GLOBAL MAGNETOHYDRODYNAMIC SIMULATION OF A COMET CROSSING THE HELIOSPHERIC CURRENT SHEET

A disconnection event (DE) of the plasma tail is one of the most spect acular phenomena observed in comets. Yet it has remained an important unsolved problem in planetary astronomy and space physics. The solar w ind is thought to play a major role in the creation of comet plasma ta il DEs. Comparison of the solar wind conditions and 16 DEs in Halley's comet shows that DEs are associated primarily with crossings of the h eliospheric current sheet (HCS) and apparently not with any other prop erties of the solar wind, such as high-speed streams [Yi et al., 1994] . In order to present a mechanism that explains the DE in terms of the local conditions at the comet, a three-dimensional resistive compress ible magnetohydrodynamic (MHD) simulation was carried out to test the effect of the HCS crossing on the comet plasma tail dynamics. We have focused on the fact that the thickness of the HCS is about 10,000 km [ Winterhalter et al., 1994], which was neglected in the previous simula tions. Such a narrow discontinuity cannot be described by a global sim ulation using large grid cells due to the intrinsic numerical diffusio n in the system. Therefore we have used an approximation to calculate the induced current across the HCS when it is compressed inside the co metosheath. This produces results different from previous simulations. However, they satisfy all the observational constraints. The results show that frontside magnetic reconnection between the reversed interpl anetary magnetic fields [Niedner and Brandt, 1978] can reproduce the e volution morphology of a DE, including ray formation, when a comet cro sses the HCS. This supports the association of DEs with HCS crossings.