 Alfvén characteristic eigenvector

For propagation along $x$, the Alfvén right eigenvector has $\delta \rho =\delta p=\delta u_x=\delta B_x=0$, ${\mathbf{B}}_{\perp }\cdot \delta {\mathbf{u}}_{\perp }=0$ and $\delta {\mathbf{B}}_{\perp }=-s\sqrt{{\mu }_0\rho }\delta {\mathbf{u}}_{\perp }$. Its speed is $u_x+s{B}_x/\sqrt{{\mu }_0\rho }$, $s=\pm 1$. When the transverse background field vanishes, both transverse polarizations are allowed.