Dispersion curves of a) propagating and b) standing sausage waves. In a) the real (k_R) and imaginary (k_I) parts of the longitudinal wavenumber are given as functions of the real angular frequency ω. In b) the real (ω_R) and imaginary (ω_I) parts of ω are shown as functions of the real wavenumber k, which is given by the vertical axis though. In both panels, the solid (dashed) curves represent the trapped (leaky) regime. Two density ratios (ρ_i/ρ_e = 4 and 100) are examined as represented by the red and blue curves, respectively.