On 29 August 1990, a 1N flare occurred in the region NOAA/USAF 6241
(
). It began at 0914UT, reached a maximum at 0920UT
and ended at 0940UT (NOAA 1990a). Companying the flare, a
tornado-shaped eruptive prominence (S12; E90) occurred over the flare.
The prominence began at 0917UT
and ended at 0959UT (NOAA 1990b). The prominence began after
the begining of the flare, and ended later than the flare. Although a
type III radio burst was
recorded for the event, we can hardly say anything more about the relation
of the prominence with the burst, because of the prominence accompanying
the flare and imperfections of the obtained data.
No X-ray radiation was detected with GOES.
A series of 
filtergrams of the prominence in the interval
was obtained at Yunnan Observatory by using -SSHG (Xuan & Lin 1993). Two-dimensional 
spectral data in the period
0927-0931UT were obtained as well. We obtained a total of 42
images. These two kinds of data are shown in Fig. 1 (click here).
Figure 1: The filtergrams (0919-0947) of the eruptive
prominence on 29 August 1990 and its partial spectral
pictures (
). The black and thick lines in the filtergrams
of 092744, 092858, and 093011UT are the images of the slit. The time order
of the spectral pictures is from the left th the right in
the pictures
Figure 1 (click here) shows a regular loop structure before 0919UT.
The axis of the loop and the solar surface are declined
about 
toward the north. The first and second filtergrams in the
figure indicate the emerging of a loop structure, i.e. a magnetic flux tube,
and seem to show a twist of the left loop leg; the flux rope emerged already
twisted. The later
spectral
diagnoses will confirm this result. The same result was obtained by Tanaka
(1991)
and Leka (1993). The emerging process
occurred before the maximum of the flare. At 0923UT, the left loop leg became
thicker, and the materials in it continuously ascended along it. After
reaching
the zenith, they dropped downwards in succession not along the right
loop leg, so the second loop structure was formed. Comparing the pictures at
0923UT
and at 0927UT, we estimate the falling velocity at 0923UT at about
90 km/s, the velocity of the materials hitting the solar surface at 0927UT
at more than 163 km/s. The picture at 092744UT shows that
the shape of the prominence
is rather complicated, but one can still discern the emerging loop structure,
the second loop structure, and what seems to be a third loop structure forming.
At 092858UT, the prominence reached climax, the third loop structure was
formed,
and the emerging loop was snapped at the top. The rupture may be due to
the decaying of the prominence, and not the result of "progressive
reconnection of elementary
flux tubes of two (or more) interacting loops" as suggested by Simberova
et al. (1993) and Yurchisin (1994). After that, the
altitude of the prominence decreased, and the
prominence gradually decayed. At 0940UT, the prominence became a regular
arch
structure, which made nearly a right angle with the solar surface.
In the decay stage, the obvious variation is the interruption at the vault of the arch, the materials descending along the two legs of the arch, but the outline of the arch not obviously changed. We measured the mean falling velocity at about 83 km/s for the left leg and 74 km/s for the right. The decay of the left leg is different from that of the right, indicating that the physical condition inside the left leg is different from that inside the right. Perhaps the material of the right is denser than the left, so that the gas pressure of the right is larger than at the left, reducing the falling velocity of the right.
We measured the altitudes of the prominence at different times, and dealt with
these data with regresssion analysis. Altitudes h (in kilometers)
and
the time t (in minutes) have the linear relation:
Equation (1) corresponds to the rising stage of the prominence, i.e. the
formation of the prominence; Eq. (2) reflects the descending stage, the
decaying
of the prominence. Figure 2 (click here) gives the comparison of the measured
values and the theoretical regression lines.
Figure 2: Variation of the prominence altitude with time. The solid line
and
the dashed line correspond to the regression line and the observational
curve respectively
Figure 2 (click here) shows that the altitude of the prominence uniformly increased with an average velocity of about 71 km/s, then evenly decreased with an average velocity of about 33.7 km/s. The formation time is about half as much as the decay time.
The problem of filament formation has always been vexations, and the situation has been exacerbated by the caution of observers and the predilections of theoreticians. A prominence is formed by the emergence of a bright knot at the solar limb, but observers have been loath to say that prominences, hence filaments, emerge from the chromosphere. In most limb observations, prominence formation could be interpreted as resulting either by the condensation of matter from the corona or by the emergence of a heavily-laden flux rope from the chromosphere (Rust & Kumar 1994).
The formation stage of the prominence indicates that the mass in the prominence was supplied during the emergence of helical flux ropes, i.e. it was lifted bodily out of the chromosphere. We do not believe that filaments or prominences condense from the corona or are formed by siphoning mass from the chromosphere, as already argued by Rust & Kumar (1994).
The net effect of flux rope emergence and spiral accumulation in filaments or prominences is to eject spiral from the Sun. This process is a necessary mechanism that releases spiral generated inside the Sun (Rust & Kumar 1994). The occurrence of the flare triggered the spiral ejection. With the development of the flare, the material ejection became more and more violent. Because the material ejection was intermittent, the ejected materials formed different trace, and the tracks became ever larger. This is why the three loop structures were formed during the flare, and the prominence reached the maximum height about 10 minutes later than the flare reached the maximum. We estimate the mean velocity of the ejected materials to be about 82 km/s (the velocity 71 km/s is its vertical component). The falling velocity is very large (about 90 km/s at 0923UT). The value is impossibly in good agreement with that expected from the simplest model: a free-fall motion along a stationary loop. We suggest that the following process exist:
The shape of a prominence reflects the trend of the magnetic field lines
or the structure of the magnetic field (Zhang 1992). According
to the morphological features of the prominence, we propose a likely
model to illustrate the observations of the prominence (see
Fig. 3 (click here)). First, a magnetic
flux tube emerged to form a loop structure (see Fig. 3 (click here)a and the
filtergram of 0919UT in Fig. 1 (click here)). The newly emerging
magnetic tube squeezed the magnetic field lines of the ambient corona. The
observations suggest that
we witnessed a reconnection process between the constantly-emerging
magnetic field lines and the squeezed magnetic field lines of the adjacent
corona. The
possible scenario is shown in Fig. 3 (click here)b. There are two reasons to
make such a suggestion. One is that the falling velocity is very large
(about 90 km/s at 0923UT). There must be another acceleration in addition
to gravity.
We suggest that the falling velocity is largely due to gravity, but
mainly due to the
acceleration caused by the reconnection of the magnetic lines.
The other reason is that the filtergrams at 092744UT and 092858UT show
a spike M
at the top of the prominence (see Fig. 1 (click here)). Maybe the spike M is
the product of the
reconnection. The reason why the spike did not escape from the prominence to
produce a CME event is that the motion of the spike was hindered by the
magnetic lines of the ambient corona. This physical scenario is analogous to
that given
by Wang et al. (1989).
Figure 3: The suggested magnetic configuration for the prominence and
the ambient corona
Schmieder et al. (1995) analysed and SXT data
of a recurrent surge and suggested that surges are produced by magnetic
reconnection between a twisted cool loop and open field lines. Our
explanation of the prominence is very similar to their results (Schmieder
et al. 1995).