We have noticed that the table 1 had some errors on its last three columns. Now the corrected values are given in the table.

Table 1. Comparison between the thermal and phase velocity for the chosen parameters.

The first line of p. 7 is corrected as ‘we have assumed that the plasma contains both $H^{+}$ and $O^{+}$ , the former being the cooler component with $\unicode[STIX]{x1D707}_{i}=0.4$ ’.