June 10, 2011, 12:00 pm - 1:00 pm
June 10, 2011, 12:00 pm - 1:00 pm
Structure in Solar Wind Turbulence: Implications for Discontinuities, Correlations, Intermittency, and Heating
Dr. William Matthaeus, University of Delaware and Bartol Research Institute
Based on observations of solar wind fluctuations, it has long been recognized that the magnetic field forms internal boundaries and characteristic correlations with other plasma variables. Frequently occurring directional discontinuities (DDs) are a familiar example of the former, while Alfv’enic fluctuations (AFs), defined by correlated velocity and magnetic fluctuations, exemplify the latter. Neither has been historically interpreted as properties of active MHD turbulence – if anything these features are viewed as contrasting or even contradicting the expectations of a turbulence description. Recently however evidence is accumulating that DDs may be dynamically formed internal boundaries, e.g., due to interacting magnetic flux tubes in MHD turbulence. In this view these structures are a manifestation of turbulence intermittency. At the same time, the appearance of Alfvenic correlations, as well as force-free and Beltrami correlations, has been shown to occur rapidly and generically in active turbulence as an integral part of the cascade process. The tendency to form patches of correlation with near discontinuous boundaries is a consequence of minimization of forces, i.e., suppression of nonlinearity. Furthermore for MHD turbulence, the coherent structures themselves are expected to be directly responsible for small scale nonGaussian statistical features, and also the site of enhanced dissipation and heating. Recent work continues to connect these ideas to the solar wind, employing conditional statistics to isolate the presence and effects of discontinuities and structures. Examples to be discussed are the signatures of heating near current sheets [1], methods for identifying candidate reconnection sites [2], and the presence of highly variable patches of Alfvenicity in the solar wind [3]. These properties are leading towards a much better understanding of the solar wind as turbulent magnetofluid plasma, as well as the prospects for understanding he dissipation mechanisms that couple the MHD activity to the kinetic microphysical plasma dynamics. [1] K. Osman et al, Astrophys. J., 727, L11 (2011) [2] S. Servidio et al, J. Geophys. Res., (2011) in press. [3] K. Osman, M. Wan et al, Astrophys. J. (submitted)