Firehose instability
= Firehose instability
{title2=$p_\parallel-p_\perp>B^2/(4\pi)$}
{wiki}
Parallel particle <pressure> can overcome <magnetic tension>. In the long-<wavelength> gyrotropic <plasma> limit, the <Alfvén wave> has $\omega^2=k_\parallel^2[B^2/(4\pi)+p_\perp-p_\parallel]/\rho$, so the condition in the title makes it unstable. The detailed kinetic threshold at shorter scales requires additional physics.