Disconnection Events in Comets

Cometary and solar wind data are compared with the purposeof 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 cometaryhead. The cometary data are fromThe International Halley Watch Atlas ofLarge-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 alongthe plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggeringmechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and themagnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentricdistance presents abimodal 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 cometsupposedly 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 solarwind speed, density and dynamic pressureduring the analysed interval. These in situ measurements arereferenced to the comet by standard co-rotationmethods. The preliminary results ofthis research reveal that the DEs onsetsof comet P/Halley are correlated with pressureeffects only in 23% of the analysed cases,therefore these effects should not be the principaltriggering mechanism of DEs.