Disconnection events (DEs) and sector boundaries: the evidence from comet Halley 1985–1986

Abstract

Cometary and solar wind data from December 1985 to April 1986 are presented for 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. , University of Colorado, Boulder, 1992). In addition, we present the kinematic analysis of four DEs, those of Dec 13.5 and 31.2, 1985, and Feb 21.7 and 28.7, 1986. The circumstances of these DEs clearly illustrate the need to analyze DEs in groups. In situ solar wind measurements from IMP-8, ICE and PVO were used to construct the variation of solar wind speed, density and dynamic pressure during this interval. Data from these same spacecraft plus Vega-1 were used to determine the time of 48 current sheet crossings. These data were fitted to heliospheric current sheet curves (Hoeksema, Adv. Space Res. 9 , 141, 1989) extrapolated from the corona into the heliosphere in order to determine the best-fit coronal source surface radius for each Carrington rotation. Comparison of the solar wind conditions and 16 DEs in Halley's comet (the four DEs discussed in this paper and 12 DEs in the literature) leaves little doubt that DEs are associated primarily with crossings of the heliospheric current sheet and apparently not with any other property of the solar wind. If we assume that there is a single or primary physical mechanism and that Halley's DEs are representative, efforts at simulation should concentrate on conditions at current sheet crossings. The mechanisms consistent with this result are sunward magnetic reconnection (Niedner and Brandt, Astrophys. J. 223 , 655, 1978) and tailward magnetic reconnection (Russell et al., J. geophys. Res. 91 , 1417, 1986), if tailward reconnection can be triggered by the sector boundary crossing.