In a recent study [1] the effect of ionization and losses on the propagation of ion acoustic solitary waves is investigated. For that purpose the corresponding source and sink terms of the form $$\begin{equation} Q_{\rm i}- \nu_{\rm L}n_{\rm i}, \label{ss}\end{equation}$$ are included in the right-hand side of the ion continuity equation. The related contribution of the source–sink effects in the ion momentum equation is then given as the above source–sink term multiplied by the ion velocity, so that the ion momentum equation used in [1]becomes $$\begin{equation}n_{\rm i} \biggl(\frac{\partial v_{\rm i}}{\partial t} + v_{\rm i}\frac{\partial v_{\rm i}}{\partial x}\biggr)= - n_{\rm i}\frac{\partial \phi}{\partial x} - \sigma_{\rm i} \frac{\partial n_{\rm i}}{\partial x} - \nu_{\rm i} n_{\rm i} v_{\rm i} - (Q_{\rm i}- \nu_{\rm L} n_{\rm i}) v_{\rm i}.\end{equation}$$ The objective of this Comment is to show that the last term in (2) cancels out exactly, as will be shown in the further text, so that any effect originating from this term is spurious. In addition, this term implies that the velocity of the neutrals that become ions, before the impact ionization (or whatever the cause of its ionization may be), is equal to the velocity of ions that become neutrals, since this source–sink term is multiplied by the same velocity. A similar improper model has been used previously in the literature (see the references cited in [1]). It is remarked that the proper inclusion of the source–sink effects has been described in the literature before, although it seems to remain unnoticed. Therefore, it is timely to address this issue in order to provide a formally correct model for future studies.

The main objective of the Comment made by J. Vranjes et al. is that the source–sink approach in studying physical phenomena, such as the ionization instability, is suspect. Also the term $(Q_{\rm i}-\nu_{\rm L} n_{\rm i})v_{\rm i}$ in the ion momentum conservation equation of the source–sink model used by [1] is improper.

This issue of the Journal of Plasma Physics is dedicated to Radu Balescu on the occasion of his 65th birthday.

Of the more than 170 articles published worldwide by Professor Balescu to date, more than 25 have been published in this journal. Readers of the Journal of Plasma Physics will thus be well acquainted with his contributions to the study of the interplay between plasma turbulence and the theory of transport phenomena in magnetically confined plasmas. A vast community of students, teachers and researchers will also have read or consulted the many monographs, five to date, written by Balescu and world-famous for their clarity. In his first monograph, Statistical Mechanics of Charged Particles (Interscience, New York), published in 1963, the whole of plasma physics, including the transport equation to which his name will remain attached, was put into the broader context of statistical physics. The monograph Equilibrium and Non-Equilibrium Statistical Mechanics (Wiley-Interscience, New York) followed in 1975, and still inspires many teachers today.