Global magnetohydrodynamic simulation of a comet crossing the heliospheric current sheet

Abstract

A disconnection event (DE) of the plasma tail is one of the most spectacular phenomena observed in comets. Yet it has remained an important unsolved problem in planetary astronomy and space physics. The solar wind is thought to play a major role in the creation of comet plasma tail DEs. Comparison of the solar wind conditions and 16 DEs in Halley's comet shows that DEs are associated primarily with crossings of the heliospheric current sheet (HCS) and apparently not with any other properties 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 compressible 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 simulations. Such a narrow discontinuity cannot be described by a global simulation using large grid cells, due to the intrinsic numerical diffusion in the system. Therefore, we have used an approximation to calculate the induced current across the HCS when it is compressed inside the cometosheath. This produces results different from previous simulations. However, they satisfy all the observational constraints. The results show that frontside magnetic reconnection between the reversed interplanetary magnetic fields [Niedner and Brandt, 1978] can reproduce the evolution morphology of a DE, including ray formation, when a comet crosses the HCS. This supports the association of DEs with HCS crossings.