Intermittency and Alignment in Strong Reduced Magnetohydrodynamic Turbulence

Abstract:

We develop an analytic model of intermittent, three-dimensional, strong, reduced magnetohydrodynamic turbulence with zero cross helicity. We take the fluctuation amplitudes to have a log-Poisson distribution and incorporate into the model a new phenomenology of scale-dependent dynamic alignment. The log-Poisson distribution in our model is characterized by two parameters. To calculate these parameters, we make use of three assumptions: that the energy cascade rate is independent of scale within the inertial range, that the most intense coherent structures at scale lambda are sheet-like with a volume filling factor proportional to lambda, and that most of the cascade power arises from interactions between exceptionally intense fluctuations and much weaker fluctuations. We then compute the scalings of the power spectrum, the kurtosis, higher-order structure functions, and three different average alignment angles. These scalings appear to be consistent with a number of results from direct numerical simulations, as well as recently published solar-wind observations.