The striae in the dust tails of great comets — a comparison to various theories

We reanalyse positions and motion of the striae in the dust tails of the bright comets Mrkos 1957 V and West 1976 VI. Two theories are compared to the data: the high speed particle ejection theory of NOTNI (1964) and the two-step kinematical sequence proposed by SEKANINA and FARRELL (1980) in their particle fragmentation theory. The final decision is in favour of the two step sequence, though the real situation may be a mixtum compositum of both. The parent clouds (first step) are found to have low lateral velocity dispersion and higher than expected acceleration. The question how these clouds originate remains unsolved. It is suggested that they comprise particles which left the coma into a tailward region of high magnetic field and plasma density. Another possibility, additionally accounting for the sometimes apparent duplicity, is an origin out of an optically thick coma at angles orthogonal to the radius vector.