Refined critical balance in strong Alfvénic turbulence

Abstract

Abstract We present numerical evidence that in strong Alfvénic turbulence, the critical balance principle – equality of the non-linear decorrelation and linear propagation times – is scale invariant, in the sense that the probability distribution of the ratio of these times is independent of scale. This result only holds if the local alignment of the Elsasser fields is taken into account in calculating the non-linear time. At any given scale, the degree of alignment is found to increase with fluctuation amplitude, supporting the idea that the cause of alignment is mutual dynamical shearing of Elsasser fields. The scale-invariance of critical balance (while all other quantities of interest are strongly intermittent, i.e. have scale-dependent distributions) suggests that it is the most robust of the scaling principles used to describe Alfvénic turbulence. The quality afforded by situ fluctuation measurements in the solar wind allows for direct verification of this fundamental principle.