Pressure-balanced plasma perturbation ($\delta p_{\perp }+B_0\delta B/(4\pi )=0$)

A low-frequency, strongly oblique plasma perturbation in which perpendicular magnetic pressure equilibrates rapidly compared with the retained dynamics. In Gaussian units, its leading total transverse pressure perturbation vanishes as shown. Thus $\delta B/B_0=-(\beta /2)\delta p_{\perp }/p_0$, where $\beta$ is plasma beta. A small fractional magnetic field perturbation can still exert a leading force.