The visibility of “Halo” coronal mass ejections

Abstract The January 6, 1997 halo CME is the subject of continuing research due to the large geomagnetic effects of this event. This paper presents a very simplified model which explains the key observational features. The CME is modelled as a hemispherical shell with a cone shaped extension toward the sun. The observed circular arc is the projection of the “shoulders” of the CME onto the image plane. The low surface brightness and the fuzzy appearance of the halo CME are naturally explained.