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 JO06: Magnetospheric PhysicsOn Demand
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Chair: Earl Scime, West Virginia University Room: Rooms 310-311 |
Tuesday, November 9, 2021 2:00PM - 2:12PM |
JO06.00001: Machine learning algorithms for detection and classification of plasma structures in multiple-X-line collisionless reconnection regions Kendra A Bergstedt, Hantao Ji, Jonathan M Jara-Almonte A crucial component of magnetic reconnection research is the analysis of in-situ data from spacecraft that study naturally occurring reconnection regions such as Earth's magnetotail. However, spacecraft can only sample a single point in space for each timestep, and trace a 1D path through the plasma. This limitation makes detection and identification of dynamic plasma structures difficult. Correctly identifying structures in multiple-X-line reconnection regions is crucial for understanding the physics of the coupling of the microscale to the macroscale, such as the potential role that the plasmoid instability plays in reconnection dynamics and energy transfer. Previous work investigating this physics used simple hand-tuned algorithms for detection and classification (Bergstedt et al. 2020). This work develops a more nuanced and robust classification algorithm which utilizes a set of simulated 'spacecraft' trajectories through 2D PIC simulations of reconnection to train a machine learning model to identify regions of data corresponding to plasmoids and current sheets. A range of models from Random Forest Classifiers to Convolutional and Recurrent Neural Networks are implemented, and their efficacies are compared. |
Tuesday, November 9, 2021 2:12PM - 2:24PM |
JO06.00002: Global Asymmetry of Hot Flow Anomalies Yu Lin, Xueyi Wang, Sun-Hee Lee, David G Sibeck We conduct a 3-D global hybrid simulation for the interaction of the Earth’s bow shock with a solar wind directional tangential discontinuity (TD) whose normal lies in the equatorial (z=0) plane. Although the convection electric field points into the TD from one side of the discontinuity on both sides of the equator, the interaction only generates an HFA in the southern hemisphere. HFA generation results from bow shock-reflected ions energized by the inward-pointing electric field on one side of the TD and inward ion gyration on the other side. The latter phenomenon only occurs south of the equator due to the global bow shock geometry. The global HFA asymmetry is driven by a reversal in the north-south component of the ion velocities at the bow shock in the two hemispheres, such that the ions gyrate into (away from) the TD in the southern (northern) hemisphere. Our results indicate that HFAs must generally exhibit north-south and dawn-dusk asymmetries. |
Tuesday, November 9, 2021 2:24PM - 2:36PM |
JO06.00003: Exact solutions of the Vlasov-Maxwell system of equations and a two-scale structure of a magnetic dip Naoki Bessho, Jason Shuster, Shan Wang, Li-Jen Chen In the Earth's magnetosphere, NASA's Magnetospheric Multiscale (MMS) has been observing kinetic structures with current sheets, including magnetic dip structures. Nongyrotropic electrons are often detected, and a typical thickness is several electron skin depths. To explain those observations, we study the Vlasov-Maxwell system of equations considering nongyrotropy of particles. We find exact one-dimensional solutions for a magnetic dip, where the nongyrotropy produces electric currents perpendicular to the magnetic field. When we impose zero electric potential, the solutions become the same as obtained by Nicholson [1963], in which the electron nongyrotropy is negligibly small, and the thickness of the magnetic dip is several skin depths. Introducing a finite electric potential allows us to use a large electron nongyrotropy. When the electron nongyrotropy is large and ions are gyrotropic, the thickness of a magnetic dip becomes several electron skin depths. In contrast, when both ions and electrons show large nongyrotropy, the magnetic dip shows two scales: The inner region shows an electron-scale thin magnetic dip, which is surrounded by the outer region of an ion-scale thick magnetic dip. We compare these solutions with a magnetic dip structure observed by MMS. |
Tuesday, November 9, 2021 2:36PM - 2:48PM |
JO06.00004: Collisionless relaxation of a disequilibrated current sheet and implications for bifurcated structures Young Dae Yoon, Gunsu S Yun, Deirdre E Wendel, James L Burch Current sheets are ubiquitous plasma structures that are important in magnetospheric contexts. Although numerous current sheet equilibrium solutions have been found, the process in which an initially disequilibrated sheet collisionlessly relaxes or equilibrates remains unknown. It is first shown that particle orbits in a sheared magnetic field profile can be classified into four orbit classes. Phase-space distributions of the classes are then analyzed and compared to particle-in-cell simulations. It is found that a current sheet equilibrates through collisionless transitions among the orbit classes. Bifurcated current sheets, which are frequently observed in geospace but whose origins have been elusive, naturally arise from this process. It is thus suggested that current sheet equilibration is responsible for such bifurcated structures; comparisons of particle-in-cell simulations to spacecraft observations support this fact. |
Tuesday, November 9, 2021 2:48PM - 3:00PM |
JO06.00005: Laboratory Studies of Laser-Driven, Ion-Scale Magnetospheres on the Large Plasma Device Derek B Schaeffer, Fabio Cruz, Robert S Dorst, Filipe D Cruz, Peter V Heuer, Carmen G Constantin, Patrick Pribyl, Christoph Niemann, Luis O Silva, Amitava Bhattacharjee Ion-scale magnetospheres have been observed around comets, weakly-magnetized asteroids, and localized regions on the Moon. These mini-magnetospheres provide a unique environment to study kinetic-scale plasma physics, in particular in the collisionless regime, but are difficult to study directly. Laboratory experiments can thus provide a controlled and reproducible platform for understanding fundamental magnetospheric physics and helping validate models of larger, planetary magnetospheres. In this work, we present preliminary experiments of ion-scale magnetospheres performed on a unique high-repetition-rate (1 Hz) platform developed for the Large Plasma Device (LAPD) at UCLA. The experiments utilize a high-repetition-rate laser to drive a super-Alfvénic plasma flow into a pulsed dipole magnetic field embedded in a uniform background magnetic field. 2D maps of magnetic field with high spatial and temporal resolution are measured with magnetic flux probes and examine the evolution of local and global magnetosphere and current density structures for a range of dipole and upstream parameters. The results are compared to PIC simulations to further identify the magnetospheric structure, kinetic-scale structures of the plasma current distribution, and dynamics of the laser-driven plasma. |
Tuesday, November 9, 2021 3:00PM - 3:12PM |
JO06.00006: Particle-in-cell simulations of laser-driven, ion-scale magnetospheres in laboratory plasmas Filipe D Cruz, Derek B Schaeffer, Fabio Cruz, Luis O Silva Ion-scale "mini" magnetospheres provide a unique environment for studying kinetic-scale plasma physics, and have been observed around comets, weakly-magnetized asteroids, and localized regions of the moon. In this work, we present collisionless particle-in-cell (PIC) simulations of ion-scale magnetospheres that reproduce recent laboratory experiments performed on the Large Plasma Device (LAPD) at UCLA. In the PIC simulations, a super-Alfvènic plasma flow is driven into a dipole magnetic field that is embedded in a uniform background magnetic field. The simulations show how the magnetosphere structures evolves, the location of the magnetopause, and kinetic-scale structures of the plasma current distribution. Different dipole and upstream plasma parameters are exploited to interpret their effect on these systems. The coupling between the driver and background plasmas is characterized using both simulations and semi-analytical models. |
Tuesday, November 9, 2021 3:12PM - 3:24PM |
JO06.00007: Numerical modeling of LAPD experiments of auroral electron acceleration Gregory G Howes, Jim Schroeder, Fred N Skiff, Craig A Kletzing, Troy A Carter, Stephen T Vincena, Seth E Dorfman The detailed chain of physical effects leading to the glowing of the aurora represents one of the longest standing, unanswered questions in space physics. The mechanism in the auroral magnetosphere by which electrons are accelerated downward toward the ionosphere below is a critical step that remains incompletely understood. Over the last two decades, an ongoing experimental project at the Large Plasma Device (LAPD) has sought to measure the acceleration of electrons by inertial Alfven waves under plasma conditions relevant to the auroral magnetosphere. To understand how electrons are energized by Alfven waves in the experiment, we use the recently devised field-particle correlation technique to determine the electron energization by the parallel component of the electric field as a function of the parallel electron velocity. This new analysis method generates a velocity-space signature that can be used to identify Landau-resonant acceleration. Here we present a field-particle correlation analysis of both self-consistent gyrokinetic numerical simulations and non-self-consistent Vlasov mapping of the electron velocity distribution through the Alfven wave fields, providing a means to interpret the velocity-space signature of electron energization arising from the experimental measurements. Comparison to results from our most recent LAPD experimental campaign demonstrates the we have definitively confirmed the hypothesis that Alfven waves can accelerate auroral electrons under plasma conditions corresponding to the auroral magnetosphere. |
Tuesday, November 9, 2021 3:24PM - 3:36PM |
JO06.00008: Kinetic Simulations of Particle Acceleration in the Magnetotail Samuel R Totorica, Amitava Bhattacharjee Magnetospheric substorms are frequent disruptions of the Earth's magnetotail associated with transient features including bursty flows of magnetized plasma and energetic particle acceleration. Using fully kinetic, two-dimensional (2D) and three-dimensional (3D) particle-in-cell simulations starting from an exact magnetotail equilibrium, we model the onset of substorms and reproduce prominent features of observations and models including magnetic reconnection onset, a near-Earth ballooning instability, current sheet flapping, and nonthermal particle acceleration. For the first time in a realistic magnetotail configuration, we use self-consistent particle tracking to analyze the trajectories of nonthermal electrons and ions. We identify the dominant acceleration mechanisms including X-point acceleration, plasmoid mediated acceleration, and betatron acceleration, and compare the energetic particle spectra with observations. We compare the acceleration in 2D and 3D simulations to determine the roles of intrinsically 3D mechanisms, providing new insight into the 3D dynamics of substorms. |
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