Dynamic Alignment as a Statistical Survival Effect

Abstract

Dynamic alignment in magnetohydrodynamic turbulence is often interpreted as scale-dependent alignment of counterpropagating Els"asser increments (δ_r z^\pm), with consequences for inertial-range spectra. We show that standard amplitude-weighted measurements do not establish progressive alignment of typical fluctuations. We separate angular statistics from Els"asser-amplitude weighting and interpret the signal as finite-time retention of amplitude--angle states, tested with Johns Hopkins Turbulence Database simulations and NASA Wind measurements. In the simulations, the unweighted folded angle (θ_r) between (δ_r z^+) and (δ_r z^-), with alignment and anti-alignment folded together, remains only moderately below the random 3D baseline and shows no monotonic decrease across inertial-range separations. Smaller angles in weighted diagnostics are produced mainly by large (A_r=|δ_r z^+||δ_r z^-|) events, giving a negative covariance between (A_r) and (\sinθ_r) that is removed by shuffled controls. Transition measurements show that high-amplitude large-angle states deplete faster than high-amplitude small-angle states. The source--depletion balance reconstructs second-order Els"asser amplitudes and gives an effective rms increment scaling close to (\ell_\perp^{1/4}), although the typical folded angle is nearly scale independent. Mean-log increment-amplitude checks give larger slopes than second-order-amplitude fits in both simulation and Wind data, consistent with stronger intermittent-event weighting of second-order statistics. Wind measurements reproduce the same amplitude--angle hierarchy and negative covariance under Taylor sampling. Conventional dynamic-alignment diagnostics therefore measure selective retention of intense Els"asser fluctuations, not progressive alignment of typical fluctuations.