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

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10. A corotating interaction region

The transition from slow to fast speed is not discontinuous. The fast and slow flows interact on their travel through the interplanetary space. What is observed at 1AU distance from the sun is described by Gosling et al. (1978). The behaviour of density, flow speed, flow angle and proton temperature across the interaction region is shown in Fig. 22 which is adapted from Fig. 1 of Gosling et al. (1978).

[FIGURE] Fig. 22. Density, flow speed, flow angle and proton temperature across a CIR (adapted from Fig. 1 of Gosling et al. 1978). Abscissa = time in hours.

The physical parameters change discontinuously at the stream interface. But they change already 1-2 days before, and continue to change after the interface. These changes on timescales of days should make most interesting effects on the tail of a big Halley type comet. We started a time-dependent calculation from the model 'big1' and varied the solar wind conditions as a function of time as shown in Fig. 22. This is complemented by a magnetic field which is constant [FORMULA] nT for [FORMULA] and [FORMULA] nT for [FORMULA]h, and increases linearly with time in the interval [FORMULA] according to Fig. 3 of Gosling et al. 1978. Time t=0 marks the instant when the interface first enters the computational grid. The computational region covers 1.56 [FORMULA] km in front and on the sides of the nucleus and 3.5 [FORMULA] km in the tail.

The most dramatic changes in the tail occur in the hours after the passage of the interface. Fig. 23 shows that after 12h the tail has the signature of an HSS interaction as described in Sect. 3. There is a kink in the tail where the tail appears to be torn apart. Fig. 24 shows in its lower panel this kink at a distance of nearly 3 [FORMULA] km. The middle panel shows that the coma dims, and that a pronounced second brightness maximum in the tail develops. The upper panel demonstrates, that a substantial part of the ions is detached from the coma and moves downward. The tail becomes disconnected about 5.5h before the interface enters the grid. It starts from a position 120 000 km behind the nucleus and is accelerated with [FORMULA] 2 m s-2.

[FIGURE] Fig. 23. Logarithms of the column densities [cm-2] in a projection to the IMF plane [FORMULA] hours after a stream interface has entered at the lower left corner. The length scale is 500 000 km

[FIGURE] Fig. 24. The same as Fig. 3 for a CIR.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000