4 Discussion and conclusions

The distribution of DEs in time and heliocentric distances shows a bimodal character, possibly related to the spatial distribution of the magnetic sector boundaries in the interplanetary medium (Voelzke & Matsuura 1998). This result corroborates the idea that DEs are associated with sector boundary crossings and that such an association is an essential element in the morphological development of DEs in plasma tails (Niedner & Brandt 1978; Brandt & Niedner 1987; Brandt et al. 1999).

Although this work assumes that the disconnected plasma moves away from the nucleus at constant velocity for a given DE (Voelzke & Matsuura 1998), the onsets of DEs calculated in this way are in good agreement with the onsets calculated by Yi et al. (1994b), who determined the time of disconnection by kinematic analysis assuming constantly accelerated linear motion. Although the kinematic analysis was different and the definition and classification for one DE differs a little from one author to another, the similarity of the results from these two different analyses suggests, that magnetic reconnection plays an important role in the formation of DEs and can be considered as the triggering mecanism.

The solar wind plays a major role in plasma tail DEs, which are characterized by the plasma tail being uprooted from the comet head and convected downstream in the solar wind being replaced by a new tail constructed from folding rays. The comparison of the solar wind conditions and the DEs of P/Halley's comet on 1985 December 13-14 and 1986 February 22 shows that these DEs are associated primarily with crossings of the heliospheric current sheet and apparently not with any other properties of the solar wind such as high-speed streams, dense regions, or dynamic pressure increases.

Of course better heliospheric current sheet and solar wind data, more comet images from the ground and in situ measurements from cometary flyby missions will greatly contribute our understanding of the triggering mechanism of cometary DEs.

Acknowledgements

The support obtained by M.R.V. from Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP, under grant 98/00246-8 and from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, CAPES, under grant BEX2582/97-0, are gratefully acknowledged. The authors thank Dr. J.C. Brandt for helpful comments.