Intermittent MHD structures and classical discontinuities

Abstract

[1] We examine statistics of rapid spatial variations of the magnetic field in simulations of magnetohydrodynamic (MHD) turbulence, by analyzing intermittency properties, and by using classical methods for identifying discontinuities. The methods identify similar structures, and give very similar event distribution functions. When the results are scaled to the correlation length, the average waiting times agree with typically reported waiting times between solar wind discontinuities. Thus discontinuities may be related to flux tube boundaries and intermittent structures that appear spontaneously in MHD turbulence.

Intermittency, nonlinear dynamics and dissipation in the solar wind and astrophysical plasmas

“…If, alternatively, the discontinuities are features of turbulence, then a methodology is needed that can compare properties, preferably statistical properties, of discontinuities in a known turbulence environment, such as MHD simulation, with the observed discontinuities in the solar wind. One such approach is the partial variance of increments (PVI) method, which was designed precisely for this purpose [36,63]. The PVI time series is defined in terms of the vector magnetic field increment B(s, τ ) = B(s + τ ) − B(s), which is evaluated along a linear trajectory labelled by s (in space or time) with a lag τ .…”

Multipoint observations of plasma phenomena made in space by Cluster

“…They may represent the borders in between 'spaghetti-like' magnetic flux tubes of a discontinuous (solar wind) plasma (Burlaga 1969;Bruno et al 2001;Borovsky 2006), or, again in the solar wind, may be fossils of the solar coronal origin (Burlaga 1968;Borovsky 2006). In addition, observed discontinuities often coincide with current sheets that may form as a consequence of an MHD turbulent cascade (Matthaeus and Montgomery 1980;Veltri 1999) and recently, it has been pointed out that their statistical properties are similar to distributions obtained from numerical simulations of MHD turbulence (Greco et al 2008(Greco et al , 2009. In this interpretation, current sheets are characteristic coherent structures expected in active intermittent MHD turbulence and are therefore integral to the dynamical couplings across scales.…”

“…Intermittency at the MHD scales has also been observed in other Cluster datasets (Alexandrova et al 2008a;Kiyani et al 2009), and appears to be a fundamental feature of the cascade. Greco et al (2008) proposed another method for investigating the intermittent behavior of solar wind turbulence using the Partial Variance of Increments (PVI) technique. This method analyzes the signal 5) where B(t, τ ) are the magnetic field (vector) increments in (2.4), and the brackets • T represent a time average.…”

Evidence for Nonlinear Development of Magnetohydrodynamic Scale Intermittency in the Inner Heliosphere

“…The square of the above quantity has been called the PVI (Greco et al 2008) and the method abbreviated as the PVI method. The performance of the PVI to identify discontinuities has been shown to be comparable to standard methods (e.g., Tsurutani & Smith 1979) in both MHD simulations and solar wind observations (Greco et al 2008(Greco et al , 2009a. Note that by construction PVI 2 = 1, while PVI 4 is related to the kurtosis of the magnetic field component increments.…”

“…We make use of the partial variance of increments (PVI) method (Greco et al 2008). Instead of employing this diagnostic only to identify events above a certain threshold, we introduce an alternative strategy based on the computation of the cumulative probability of finding the PVI statistics above a specified threshold.…”