Comment on "Heating of ions by low-frequency Alfven waves in partially ionized plasmas" [Phys. Plasmas 18, 030702 (2011)]

Some comments on a study conducted by Dong and Paty investigating heating of ions by low-frequency Alfvén waves in partially ionized plasmas are presented. The heating efficiency is found to be lower in partially ionized plasmas than that in plasmas without ion-neutral collision. The results also showed that parallel propagating Alfven waves can only heat pickup ions, which initially do not follow the coherent particle motion dictated by these Alfvén waves. The randomization or heating process saturates when phase differences are sufficiently large. It is thus concluded that in partially ionized plasma, such as solar chromosphere, the pickup ions ionized from the neutral particles may be heated by the Alfven waves only in a time scale of a collision period.