Five main tail streamers of Comet Halley

It is of much interest that Comet Halley displayed long and straight main tails (tail streamers) even at large heliocentric distances. Streamers are usually thought to be plasma in nature. However, a dust streamer may be seen superimposed on the main tail when the Earth is close to the cometʹs orbital plane. Because of the broad sensitive wave range of the photographic plate and of projection effect, both plasma and dust streamers may be superimposed in the image. In this paper, photometric and geometric measurements of five main tail streamers of Comet 1P/Halley at different heliocentric distances are made and compared. The brightness profiles along streamer axes, the full widths at half maximum (FWHMs) of several transverse cuts, the apparent and actual aberration (or lag) angles of the axes as well as the tail lengths are obtained. Among the streamers recorded on blue plates, two before the perihelion passage are definitely plasma streamers. The similarity of the photometry and geometric features of the five streamers and other lines of evidence suggest that the other three streamers are also mainly plasma streamers, while any contamination by dust would be secondary.