Isothermal coplanar magnetohydrodynamic characteristic matrix ($v=u_x\pm c_f,u_x\pm c_{\mathrm{slow}}$)

For one-dimensional flow obeying an isothermal equation of state with $u_z=B_z=0$, the primitive variables $(\rho ,u_x,u_y,B_y)$ form a four-variable hyperbolic system and $B_x$ is constant. The speeds relative to $u_x$ satisfy $c^4-(c_s^2+v_a^2)c^2+c_s^2{v}_{ax}^2=0$. These are the fast and slow magnetosonic waves. The independent out-of-plane Alfvén wave polarization and a freely advected entropy mode are absent from this reduced isothermal system.