Morphological analysis of the plasma structures of comet P\Halley

Five hundred and thirty-one images from September 1985 to July 1986 are analysed for the purpose of identifying, measuring and correlating the morphological structures along the plasma tail of P\Halley. These images are from The International Halley Watch Atlas of Large-Scale Phenomena (Brandt et al., 1992). A systematic visual analysis revealed 124 wavy structures along the main tail and 27 along the secondary tails, 109 solitary waves (solitons) along the main tail and 37 along the secondary tails, 12 Swan-like (Hyder et al., 1974; Niedner and Brandt, 1980; Jockers, 1985) tails, 47 disconnection events (DEs) (Niedner and Brandt, 1979; Jockers, 1985; Celnik et al., 1988; Delva et al., 1991) and 23 knots (Matsuura and Voelzke, 1990; Voelzke, 1996; Voelzke et al., 1997). While the wavy structures denote undulations or a train of waves, the solitons refer to the formations usually denominated kinks (Tomita et al., 1987). The distribution of the DEs in time or heliocentric distance presents a bimodal character possibly associated with the spatial distribution of the magnetic sector boundaries in the interplanetary medium. In general, it is possible to associate the occurrence of a knot and\or a Swan-Tail with the occurrence of a DE, but the last one may occur independently. The 47 DEs documented in 47 different images allowed the derivation of 19 onsets of DEs (Table 1), i.e., the time when the comet supposedly crossed a frontier between magnetic sectors of the solar wind. The present analysis indicates that in a given DE the disconnected plasma moves away from the comet nucleus at constant velocity (this conclusion differs from results in the literature). Such a velocity, however, varies broadly from one DE to another. Nine onsets of DEs were determined before the perihelion passage with an average of the corrected velocities Vc equal to (260±87) km s−1, and ten after the perihelion with an average equal to (130±37) km s−1. This asymmetric behaviour of Vc in relation to the perihelion may be due to the asymmetric way by which the radial component of the orbital velocity of the comet combines with the velocity of the solar wind in the cometocentric frame of reference. The mean value of the corrected wavelength λc measured in 16 different wavy structures is equal to (2.2 ± 0.2)×106 km. The mean value of the corrected cometocentric phase velocity Vpc is equal to (114±31) km s-1 and the mean amplitude A of the wave is equal to (2.8±0.5)×105 km. There is a tendency for A andλc to increase with increasing cometocentric distance.