The critical velocity effect as a cause for the Hα emission from the Magellanic Stream

Observations show significant H alpha emissions in the Galactic halo near the edges of cold gas clouds of the Magellanic Stream. The source of the ionization of the cold gas is still a wide-open question. In our paper we discuss the critical velocity effect as a possible explanation for the observed H alpha emission. The critical velocity effect can yield a fast ionization of cold gas if this neutral gas passes through a magnetized plasma under suitable conditions. We show that for parameters that are typical for the Magellanic Stream, the critical velocity effect has to be considered as a possible ionization source of high relevance.