Doppler-shifted dispersion relations: (a,c) show the sense of polarization and (b,d) compressibility ratio ξ_e. We mark the outward propagating waves with “+” and inward-propagating by “−”. We also indicate the frequency ranges for the observed RH and LH bands (green dashed lines), so that we are looking for the “blue” branches in the LH band, and the “red” branches in the RH band.