This work compares cometary and
solar wind data with the purpose of determining the solar wind conditions
associated with comet plasma tail disconnection events (DEs). The cometary
data are from
The
International Halley Watch Atlas of Large-Scale Phenomena
(Brandt et al.
1992a).
A systematic visual analysis of the atlas images
(Voelzke & Matsuura
1998)
revealed, among other morphological
structures, 47 DEs.
The solar wind data are in situ measurements
from IMP-8, ICE and PVO, which are used to construct the variation of solar wind
speed, density and dynamic pressure
during the analysed interval. Data from these same spacecraft plus Vega 1 were
used to determine the times of current sheet crossings. These data were fitted to
heliospheric current sheet curves
(Hoeksema
1989)
extrapolated from the corona
into the heliosphere in order to determine the best-fit coronal source-surface
radius for each Carrington rotation.
This work presents the kinematic analysis for two onsets of DEs (1985 December
13.5 and 1986 February 22.2). The DE onset time, when the comet supposedly
crossed a magnetic sector boundary, is determined assuming that the
disconnected plasma moves away from the nucleus at constant
velocity
(Voelzke & Matsuura
1998).
This is compared with the analysis of
(Yi et al.
1994b)
which determined the time of disconnection assuming
constantly accelerated linear motion. The velocity varies broadly from one DE to
another. In this paper, we report the results of an analysis of four DEs
observed from 13 through 14 December 1985 and on 22 February 1986. From a
kinematic analysis of the wide-field photographic images we calculated the
disconnection time of the 1985 December 13-14 DEs to be December 13.5 and of
the 1986 February 22 to be February 22.2, which are in good agreement with
Yi et al. (
1994b).
The solar wind conditions around comet P/Halley, inferred
by corotation of IMP-8 data to the comet, were such that comet P/Halley had
just crossed the interplanetary magnetic field (IMF) sector boundary; the
solar wind density was, respectively, 15 cm
-3 and 7 cm
-3; the solar
wind speed was, respectively, 370 km s
-1 and 500 km s
-1. We conclude
that our results corroborate the idea that DEs are associated with sector
boundary crossings and that the magnetic reconnection plays an important role
in the formation of DEs and can be considered as the triggering mechanism
(Niedner & Brandt
1978; Brandt et al.
1999).
Key words: comets -- comet P/Halley plasma dynamics -- disconnection events
