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Astron. Astrophys. 354, 321-327 (2000)

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5. Discussion

In Sect. 4.1 we calculated the density profile for pick-up ions as a function of [FORMULA] and [FORMULA], furthermore it was shown that our results reduce to those presented by PD96, for the particular case in which the magnetic field is perpendicular to the plane of the flow. In such study the authors compared the density profile obtained from Eq. (44) with the observations of ICE spacecraft through the plasma tail of the comet Giacobinni-Zinner. However, a better fit to the measurements can be achieved by considering that the main component of the magnetic field is in the direction parallel to the flow. In this case the density profile is obtained from Eq. (43) and is illustrated in Fig. 4 when [FORMULA], [FORMULA], [FORMULA], these values are typical in the interaction of the solar wind with a comet. The different curves in Fig. 4 represent the density profiles for different angles between the magnetic field and the direction of the flow.

One of the more enigmatic issues related to the physics of comets is the formation of very long and narrow structures that extend throughout the plasma tail. These structures known as rays , are the result of the accumulation of ions of cometary origin in these regions. Observations carried out by Miller (1979) allow us to know the principal characteristics of these structures. Most important is the fact that as a whole they form a structure that exhibits a converging pattern directed toward the antisolar direction. As a function of time the tail rays are seen to first appear as short and wide structures upstream from the comet and then evolve by increasing their length and rotating toward the antisolar direction. Despite the similarity in the overall configuration of cometary tail rays with respect to the expected accumulation of assimilated ions in structures expected along their cycloidal trajectory in a velocity shear (Fig. 8) it should be noted that our calculations are not related to a time-evolution of their distribution. Instead, the most populated concentration of ions near the apex of their cycloidal trajectories provides only a steady state view of the manner in which they are distributed across the velocity shear.

Within the context of our study it is possible to argue that conditions suitable for the existence of a velocity shear are present around comets and also that cometary ions accelerated by the solar wind move across regions compatible with the extent of a velocity shear. For example, the ICE measurements in comet Giacobini-Zinner show a velocity boundary layer with [FORMULA] at about [FORMULA] downstream from the nucleus (Bame et al. 1986). Since the solar wind speed is [FORMULA] an the interplanetary magnetic field intensity is [FORMULA] the cometary ions (formed mostly by [FORMULA] particles) will execute cycloidal trajectories in which the distance between neighboring apexes given by Eq. (23) is nearly 105 km. This value is one order of magnitude larger than the size of the region where there is a strong population of neutral CO particles around the comet (Beard 1980) and thus complies with the assumption given in Eq. (45) in which the source of cometary ions is to be much smaller than the length of the cycloidal trajectory. As discussed in Sect. 4.1.2 a finite source should produce sharp accumulations of assimilated ions rather than the more uniform patterns expected when the source size is infinite.

A useful calculation of the pattern of accumulated structures across a cometary velocity shear can be conducted by estimating their angle and length from Eqs. (47) and (48). The values obtained for the first 6 structures are presented in Table 1 and are plotted in Fig. 11.

[FIGURE] Fig. 10. Trajectories for [FORMULA] ions born a different initial positions. In this case [FORMULA], [FORMULA], [FORMULA].

[FIGURE] Fig. 11. Distribution of the first 7 regions of acumulated particles that predicted by the model.


Table 1. Length and angle with respect to the direction of the flow for the first 7 rays predicted by the model

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000