A previous analysis of anomalous width variation for a single-ion
rarefactive solitary wave [S. S. Ghosh and A. N. Sekar Iyengar, Phys. Plasmas,
4, 3204 (1997)] has been extended to a multi-ion species, and shows that the corresponding width–amplitude characteristics depend crucially on the respective ion
concentrations; for example, with increasing light-ion concentration, an ‘increasing-
width’ rarefactive solitary wave transforms to a ‘decreasing-width’ one for a
constant Mach number exhibiting an overall U-shaped variation profile. The influence of the second ion species generally decreases with increasing amplitude,
being maximum for the single to multi-ion transition. However, in the neighbourhood of double-layer-like solutions, the influence again becomes prominent, which
may be associated with the corresponding energy–amplitude relations for both the
ions.