SH42A-01 INVITED
Magnetic reconnection in turbulent plasmas
With the progress of collisionless reconnection models a really burning question arises. Is the reconnection rate fast in collisionless environments and slow when collisions are important? The positive answer to that question means that all MHD simulations of collisional environments do not make sense, as high diffusivity of codes makes numerical reconnection fast. An alternative scheme of reconnection was proposed several years ago and it invokes field wandering induced by MHD turbulence. In the presence of collisionless effects or anomalous resistivity the scheme allows to address the second question above, as it solves the mass and flux outflow constraint without imposing the necessity of having a global X-point. However, our study has shown that the effect of the small- scale magnetic field structure on the rate of magnetic reconnection may be even more profound. The turbulence may affect the rate of reconnection by reducing the transverse scale for the reconnection flow and by allowing many independent reconnection events to occur simultaneously. A non-trivial prediction of the model is that, in the presence of turbulence, the reconnection may be fast even for normal Ohmic resistivity. I shall show our first results on turbulent reconnection, discuss the dependence of the reconnection on the level turbulence, injection scale of turbulence in the system and the dimensionality of the simulations. I shall show that the reconnection is fast, i.e. does not depend on resistivity only in 3D simulations.
SH42A-02 INVITED
The Viscosity of the Collisionless Solar Wind and the Reynolds Number of Its MHD Turbulence
Every fluctuation of MHD turbulence has a flow nature and an Alfven-wave nature. Further, all Alfven waves in a collisionless plasma are subject to Landau damping Hence, for the solar wind all turbulent fluctuations experience dissipation, which can be couched as a shear viscosity of the solar-wind plasma and this shear viscosity can be used to construct Reynolds numbers. The expressions obtained for viscosity are quite simple. For the solar-wind plasma, the Landau-damping viscosity is much stronger than the Braginskii shear viscosity, yielding turbulence Reynolds numbers R of 500,000 to 50,000,000, depending on the model used to describe the solar-wind turbulence. These R values are compared with effective Reynolds numbers obtained from the ratios of the measured correlation lengths to the measured Taylor scales of the solar-wind turbulence. A discussion follows as to whether MHD turbulence obeys the same scalings with Reynolds number as does Navier-Stokes turbulence.
SH42A-03
3D Simulations of Solar Wind Plasma Turbulence
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 Alfvén 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.
SH42A-04
A Reassessment of the Waves + Q2D Model of Solar Wind Fluctuations Based on Simulations with a Turning Magnetic Field
We examine a popular "two-component" model of the solar wind in light of the necessity for the assumed alignment of the components to turn along with the Parker spiral. In particular, we use a 3-D MHD spectral code to show that neither Q2D nor slab-wave components will turn their wavevectors in a turning Parker-like field. This is consistent with the need for transverse inhomogeneities in the background to create the required changes in the radial field. Combining the two components does result in a turning, but it is only through a nonlinear interaction between the components that no longer allows a clear identification of either component. Thus, we argue that while a "slab + Q2D" model may be a useful approximation for some purposes, it does not represent the underlying nature of the fluctuations and their anisotropy. Results to date are based on a static "mis-aligned" mean field, but we believe the results will hold for the turning case that we plan to implement soon.
SH42A-05
Diffusion Confusion: Field Line Random Walk in Magnetic Turbulence
Magnetic field lines in a plasma with both a mean field and a turbulent component will generally spread away
from the mean field direction in a process called field line random walk (FLRW). Theoretical treatments
generally agree that the character of this spreading is governed by the behavior of the turbulence spectrum
at very large scales. In slab turbulence, subdiffusion, superdiffusion, and ordinary classical diffusion are all
mathematically possible, while in two-dimensional (2D) turbulence diffusive and superdiffusive regimes occur.
This work aims to delimit the possible behaviors of FLRW in terms of the turbulence correlation function and
wave spectrum. In the case of slab turbulence, a magnetic correlation function that has bounded support
(i.e., correlations vanish beyond some sufficiently large distance) rules out the superdiffusive regime of
FLRW. Subdiffusion remains technically possible, but only for correlation functions with special properties. In
the case of 2D turbulence, the homogeneity condition suffices to place FLRW in the diffusive regime.
Further, even for inhomogeneous systems that display asymptotic superdiffusive behavior, this behavior is
obtained only for the idealized case of an infinitely large system with turbulence wave modes populating
arbitrarily large scales. Imposing a finite system size (or an upper wavelength limit) restores asymptotic
diffusive behavior to FLRW. Supported by NASA Heliophysics Guest Investigator grant NNX07AH73G, by
NASA Heliophysics Theory grant NNX08AI47G, and by the Thailand Research Fund.
http://neutronm.bartol.udel.edu/
SH42A-06
Spatial and Temporal Analysis of Magnetic Helicity in the Solar Wind
Magnetic helicity Hm is one of the three quadratic invariants of the ideal MHD equations of motion. An important property is that it is a pseudoscalar and changes sign under coordinate inversion x→ -x. Then, Hm represents a measure of lack of mirror symmetry of the magnetic field or, in other words, it estimates the "knottedness" of magnetic field lines. A positive/negative Hm would indicate a right/left- hand sense of polarization. A standard analysis of reduced Hm, based on Fourier decomposition and performed in interplanetary space, would show that the handedness of the fluctuations in a given wave number is uncorrelated with the handedness of fluctuations at nearby wavenumbers. As a matter of fact, Hm would continuously fluctuate between positive and negative values throught the whole frequency range. However, this kind of analysis provides only part of the information needed to fully characterize the topology of magnetic field lines. A remarkable help comes out from a new technique, which uses a wavelet decomposition, able to unravel interesting features of the magnetic field fluctuations probably related to the filamentary structure of turbulence. Results from a recent Sun-Earth-Ulysses alignment and, more in general, statistical studies performed in fast and slow wind within the inner heliosphere will be discussed. Moreover, single case studies focussing on interplanetary flux ropes will be reported and possible implications with persistence and sharpness of observed energetic particles dropouts phenomenon at the Earth orbit will be addressed.
SH42A-07
Linear and non linear tearing and Kelvin-Helmholtz driven instabilities in current-sheets with velocity shears: three-dimensional compressive MHD simulations.
Magnetic shear driven instabilities play a major role both in the dynamics of astrophysical objects and, in particular, in the evolution of several structures in the heliosphere. Although tearing-driven dynamics in two dimensions are relatively well understood, in three dimensions the overall dynamics can be highly complex due to the onset of secondary instabilities. The presence of sheared flows, and the resulting stream plus current-sheet interaction, adds to this complexity. Considering two different perturbed equilibrium configurations of a current-sheet, a pressure-balanced and a force-free configuration, we present the three dimensional evolution of a tearing instability driven current- sheet in the presence of velocity shears, in the framework of compressible and resistive MHD. The large scale structure of the initial configuration determines the linear and non linear evolution of the system: primary (resistive and/or Kelvin-Helmholtz like) modes are selected according to the geometry of the magnetic field, secondary instability development depends on the initial equilibrium configuration with the strongest modes characterized by a specific direction in Fourier space. The competition between primary and secondary modes determines the global plasma structure in the non linear regime and, in all cases, the magnetic energy spectrum is observed to be highly anisotropic.