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 Magnetospheric Multiscale (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 Alfvén 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.1 MoreReceived 14 June 2018Revised 3 June 2019DOI:https://doi.org/10.1103/PhysRevX.9.031037Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic waves & oscillationsPlasma turbulencePhysical SystemsSpace & astrophysical plasmaTechniquesParticle-in-cell methodsSatellite data analysisPlasma PhysicsGravitation, Cosmology & AstrophysicsParticles & Fields
Highlights
A wide range of turbulent systems, in different areas of physics, supports linear waves [1,2,3,4,5,6,7]
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
Subsequent numerical studies showed that current sheets emerge in a more realistic configuration with two counterpropagating, initially separated wave packets [164], and that the modes produced during nonlinear wave interaction can be themselves characterized as Alfven waves [165]
Summary
A wide range of turbulent systems, in different areas of physics, supports linear waves [1,2,3,4,5,6,7]. Turbulence exhibits striking features of self-organization in the form of coherent structures, such as electric current sheets in magnetized plasmas or columnar vortices in rotating fluids [11,17,18,19,20,21,22,23,24,25]. These intense structures are believed to significantly impact the turbulence statistics and the rate
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