  Context. In 1985, at the end of the active phase 1977-1986, a broad (4000
km/s) Ly alpha line appeared in the symbiotic system CH Cygni that had never
been observed previously. Aims. In this work we investigate the origin of this
anomalous broad Ly alpha line. Methods. We suggest a new interpretation of the
broad Ly alpha based on the theory of charge transfer reactions between ambient
hydrogen atoms and post-shock protons at a strong shock front. Results. We have
found that the broad Ly alpha line originated from the blast wave created by
the outburst, while the contemporary optical and UV lines arose from the nebula
downstream of the expanding shock in the colliding wind scenario.
