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.

Plasma-neutral gas simulations of reconnection events in cometary tails

“…The main difference to a 3D configuration is the suppression of field line slippage. The slippage is expected to slow down reconnection rather than to hinder it completely (Yi et al 1996;Konz et al 2004) We neglect mass loading of the solar wind, since it is a kinetic effect that cannot be treated consistently in a fluid description. The main effect of mass loading is a broadening of the bow shock (Flammer et al 1997).…”

“…Numerical magnetohydrodynamic studies have shown that the response of cometary plasmas to magnetic field variations in the ambient medium, as, e.g., tangential discontinuities and variations in solar wind parameters, is characterized by strong dynamic features (Ogino et al 1986;Schmidt-Voigt 1989;Rauer et al 1995;Gombosi et al 1996;Yi et al 1996;Wegmann 1995). Gombosi et al performed sophisticated threedimensional ideal MHD simulations of a comet in the solar wind using an adaptive numerical mesh.…”

“…They succeeded in reproducing density, temperature, and magnetic field profiles measured for Halley's comet by the Giotto probe. Ogino et al (1986), Schmidt-Voigt (1989), Rauer et al (1995), and Yi et al (1996) simulated two-and three-dimensional cases where a comet crosses an interplanetary sector boundary. While Schmidt-Voigt and Rauer et al did not find indications for magnetic reconnection, Ogino et al and Yi et al found clear evidence for magnetic reconnection.…”

The onsets of disconnection events of comet P/Halley on 1985 December 13-14 and 1986 February 22

“…This time is supposedly related, with a certain delay, to the time when the comet crossed the boundary between interplanetary magnetic sectors of the solar wind. Magnetohydrodynamic simulations (Yi et al 1996) of a comet crossing the heliospheric current sheet have confirmed the frontside magnetic reconnection between the reversed interplanetary magnetic fields (Niedner & Brandt 1978), and were able to reproduce the typical morphological evolution of a DE. They also strongly support the association of comet P/Halley DEs with the heliospheric current sheet crossings (Yi et al 1994b;Yi et al 1996).…”

“…Many ideas were suggested to explain the cyclic phenomena of DEs (Brandt 1990). The current competitive theories to explain the onset of DEs can be grouped into three classes based on the triggering mechanisms: (a) Ion production effects (Wurm & Mammano 1972), (b) Pressure effects (Ip & Mendis 1978;Ip 1980;Jockers 1985;Wegmann 1995;Wegmann 1998), and (c) Magnetic reconnection (Niedner & Brandt 1978;Russell et al 1986;Brandt et al 1992c;Yi et al 1996;Yi et al 1998;Brandt et al 1999). In this work the comet P/Halley DEs are interpreted in terms of the location of the magnetic sector boundaries derived from solar magnetic field data on the "source-surface" (Hoeksema et al 1983) and the expansion of the solar wind.…”