The Alfvenic nature of energy transfer mediation in localized, strongly nonlinear Alfven wavepacket collisions

TL;DR

This study demonstrates that in localized Alfven wave collisions, all resulting fluctuations are Alfven waves, confirming their fundamental role in mediating energy transfer in plasma turbulence.

Contribution

It provides a direct comparison showing that localized Alfven wave collisions produce only Alfven waves, highlighting their key role in energy cascade processes.

Findings

Localized collisions produce only Alfven waves.

All nonlinear fluctuations in localized collisions are Alfven waves.

Energy transfer to smaller scales is mediated by Alfven waves.

Abstract

In space and astrophysical plasmas, violent events or instabilities inject energy into turbulent motions at large scales. Nonlinear interactions among the turbulent fluctuations drive a cascade of energy to small perpendicular scales at which the energy is ultimately converted into plasma heat. Previous work with the incompressible magnetohydrodynamic (MHD) equations has shown that this turbulent energy cascade is driven by the nonlinear interaction between counterpropagating Alfven waves - also known as Alfven wave collisions. Direct numerical simulations of weakly collisional plasma turbulence enables deeper insight into the nature of the nonlinear interactions underlying the turbulent cascade of energy. In this paper, we directly compare four cases: both periodic and localized Alfven wave collisions in the weakly and strongly nonlinear limits. Our results reveal that in the more realistic case of localized Alfven wave collisions (rather than the periodic case), all nonlinearly generated fluctuations are Alfven waves, which mediates nonlinear energy transfer to smaller perpendicular scales.