6. From fast to slow wind
The transition from fast to slow wind is in general gradual. A sudden decrease in velocity is rarely observed. Nevertheless, it is interesting to model the reaction of the cometary tail to such an event.
We start from the model 'fast1' and switch at time t=0 the solar wind conditions to 'slow1'. The discontinuity is not in equilibrium. It is resolved into several waves according to the Riemann problem. Since the magnetic field is transversal, there are only three waves namely two rarefaction waves moving approximately with the fast magnetosonic speed and a contact discontinuity moving with the flow speed. When calculated hydrodynamically the fast and the slow flow are connected by two intermediate states which are listed in Table 3. The state after 1 second is shown in Fig. 10. The rarefaction fans are approximated by inverse shocks.
Fig. 11 shows that the tail is rotated by and becomes brighter. The outer parts of the tail attain the new position faster than the central parts. In addition they brighten faster. This can be seen in the ion density distribution in a plane cut 600 000 km behind the nucleus (Fig. 13).
The originally kidney shaped contour lines are deformed after two hours to hearts. Only gradually they attain their final kidney shapes at the shifted position.
In view of this morphology, after three hours, the new tail, although only partly established, competes in brightness with the old tail. The tail splits into two well defined separated branches. The position of the tail shown in the lower panel of Fig. 12 after 3 hours is the position of the upper branch, which is distinguished as a clear maximum in brightness in cuts across the tail.
The branchpoint is not at the nucleus as one would expect for tail rays. The tail branches at a distance of about 200 000 km behind the nucleus. The tail is still split into two branches at later times. The branchpoint moves farther downstream and arrives after 5 hours real time at a distance of 400 000 km (see Fig. 11). Note that there is no condensation at the branchpoint! This is different from the morphology induced by a density enhancement (see Sect. 8).
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000