Kinetic Turbulence in Astrophysical Plasmas: Waves and/or Structures?

TL;DR

This study investigates the nature of kinetic turbulence in astrophysical plasmas, revealing that both structures and waves coexist as localized wave packets that retain linear wave features during nonlinear evolution.

Contribution

It provides quantitative evidence that kinetic turbulence comprises localized wave packets maintaining linear wave properties, challenging the view of mutually exclusive waves and structures.

Findings

Both structures and fluctuations preserve linear kinetic Alfven wave features.

Kinetic turbulence can be viewed as an ensemble of localized, anisotropic wave packets.

Linear wave properties are retained during nonlinear evolution of turbulence.

Abstract

The question of the relative importance of coherent structures and waves has for a long time attracted a great deal of interest in astrophysical plasma turbulence research, with a more recent focus on kinetic scale dynamics. Here we utilize high-resolution observational and simulation data to investigate the nature of waves and structures emerging in a weakly collisional, turbulent kinetic plasma. Observational results are based on in situ solar wind measurements from the Cluster and MMS spacecraft, and the simulation results are obtained from an externally driven, three-dimensional fully kinetic simulation. Using a set of novel diagnostic measures we show that both the large-amplitude structures and the lower-amplitude background fluctuations preserve linear features of kinetic Alfven waves to order unity. This quantitative evidence suggests that the kinetic turbulence cannot be described as a mixture of mutually exclusive waves and structures but may instead be pictured as an ensemble of localized, anisotropic wave packets or "eddies" of varying amplitudes, which preserve certain linear wave properties during their nonlinear evolution.