Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session GM9: Mini-Conference: Nonlinear Effects in Geospace Plasmas I |
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Chair: Evgeny Mishin, Air Force Research Laboratory Room: 553AB |
Tuesday, October 30, 2012 9:30AM - 9:42AM |
GM9.00001: Meteor Plasmas in the E-Region Ionosphere Y.S. Dimant, M.M. Oppenheim Every day billions of tiny meteoroids impact the Earth's atmosphere at hypersonic speeds, creating dense plasmas between 80 and 130 km altitude. In this part of the E-region ionosphere electrons are magnetized by the geomagnetic field while ions are largely unmagnetized due to their frequent collisions with neutral atmosphere. This discrepancy leads to a variety of inhomogeneous, unstable, and nonlinear plasma phenomena. Among them is the formation of field-aligned irregularities in the slowly diffusing dense meteor plasma trails which are important for radar observations of mostly optically invisible meteors. We will present a quantitative model of the evolution of a plasma trail density and its ambipolar electric fields. Our theory predicts that plasma trail diffusion induces electric currents through a large volume of the background ionosphere with important consequences for plasma trail diffusion. Also, strong electric fields propagate long distances along the magnetic field lines from the dense plasma trail deep into the tenuous ionosphere and create significant disturbances of the background density. This may explain radar and rocket observations of extensive nighttime E-region density structures. [Preview Abstract] |
Tuesday, October 30, 2012 9:42AM - 9:54AM |
GM9.00002: Whistler Turbulence Forward Cascade: Three-Dimensional Particle-in-Cell Simulations S. Peter Gary, Ouliang Chang, Joseph Wang Three-dimensional particle-in-cell (PIC) simulations of whistler turbulence in a magnetized, homogeneous, collisionless plasma have been carried out. An initial relatively isotropic spectrum of long-wavelength whistler mode fluctuations is imposed upon the system. The simulations follow the temporal evolution of the field fluctuations as they decay into a broadband, turbulent spectrum at shorter wavelengths via a forward cascade with an anisotropy in the sense of stronger fluctuation energy at $k_{\perp}$ than at comparable $k_{\parallel}$. Reduced $k_{\perp}$ magnetic fluctuation spectra show a clear break near the inverse electron inertial length scale, from steep spectra at long wavelengths to still steeper spectra at shorter wavelengths, similar in character to spectra at electron scales recently measured in the solar wind. Computations have been done at various values of the electron beta, where $\beta_e$ is changed by altering the ratio of the background plasma density to the background magnetic field energy density. Preliminary results show that an increasing $\beta_e$ corresponds to a faster cascade, more nearly isotropic spectra, stronger electron heating, and greater damping of the whistler fluctuations. [Preview Abstract] |
Tuesday, October 30, 2012 9:54AM - 10:06AM |
GM9.00003: New Breakthroughs and Challenges in Kinetic Simulations of the Magnetosphere Yuri Omelchenko, H. Karimabadi, X. Vu, V. Roytershteyn, B. Loring Global magnetospheric simulations have long been carried out with MHD. These simulations have proven useful in studies of the global dynamics of the magnetosphere with the goal of predicting eminent features of substorms and other global events. However, it is well known that the magnetosphere is dominated by kinetic ion and electron effects, and many key aspects of the magnetosphere relating to transport and structure of boundaries await global kinetic simulations. With the advent of petascale computing and a number of recent algorithmic innovations, we have been able to conduct first-ever 3D global hybrid (electron fluid, kinetic ions) and 2D global full PIC simulations. Here we show several specific science issues that we have been able to address for the first time. This includes formation of flux transfer events at the dayside magnetopause and associated flows, plasma depletion layer, and flux ropes in the magnetotail. We also discuss new models for extended hybrid simulations as well as the proposed coupling of global hybrid simulations with physically driven ionospheric models which include ion outflow. [Preview Abstract] |
Tuesday, October 30, 2012 10:06AM - 10:18AM |
GM9.00004: Quasilinear Evolution from Whistler and KAW Turbulence in the High Beta Solar Wind Manish Mithaiwala, Leonid Rudakov, Gurudas Ganguli, Chris Crabtree The electron and ion distribution functions resulting from quasi-linear diffusion in the turbulent solar wind plasma is calculated using the measured spectrum of the kinetic Alfven wave (KAW) fluctuations. Quasi-linear diffusion establishes a step-like profile on the distribution function over parallel velocity [1]. It is shown that the dispersion relation for whistler waves is identical for a high or low beta plasma. Furthermore in the high-beta solar wind plasma whistler waves meet the Landau resonance with electrons for velocities less than the thermal speed, and consequently the electric force is small compared to the mirror force. However, the whistlers are not damped since the background kinetic Alfven wave turbulence creates a plateau by quasilinear diffusion in the solar wind electron distribution at small velocities. The diffusion coefficient for whistlers in a high beta plasma is determined from mirror force, while the KAW diffusion is determined from the electric and mirror force. The size of ``plateau'' $v_{me} $, which can be created within the time of travel of solar wind plasma to the Earth $>$ 10$^{5}$ s, is estimated for electrons as \textit{vme/ve}$\sim $0.5. For a whistler spectrum similar to that of KAW, it is found that for whistler energy density of only $\sim $10$^{-3}$ of the kinetic Alfven waves, the quasilinear diffusion rate due to whistlers and KAW are comparable. Thus very small amplitude whistler turbulence can have a significant consequence on the evolution of the solar wind electron distribution function. [1] L. Rudakov, M. Mithaiwala, G. Ganguli, and C. Crabtree. Phys. Plasmas \textbf{18}, 012307 (2011) [Preview Abstract] |
Tuesday, October 30, 2012 10:18AM - 10:30AM |
GM9.00005: Chorus waves: a high-gain free-electron laser in the Earth's magnetosphere A. Bhattacharjee, R. Soto-Chavez Chorus waves in the magnetosphere are very-low-frequency (VLF) phenomena that arise due to the interaction of gyro-resonant electrons with whistler waves. These waves typically have frequencies equal to half the electron gyro-frequency at the geomagnetic equator, are amplified to amplitudes of more than 30dB, and exhibit a continuous frequency chirp. The study of these waves, and their role in energizing particles, is one of the principal objectives of the upcoming Radiation Belt Storm Probes (RBSP) mission. Here we present a new model of chorus waves based on the high-gain free-electron laser mechanism. We derive a new closed set of self-consistent relativistic equations that couple the Hamiltonian single-particle equations with Maxwell's equations for the radiation field, assuming that the latter is slowly varying. We demonstrate, neglecting slippage between the electrons and the radiation field, that in the exponential high-gain regime, these differential equations yield an exact cubic algebraic equation that predicts whistler wave amplification levels in good agreement with observations. When slippage is included in the theory, the radiation field phase evolves in time, predicting a frequency chirp. [Preview Abstract] |
Tuesday, October 30, 2012 10:30AM - 10:42AM |
GM9.00006: Observations and Simulations of nonlionear energtic signatures in reconnection Giovanni Lapenta, Stefano Markdis, David Newman, Martin Goldman, Laila Andresson, Stefan Eriksson The solar wind and magnetospheric plasmas provide great information about non-linear processes and feedbacks among different waves. Microinstabilities can lead to turbulence that affect reconneciton. We focus here on understanding: 1) The triggering of turbulence at the fluid and kinetic level in regions where reconnection develops at the macroscopic level. Recent results from the study of the lower hybrid instability and the plasmoid instability will out in context of observational data. How do these processes affect reconnection? 2) Reconnection itself produces changes in its environment, promoting flows and anisotropies that in turn produce instabilities, in the inflowing plasma entering the reconnection region and in the outflowing plasma. How does this feedbacks then on reconnection and what impacts it has on the global evolution. 3D massively parallel simulations of kinetic processes and of fluid processes will be shown to demonstrate the different non-linear interplay of small and large scales in the two models. [Preview Abstract] |
Tuesday, October 30, 2012 10:42AM - 10:54AM |
GM9.00007: Scattering of High Frequency Electromagnetic Waves in the Presence of Low Frequency Density Irregularities V. Sotnikov, T. Kim, W.E. Amatucci, G. Ganguli, E. Tejero, T.A. Mehlhorn Presence of plasma can strongly influence propagation properties of electromagnetic signals used for surveillance and communication. In particular, we are interested in mechanisms of generation of low frequency plasma turbulence in the ionosphere and inside a plasma sheath of reentry and hypersonic vehicles and in similar applications. We will discuss generation of low frequency density irregularities due to the presence of plasma flows with velocity shear and interchange instability. Next, influence of excited wave turbulence on scattering of high frequency electromagnetic waves used for communication purposes will be presented. Finally, scattering cross-sections due to interaction of high frequency EM waves with density irregularities produced by different types of low frequency plasma turbulence will be discussed. [Preview Abstract] |
Tuesday, October 30, 2012 10:54AM - 11:06AM |
GM9.00008: Laboratory observations of electron phase-space holes driven during magnetic reconnection W. Fox, M. Porkolab, J. Egedal, N. Katz, A. Le We report studies of electrostatic turbulence during spontaneous reconnection events on the Versatile Toroidal Facility reconnection experiment [1]. Electrostatic fluctuations are observed by small, high-bandwidth, impedance-matched Langmuir probes. Fluctuations observed include broadband lower-hybrid fluctuations and large-amplitude, positive potential spikes, identified as electron phase space holes [2,3]. The properties of the holes are studied with cross-correlation techniques between closely spaced probes. For the holes, the parallel and perpendicular sizes are roughly equal, approximately 1--2~mm (50--100$~\lambda_D$, or 5--10$~\rho_e$), and the holes are observed to travel equal or faster than the electron thermal speed. Based on the observations and scaling arguments, the holes can be shown to be predominantly electrostatic. Finally, holes are observed to be confined to highly localized regions (1--2 cm, $\sim \rho_i$). These observations will be connected to recent space observations and theory. This work was funded in part by DOE Grant DE-FG02-06ER54878 and CMPD Grant DEFC02-04ER54786.\\[4pt] [1] J. Egedal, et al., PRL 98, 015003 (2007). \\[0pt] [2] W. Fox, et al., PRL 101, 255003 (2008). \\[0pt] [3] W. Fox, et al., Phys. Plasmas 19, 032118 (2012). [Preview Abstract] |
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