Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session QO1: Plasma Astrophysics |
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Chair: Joseph Borovsky, Los Alamos National Laboratory Room: Adam's Mark Hotel Governor's Square 10 |
Thursday, October 27, 2005 9:30AM - 9:42AM |
QO1.00001: Effect of the Interstellar Magnetic Field on the Termination Shock: Explaining the Voyager 1 Results Paulett Liewer, Merav Opher, E. C. Stone After a 27 year journey, Voyager 1 crossed the solar wind termination shock, the first boundary separating the solar system from the rest of the galaxy, and is now exploring the heliosheath. In 2002, before crossing the shock, Voyager 1 observed strong beams of energetic particles coming outward along the spiral magnetic field, the opposite of the direction expected for particles accelerated at the shock. This can be explained if the shock is non-spherical so that the interplanetary magnetic field lines cross the shock into the heliosheath and then reenter the solar wind before reaching Voyager 1. This configuration has been invoked recently (Jokipii et al., 2004; Stone et al., 2005) to explain how Voyager 1 can detect energetic particles accelerated at the shock several years before crossing it. Using 3D MHD models of the global heliosphere, we show that the termination shock is, in fact, non-spherical due the distortion caused by an inclined interstellar magnetic field. We use values for the direction of the interstellar magnetic field based on observations and show that the shape of the termination shock depends strongly on the direction of the interstellar magnetic field. [Preview Abstract] |
Thursday, October 27, 2005 9:42AM - 9:54AM |
QO1.00002: WITHDRAWN--On the dynamics of transition to incompressibility from compressible MHD turbulence Dastgeer Shaikh, Gary P. Zank Compressibility is regarded as an essential characteristic of interplanetary, interstellar, and laboratory magnetohydrodynamic (MHD) plasmas, yet small scale low-frequency phenomena, and interplanetary or laboratory turbulence are frequently described on the basis of incompressible MHD. Understanding why magnetofluids observed in the solar wind or interstellar medium frequently behave as though they are incompressible has proved a major challenge to our understanding of small-scale dynamical processes in a plasma. The past 15 years have witnessed an effort to understand this apparent paradox of compressible MHD behaving as though it were incompressible in a variety of environments ranging from the solar wind to the interstellar medium. On the basis of 3D time dependent numerical simulations, we find that compressible magneto hydrodynamic fluids describing super-Alfvenic supersonic and strongly magnetized space and laboratory plasmas decay progressively to a state of near incompressibility. This transition is mediated dynamically by disparate spectral energy dissipation rates in compressible magnetosonic and shear Alfvenic modes. Dissipation leads to super-Alfvenic turbulent motions decaying to a sub-Alfvenic regime that couples weakly with (magneto) acoustic cascades. Consequently, the supersonic plasma motion dissipates into highly subsonic motion and density fluctuations experience a passive convection. [Preview Abstract] |
Thursday, October 27, 2005 9:54AM - 10:06AM |
QO1.00003: Global Hybrid Simulations of the Earth Magnetosphere: Nuts and Bolts H. Karimabadi, H.X. Vu, D. Krauss-Varban, Y.A. Omelchenko, J. Raeder We present results of global 2-D and 3-D hybrid (electron fluid, kinetic ions) simulations of the Earth magnetosphere and compare them with equivalent MHD simulations. There are a number of numerical and modeling issues that may affect simulation results and special care must be taken to ensure proper modeling. We discuss these pitfalls through examples and present the appropriate choice of the boundary conditions, resistivity model, and hybrid algorithm, along with a number of new physical effects we have discovered. We have also performed the highest resolution global MHD simulations of the magnetosphere to date. We compare the computational hybrid and MHD models and show how these two tools can be used in a complementary fashion to advance our understanding of the underlying magnetospheric physics. Finally, we briefly discuss our progress in developing discrete-event simulation technology with the eventual goal of applying it to the 3-D global hybrid simulations. [Preview Abstract] |
Thursday, October 27, 2005 10:06AM - 10:18AM |
QO1.00004: Vlasov Simulations of the Lower Boundary of the Upward Current Region Daniel Main, David Newman, Robert Ergun The lower boundary of the upward current region has been modeled as a BGK double layer (DL) using FAST data to model the distribution functions on the ionospheric and auroral cavity sides of the DL. The evolution of the DL has been studied using a 1-D open boundary Vlasov simulation. We present results that show that ion holes form only if both $H^+$ and $O^+$ are included in the ionospheric ion beam population. If only $H^+$ is included in the ionospheric ion beam, the DL does not evolve, and the BGK DL is stable. We compare the linear stages of the Vlasov simulation with linear kinetic theory and show that the dominant instability is between the $O^+$ and $H^+$ beams. In the 1-D simulation, we show that ion holes form in the $H^+$ but not the $O^+$. In addition, we explain how the electrons respond by saturating the growth, thus keeping the ion holes from exploding in size. We also present periodic magnetized 2-D results which shows that ion holes form as well as Berstein modes. However, the Berstein modes form only after the formation of the ion holes. These numerical results are consistent with observations from the FAST spacecraft which has observed ion holes and Berstein waves in the auroral cavity [Preview Abstract] |
Thursday, October 27, 2005 10:18AM - 10:30AM |
QO1.00005: Numerical Simulations of the MHD Plasma Equilibrium Using Block Adaptive Grids Igor Sokolov, Tamas Gombosi, Ilia Roussev A numerical simulation of the MHD equilibrium in toroidal plasma devices is a central problem of the magnetic confinement fusion. This is also an important problem for solar physics, because active regions on the Sun prior to coronal mass ejection are believed to include a magnetic configurationthat very closely resembles that of a half toroid. For both versions of the problem, the large variety of spatial scales ispertinent. To simulate them numerically we applied a block adaptive technology, what allows us to reach the higher resolution in the regions of interest (like magnetic islands or reconnetion sites), with a reasonably low total amount of computational cells. Assuming the outer plasma boundary to be an axially symmetric magnetic surface which satisfies the Grad-Shafranov equation, we constructed the vortex-based grid, which exactly fits the plasma boundary. We studied the equilibrium in the tokamak of a D-shaped cross-section. For solar physics application, we analyzed the quasi-steady-state configurations in the region having a semi-toroidal form by applying the "shearing motion" at the ending cross-sections, which are to represent the photospheric motions at the Sun. [Preview Abstract] |
Thursday, October 27, 2005 10:30AM - 10:42AM |
QO1.00006: Microinstabilities and Electron Holes in Current Sheets Martin V. Goldman, David L. Newman, James F. Drake Magnetic reconnection simulations (3D PIC) reveal electric field structures explained in terms of Buneman instabilities.$^1$ The waves driven by the current sheet trap the electrons forming electron phase space holes (localized bipolar $E_\parallel$). Under certain conditions the holes can provide the small-scale dissipation necessary for fast magnetic reconnection.$^1$ Current sheets inevitably exhibit shear near their edges. Using 2D Vlasov simulations and linear global theory, we address the role of shear (variation in $v_\parallel$ across $B$) on both the linear and nonlinear evolution of 2-D Buneman instabilities assuming conditions similar to those employed in the 3D reconnection simulations. The Vlasov simulations are initialized with strongly magnetized Maxwellian electrons drifting relative to unmagnetized ions. The 2D Vlasov simulation results (with and without shear) are compared with the 3D reconnection results. In the weakly sheared linear regime, Poisson's eqn becomes a Mathieu equation for the eigenpotentials, which compare well with simulation results. A small amount of shear can significantly alter nonlinear hole evolution.$^2$ \newline $^1$Drake, et al., \textit{Science}, 299, 873-877 (2003) \newline $^2$Goldman, et al., COSPAR04-A- 02395; D3.5-0015-04, (2004). [Preview Abstract] |
Thursday, October 27, 2005 10:42AM - 10:54AM |
QO1.00007: Quasineutral Particle/Fluid Simulation Techiques for Whistlers Martin Lampe, Glenn Joyce, Wallace Manheimer, Anatoly Streltsov, Guru Ganguli We present a new hybrid fluid/PIC simulation scheme for whistlers, which eliminate both speed of light and electron plasma oscillation time scale, and concentrates simulation resources on the resonant parts of the electron phase space that control whistler evolution. The fluid part advances the velocity and magnetic field; the electric field is determined by the solution of a Poisson like equation. The code runs with time steps on the order of the electron gyro frequency, with nearly perfect conservation of energy and numerical stability. Examples are shown of application to whistler instability growth and saturation, and on the ducting of whistlers in density channels. [Preview Abstract] |
Thursday, October 27, 2005 10:54AM - 11:06AM |
QO1.00008: Whistlers in Inhomogeneous Plasmas Anatoly Streltsov, Martin Lampe, Wallace Manheimer, Guru Ganguly, Glenn Joyce This paper examines ducting of whistlers in density channels perpendicular to an ambient magnetic field, focusing on the case where the transverse scale-sizes of the ducts are comparable to the perpendicular wavelength. Here analysis of the whistler ducting problem based on the geometrical optics becomes invalid, and numerical simulations of the full wave model should be done. Numerical model used in this study is based on the quasi-longitudinal, electron MHD model. Simulations confirms some of the classical results related to the guiding of the whistler waves by the density channels and also reveals some new effects. In particular, our results demonstrates that whistlers can be trapped not only in symmetrical density channels but also on a single transverse gradients in the background density, which is an important finding in the application to the magnetosphere. Also for high density ducts, we find that the wave energy can leak out due to coupling to modes propagating outside the duct. [Preview Abstract] |
Thursday, October 27, 2005 11:06AM - 11:18AM |
QO1.00009: Non-local wave-particle interactions of kinetic Alfven waves on auroral field lines Robert Lysak, Yan Song Recent observations from the FAST satellite as well as a number of sounding rocket missions have shown two distinct modes of auroral electron acceleration: the classic inverted-V signature consisting of a beam broad in pitch angle but narrowly confined in energy, and a lower energy, field-aligned acceleration that has been attributed to kinetic Alfven waves. Moreover, observations indicate that these two particle populations often co-exist, suggesting that Alfvenic acceleration can occur on field lines with a quasi-static potential drop. Electrons of appropriate energy and magnetic moment can be trapped between this potential drop and their magnetic mirror points. Calculations indicate that trapped electrons of a few hundred electron volts have bounce periods of 1-5 seconds, comparable to the period of waves in the ionospheric Alfven resonator, a structure produced by the gradients in the Alfve n speed above the auroral ionosphere. This suggests that a bounce resonant instability may occur that would excite waves in the resonator that could play a role in the acceleration of low-energy field-aligned electrons. A non-local kinetic theory including trapped electrons has been developed to determine what role such bounce resonance plays in the auroral acceleration process. [Preview Abstract] |
Thursday, October 27, 2005 11:18AM - 11:30AM |
QO1.00010: Formation of double layer in active space plasma and auroral particle acceleration Yan Song, Robert Lysak Mechanisms for magnetic reconnection and for the acceleration and energization of charged particles are two of the most important and long-standing questions in space plasma physics. The existence of a non-zero parallel electric field, which is often in the form of a double layer, is a necessary condition for auroral particle acceleration as well as the breakdown of the frozen-in condition required for reconnection. However, the basic dynamical theory of the generation of parallel electric fields has yet to be established. Models based mainly on the generalized Ohm's law yield only a force balance, not the generation of parallel electric field itself. Most models and theories of double layer are based on an assumed existence of the parallel potential drop, without explaining how the parallel potential drops are formed. The first dynamical double layer model will be given, based on the combination of a localized dynamo and parallel potential drop. The formation of double layers is often the result of nonlinear interaction of MHD wave packets rather than sinusoidal waves. Energy to support a significant double layer and the energization of charged particles comes from releasing the local free magnetic energy and/or kinetic energy during the nonlinear wave packet interaction. [Preview Abstract] |
Thursday, October 27, 2005 11:30AM - 11:42AM |
QO1.00011: Temporal Evolution of Directly Driven Hydrodynamic Jets Relevant to Astrophysics S. Sublett, J.P. Knauer, I.V. Igumenshchev, A. Frank, D.D. Meyerhofer A hydrodynamic jet is formed when a strong laser shock drives material from a metal plug in a dense, high-$Z$ washer through its hole into a low-density, foam ambient medium.~The jet is about ten times as dense as the medium, a ratio important for scaling to astrophysical phenomena.~The plug material and backlighter x-ray energy are varied to radiograph either the jet's core or its interaction with the ambient medium.~Temporal evolution of the lateral expansion of the bowshock, contact discontinuity, and Mach disk is also tracked at several times during the evolution.~The mass of the jet is determined. Quantitative comparisons with simulations are presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Thursday, October 27, 2005 11:42AM - 11:54AM |
QO1.00012: Effect of Ambient Medium Inhomogeneity on Jet Evolution B.E. Blue, S.G. Glendinning, T.R. Dittrich Experimental studies of hydrodynamic instabilities of interfaces related to inertial confinement fusion often utilize low-density foams. A detailed understanding of these instabilities necessitates an accurate knowledge of the materials in which they are evolving. Theoretical predictions and simulations of these instabilities often assume the foam to be a continuous solid medium. However, the foams are not a solid material; rather they are porous with cell sizes ranging from nanometers to microns. An experiment was performed on the OMEGA Laser to measure the temporal evolution of hydrodynamic jets into foams with the same density but different cell sizes. A 1 ns 5.5 kJ laser pulse was used to drive a 20 Mbar shock into an Al target backed by a 100 mg/cm$^{3}$ foam. An RF foam with 100 nm cell size was tested against a CH foam with 2 $\mu $m cell size. Snapshots of the jet's evolution were recorded with point-projection radiography at two different times. Results and simulations of the experiment will be presented. This work is performed under the auspices of the U. S. DOE by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. [Preview Abstract] |
Thursday, October 27, 2005 11:54AM - 12:06PM |
QO1.00013: Supersonic jet experiments on Sandia National Laboratories' Z accelerator G.R. Bennett, D.B. Sinars, M.E. Cuneo, D.K. Lobley (Ktech Corp.), T.A. Mehlhorn, J.L. Porter, D.G. Schroen (Schafer Corp.), R.A. Vesey, D.F. Wenger, B.H. Wilde, R.F. Coker, P.A. Rosen, J.M. Foster, T.S. Perry An x-ray-driven supersonic jet experiment has been performed on Sandia National Laboratories' Z accelerator. ~The 130-140 eV drive of a z-pinch-driven hohlraum ablates a cylindrical Al pin (300-micron-length, 600-micron-diameter) that is embedded half way into a 150-micron-thick Au washer. ~A strong convergent shock is formed on axis, and the dense Al plasma propagates into a 300 mg/cc RF foam, on the opposite side, and a jet is formed. ~The jet evolution is imaged by a 6.151 keV curved-crystal imaging system with 10-11 micron spatial resolution. ~This allows the bow shock, Kelvin-Helmholtz roll-up, and other jet features to be studied in detail and then compared with various radiation hydrodynamics codes. ~Radiographs can be reduced to running integral ``1-T'' plots (T=6.151 keV transmission) through various sections (of chosen width) of the jet along the direction parallel to the foam's z-axis, to provide metrics for direct comparison with simulations. Results are presented in this and the following paper. [Preview Abstract] |
Thursday, October 27, 2005 12:06PM - 12:18PM |
QO1.00014: Modeling of Supersonic Jet Experiments Performed on the Sandia National Laboratory Z Accelerator B.H. Wilde, R.F. Coker, G.R. Bennett, D.B. Sinars, P.A. Rosen, J.M. Foster, T.S. Perry As a follow on to the previous talk, we present the results of two-dimensional simulations for the supersonic jet experiments performed on the Sandia National Laboratory Z Accelerator. Since the jet is effectively driven by the gold hohlraum surrounding the Z tungsten wire array, the calculations are driven with the measured temperature profile obtained with a transmission grating spectrometer and an imaging x-ray silicon diode array. In addition to the peak temperature of $\sim $140 eV with a full-width half maximum of 20 ns, the ablatively-driven pin also sees $\sim $100 ns of a low temperature foot generated during the collapse of the wire array onto the symmetry axis. We have used the continuous-adaptive-mesh-refinement radiation-hydrodynamics code RAGE to design and analyze these experiments. Although the agreement of the jet evolution with the data is good, the material following the jet is not matched as well. This may be due to the limitation of using radiation-diffusion instead of transport for the simulations and a lack of a complete understanding of the asymmetries introduced during the pinch implosion. [Preview Abstract] |
Thursday, October 27, 2005 12:18PM - 12:30PM |
QO1.00015: Astrophysical Connections to Collapsing Radiative Shock Experiments A.B. Reighard, R.P. Drake, J.F. Hansen, B. Blue, S. Bouquet, L. Boireau, M. Koenig, T. Vinci Radiative shocks occur in many high-energy density explosions, but prove difficult to create in laboratory experiments or to fully model with astrophysical codes. Low astrophysical densities combined with powerful explosions provide ideal conditions for producing radiative shocks. Here we describe an experiment significant to astrophysical shocks, which produces a driven, planar radiative shock in low density Xe gas. Including radiation effects precludes scaling experiments directly to astrophysical conditions via Euler equations, as can be done in purely hydrodynamic experiments. We use optical depth considerations to make comparisons between the driven shock in xenon and specific astrophysical phenomena. This planar shock may be subject to thin shell instabilities similar to those affecting the evolution of astrophysical shocks. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grants DE-FG52-03NA00064, DE-FG53-2005-NA26014, and other grants and contracts. [Preview Abstract] |
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