The velocity field of a coronal mass ejection: The event of September 1, 1980

A coronal mass ejection with the appearance of two sets of overlapping loops occurred at about 0600 UT on September 1, 1980, over the northwest limb of the sun. It was one of the fastest events observed by the Solar Maximum Mission coronagraph during the 1980 epoch, with apparent radial velocity components of several features approaching 1000 km s−1. A study of the slow evolution of Hα prominence filaments and coronal structures in the northwest solar sector suggests that the mass ejection resulted from the disruption of a helmet streamer in association with, possibly, two filaments to give rise to the double-loop structure. This event is well covered by 10 coronagraph images of good quality so that the complex velocity field, defined by the apparent motions of many different parts of the mass ejection, can be mapped out as a function of space and time. The results of such an analysis are presented and related to current concerns in the theoretical understanding of mass ejections. In particular, it is concluded that a self-similar description of the velocity field is a gross oversimplification and that although some evidence of wave propagation can be found, the bright features in this mass ejection are plasma structures moving (presumably) with frozen-in magnetic fields, rather than waves propagating through plasmas and magnetic fields.