  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.

Space Physics;Plasma Physics