= Pressure-anisotropy-modified magnetic tension
{title2=$\mathcal T=B^2/(4\pi)+p_\perp-p_\parallel$}
In <Gaussian units>, the gyrotropic <pressure tensor> adds directional <stress> to the <magnetic tension>. A transverse displacement of a uniform <magnetic field> obeys $\rho\partial_t^2\boldsymbol\xi_\perp=\mathcal T\partial_\parallel^2\boldsymbol\xi_\perp$. Positive $\mathcal T$ gives a restoring <force>; negative $\mathcal T$ gives the fluid <firehose instability>. This follows by combining $-\nabla\cdot\mathsf P$ with the divergence of the <Maxwell stress tensor>. In <SI> replace $B^2/(4\pi)$ by $B^2/\mu_0$.
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