Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session PM9: Mini-Conference: Physics of the Radiation Belts: Collaboration between Laboratory, Theory and Satellite Observations IIMini-Conference
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Chair: Evgeny Mishin, Air Force Research Laboratory Room: 211 CD |
Wednesday, November 2, 2016 2:00PM - 2:25PM |
PM9.00001: Understanding Earth's radiation belt electron dynamics: Van Allen Probes observations and simulations Wen Li, Qianli Ma, Richard Thorne, Jacob Bortnik, Xiaojia Zhang Various physical processes are known to cause acceleration, loss, and transport of energetic electrons in the Earth's radiation belts, but their quantitative roles in different time and space need further investigation. In the present paper, we evaluate the relative roles of various physical processes during geomagnetic storms using a 3D diffusion simulation. By quantitatively comparing the electron evolution observed by Van Allen Probes and simulation, we found that whistler-mode chorus waves play a critical role in accelerating electrons up to several MeV through efficient energy diffusion. By only including radial diffusion driven by ultra-low-frequency waves, the simulation underestimates the observed electron acceleration, while radial diffusion plays an important role in redistributing electrons. Although an additional loss process is required to fully explain the overestimated electron fluxes at multi-MeV, the combined physical processes of radial diffusion and scattering by whistler-mode waves reproduce the observed electron dynamics remarkably well, suggesting that quasi-linear diffusion theory is reasonable to evaluate radiation belt electron dynamics, and the importance of nonlinear wave-particle interaction may still remain as an open question. [Preview Abstract] |
Wednesday, November 2, 2016 2:25PM - 2:50PM |
PM9.00002: Laboratory experiment on the excitation of whistler-mode chorus waves Xin An, Bart Van Compernolle, Jacob Bortnik, Viktor Decyk, Richard Thorne Whistler-mode chorus waves play an important role in accelerating electrons to relativistic energies in the heart of the outer radiation belt, as well as in precipitating electrons to the atmosphere. An experiment in the Large Plasma Device at UCLA generates both broadband and discrete chirping whistler-mode emissions using a gyrating electron beam injected into a cold background plasma. The mode structure of these emissions is identified using a phase-correlation technique. The emission forms of the whistler waves depend on plasma density, beam density and magnetic field profiles. A kinetic simulation in accordance with the experiment shows an initial relaxation of the electron beam by Langmuir waves and subsequently growing whistler waves through cyclotron resonance and Landau resonance. [Preview Abstract] |
Wednesday, November 2, 2016 2:50PM - 3:15PM |
PM9.00003: Laboratory Study of Triggered Emissions and Nonlinear Wave-Particle Interactions Erik Tejero, Lon Enloe, Bill Amatucci, Chris Crabtree, Guru Ganguli Experiments conducted in the Space Physics Simulation Chamber at the Naval Research Laboratory using an electron beam propagating in a non-uniform magnetic field and an antenna launching counter-propagating Whistler waves have demonstrated nonlinear Whistler amplification and triggered emissions due to nonlinear wave-particle interactions. When the antenna was not used, chorus-like chirped Whistler waves were observed. These experiments provide a good testbed for understanding the generation mechanism for nonlinear wave-particle interactions and resulting wave phenomena. Recent results from these experiments will be presented. [Preview Abstract] |
Wednesday, November 2, 2016 3:15PM - 3:40PM |
PM9.00004: Identify the nonlinear wave-particle interaction regime during chorus wave generation Xin Tao, Liu Chen, Fulvio Zonca Nonlinear wave particle interaction during chorus wave generation was assumed to be in the adiabatic regime in previous studies; i.e., the particle phase-space trapping time scale ($\tau_\text{tr}$) is considered to be much smaller than the nonlinear dynamics time scale $\tau_\text{NL}$. In this work, we use particle-in-cell simulations to demonstrate that $\tau_\text{tr} \sim \tau_\text{NL}$; i.e., the interaction regime during chorus generation is in the non-adiabatic regime. The time scale for nonlinear evolution of resonant particle phase space structures is determined by making the time averaged power exchange plot, which clearly demonstrates that particles with pitch angle near $70\degree$ make the most significant contribution to wave growth. The phase-space trapping time scale is also comparable to the amplitude modulation time scale of chorus, suggesting that chorus subpackets are formed because of the self-consistent (non-perturbative) evolution of resonant particle phase-space structures and spatiotemporal features of the fluctuation spectrum. [Preview Abstract] |
Wednesday, November 2, 2016 3:40PM - 4:05PM |
PM9.00005: Nonlinear wave-particle interactions in the outer radiation belts: Van Allen Probes results Oleksiy Agapitov, Forrest Mozer, Anton Artemyev, James Drake, Ivan Vasko Huge numbers of different nonlinear structures (double layers, electron holes, non-linear whistlers, etc. referred to as Time Domain Structures - TDS) have been observed by the electric field experiment on board the Van Allen Probes. A large part of the observed non-linear structures are associated with whistler waves and some of them can be directly driven by whistlers. Observations of electron velocity distributions and chorus waves by the Van Allen Probe B provided long-lasting signatures of electron Landau resonant interactions with oblique chorus waves in the outer radiation belt. In the inhomogeneous geomagnetic field, such resonant interactions then lead to the formation of a plateau in the parallel (with respect to the geomagnetic field) velocity distribution due to trapping of electrons into the wave effective potential. The feedback from trapped particles provides steepening of parallel electric field and development of TDS seeded from initial whistler structure (well explained in terms of Particle-In-Cell model). The decoupling of the whistler wave and the nonlinear electrostatic component is shown in PIC simulation in the inhomogeneous magnetic field system and are observed by the Van Allen Probes in the radiation belts. [Preview Abstract] |
Wednesday, November 2, 2016 4:05PM - 4:30PM |
PM9.00006: Scalings for the Alfven-cyclotron Instability: Linear Dispersion Theory and Hybrid Particle-in-Cell Simulations S. Peter Gary, Xiangrong Fu, Misa M. Cowee, Dan Winske, Kaijun Liu The Alfv\`{e}n-cyclotron instability is driven by an ion temperature anisotropy such that T$_{\mathrm{\bot }}$/T$_{\mathrm{\vert \vert }}$ \textgreater 1 where $\bot $ and \textbar \textbar denote directions perpendicular and parallel to a uniform background magnetic field \textbf{B}$_{\mathrm{o}}$. The theory used here considers a magnetized, homogeneous, collisionless plasma with a magnetospheric-like configuration of two proton components, a more dense, relatively cool, isotropic component and a less dense, relatively hot, anisotropic component which drives the instability. Only wave propagation parallel to \textbf{B}$_{\mathrm{o\thinspace }}$is considered. Using numerical solutions of the full kinetic linear dispersion equation, concise analytic expressions for the scaling of the maximum instability growth rate and the corresponding real frequency are derived as functions of three dimensionless variables: the hot proton temperature anisotropy, the dimensionless hot proton density, and the hot proton $\beta_{\mathrm{\vert \vert }}$. Furthermore, using one-dimensional hybrid particle-in-cell simulations of this same instability, a third relation for the scaling of the maximum amplitude of the fluctuating magnetic field energy density is derived. [Preview Abstract] |
Wednesday, November 2, 2016 4:30PM - 4:55PM |
PM9.00007: Effects of Magnetic Reconnection Processes in the Near Magnetosphere* B. Coppi, A. Fletcher Magnetic reconnection processes in collisionless plasmas on the Earth’s dayside and nightside are shown to be capable of producing high energy populations of both ions and electrons. These particles can interact with the Radiation Belts and reach the regions close to the Earth where auroral substorms can be produced [1]. The main theoretical issues faced in identifying plasma modes with realistic characteristics, given the scale distances and time scales, that can be responsible for the needed reconnection processes are pointed out. Solution of relevant equations obtained by a combined analytical and numerical approach. *Sponsored in part by the U.S. D.O.E. and the N.S.F.\\ $[1]$ B. Coppi, G. Laval, R. Pellat, $\it{Phys. Rev. Lett.}$ $\bf{16}$, 1207 (1966). [Preview Abstract] |
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