Propagating inhomogeneities in the dust tail of comet West 1975

Abstract

COMET West has displayed some remarkable properties. Sekanina¹ has studied the multiple splitting of its nucleus and also discussed the strange structure of its dust tail. This tail is characterised by a system of bright bands which do not correspond to the true synchronous band² and which are shown by very few comets. We call these striae (striated tail) and to study them we have examined numerous original and duplicate pictures, and selected four observations (Table 1) which reveal that the morphology was relatively well conserved during the evolution of the dust tail over a period of more than 4 days. We have identified three striae (Fig. 1) which seem to propagate in space while retaining their basic form. This phenomenon is not unusual in plasmas, such as gas tails of comets³ or streamers in the solar K-corona^4,5. However, this is apparently the first time that it has been observed in dust ‘clouds’. These striae, which we describe here, are made of dust grains which can be readily demonstrated by the wide-field colour photographs made by comet observers: the gas tail (type I) appears distinctly blue because of CN emission and is directly in the anti-solar direction as well as being well separated from the large dust tail (type II) which has a neutral colour.