Parallel plasma pressure ($p_{\parallel }=m\int (v_{\parallel }-u_{\parallel }{)}^{2}f{d}^{3}v$)

The normal stress along the local magnetic field, evaluated using the random parallel velocity. At zero equilibrium mean flow, its linear perturbation is obtained from $m\int v_{\parallel }^2\delta f{d}^{3}v$ together with the perturbation of the velocity-space measure.