Energy absorption length, L_A, as a function of the wave period, P = 2π/ω, for an Alfvén wave propagating in a prominence plasma when different values of the hydrogen ionization ratio, ξ_i, are assumed. The meaning of the various line styles is indicated within the figure. The parameters used in the computations are ρ = 5 × 10^-11 kg m^-3, T = 8000 K, B = 10 G, and A_He/A_H = 0.1. Logarithmic scale has been used in both axes.