A gyrokinetic approach to modeling mirror and firehose instabilites in the solar wind

Abstract:

Observational surveys of temperature anisotropy in the solar wind indicate that anisotropy is bounded over a wide range of plasma beta and the anisotropy bounds appear to be predominately controlled by wave-particle interactions associated with mirror and oblique firehose instabilities. We present a reduced kinetic description that exploits gyrosymmetry (a symmetry associated with the gyromotion), providing an efficient, self-consistent approach that can be utilized in global models of the solar wind. We discuss the underlying physics of the mirror and firehose instabilities that allow for a reduced gyrokinetic description, and we verify the approach through comparisons of theory and simulations using gyrokinetic, hybrid, and fully kinetic descriptions. We present simulations showing the nonlinear development and saturation of the mirror instability and explain the amplitude and structure of the nonlinear state in terms of particle trapping. Finally, we present new insights into the nature of the parallel and oblique firehose instability by considering how the topology of the dispersion surfaces change as an anisotropic population is added to an isotropic plasma. We discuss the role of resonant and nonresonant particles in the instability and show that a gyrokinetic description is in good agreement with a fully kinetic description.