Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session YO3: Astrophysical and Space Plasmas |
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Chair: Robert Lysak, University of Minnesota Room: Reunion A |
Friday, November 21, 2008 9:45AM - 9:57AM |
YO3.00001: Dimensionless, Rare, Single Spacecraft, Scalar, Observable Properties of the Electron Diffusion Region of Collisionless Magnetic Reconnection Jack Scudder, William Daughton PIC simulations are used to demonstrate five observable, single spacecraft, dimensionless, scalar properties of the Electron Diffusion Region (EDR) of Collisionless Magnetic Reconnection (CMR). These properties are the (i) the demagnetization of the thermal electrons exceeds unity; (ii) the perpendicular electron mach number exceeding unity; (iii) the electron anisotropy exceeding 2.5; (iv) the electron agyrotropy exceeding 0.5; (vi) near alignment ($<$5$^{\circ})$ of one of the non-gyrotropic electron pressure tensor's eigenvectors with the direction of the perpendicular electric field. These scalar dimensionless properties are shown to characterize the electron diffusion regions identified by the standard theoretical diagnostics that would not be readily possible (even with finite differencing) from multiple spacecraft measurements. Similar diagnostics may be useful in identifying EDR's in 3D PIC codes and for use in triggering high data recovery on future space missions. [Preview Abstract] |
Friday, November 21, 2008 9:57AM - 10:09AM |
YO3.00002: Magnetic Turbulence Cascade and Electron Heating in Relativistic Plasmas Edison Liang, Guy Hilburn, Hui Li, Siming Liu Magnetorotational instability (MRI) generates MHD turbulence, which transports angular momentum and drives accretion flows onto compact astrophysical objects. High resolution MHD simulations show that this turbulence is highly anisotropic, with complex current sheet structures and a spectrum consistent with Kolmogorov. Using Particle-in-Cell (PIC) codes, we have simulated the kinetic dissipation of magnetic turbulence cascade in relativistic plasmas, using as initial conditions magnetic field and current sheet patterns similar to those found in MRI MHD simulations, but extrapolated down to the kinetic scale, assuming that self-similarity holds in the inertial range. We find that electron heating is much more effective when the wave turbulence is combined with thin current sheets in the transverse dimension. In this case the wave turbulence induces and enhances magnetic reconnection along the current sheets, producing a superthermal electron component on top of thermal heating. A significant fraction of the magnetic energy is converted to hot electrons with energies exceeding tens of MeVs. [Preview Abstract] |
Friday, November 21, 2008 10:09AM - 10:21AM |
YO3.00003: 3D Simulations of Turbulent Spectra in Compressible Hall-MHD Plasmas Dastgeer Shaikh, P.K. Shukla Turbulent spectral cascades are investigated by means of fully three-dimensional (3D) simulations of a compressible Hall-magnetohydrodynamic (HMHD) plasma in order to understand the observed spectral break in the solar wind turbulence in the regime where the characteristic length-scales associated with electromagnetic fluctuations are smaller than the ion gyroradii. In this regime, the results of our 3D simulations show that the turbulent spectral cascades follow an omnidirectional anisotropic inertial range spectrum close to $k^{-7/3}$, which is associated with the Hall current arising from non-equal electron and ion fluid velocities in our HMHD plasma model. Furthermore, we find that short wavelength (in comparison with the ion skin depth high-frequency kinetic Alfv\'en waves play a crucial role in producing the density perturbations in the solar wind plasma (SWP), and they lead to a turbulent equipartition between the ion fluid velocity and magnetic field fluctuations. The density perturbations in the SWP are associated with the magnetic and velocity field perturbations, as evident from their respective inertial range spectra. [Preview Abstract] |
Friday, November 21, 2008 10:21AM - 10:33AM |
YO3.00004: A paradigmatic flow for small-scale MHD and the collision of current sheets Annick Pouquet, Ed Lee, Pablo Mininni, Marc-Etienne Brachet, Duane Rosenberg We propose a new flow in which the velocity and magnetic fields have symmetries that are preserved by the dynamical equations, allowing for substantial savings in CPU time and memory for a given Reynolds number when implemented numerically. Basic properties of this Taylor-Green flow generalized to MHD are studied up to a 2048**3 grid point equivalent. The temporal evolution of the logarithmic decrement of the energy spectrum remains exponential with no sign of singularity in the ideal case; at the highest resolution, an acceleration corresponding to the near collision of two current sheets driven together by magnetic pressure, with a fast rotation of the direction of the magnetic field is seen, a feature also observed in the solar wind. With dissipation at a unit magnetic Prandtl number, statistical properties of the decaying case are analyzed after averaging over a sizable time interval during which the flow is stationary. An investigation of energy spectra gives a clear tendency toward anisotropic weak MHD turbulence. [Preview Abstract] |
Friday, November 21, 2008 10:33AM - 10:45AM |
YO3.00005: Large-Scale 2D and 3D Simulations of Plasma Turbulence in the Lower Ionosphere Y.S. Dimant, M.M. Oppenheim For five decades, radars and rockets have observed plasma turbulence in the weakly ionized, highly collisional plasma of the E-region ionosphere. This turbulence is caused by the Farley-Buneman, gradient drift, and thermal instabilities. In the high-latitude electrojet, during strong magnetospheric perturbations (storms and sub-storms), radars have observed anomalous electron heating (AEH) caused by turbulent electric fields. We will present results of recent 2D and 3D fully kinetic, particle-in-cell, simulations that reproduce many of the observational characteristics of radar signals. As predicted by theory, the 3D simulations show the development of waves having a turbulent electric field with a small component parallel to the geomagnetic field. This field component is mainly responsible for the majority of AEH. For the first time, we can now quantify this effect using accurate simulations. These simulations provide information useful in accurately modeling plasma turbulence and demonstrate the significant progress we have made simulating physical processes in E-region electrojets. [Preview Abstract] |
Friday, November 21, 2008 10:45AM - 10:57AM |
YO3.00006: Physics of the Alfv\'{e}nic Aurora Robert Lysak, Yan Song, Jesse Woodroffe Many measurements of auroral particles, in particular recent measurements from the FAST satellite, indicate that in many cases the auroral energy distribution is broad in energy and strongly field-aligned in pitch angle. These observations suggest that the cold ionospheric electrons are being accelerated in time-dependent fields. Such electrons are seen in conjunction with strong kinetic Alfv\'{e}n waves, and so have been termed the ``Alfv\'{e}nic aurora.'' The Alfv\'{e}nic aurora is predominant at the polar cap boundary of the aurora as well as in the auroral arc that brightens during substorm onset, suggesting it is a transitional phase to the quasi-static aurora. Kinetic Alfv\'{e}n waves are accompanied by a parallel electric field when the perpendicular wavelength is the order of the electron inertial length or the ion acoustic gyroradius. These scales are a few kilometers for auroral parameters, comparable in size to the narrowest auroral arcs. The critical question in understanding this mechanism is accounting for this narrow scale. Phase mixing, ionospheric feedback, and nonlinear interactions will be investigated to determine their potential roles in this development. [Preview Abstract] |
Friday, November 21, 2008 10:57AM - 11:09AM |
YO3.00007: Breakdown of the Frozen-in condition and Plasma Energization in Cosmic Plasmas Yan Song, Robert Lysak A dynamical theory shows that the generation of a sustained parallel electric fields is associated with the release of localized enhanced magnetic or mechanical stresses, and favors a low plasma density (\textit{Song and Lysak}, \textit{PRL}, 2006). This result suggests that reconnection and the energization and acceleration of charged particles in cosmic plasmas are natural consequences of Alfvenic dynamical interactions and should often occur when the plasma density is low. We demonstrate how the Alfvenic interaction of MHD wave packets at current sheets can generate parallel electric fields causing the breakdown of the frozen-in condition and plasma energization. In the auroral acceleration region, this interaction can create and sustain parallel potential drops in the form of double layers. These Alfvenic interactions are reactive in nature. The reactance associated with these interactions leads to a dynamo effect, and is often many orders of magnitudes larger than classical dissipative-type transport coefficients. The Alfvenic interaction leads to fast and efficient reconnection and plasma energization. [Preview Abstract] |
Friday, November 21, 2008 11:09AM - 11:21AM |
YO3.00008: Kinetic modeling of particle kinetics close to low orbit satellites Richard Marchand, Jean-Jacques Berthelier, David Knudsen Some satellites are equipped with ion analyzers capable of measuring ion distribution functions with very high sensitivity. This is the case, for example, with DEMETER, launched in 2004. Similar ion analyzers are also planned for Swarm, to be launched in 2010. The interpretation of ion distribution functions measured by ion analyzers onboard satellites often relies on the assumption that the surrounding electrostatic sheath does not significantly perturb the distribution of the incoming ions. Yet, observations show that electrostatic sheaths can significantly affect the distribution of particles in their energy, and in their direction. We use a three dimensional PIC code to simulate the interaction between ionospheric plasmas and satellites, using DEMETER and Swarm as examples. The computed resulting fields are used as input in a particle backtracking code to infer measured ion distribution functions, in terms of assumed distribution far from the sheath region. The results are used to interpret anomalies in DEMETER measurements, and assess possible similar implications on Swarm. [Preview Abstract] |
Friday, November 21, 2008 11:21AM - 11:33AM |
YO3.00009: Triggering, nonlinear amplification and absolute instability of whisters of whistlers Martin Lampe, Gurudas Ganguli, Glenn Joyce, Wallace Manheimer We consider the instability of a whistler wave packet propagating from north to south along a geomagnetic field line, in the presence of a cold plasma backgrounds as well as a distribution of high energy electrons. For simplicity of analysis, we first consider the case of a ring distribution of high energy electrons, moving from south to north. We present simulations and analysis showing that instability begins at two locations, symmetrically located in the northern and southern hemispheres, where the electrons are cyclotron resonant with the wave. After trapping sets in, nonlinear growth continues, due to the geomagnetic field gradient, only in the southern hemisphere. The stream of resonant electrons which has passed through, and become modulated at the resonant point, drives new waves at a lower frequency which is locally resonant with the electrons, permitting growth all the way from the linearly resonant point to the equator. Due to these wake electrons, the wave grows in the opposite direction to linear wave propagation, leading to an absolute instability. We find that a similar effect occurs for the more physical case of an electron distribution function that has step discontinuities. [Preview Abstract] |
Friday, November 21, 2008 11:33AM - 11:45AM |
YO3.00010: A Dynamic Model of the Radiation-Belt Electron Phase Space Density driven by the Inner-Magnetospheric ULF Wave Activity Dimitris Vassiliadis, Mark Koepke, Mattias Tornquist The phase space density fe of the radiation-belt relativistic-electron population is reconstructed based on measurements made by the High Sensitivity Space Telescope on board the POLAR spacecraft. The density peaks in invariant space (mu, K, L*) are shown to be responding to changes in the solar wind velocity and density, and the interplanetary magnetic field. We have associated specific types of storms with the appearance of peaks thereby producing a climatology of fe. We show that there is a strong similarity between phase space density changes during these storms and the ULF wave power in the inner magnetosphere remote-sensed by the GOES geosynchronous spacecraft and the IMAGE ground magnetometer array. We discuss numerical simulations of the particle energization via diffusive-convective models. [Preview Abstract] |
Friday, November 21, 2008 11:45AM - 11:57AM |
YO3.00011: nMHDust: A 4-Fluid Partially Ionized Dusty Plasma Code Samuel Lazerson nMHDust is a next generation 4-fluid partially ionized magnetized dusty plasma code, treating the inertial dynamics of dust, ion and neutral components. Coded in ANSI C, the numerical method is based on the MHDust 3-fluid fully ionized dusty plasma code. This code expands the features of the MHDust code to include ionization/recombination effects and the netCDF data format. Tests of this code include: ionization instabilities, wave mode propagation (electromagnetic and acoustic), shear-flow instabilities, and magnetic reconnection. Relevant parameters for the space environment are considered, allowing a comparison to be made with previous dusty plasma codes (MHDust and DENISIS). The utility of the code is expanded through the possibility of a small dust mass. This allows nMHDust to be used as a 2-ion plasma code. nMHDust completes the array of fluid dusty plasma codes available for numerical investigations into nonlinear phenomena in the field of astrophysical dusty plasmas. [Preview Abstract] |
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