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

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8. Non compressive density enhancement

High densities are sometimes observed when bulk speed is nearly constant and slow (less than 450 km/s). Since these density enhancements are not the result of a compression by stream interaction they are called non compressive density enhancements (NCDE) (Gosling et al. 1977). The magnetic field is not enhanced, proton and electron temperatures are low and vary in opposition to the density. Approximately one third of these events contain IMF reversals which are apparently not sector boundaries. The NCDEs occur typically as precursors of HSS (Gosling et al. 1977, p. 5005). The average occurence of NCDEs is 2.7 events per month. NCDEs are apparently non recurrent, but primarily transient in nature. NCDE often appear to be discrete with recognizable beginnings and ends (Gosling et al. 1977, p. 5008).

The heliospheric current sheet is embedded in a plasma sheet (HPS) with a typical thickness of 320 000 km (Winterhalter et al. 1994). This HPS is characterized by high density. These high density peaks are of coronal origin. The coronal streamers (often of helmet-like appearance) are on eclipse photographs brighter than the background corona (Gosling et al. 1981, p. 5444). Superposed epoch plots of the solar wind proton density for 23 sector boundaries show a distinct density peak at the position of the sector boundary which is well ahead and well separated from the density peak of the following stream interaction (Gosling et al. 1981, Fig. 1).

For modeling an NCDE we start from model 'slow2', enhance at time t=0 the solar wind density by a factor of three from 5 to 15 amu cm-3 (the values of 'slow2' in brackets), and switch back to the original density after 1 hour.

In an NCDE only the density [FORMULA] increases with all other quantities unchanged. This implies that [FORMULA] decreases, and the dynamic pressure increases. It follows from Eq. (5) that density [FORMULA] and column density [FORMULA] both decrease proportional to [FORMULA]. This means that the brightness would finally be reduced by a factor of three if the high density solar wind would persist.

We see in Fig. 17 that in the first hour the tail is compressed and attains higher maximal brightness, while at the same time the ions are moved from the coma into the tail. The point of maximum brightness starts to move into the tail after 40 minutes. This brightness peak is accelerated. After 80 minutes, shortly after the solar wind has returned to its original state, the density in the coma recovers and develops a new brightness peak. But a gap between the renewed coma and the receding old tail opens and widens. After 3 hours the brightness maximum in the tail is no longer accelerated but continues to move with a constant speed of about 75 km/s.

Fig. 16 shows that after 3 hours downstream of this cloud the tail fans out. This is due to the fact that the low total pressure in the sheet corresponding to the high density solar wind allows the tail to expand laterally. The new tail first appears as a crescent near the nucleus. The outer parts of this crescent move fast and close up to the old receding tail. The merging tails contribute to the fan like appearance of the far tail.

[FIGURE] Fig. 16. The same as Fig. 2 for a noncompressive density enhancement of 1h duration.

[FIGURE] Fig. 17. The same as Fig. 3 for a noncompressive density enhancement of 1h duration.

In the IMF plane the tail is still well confined by the magnetic field. The tail can expand more easily perpendicular to the field. Therefore, the lateral expansion of the tail is much more pronounced in a projection onto the perpendicular xz-plane (see Fig. 18).

[FIGURE] Fig. 18. The same as Fig. 16 but in a projection onto the perpendicular plane.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000