Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session UO5: Solar Wind, Magnetosphere, and Plasma Astrophysics Theory |
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Chair: Steven Spangler, University of Iowa Room: Ballroom F |
Thursday, November 17, 2011 2:00PM - 2:12PM |
UO5.00001: Modeling of resonant sweeping of Alfv\'{e}n waves in divergent solar wind Valentin Shevchenko, Vitaly Galinsky Sweeping mechanism of absorption of Alfv\'{e}n wave energy in the divergent solar wind is modeled using a scale-separation approach.\footnote{Galinsky V.L. and V.I. Shevchenko, {\it Phys. Rev. Lett}, \textbf{85}, 90, (2000)} MHD waves are excited through reconnection at the sun with frequencies below the local ion cyclotron frequency. The fluctuations with highest frequencies will be absorbed by heavy ions as the wave packet propagates within the lower corona in the outward direction in decreasing magnetic field. At larger distances smaller frequencies will fall in cyclotron resonance with heavy ions but the wave absorption can eventually stop as the heavy ions form the shell-like distribution. Amplitude at these frequencies can even grow due to secondary cyclotron instability\footnote{V. Shevchenko et al., {\it Phys. Plasmas}, \textbf{11}, 4290, (2004)}. Part of the wave spectrum can reach at some distance the cyclotron frequency of $\alpha$-particles where the same picture will take place. The remaining low frequency part of the spectrum that falls in cyclotron resonance with protons at larger distances from the sun will be responsible for the solar wind heating and acceleration. We answer the questions: ``Will the wave spectrum be completely absorbed by minor heavy ions and $\alpha$-particles?'' and ``Do we need some additional mechanism of generation of the MHD waves interacting with protons?'' [Preview Abstract] |
Thursday, November 17, 2011 2:12PM - 2:24PM |
UO5.00002: Residual energy in magnetohydrodynamic turbulence and in the solar wind Stanislav Boldyrev, Jean Carlos Perez, Vladimir Zhdankin, Yuxuan Wang Recent observations of the solar wind turbulence reveal puzzling breakdown of equipartition between kinetic and magnetic energies. This phenomenon is studied in the framework of weak magnetohydrodynamic (MHD) turbulence consisting of Alfven waves propagating in opposite directions along the guide magnetic field. It is demonstrated that nonlinearly interacting Alfven waves spontaneously generate imbalance of magnetic and kinetic energies, leading to the accumulation of residual energy Er=Ev-Eb at small field-parallel wavenumbers. The effect is also studied phenomenoligically and numerically for the case of strong MHD turbulence. The generation of the residual energy may be a manifestation of magnetic self-organization in a driven turbulent system. Implication of this phenomenon for the theory of MHD turbulence and for practical applications is discussed. [Preview Abstract] |
Thursday, November 17, 2011 2:24PM - 2:36PM |
UO5.00003: Scaling Laws in Magnetohydrodynamics: Simulations vs Observations Jean C. Perez, Stanislav Boldyrev, John Podesta, Joseph Borovsky A comparison between spectral scaling laws in MHD turbulence simulations and solar wind observations at 1 AU is presented. Distributions of spectral indices for the velocity, magnetic field, and total energy, computed both from high resolution numerical simulations and solar wind observations, show remarkable agreement. The results show the magnetic field spectrum Eb is steeper than the velocity spectrum Ev, while the residual energy Eb-Ev decreases more rapidly following a $k_\perp^{-2}$ scaling. The agreement between simulations and observations persists for both balanced and imbalanced turbulence, that is, when one considers regions with and without cross helicity. In light of these results, it will be discussed to what extent incompressible MHD can adequately describe random magnetic and velocity fluctuations measured in the solar wind at 1 AU. [Preview Abstract] |
Thursday, November 17, 2011 2:36PM - 2:48PM |
UO5.00004: Current Sheet Boundaries in MHD Turbulence and in the Solar Wind Vladimir Zhdankin, Stanislav Boldyrev, Jean Perez, Joanne Mason Current sheets inside of plasmas are characterized by strong changes in the magnetic field direction. We study this property of current sheets by measuring the angular change of the magnetic field direction across fixed spatial increments throughout the plasma domain. Using data from turbulent MHD simulations, we find that the probability distribution of angular change obeys an exponential law, with a scaling that is largely independent of the choice of spatial increment. In the first case, reduced MHD is used with a strong guide field ($\frac{\delta b}{B_0} = 1/5$), and the scaling is approximately fit by exp($-\theta/6.5$). In the second case, full MHD is used with a weak guide field, and the fit is exp($-\theta/21.7$). It is proposed that the difference in scaling parameters between the two regimes is due to the strength of the background magnetic field. This may explain the observations of spacecraft in the solar wind, which found two distinct populations of magnetic discontinuities with different exponential distributions of angular change in magnetic field, e.g. Borovsky (2008) and Miao et al. (2011). [Preview Abstract] |
Thursday, November 17, 2011 2:48PM - 3:00PM |
UO5.00005: Observations of parallel propagating EMIC and whistler waves in the solar wind with wavenumbers $kc/\omega_{pi} \sim 1$ John J. Podesta Parallel propagating electromagnetic
ion cyclotron (EMIC) waves and electron cyclotron
(whistler) waves can be generated through proton pressure
anisotropy
instabilities in the solar wind which are believed to
play an important role in the regulation of proton distribution
functions.
However, observations that provide a positive identification of
these waves are rare in the relevant wavenumber range $kc/
\omega_{pi} \sim 1$.
Here I report observations obtained using wavelet analysis
techniques which
indicate that these waves are ubiquitous in high speed
streams.
The observations show the persistent presence of parallel
propagating
EMIC waves propagating predominantly {\it away} from the sun
along the magnetic
field and/or whistler waves propagating predominantly {\it
toward} the sun
along the magnetic field. The average power of these parallel
propagating
waves is comparable to the trace magnetic power at the same
wavenumber. When the observed differential streaming of alpha
particles
is taken into account in calculations of the growth rates, it is
found that proton pressure anisotropy instabilities
preferentially generate
EMIC waves propagating away from the sun when $T_{\perp p}>T_
{\parallel p}$ and
whistler waves propagating toward the sun when $T_{\perp p} |
Thursday, November 17, 2011 3:00PM - 3:12PM |
UO5.00006: Helicity and Super-Helicity in Homogeneous Turbulent Shear Flow Frank Jacobitz, Kai Schneider, Wouter Bos, Marie Farge Initial mean helicity is imposed on direct numerical simulations of homogeneous turbulent shear flow. As time is advanced in the simulations, an approximately exponential growth of the turbulent kinetic energy is found, but the mean helicity is observed to decay. Distributions of helicity are skewed according to its initial value. A wavelet-based scale-dependent analysis shows that this skewness is largest for large scales of the turbulent motion and decreases for smaller scales. In addition, a trend to two-dimensionalization for large scales of motion and a preference for helical motion at small scales is found. Joint probability distribution functions show a strong correlation of the signs of helicity and super-helicity for all cases, including Gaussian fields. This correlation supports the conjecture that super-helicity dissipates helicity. [Preview Abstract] |
Thursday, November 17, 2011 3:12PM - 3:24PM |
UO5.00007: Intermittency and geometrical statistics of 3d homogeneous magnetohydrodynamic turbulence using wavelets Kai Schneider, Katsunori Yoshimatsu, Naoya Okamoto, Yasuhiro Kawahara, Marie Farge Scale-dependent intermittency and geometrical statistics of 3d incompressible homogeneous magnetohydrodynamic turbulence without mean magnetic field are examined by means of the orthogonal wavelet decomposition. The field is computed by direct numerical simulation with a Fourier spectral method at resolution $512^3$, and the magnetic Prandtl number is taken to unity. Scale-dependent second- and higher-order statistics of the velocity and magnetic fields allow to quantify the intermittency of the fields in terms of spatial fluctuations of the energy spectrum, the flatness and the probability distribution functions of both fields at different scales. Scale-dependent different relative helicities, yield geometrical information on alignment between different scale-dependent fields. At each scale, the scale-dependent alignment between the velocity and magnetic field is found to be more pronounced than the other alignments considered here. Finally, statistical scale-dependent analyses of both Eulerian and Lagrangian accelerations and the corresponding time-derivatives of the magnetic field are performed. [Preview Abstract] |
Thursday, November 17, 2011 3:24PM - 3:36PM |
UO5.00008: Multiscale LES Models for Incompressible Magnetohydrodynamics David Sondak, Assad Oberai We develop novel LES models for incompressible magnetohydrodynamics (MHD). Energy equations for the resolved magnetic induction and the velocity fields are presented and analyzed to gain insight into the nature of the new model terms. In particular, the ability of the multiscale technique to capture the small scale dynamo is assessed. Following this \textit{a priori} energy analysis, energy spectra of the magnetic induction and the velocity field are compared to those obtained via direct numerical simulations in order to gauge the performance of the new models. [Preview Abstract] |
Thursday, November 17, 2011 3:36PM - 3:48PM |
UO5.00009: Numerical study of laboratory MRI experiment Christophe Gissinger, Jeremy Goodman, Hantao Ji Theoretical studies of the MagnetoRotational Instability (MRI) generally rely on a local description, or computations between axially infinite (or periodic) cylinders. Since laboratory MRI experiments involve finite geometries, it is important to understand the effect of boundaries on the MRI. We investigate numerically the flow of a conducting fluid in a Taylor-Couette flow when an axial magnetic field is applied. To minimize Ekman recirculation due to vertical no-slip boundaries, two rotating rings are used in the vertical endcaps, approximating setup used in the Princeton MRI experiment. Our 3D global simulations show that, in presence of boundaries, the nature of the bifurcation, the saturation and the structure of axisymmetric MRI modes are significantly affected by the resultant recirculation. In addition, large scale non-axisymmetric modes are obtained when the applied field is sufficiently strong. We show that these modes are related to destabilization of a free shear layer created by the conjugate action of the applied field and the rotating rings. Finally, we compare our calculations in cylindrical and spherical geometries to recent experimental results obtained in the Maryland experiment and the Princeton MRI experiment. [Preview Abstract] |
Thursday, November 17, 2011 3:48PM - 4:00PM |
UO5.00010: The Role of Magnetic Structures in Dynamo Action J. Pratt, W.-C. Mueller By tracking the movements of Lagrangian particles, we examine the formation of large-scale magnetic structures and current filaments in steady-state 3D turbulent magnetoconvection maintained by dynamo action. The movement of fluid particles evolves differently when large structures develop in the convecting plasma, and this difference is reflected in Lagrangian statistics. We discuss the role that magnetic structures in combination with convective motions play in the dynamo process. Our simulation employs the Boussinesq approximation to the MHD convection equations to allow for the effect of temperature fluctuations on the flow {\it via} buoyancy forces. Pseudo-spectral simulations performed at resolutions of $512^3$ and $1024^3$ solve these equations for a geometric cube of plasma with an imposed mean temperature gradient. Boundary conditions are fully periodic and disallow vertical streamers, specifically $k_z=0$ velocity or temperature modes. [Preview Abstract] |
Thursday, November 17, 2011 4:00PM - 4:12PM |
UO5.00011: Propagation of radiation in fluctuating multiscale plasmas: Kinetic theory and simulations Kunwar Pal Singh, P.A. Robinson, Yu. Tyshetskiy, Iver H. Cairns Propagation of radiation in plasmas with small scale fluctuations is very important in many applications, especially in the systems involving multiscale physics where plasma nonuniformities are highly prevalent. A theory of radiation propagation in a large scale plasma with small scale fluctuations is developed using a kinetic description of the radiation in terms of the probability distribution function of the radiation in space, time, and wave vector space. Large scale effects associated with the refractive index of the plasma and small scale effects such as scattering of radiation by density clumps in fluctuating plasma, spontaneous emission, damping, mode coupling, and mode conversion are included in the multiscale kinetic description of the radiation. A finite difference algorithm is developed to solve the kinetic equation governing propagation of radiation. The algorithm is tested and verified for diffusion in wave-vector space, nonuniform plasma and small scale density fluctuations. The multiscale simulations verify the main physical effects in plasma profiles that approximate those in space plasmas, and demonstrate that realistic simulations can be carried out in a feasible amount of computational time. [Preview Abstract] |
Thursday, November 17, 2011 4:12PM - 4:24PM |
UO5.00012: Magnetospheric Plasma Conditions During Periodic Substorms W. Horton, E. Spencer Sawtooth events in the period between 1998-2006 are input into the low-dimensional WINDMI physics model to obtain the statistical average of the energy content in the earth's ring current, central plasma sheet and geotail lobes. The input to the model are solar wind parameters obtained from the ACE spacecraft, while the output are the AL index and Dst index. We use a database of events to constrain the WINDMI model physics parameters. The model predicts the geotail lobe magnetic energy, the plasma sheet electric field and plasma sheet pressure that results in the triggering of periodic substorms. The relative timing between the electric field, magnetic field and plasma sheet pressure during the triggering and the growth phase of the substorms are presented. We show that for certain fixed states, the response of the magnetosphere depends on the level of solar wind forcing. We also present the differences in magnetospheric plasma conditions between periodic substorms and isolated substorm events, all under southward IMF conditions. [Preview Abstract] |
Thursday, November 17, 2011 4:24PM - 4:36PM |
UO5.00013: Whistler amplification: a free electron laser in the Earth's magnetosphere A.R. Soto-Chavez, A. Bhattacharjee, C.S. Ng Whistler mode emissions are very low frequency (VLF) waves in the Earth's magnetosphere that arise due to the interaction of whistler waves with radiation belt electrons. They have the characteristic feature of having typical frequencies of half the electron gyro-frequency at the geomagnetic equator and with saturation amplitudes of more than 30 dB. They also chirp in frequency. The amplification of these VLF waves has been studied both analytically and with simulations. However, while the analytic approaches have made use of the Hamiltonian equations for the electron motion, what has been missing is an analytical equation for the radiation field that brings closure to the problem of amplification of the whistler wave. Based on the similarities between free electron lasers (FELs) and whistler mode emissions, we present here a new set of closed relativistic equations. We show that these equations predict, through a cubic equation, whistler amplification levels well in agreement with those observed in the Earth's magnetosphere. We also discuss the implications of our formulation on the phenomenon of chirping of these modes. [Preview Abstract] |
Thursday, November 17, 2011 4:36PM - 4:48PM |
UO5.00014: How do double layers form inside the auroral cavity? Daniel Main, David Newman, Clark Scholz, Robert Ergun One of the unresolved questions in auroral physics is how the auroral potential drop is distributed. One view is that a near-uniform ambipolar electric field (with $\sim$ mV/m electric field amplitudes) exists along auroral magnetic field lines which, when integrated, leads to auroral potential drops of $\sim 10^4$ V. Another view is that the field lines are populated by a number of discrete double layers (with amplitudes of a few hundred mV/m) which, when added up, can also leads to auroral potential drops of $\sim 10^4$ V. The actual field distribution may combine elements of both models. Here, we consider the second model focusing on the upward current region. We present results from one and two-dimensional Particle-in-Cell simulations of double layers (DLs) in the interior of the auroral cavity, known as ``mid-cavity'' DLs (Ergun et. al.,2004). The simulations include hot H$^+$ magnetospheric ions and electrons, cold dense ionospheric electrons, and H$^+$ and O$^+$ beams. We show that upon the formation of a DL at the ionosphere-auroral cavity boundary, the non-linear evolution of the ion beams in the auroral cavity leads to an earthward traveling H$^+$ beam. This H$^+$ beam interacts with the anti-earthward H$^+$ beam forming an ion acoustic soliton and a candidate mid-cavity DL. FAST data in support of this interpretation are presented. [Preview Abstract] |
Thursday, November 17, 2011 4:48PM - 5:00PM |
UO5.00015: Spatial Intermittency in Electron Magnetohydrodynamic Turbulence Bhimsen Shivamoggi Spatial intermittency in the energy cascade of electron magnetohydrodynamic (EMHD) turbulence is considered. A multi- fractal model for the energy dissipation field is considered to determine intermittency corrections to the scaling behavior in the high-wavenumber (hydrodynamic limit) and low-wavenumber (magnetization limit) asymptotic regimes of the inertial range. Extrapolation of the multi-fractal scaling down to the dissipative microscales does seem to confirm in these asymptotic regimes a dissipative anomaly previously indicated by the numerical simulations of EMHD turbulence. [Preview Abstract] |
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