Non-linear mirror instability

• Published in: Monthly notices of the Royal Astronomical Society. Letters Vol. 447; no. 1; pp. L45 - L49
• Main Authors: Rincon, F., Schekochihin, A. A., Cowley, S. C.
• Format: Journal Article
• Language: English
• Published: 11.02.2015
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1093/mnrasl/slu179

Slow dynamical changes in magnetic-field strength and invariance of the particles’ magnetic moments generate ubiquitous pressure anisotropies in weakly collisional, magnetized astrophysical plasmas. This renders them unstable to fast, small-scale mirror and firehose instabilities, which are capable of exerting feedback on the macroscale dynamics of the system. By way of a new asymptotic theory of the early non-linear evolution of the mirror instability in a plasma subject to slow shearing or compression, we show that the instability does not saturate quasi-linearly at a steady, low-amplitude level. Instead, the trapping of particles in small-scale mirrors leads to non-linear secular growth of magnetic perturbations, δB/B ∝ t2/3. Our theory explains recent collisionless simulation results, provides a prediction of the mirror evolution in weakly collisional plasmas and establishes a foundation for a theory of non-linear mirror dynamics with trapping, valid up to δB/B = O(1).