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Astron. Astrophys. 358, 759-775 (2000)

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1. Introduction

The plasma tail of a comet is shaped by the solar wind. Therefore, it is sensitive to changes and disturbances in the solar wind. We could use the cometary tail as a measuring instrument (free of charge) if we were only able to understand what it is telling us. Magnetohydrodynamic (MHD) model calculations are an ideal means to study the response of the comet to well specified solar wind conditions. Such models are therefore a valuable tool to decipher the comet's message.

Most spectacular are tail disconnections events (DE). Although there is no consensus in the literature on the formal definition of a DE, the main feature is that "a major portion of the tail disconnects from the coma and falls away" (Farnham & Meech 1994, p. 420). Several researchers have identified the DEs during comet Halley's 1986 apparition (Niedner 1986; Celnik & Schmidt-Kaler 1987; Delva et al. 1991; Yi et al. 1994; Voelzke & Matsuura 1998; Brandt et al. 1999). Much effort has been spent to demonstrate that there is a close association between DEs and crossings of the heliospheric current sheet (HCS) (Niedner 1986; Yi et al. 1994; Voelzke & Matsuura 1998; Brandt et al. 1999). It is now widely accepted that such an association exists, but it is less obvious that this correspondence is one-to-one.

Delva et al. (1991) correlate solar wind events in the VEGA data with events in comet Halley's tail. Their finding: "For the 22 DEs ... we find a possible correlation with SB [= sector boundaries] in four cases, a likely one in six cases and in two cases a SB as well as a high speed stream may be correlated with the DE. ... In the other 10 cases, a DE was observed although no SB was registered at the corresponding time. This indicates that a SB crossing is certainly not a necessary condition for the generation of a DE. On the other hand ... [SB crossing] is not a sufficient condition for DE. ... There is an indication that HSS also play a role." (Delva et al. 1991, p. 707).

The model of Niedner and Brandt (1978) assumes that the magnetic field in the HCS reconnects when caught in the comet and so tears off the tail. Several efforts to corroborate this disconnection model by MHD model calculations failed (Fedder et al. 1984; Schmidt-Voigt 1989; Yi et al. 1996).

It has been shown in MHD model calculations (e.g. Schmidt & Wegmann 1982; Fedder et al. 1984) and confirmed by the ICE spacecraft at comet Giacobini-Zinner (see Slavin et al. 1986) that the cometary plasma is diamagnetic. The plasma tail is a narrow ribbon confined by two magnetic flux lobes. Therefore, the reconnected magnetic field could take only a small fraction of the tail with it.

It is discomforting, that the HCS sometimes seems to succeed in disconnecting the tail, but sometimes fails. The field reversal in the HCS is always present and should lead to field reconnection and tail disconnection. The statistical evidence seems to be more in favour of something which is more loosely coupled with the HCS. In view of this observational and theoretical evidence it may be worthwile to search for other mechanisms by which the HCS and its companions could trigger a DE.

In a previous paper (Wegmann 1995) we have demonstrated how an interplanetary shock wave can generate a DE. It is clear, however, that not all observed DEs can be caused by shocks, since comets are not hit by shocks frequently enough.

In view of the close association of the HCS with high speed streams, the statistical evidence could be also in favour of high speed streams as agent. Statistical analysis shows that the DE starts about .75 days (Niedner & Brandt 1978) or almost 1 day (Yi et al. 1994; Brandt et al. 1999) after the HCS has met the comet. At this time most of the HCS has passed the comet, and there is only little field left in the comet to reconnect. The time delay of the order of a day is an indication that the DE is caused by something which comes after the HCS. The HSS is the most likely candidate.

In this paper we investigate the effect of an HSS on the plasma tail. The result is a tail disconnection which is accompanied by a kink in the tail as it is frequently observed. We show that a simple rotation of the solar wind's flow direction produces a gradual turning of the tail, but no kink. Another test calculation demonstrates that the magnetic field enhancement in the HSS is important for the generation of a DE.

The HCS is often associated with a noncompressive density enhancement (NCDE) (Winterhalter et al. 1994). Motivated by this observational finding we investigate the effect of an NCDE on the plasma tail. It is shown that this also leads to a DE. This may explain the mechanism how the HCS itself can affect the tail.

The DEs generated by a shock, an HSS or an NCDE have different properties. These can be used to diagnose from an observed DE its most likely cause.

The plan of this paper is as follows: In Sect. 2 we recall some basic facts of cometary MHD, in particular the scaling law for densities and column densities. In Sect. 3 we investigate the effect of a high speed stream, in Sect. 4 that of a change in solar wind flow direction. In Sect. 5 we consider a HSS without magnetic field enhancement, and in Sect. 6 a sudden transition from fast to slow wind. In Sect. 7 we discuss the effect of a current sheet and in Sect. 8 that of an NCDE. We recall our calculations for an interplanetary shock wave in Sect. 9. Finally, we study in Sect. 10 the effect of a full corotating interaction region. In Sect. 11 we discuss some observations and their relation to our model calculations. In Sect. 12 the results are compiled in a sort of `dictionary' which allows to translate solar wind disturbances into tail events (and vice versa). The paper is summarized in the concluding Sect. 13.

Several other solar wind discontinuities have already been treated. We mention in particular the rotation of the magnetic field orientation, which can generate tail rays (Schmidt & Wegmann 1982; Rauer et al. 1995; Wegmann et al. 1996). Other solar wind features, such as coronal mass ejections, will be studied in the future.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000