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

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11. Observations

High speed streams have been caught `with smoking gun' several times.

Niedner et al. (1978) found an association between an event in comet Kohoutek's tail and a corotating HSS. A beautiful image of the comet on Jan 20 1974 is reproduced by Niedner et al. (1978) on plate 18. A schematic drawing of the tail (Niedner et al. 1978, Fig. 4) at this date shows two nearly straight segments connected by a 'joint'. "The results [of Niedner et al. 1978] indicate that, at the time of formation of the perturbed tail structure, the comet was entering the compression region of [a] strong high-speed solar-wind ... " (Niedner et al. 1978, p. 1014). This is elaborated further by Jockers (1981 , 1985).

Later Niedner and Brandt (1978) noticed that also the HCS was close to the comet at the right time to trigger this event. Comparison of the two papers of Niedner and coworkers (1978) demonstrates that the association of cometary and solar wind events is not always unambiguous.

Jockers and Lüst (1973), p. 116, interprete a kink in the tail of comet Bennett on April 6 1970 as being caused by the April 5 Solar Wind HSS. They also describe the three regions: The far tail has not yet noticed the fast wind, the near tail is in the new direction, the transition region connects both.

Brandt et al. (1980) investigate the rapid turning of the tail of comet Bradfield on 1980 February 6. They consider as cause a rapid change in velocity direction and note that "the likely location of these sharp gradients in flow are the leading edges of HSS ... and interplanetary shock fronts ..." (p. L54).

The collections of images (C88) of comet Halley's 1986 apparition made by Celnik et al. (1988), and the International Halley Watch Atlas (IHW) compiled by Brandt et al. (1992) provide us with ample material to study the morphology of a cometary plasma tail. Some prominent examples will be discussed here.

March 9: Image 4993 in C88, and p. 195 of IHW show a well developed kink in the tail. We suspect an HSS as cause.

March 10: On image 5028 in C88, and on p. 198 of IHW the tail looks diagonally split. This event is discussed by Wu & Qiu (1987), where it is associated with a shock wave generated by a solar flare on March 6 at 16:37 UT. This fits very well to our model calculations (Wegmann 1995, see also Fig. 19).

March 17: Image 5168 in C88 and p. 249 of IHW give another nice example of a kink.

March 20: On image 5241 in C88, and on p. 287 of IHW one can see a tail which is narrow close behind the nucleus but fans widely out at a position far behind the nucleus. This looks similar to what we obtained in our model calculations for the effect of an NCDE (see Figs. 16 and 18). On March 18 the spacecraft Sakigake, at that time still close to comet Halley, measured a solar wind density of about 40 cm-3, the highest value in the five months interval reported by Oyama et al. (1986). This lends some support to our interpretation that this event was generated by enhanced solar wind density. The front edge of the disconnected tail moved from 2.43 to 6.46 [FORMULA] 106 km distance with the nearly constant velocity of 37.0-40.4 km/s (Brosius et al. 1987, Table 3). This fits very nicely to the model results of Sect. 8 where we found that the tail structure triggered by an NCDE travels finally with constant velocity. According to Celnik et al.(1988), p. 106, the position of the plasma tail structure, labeled [FORMULA], at three consecutive nights March 21.33, 22.31, and 23.29 was 5.73, 12.19, and 19.4 [FORMULA] km behind the nucleus. This means that the structure moved after an intermediate acceleration again with a nearly constant velocity of 76-85 km s-1.

April 8-9: Image 5771 in C88, and p. 377 of IHW provide an example of an event where apparently the whole tail is severed. The tail looks similar to what we see in our model calculation for the action of an interplanetary shock when the event is observed parallel to the IMF plane (see Fig. 21).

April 11-12: This event shown in Fig. 2e of Brosius et al. (1987), and on page 429 in IHW can be correlated with solar wind data obtained by the ICE spacecraft. These data show in the 'arrival window' a magnetic field reversal accompanied with a density enhancement of more than a factor of 2 (Brosius et al. 1987, Fig. 4). The tail on the image on page 429 in IHW looks like a cone with apex far behind the nucleus. The front edge of the disconnected tail moved from 1.08 to 4.12 [FORMULA] 106 km distance with the nearly constant velocity of 42.7-54 km/s (Brosius et al. 1987, Table 3). This phenomenology resembles closely to what our model calculations predict for the effect of an NCDE. We see here a current sheet in action, but it acts not via reconnection but via the associated density enhancement.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000