Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session GM9: Mini-Conference: Van Allen 100: Radiation Belt Physics |
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Chair: Stanslav Boldryev, University of Wisconsin Room: Salon ABC |
Tuesday, October 28, 2014 9:30AM - 10:05AM |
GM9.00001: Frontiers of Radiation Belt Physics Louis Lanzerotti The discovery of trapped radiation around Earth by James Van Allen in 1958 revolutionized concepts of Earth's space environment, and its relationship to solar activity. Coming in the same era as the declassification of research in laboratory plasma physics, concepts and theories in space and laboratory plasma environments have grown in parallel, sometimes building upon one another and at times diverging with little overlap. The launch of the dual spacecraft NASA Van Allen Probes mission (August 2012) has opened a fresh era in understanding of Earth's space plasma environment, and has stimulated new opportunities for collaborative interactions between laboratory and space plasma researchers. This talk will outline some past history of space plasma research, and will describe some of the latest developments in new understandings achieved by data from the Van Allen Probes. [Preview Abstract] |
Tuesday, October 28, 2014 10:05AM - 10:40AM |
GM9.00002: Observations of Whistler-Mode Chorus with Van Allen Probes William Kurth, George Hospodarsky, Ondrej Santolik, Craig Kletzing, Scott Bounds The Van Allen Probes mission provides an excellent opportunity to observe whistler-mode chorus and its role in the radiation belts. The plasma wave instrument on the two probes, called Waves, includes six identical waveform receivers covering the frequency range from 10 Hz to 12 kHz. The instrument measures three orthogonal magnetic field components and three orthogonal electric field components of waves. This complement supports wave-normal and Poynting flux analyses of chorus as well as other wave modes that interact with radiation belt particles. Extensive use of burst modes provides multicomponent waveforms enabling the study of individual chorus elements, including their substructure. The early-mission publications confirm the importance of chorus to the local acceleration of electrons in the outer radiation belts. The orbital precession of the twin Van Allen Probes through a complete range of local times now allows for a new survey of the distribution of chorus emissions. Hence, we now have the tools to study chorus from the nonlinear growth in chorus element substructures through synoptic studies of the near-equatorial occurrence of chorus out to a distance of approximately 5.8 Earth radii. [Preview Abstract] |
Tuesday, October 28, 2014 10:40AM - 11:15AM |
GM9.00003: Generation mechanism of whistler-mode chorus emissions Yoshiharu Omura There has been significant progress in understanding the generation mechanism of whistler-mode chorus emissions in recent years. This is partly due to the successful reproduction of chorus emissions by computer simulations and partly due to precise observations of the emissions by spacecraft. We review nonlinear theory and simulations on the generation mechanism of chorus emissions that have been revealed by the simulations and observations. We describe the nonlinear dynamics of resonant electrons and the formation of electromagnetic electron holes or hills that result in resonant currents generating rising-tone emissions or falling-tone emissions, respectively. Each chorus element comprises many sub-packets in which nonlinear wave growth takes place above the threshold amplitude and saturates at the optimum wave amplitude for triggering emissions. We also describe the mechanism of nonlinear wave damping due to quasi-oblique propagation, which results in formation of a gap at half the electron cyclotron frequency, separating a single chorus element generated at the magnetic equator into upper band and lower band elements in off-equatorial regions. [Preview Abstract] |
Tuesday, October 28, 2014 11:15AM - 11:40AM |
GM9.00004: A model for falling tone chorus in the Earth's magnetosphere A. Rualdo Soto-Chavez, Ge Wang, Amitava Bhattacharjee, Guo-yong Fu, Hakan Smith Motivated by the fact that geomagnetic field inhomogeneity is weak close to the chorus generation region and the observational evidence that falling-tone chorus tend to have large oblique angles of propagation, we present a new model for falling-tone chorus in which we propose that they start as a marginally unstable mode. The marginally unstable mode requires the presence of a relatively large damping, which has its origins in the Landau damping of oblique waves in this collision-less environment. A marginally unstable mode produces phase-space structures that release energy changing its frequency content. We show that the present model produces results in reasonable agreement with observations. [Preview Abstract] |
Tuesday, October 28, 2014 11:40AM - 12:05PM |
GM9.00005: Nonlinear Scattering of VLF Waves in the Radiation Belts Chris Crabtree, Leonid Rudakov, Guru Ganguli, Manish Mithaiwala Electromagnetic VLF waves, such as whistler mode waves, control the lifetime of trapped electrons in the radiation belts by pitch-angle scattering. Since the pitch-angle scattering rate is a strong function of the wave properties, a solid understanding of VLF wave sources and propagation in the magnetosphere is critical to accurately calculate electron lifetimes. Nonlinear scattering (Nonlinear Landau Damping) is a mechanism that can strongly alter VLF wave propagation [Ganguli et al. 2010], primarily by altering the direction of propagation, and has not been accounted for in previous models of radiation belt dynamics. Laboratory results have confirmed the dramatic change in propagation direction when the pump wave has sufficient amplitude to exceed the nonlinear threshold [Tejero et al. 2014]. Recent results show that the threshold for nonlinear scattering can often be met by naturally occurring VLF waves in the magnetosphere, with wave magnetic fields of the order of 50-100 pT inside the plasmapause. Nonlinear scattering can then dramatically alter the macroscopic dynamics of waves in the radiation belts leading to the formation of a long-lasting wave-cavity [Crabtree et al. 2012] and, when amplification is present, a multi-pass amplifier [Ganguli et al. 2012]. By considering these effects, the lifetimes of electrons can be dramatically reduced. [Preview Abstract] |
Tuesday, October 28, 2014 12:05PM - 12:30PM |
GM9.00006: Nonlinear Generation of Electromagnetic Waves Through Induced Scattering by Thermal Electrons Erik Tejero, Chris Crabtree, David Blackwell, Bill Amatucci, Manish Mithaiwala, Guru Ganguli, Leonid Rudakov Nonlinear interactions involving whistler wave turbulence are important contributors to radiation belt dynamics. Given sufficient whistler energy density, nonlinear scattering from thermal electrons can substantially change the wave normal angle, while inducing a small frequency shift. This nonlinear process is being studied in the NRL Space Physics Simulation Chamber (SPSC) in scaled magnetospheric conditions. Experiments conducted in the SPSC have demonstrated induced nonlinear scattering of quasi-electrostatic pump waves by thermal electrons. Measurements of the magnetic field vectors for the pump and daughter waves allow for the determination of wave distribution functions, which indicate the direction of propagation for each of these waves. The experiment supports the theory of electromagnetic whistler wave generation through nonlinear scattering of electrostatic lower hybrid waves by thermal electrons in the Earth's magnetosphere. [Preview Abstract] |
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