Intermittency scaling laws in the fast solar wind and magnetohydrodynamic turbulence simulations.

Abstract:

Turbulence is an irregular state characterized by the excitation of fluid motions over a wide range of length-scales. Intermittency is a process caused by the stochastic dissipation of the energy cascade as it proceeds from large to small scales, developing characteristically non-Gaussian statistics from the inertial range to dissipation scales. The spatial structures generated by this phenomenon, which has been observed in simulations as well as observations in fluids and plasmas, influence dissipation, heating, transport and acceleration of charged particles. In this work, the structure and scaling properties of the statistical distribution of field increments are investigated using solar wind observations and numerical simulations of magnetohydrodynamic (MHD) turbulence. The moments of these distributions, also known as structure functions, satisfy universal power law scalings in the inertial range. However, lack of ergodicity and finite sample size are the main complications when calculating these functions, for both experimental data and results from simulations, especially for high order structure functions where rare events play a very important role. Our approach aims to overcome these limitations by instead modeling the scale-dependent Probability Distribution Functions (PDFs) of field increments of various turbulent quantities and infer their scaling law properties from these PDFs. For observations we used 23 years of data from the WIND spacecraft located near 1 AU, carefully selected to represent periods of homogeneous and incompressible turbulence in the fast solar wind. For simulations, high resolution simulations of homogeneous RMHD (2048^3 grid points) were analyzed. Results show very good agreement between observations and simulations when comparing PDFs, allowing us to obtain better estimates of the scaling exponents of structure functions for homogeneous and incompressible turbulence in the fast solar wind.