Nonlinear and Linear Timescales near Kinetic Scales in Solar Wind Turbulence

TL;DR

This paper compares linear kinetic timescales with nonlinear turbulence timescales in the solar wind, finding that nonlinear timescales are often comparable to or faster than instability timescales, questioning the applicability of linear theory.

Contribution

It provides observational and simulation evidence that nonlinear turbulence timescales near kinetic scales are comparable to or faster than instability timescales in the solar wind.

Findings

Nonlinear cascade times are slower than cyclotron periods.

Turbulence timescales are comparable to or faster than instability timescales.

Linear theory assumptions may be invalid near kinetic scales.

Abstract

The application of linear kinetic treatments to plasma waves, damping, and instability requires favorable inequalities between the associated linear timescales and timescales for nonlinear (e.g., turbulence) evolution. In the solar wind these two types of timescales may be directly compared using standard Kolmogorov-style analysis and observational data. The estimated local nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic scales are approached, remain slower than the cyclotron period, but comparable to, or faster than, the typical timescales of instabilities, anisotropic waves, and wave damping. The variation with length scale of the turbulence timescales is supported by observations and simulations. On this basis the use of linear theory - which assumes constant parameters to calculate the associated kinetic rates - may be questioned. It is suggested that the product of proton gyrofrequency and nonlinear time at the ion gyroscales provides a simple measure of turbulence influence on proton kinetic behavior.