Mirror instability: 1. Physical mechanism of linear instability

Abstract

The mirror instability is prevalent in planetary and cometary magnetosheaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived from 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 hinges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with other uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other instabilities driven by resonant particles. We comment on the implications of these results for our understanding of the observations of mirror instability-generated signals in space.

Compressional Pc5 type pulsations in the morningside plasma sheet

“…Thus, for the magnetic and thermal pressure to be in antiphase, plasma should be anisotropic, T ⊥ > T , and only in special cases, magnetic and thermal pressures should balance each other. However, Southwood and Kivelson (1993) show that if the contribution from resonant particles is taken into account during the growth of the mirror mode, magnetic and thermal pressures balance each other. A similar result was obtained earlier by Southwood (1976).…”

Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

“…If the temperature anisotropy | | -R 1 p of the protons exceeds a certain threshold, then the plasma becomes unstable, and various kinds of plasma waves and/or nonpropagating structures grow, while the distribution function relaxes toward a stable state. If > R 1 p , the plasma can excite parallel-propagating Alfvén/ion-cyclotron (A/IC) waves or nonpropagating mirror modes (Rudakov & Sagdeev 1961;Sagdeev & Shafranov 1961;Tajiri 1967;Southwood & Kivelson 1993;Gary & Lee 1994;Kunz et al 2014;Riquelme et al 2015;Gary et al 2016). If < R 1 p , the plasma can excite parallel-propagating fastmagnetosonic/whistler (FM/W) waves or nonpropagating oblique firehose modes (Quest & Shapiro 1996;Gary et al 1998;Hellinger & Matsumoto 2000;Hellinger & Trávníček 2008;Rosin et al 2011).…”

The Near-Saturn Magnetic Field Environment

“…A common signature in the magnetosheath is a periodic anti-correlation between magnetic field strength and density fluctuations, which is indicative of so-called mirror mode waves [Violante et al, 1995]. These nonpropagating and compressive MHD waves are generated by the mirror instability which occurs in anisotropic plasmas when T⊥/T∥ > 1+1/β⊥ [Southwood and Kivelson, 1993]. This condition is more easily met at Saturn's magnetosheath where β is larger than at Earth.…”