There is no theory that can explain the structures of the which is still unknown. A dynamic model for the formation SYBs perfectly. However, we review some of the theories mechanism of synchronic bands, which is based on the following process, is proposed. presented so far and point out the difficulties with them The complex particles of the aggregates of the unit particles (Nishioka et al. 1992).
are ejected from the nucleus of the comet and disintegrate repeatedly into individual unit particles at various disintegra-2.1. Fragmentation Model tion speeds. Then, these unit particles break up and their fragments are observed as synchronic bands. These fragments con-It is generally believed that the particles of the type II tinue to disintegrate or sublimate into smaller pieces and finally tail of a comet move around the Sun along their Keplerian they become too small to be seen at a certain normalized orbits, receiving both solar gravitational force and solar lifetime.radiation pressure. Hereafter, the ratio of solar radiationThe structures calculated with this theory fit well the obpressure to solar gravitational force is denoted by ͱ as served shape and orientation of the synchronic bands of Comet in conventional use. The fragmentation model (Sekanina West and Comet Seki-Lines. This dynamic model suggests 1976) (hereafter SFM) explains the formation of a SYB that the radii of the complex particles and the radii of the unit by using a time dependent ͱ value of the particles. The particles are of less than visible wavelength.
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