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

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4. Rotation of the velocity vector

Jockers and Lüst (1973, p. 120) interprete two kink events at comet Bennett on April 2 and April 6, 1970 "as being caused primarily by a change in the solar wind direction". In this section we single out the effect of a change in the flow direction and show that this is not quite sufficient to knock a kink into the tail.

The time dependent calculation is started from model 'slow1'. At time t=0 the solar wind velocity vector is rotated by 10o to the values of 'slow1' in brackets. A similar calculation has been done by Rauer et al. (1995) for different physical parameters and with a different numerical code but with basically the same result.

The result of our calculation is shown in Fig. 6. The velocity discontinuity is not quite in equilibrium. On one side (the lower side in Fig. 6) fast flow tries to overtake slow flow. This generates a compression which runs through the tail. On the opposite side the slow flow lags behind and gives way to a rarefaction wave. These discontinuities pass the ambient plasma in little more than half an hour. The main tail is compressed from one side. This enhances the brightness in the first two hours (see middle panel of Fig. 7).

[FIGURE] Fig. 6. The same as Fig. 2 for a change in flow direction.

[FIGURE] Fig. 7. The same as Fig. 3 for a change in flow direction.

After three hours the brightness decreases to new lower values. This dimming is due to the fact, that the new flow direction forms an angle of [FORMULA] with the magnetic field, larger than the old angle of [FORMULA]. The magnetic field relaxes its pressure from the lower side, the tail expands, becomes more diffuse and so reduces the maximum brightness.

The upper panel of Fig. 7 shows that the ion content remains nearly unchanged. There is no disconnection in this case. The lower panel of Fig. 7 shows that the tail is shifted gradually to the new position. This is a consequence of the fact, that the outer parts of the tail turn faster than the central part. The tail becomes, while turning, more diffuse. This is seen in Fig. 6 where after three hours a spray develops on the lee side of the unrotated parts of the tail. There is no indication of a kink.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000