http://arxiv.org/abs/1402.0010
The first hybrid-kinetic numerical simulations of the firehose and mirror instabilities in a collisionless plasma are performed in which pressure anisotropy is driven as the magnetic field is changed by a persistent linear shear S. For a decreasing field, it is found that mostly oblique firehose fluctuations grow at Larmor scales and saturate with energies ~S^{1/2}; the pressure anisotropy is pinned at the stability threshold by anomalous particle scattering. In contrast, nonlinear mirror fluctuations are large compared to the Larmor scale and grow secularly in time; marginality is maintained by an increasing population of resonant particles trapped in magnetic mirrors. After one shear time, saturated order-unity magnetic mirrors are formed and particles scatter off their sharp edges. Both instabilities drive sub-Larmor-scale fluctuations, which appear to be kinetic-Alfven-wave turbulence. Our results impact theories of momentum and heat transport in astrophysical and space plasmas, in which the stretching of a magnetic field by shear is a generic process.
Tue, 4 Feb 14
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