KINETIC PHYSICS OF THE MIRROR INSTABILITY

Kinetic mechanisms for the growth and saturation of the mirror instability are described using one-dimensional hybrid simulations. Two parameter regimes are considered. In the first regime, a relatively small ion anisotropy excites a slowly growing instability that produces small-amplitude waves; most ions respond to the waves as an adiabatic fluid. In the second regime a large anisotropy excites a rapidly growing instability that generates large-amplitude waves; the response of many ions in this case is nonactiabatic. The difference in ion response is due to the relative importance of two ion populations, resonant and nonresonant. Resonant ions, those ions with low velocities parallel to the background magnetic field, contribute to the growth of the instability as a result of their gyrointeractions with the noncoplanar component of the wave electric field and respond to the mirror waves nonadiabatically. Nonresonant ions, those with large parallel velocities, respond as an adiabatic fluid. In both regimes, ion anisotropy is reduced by means of the magnetic mirror force; in the second regime, the anisotropy is further reduced as a consequence of the field rotation at magnetic minima. The anisotropy reduction reduces the free energy available for wave growth and leads to the saturation of the mirror instability.