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The link between phase coherence and non-Gaussian statistics is investigated using magnetic field data observed in the solar wind turbulence near the Earth's bow shock. The phase coherence index C , which characterizes the degree of phase correlation ͑i.e., nonlinear wave-wave interactions͒ among scales, displays a behavior similar to kurtosis and reflects a departure from Gaussianity in the probability density functions of magnetic field fluctuations. This demonstrates that nonlinear interactions among scales are the origin of intermittency in the magnetic field turbulence. Solar wind is a good laboratory for the study of collisionless magnetohydrodynamic ͑MHD͒ turbulence ͑see, e.g., ͓1͔ and references therein͒. In particular, the intermittent nature of turbulence is one of the fundamental problems for understanding the complex behavior of fluids ͓2,3͔ and other dynamical systems ͓4͔. Solar wind intermittency can be characterized by the probability density functions ͑PDFs͒ of velocity ͑or magnetic͒ field fluctuations over a range of scales. For large scales the PDFs are approximately Gaussian. As the scale decreases, the tails of the distribution gradually become fatter ͓5͔.Since MHD turbulence is governed by nonlinear MHD equations, the turbulent fields may display non-Gaussian fluctuations where the phases among scales ͑e.g., phases of the Fourier modes͒ are not random. In some previous studies of MHD turbulence the so-called random-phase approximation has been adopted to describe random-phase mixing among scales ͓6͔. However, in solar wind turbulence coherent structures such as solitonlike waves are often observed, especially near the planetary bow shock ͓7͔. Therefore, in real situations, the description of MHD turbulence as a superposition of random-phase fluctuations may not be valid and a finite-phase correlation among scales is to be expected due to nonlinear wave-wave interactions. This paper investigates the link between non-Gaussianity ͑intermittency͒ and phase correlation ͑nonlinear interactions͒ among scales in solar wind turbulence. Previous works have revealed the nonGaussianity of PDFs in the solar wind as a signature of intermittency, but whether this departure from Gaussianity is due to nonlinear wave-wave interactions or not has not been clearly demonstrated yet. In analytic modeling and numerical simulations of intermittent turbulence based on a set of deterministic equations, it is naturally expected that the departure from Gaussianity is due to nonlinear interactions ͓2͔. In contrast, the observational data from solar wind are an admixture of deterministic signal and stochastic noise. In such a case, the demonstration of finite phase coherence is required to ascertain the nonlinear origin of non-Gaussian fluctuations. In the present work, we quantify the degree of nonlinear interactions in solar wind data using a phase coherence index and demonstrate its relation with kurtosis ͑flatness͒ in the structure function.A central assumption of the Kolmogorov 1941 ͑hereafter K41͒ theory is the sel...

Nonattracting chaotic sets (chaotic saddles) are shown to be responsible for transient and intermittent dynamics in an extended system exemplified by a nonlinear regularized long-wave equation, relevant to plasma and fluid studies. As the driver amplitude is increased, the system undergoes a transition from quasiperiodicity to temporal chaos, then to spatiotemporal chaos. The resulting intermittent time series of spatiotemporal chaos displays random switching between laminar and bursty phases. We identify temporally and spatiotemporally chaotic saddles which are responsible for the laminar and bursty phases, respectively. Prior to the transition to spatiotemporal chaos, a spatiotemporally chaotic saddle is responsible for chaotic transients that mimic the dynamics of the post-transition attractor.

We study coherent structures in solar photospheric flows in a plage in the vicinity of the active region AR 10930 using the horizontal velocity data derived from Hinode/SOT magnetograms. Eulerian and Lagrangian coherent structures are detected by computing the Q-criterion and the finite-time Lyapunov exponents of the velocity field, respectively. Our analysis indicates that, on average, the deformation Eulerian coherent structures dominate over the vortical Eulerian coherent structures in the plage region. We demonstrate the correspondence of the network of high magnetic flux concentration to the attracting Lagrangian coherent structures (a-LCS) in the photospheric velocity based on both observations and numerical simulations. In addition, the computation of a-LCS provides a measure of the local rate of contraction/expansion of the flow.