Linear Marginal Stability Research Articles

Nonlinear theory of absolute whistler instabilities in finite β plasmas

The nonlinear evolution and stabilization of absolute whistler instabilities driven by electron pressure anisotropy and propagating obliquely to the external magnetic field in finite β plasmas are investigated without introducing the assumption of random phases. Both in the firehose and the mirror instability regimes the pressure anisotropy relaxes quasilinearly toward a more stable state. The nonlinear wave equation has also been obtained. By choosing the parameters in such a way that the plasma is sufficiently close to linear marginal stability, it has been possible to show both numerically and analytically that the (firehose) instability stabilizes nonlinearly at sufficiently low levels of excitation. The fluctuation level does not asymptotically approach a constant value, but oscillates in time, the maximum amplitude being proportional to the linear growth rate. Also, close to marginal stability, finite Larmor radius effects can play a significant role even in the limit kR ≪ 1. It is found that finite Larmor radius effects are stabilizing if tan2α > 4β‖/β⊥, where α is the angle between the direction of wave propagation and the direction of the background magnetic field and β‖(⊥) is the ratio of parallel (perpendicular) kinetic to magnetic pressure.