MIRROR INSTABILITY .1. PHYSICAL-MECHANISM OF LINEAR INSTABILITY

The mirror instability is prevalent in planetary and cometary magnetos heaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived fr om magnetohydrodynamic fluid theory, later work showed that there were significant differences between the fluid theory and a more rigorous kinetic approach. Here we point out that the instability mechanism hin ges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with othe r uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other in stabilities driven by resonant particles. We comment on the implicatio ns of these results for our understanding of the observations of mirro r instability-generated signals in space.