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...

Abstract. We apply two distinct nonlinear techniques, kurtosis and phase coherence index, to analyze the modulus of interplanetary magnetic field data |B| measured by Cluster and ACE spacecraft from 1 to 3 February 2002. High degree of phase synchronization is found across a wide range of time scales, from 1 s to 10 4 s, in the magnetic field fluctuations, both in the shocked solar wind upstream of Earth's bow shock and in the unshocked ambient solar wind at the L1 Lagrangian point. This is the first direct measurement of phase coherence in the ambient solar wind turbulence. We show that phase synchronization related to nonlinear multiscale interactions is the origin of the departure from Gaussianity in the intermittent magnetic field turbulence. In particular, we demonstrate that at small scales near the spectral break the intermittency level of Cluster is lower than ACE, which may be a signature of the reflected ions from the shock.

Amplitude and phase synchronization due to multiscale interactions in chaotic saddles at the onset of permanent spatiotemporal chaos is analyzed using the Fourier-Lyapunov representation. By computing the power-phase spectral entropy and the time-averaged power-phase spectra, we show that the laminar (bursty) states in the on-off spatiotemporal intermittency correspond, respectively, to the nonattracting coherent structures with higher (lower) degrees of amplitude-phase synchronization across spatial scales.

In a recent paper, the relation between current sheet, magnetic reconnection, and turbulence at the leading edge of an interplanetary coronal mass ejection was studied. We report here the observation of magnetic reconnection at the interface region of two interplanetary magnetic flux ropes. The front and rear boundary layers of three interplanetary magnetic flux ropes are identified, and the structures of magnetic flux ropes are reconstructed by the Grad–Shafranov method. A quantitative analysis of the reconnection condition and the degree of intermittency reveals that rope–rope magnetic reconnection is the most likely site for genesis of interplanetary intermittency turbulence in this event. The dynamic pressure pulse resulting from this reconnection triggers the onset of a geomagnetic storm.

ABSTRACT The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form prominent structures as they are advected by photospheric flows. The aim of this paper is to study supergranular turbulence by detecting Lagrangian coherent structures (LCS) based on the horizontal velocity fields derived from Hinode intensity images at disc centre of the quiet Sun on 2010 November 2. LCS act as transport barriers and are responsible for attracting/repelling the fluid elements and swirling motions in a finite time. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE), and vortices are found by the Lagrangian-averaged vorticity deviation method. We show that the Lagrangian centres and boundaries of supergranular cells are given by the local maximum of the forward and backward FTLE, respectively. The attracting LCS expose the location of the sinks of photospheric flows at supergranular junctions, whereas the repelling LCS interconnect the Lagrangian centres of neighbouring supergranular cells. Lagrangian transport barriers are found within a supergranular cell and from one cell to other cells, which play a key role in the dynamics of internetwork and network magnetic elements. Such barriers favour the formation of vortices in supergranular junctions. In particular, we show that the magnetic field distribution in the quiet Sun is determined by the combined action of attracting/repelling LCS and vortices.