Role of subdominant stable modes in plasma microturbulence

In gyrokinetic simulations, thousands of degrees of freedom are available to contribute to the fluctuation spectrum. For wavevectors with a single linear instability, the unstable eigenmode accounts for only one of these degrees of freedom. Little has been known about the role of the remaining fluctuations in the turbulent dynamics. In this paper, these fluctuations are characterized as modes in mode decompositions of gyrokinetic distribution functions from nonlinear simulations. This analysis reveals the excitation of a hierarchy of damped modes at the same perpendicular scales as the driving instabilities. Two effects of these subdominant modes are described: First, these damped modes define a potent energy sink, creating a situation where energy drive and energy dissipation peak at the same perpendicular scales. Second, damped modes with tearing parity (even parity about the outboard midplane for A fluctuations) are driven to significant amplitudes and facilitate the development of magnetic stochasticity in electromagnetic gyrokinetic simulations.