Descripción: The first direct determination of the inertial range energy cascade rate, using an anisotropic form of Yaglom's law for magnetohydrodynamic turbulence, is obtained in the solar wind with multispacecraft measurements. The two-point mixed third-order structure functions of Elsässer fluctuations are integrated over a sphere in magnetic field-aligned coordinates, and the result is consistent with a linear scaling. Therefore, volume integrated heating and cascade rates are obtained that, unlike previous studies, make only limited assumptions about the underlying spectral geometry of solar wind turbulence. These results confirm the turbulent nature of magnetic and velocity field fluctuations in the low frequency limit, and could supply the energy necessary to account for the nonadiabatic heating of the solar wind. © 2011 American Physical Society.

Descripción: Correlation anisotropy emerges dynamically in magnetohydrodynamics (MHD), producing stronger gradients across the large-scale mean magnetic field than along it. This occurs both globally and locally, and has significant implications in space and astrophysical plasmas, including particle scattering and transport, and theories of turbulence. Properties of local correlation anisotropy are further documented here by showing through numerical experiments that the effect is intensified in more localized estimates of the mean field. The mathematical formulation of this property shows that local anisotropy mixes second-order with higher order correlations. Sensitivity of local statistical estimates to higher order correlations can be understood in connection with the stochastic coordinate system inherent in such formulations. We demonstrate this in specific cases, and illustrate the connection to higher order statistics by showing the sensitivity of local anisotropy to phase randomization, after which the global measure of anisotropy is recovered at all scales of averaging. This establishes that anisotropy of the local structure function is not a measure of anisotropy of the energy spectrum. Evidently, the local enhancement of correlation anisotropy is of substantial fundamental interest and must be understood in terms of higher order correlations, specifically fourth-order and above. © 2012. The American Astronomical Society. All rights reserved.

Descripción: Recent studies have compared properties of the magnetic field in simulations of Hall MHD turbulence with spacecraft data, focusing on methods used to identify classical discontinuities and intermittency statistics. Comparison of ACE solar wind data and simulations of 2D and 3D turbulence shows good agreement in waiting-time analysis of magnetic discontinuities, and in the related distribution of magnetic field increments. This supports the idea that the magnetic structures in the solar wind may emerge fast and locally from nonlinear dynamics that can be understood in the framework of nonlinear MHD theory. The analysis suggests that small scale current sheets form spontaneously and rapidly enough that some of the observed solar wind discontinuities may be locally generated, representing boundaries between interacting flux tubes. © 2010 American Institute of Physics.

Descripción: Numerical simulations of two-dimensional Magnetohydrodynamic (2D MHD) turbulence reveal the presence of a huge number of sites where magnetic reconnection locally occurs. The properties of this ensemble of reconnection events, that are spontaneously generated by turbulence, have been studied. The associated reconnection rates, computed as the electric field at the neutral points, are broadly distributed and the statistics of these events are presented. This new description of reconnection is relevant for space and laboratory plasmas, where generally turbulence is present. © 2010 American Institute of Physics.

Descripción: In this work, recent advances in numerical studies of local reconnection events in the turbulent plasmas are reviewed. Recently [1], the nonlinear dynamics of magnetic reconnection in turbulence has been investigated through high resolution numerical simulations. Both fluid (MHD and Hall MHD) and kinetic (HybridVlasov) 2D simulations reveal the presence of a large number of X-type neutral points, where magnetic reconnection locally occurs. The associated reconnection rates are distributed over a wide range of values and they depend on the local geometry of the diffusion region. This new approach to the study of magnetic reconnection has broad applications to the turbulent solar wind (SW). Strong magnetic SW discontinuities are in fact strongly related to these intermittent processes of reconnection [2, 3]. Methods employed to identify sets of possible reconnection events along a one-dimensional path through the turbulent field (emulating experimental sampling by a single detector in a highspeed flow) are here reviewed. These local reconnection/discontinuity events may be the main sites of heating and particle acceleration processes [4]. Results from hybrid-Vlasov kinetic simulations support these observations [5, 6]. In the turbulent regime, in fact, kinetic effects manifest through a deformation of the ion distribution function. These patterns of non-Maxwellian features are concentrated in space nearby regions of strong magnetic activity. These results open a new path on the study of kinetic processes such as heating, particle acceleration, and temperature anisotropy, commonly observed in astrophysics. © 2013 AIP Publishing LLC.