-shear can intensify due to plasma compression and drive broadband turbulence peaking at the lower hybrid frequency. Nonetheless, velocity shear-driven waves have largely been overlooked in relation to compressed current sheets in the magnetotail despite evidence that they play important roles in ion heating, acceleration, transport, and other anomalous dissipation processes. We use Magnetospheric Multi-Scale satellite data to analyze kinetic scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Additionally, a new kinetic equilibrium model shows an ambipolar electric field forms self-consistently and intensifies as a large scale Harris current sheet is compressed. This produces velocity shear in the current sheet near the magnetic null, indicating that velocity shear-driven waves can arise in thinning current sheets and provide anomalous dissipation to trigger the reconnection process. In addition, the distribution function becomes non-gyrotropic, which explains the observation of crescent shaped distributions, temperature gradients, and the formation of substructures in the current sheet such as embedded and bifurcated current sheets. |
Tuesday, November 9, 2021 10:30AM - 11:00AM |
GI01.00003: The Alfvén-wave acceleration of auroral electrons: laboratory measurement Invited Speaker: Jim Schroeder Since the 1970’s, Alfvén waves have been predicted to accelerate auroral electrons. Spacecraft measurements have shown that powerful descending Alfvén waves are common above the bright and active discrete auroras of geomagnetic storms. Theoretical and computational work has suggested that auroral electrons can be accelerated by Alfvén waves via Landau resonance. A direct test requires simultaneous measurements of electrons and the wave fields responsible for their acceleration. Spacecraft data have not provided such a direct test. Laboratory measurements using UCLA’s Large Plasma Device (LAPD) avoid many of the limitations of spacecraft data and offer increased control and repeatability. To produce conditions relevant to an altitude of 1-3 Earth radii where electron acceleration occurs, the LAPD is tuned so vA>vte. Inertial Alfvén waves are launched by an antenna, and the electron velocity distribution is measured using the resonant absorption of a small amplitude whistler-mode wave. Data are analyzed using the field-particle correlation technique. Experimental results show that electrons near the Alfvén wave phase velocity gain energy from the Alfvén wave, indicating electron acceleration by Landau resonance. The field-particle correlation produced by analytical kinetic theory agrees with experimental results. Two additional theoretical approaches are consistent with experimental results including Liouville mapping of a test-particle distribution through simulated Alfvén wave fields and a nonlinear gyrokinetic simulation. The agreement of direct measurements with theory and simulation shows for the first time that Alfvén waves can accelerate electrons that cause the aurora. |
Tuesday, November 9, 2021 11:00AM - 11:30AM |
GI01.00004: Generation of residual energy by many interacting Alfvén waves Invited Speaker: Seth E Dorfman Counter-propagating Alfvén wave interactions which transfer energy from large to small spacial scales lie at the heart of astrophysical plasma turbulence. An unexpected feature of the turbulence is the generation of residual energy – excess energy in the magnetic fluctuations compared to the velocity fluctuations. By contrast, an MHD Alfvén wave has equal amounts of energy in fluctuations of each type. Howes, et. al. 2013 showed that purely magnetic fluctuations develop in non-linear interactions and suggested that this may explain residual energy generation. |
Tuesday, November 9, 2021 11:30AM - 12:00PM |
GI01.00005: Kinetic physics in the solar wind: local processes and global consequences Invited Speaker: Maria Elena Innocenti Parker Solar Probe and Solar Orbiter observations have confirmed that kinetic scale processes are ubiquitous in the solar wind. The spatial and temporal scales of kinetic instabilities are smaller and shorter than system scales by several orders of magnitudes. However, they contribute to shape large-scale solar wind dynamics. |
Tuesday, November 9, 2021 12:00PM - 12:30PM |
GI01.00006: Kinetic model of pair discharges in pulsar polar caps Invited Speaker: Fabio Cruz Time-dependent discharges of electron-positron pairs have recently been proposed as a primary ingredient to explain the nature of pulsar radio emission, a longstanding open problem in high-energy astrophysics. During these discharges - positive feedback loops of gamma-ray photon emission via curvature radiation by TeV electrons and positrons and pair production -, the plasma self-consistently develops inductive waves that couple to electromagnetic modes capable of escaping the pulsar dense plasma. In this work, we present an analytical description of pair cascades relevant in pulsars, including their onset, exponential growth and saturation stages. We show that the plasma is set to inductively oscillate at the relativistic plasma frequency after the cascade saturates. These oscillations are found to be unstable to a new kinetic instability, that converts the energy in the inductive oscillations into an infinite number of electrostatic modes. All analytical results are illustrated with particle-in-cell simulations performed with OSIRIS. These results are used to interpret new multidimensional simulations including an ab initio description of the Quantum Electrodynamics (QED) effects responsible for hard photon emission and pair production in pair discharges. It is shown that the electromagnetic modes generated during pair discharges present direct imprints of QED and plasma kinetic effects in properties (e.g. frequency, polarisation and Poynting flux angular distribution) that are consistent with observations. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700