DISCONNECTION EVENTS (DES) AND SECTOR BOUNDARIES - THE EVIDENCE FROM COMET HALLEY 1985-1986

Cometary and solar wind data from December 1985 to April 1986 are pres ented for the purpose of determining the solar wind conditions associa ted with comet plasma tail disconnection events (DEs). The cometary da ta are from The International Halley Watch Atlas of Large-Scale Phenom ena (Brandt et al., University of Colorado, Boulder, 1992). In additio n, we present the kinematic analysis of four DEs, those of Dec 13.5 an d 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 sola r wind measurements from IMP-8, ICE and PVO were used to construct the variation of solar wind speed, density and dynamic pressure during th is interval. Data from these same spacecraft plus Vega-1 were used to determine the time of 48 current sheet crossings. These data were fitt ed 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 Carri ngton 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 wit h crossings of the heliospheric current sheet and apparently not with any other property of the solar wind. If we assume that there is a sin gle or primary physical mechanism and that Halley's DEs are representa tive, efforts at simulation should concentrate on conditions at curren t sheet crossings. The mechanisms consistent with this result are sunw ard 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 th e sector boundary crossing.