Disconnection Events in Comets

Abstract

Cometary and solar wind data are compared with the purpose of identifying the solar wind conditions which are associated with comet plasma tail disconnection events (DEs), i.e., when the plasma tail appears disconnected from the cometary head. 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 and Matsuura, 1998) revealed, among other morphological structures, 47 DEs along the plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggering mechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and the magnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentric distance presents a bimodal character possibly associated with the cometary passage through the magnetic sector boundaries in the interplanetary medium. The 47 DEs documented in 47 different images allowed the estimation of 19 onsets of DEs, i.e., the time when the comet supposedly crossed a frontier between magnetic sectors of the solar wind. The solar wind data are taken from in situ measurements of IMP-8 (King, 1982), which is used to construct the actual variation of solar wind speed, density and dynamic pressure during the analysed interval. These in situ measurements are referenced to the comet by standard co-rotation methods. The preliminary results of this research reveal that the DEs onsets of comet P/Halley are correlated with pressure effects only in 23% of the analysed cases, therefore these effects should not be the principal triggering mechanism of DEs.