**ON CORONAL STREAMER CHANGES**

N. Gopalswamy^1^, M. Shimojo^2^, W. Lu^1^, S. Yashiro^3^, K.
Shibasaki^2^,

and R. A. Howard^4^

^1^NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

^2^*Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano,
384-1305, Japan*

^3^*The Catholic University of America, Washington DC 20064, USA*

^4^*Naval Research Laboratory, Washington DC 20064, USA*

**ABSTRACT**

Coronal streamer represents one of the pre-eruption configurations of
coronal mass ejections (CMEs), because they overlie prominences and
often possess all the substructures of CMEs. In this paper, we report on
a study of streamer changes associated with prominence eruptions. The
prominence eruptions and streamer changes were observed by the Nobeyama
radioheliograph and Solar and Heliospheric Observatory (SOHO),
respectively. Multiwavelength data showed that at least one of the
streamer events involved heating and small-scale material ejection that
subsequently stalled. After presenting illustrative examples, we compare
the properties of the streamer-related events with those of general
population of prominence events. We find that the properties of
streamer-related prominence events are closer to those of prominence
eruptions with transverse trajectories.

**INTRODUCTION**

It has been known for a long time that coronal streamers visible in
eclipse pictures often overlie prominences and coronal cavities (Saito
and Tandberg Hanssen, 1973). At least in one class of coronal mass
ejections (CMEs), prominence core, cavity and frontal structure are
observed similar to what is seen in streamers. If we associate the outer
structure of CMEs with the white light streamers (see, e.g., Hundhausen,
1988), we can conclude that the streamers represent the pre-eruption
state of at least some CMEs. How the pre-eruption configurations evolve
may hold clues to the understanding of the eruption. In one such
scenario, slow addition of mass to the closed field regions of the
streamer may result in its subsequent eruption (Wolfson et al., 1987)
seen as "bugles" (Howard et al., 1985; Hundhausen 1993). Another aspect
of this configuration is that the mass in the streamer restrains the
cavity, and hence allows for the build up of sufficient energy to
produce eruptions (Wolfson and Saran, 1998). In this paper, we discuss
another kind of evolution in which the eruption is partial and results
in streamer changes. Full eruptions would correspond to the streamer
blowout events first described by Howard et al., (1985) using *Solwind*
data. The events presented in this paper were noticed among the
prominence eruption events observed by the Nobeyama radioheliograph
while studying their association with white-light CMEs (Gopalswamy et
al., 2003).

**ILLUSTRATIVE EXAMPLES**

The streamer-related prominence events were identified in the sample of
prominence eruptions in microwaves obtained by the Nobeyama
radioheliograph at 17 GHz. The Solar and Heliospheric Observatory (SOHO)
mission's Large Angle and Spectrometric Coronagraph (LASCO) images were
used to observe the streamer changes. During 1996-2001, a total of 226
prominence eruptions were identified from the daily images using an
automatic detection software (see, Gopalswamy et al., 2003 for details).
Out of these, 186 events overlapped with SOHO observations. Some 52
prominence eruptions lacked white-light CMEs, out of which 11 were
streamer changes (see Table 1 for the list). We study these 11 events in
this paper.

**2000 January 18 Event**

The microwave images of the Sun (southeast quadrant is shown) in Figure
1 show the 2000 January 18 prominence event. The prominence mainly moved
in the transverse direction (parallel to the limb), with no significant
radial motion. However, the overall height of the prominence increased
slightly. From the height-time measurements, we found the average speed
to be only \~ 5.2 km/s. Although the Nobeyama observations ended by
06:11 UT, we were able to track the prominence using the Pic du Midi
H-alpha coronagraph data (see Fig 1b). By 14:32, the prominence had
completely erupted. In white light, the streamer overlying the
prominence showed some change during the Nobeyama observations, but did
not erupt. Figure 2 shows the streamer as observed by LASCO
corresponding to the period of the Nobeyama observations. Note that the
streamer increased in height and width slowly. Figure 2b shows the
subsequent evolution of the streamer. The streamer was also slightly
perturbed (deflected to the south) by a nearby CME. After continued
swelling, the streamer finally erupted as a slow CME in association with
the prominence eruption observed in H-alpha by the Pic du Midi
coronagraph. The average speed within the coronagraph field of view was
only 143 km/s and showed significant acceleration. The activation of the
filament started at 01:41 UT, and it took about 11 hours for the onset
of the CME. The response of the streamer to the prominence activation or
small-scale eruption seems to be the slow swelling. It must be noted
that the streamer change is much more than the changes one would expect
from solar rotation.

![](media/image7.wmf) ![](media/image7.wmf)

Fig. 1a (left). A series of microwave (17 GHz) images (southeast
quadrant) showing the 2000 January 18 prominence event and, b (right)
subsequent evolution of the prominence in H-alpha as observed by the Pic
du Midi coronagraph (images courtesy: SOHO summary data base).

![](media/image7.wmf) ![](media/image5.wmf)

Fig. 2a (left). SOHO/LASCO images (southeast quadrant of the corona)
from the C2 telescope showing the swelling of the streamer; 2b (right)
Subsequent evolution and the eruption as a CME. The white arc represents
the solar disk superposed on the occulting disk.

![](media/image7.wmf)

Fig. 3. Microwave (two leftmost panels) and SOHO/LASCO images showing
the prominence eruption, streamer swelling (arrows) and the subsequent
eruption of the streamer as a spectacular CME (four right panels).

**The 1998 June 01 Event**

The 1998 June 1 prominence event occurred from the southwest limb.
Figure 3 shows the prominence eruption along with the streamer changes.
The microwave image at 01:06 UT shows the pre-eruption position of the
prominence. The image at 06:36 shows that the leading edge is at a
larger height. The prominence motion was again slow with an average
speed of 8 km/s. The streamer had corresponding increase in the height
of its helmet (compare the 01:30 and 06:06 LASCO images in Fig. 3). The
apex of the streamer continued to increase (see the 13:31 UT image in
Fig. 3) and finally resulted in a CME the next day at 08:08 UT. The
fully formed CME can be seen in the 10:30 UT image in Fig. 3. The
Nobeyama radioheliograph stopped observations for the day around 06:30
UT. Fortunately, the filament was also observed by SOHO's
extreme-ultraviolet imaging telescope (EIT), which showed indications of
a EUV eruption from the northern part of the filament. The EUV eruption
started at 05:00 and reached its maximum height of \~ 0.3 Rs above the
surface. After reaching the maximum height, the

EUV ejecta started fading away until about 06:56 UT. Movies of EUV
images clearly showed that it was a "failed eruption." It appears as
though the EUV ejecta hit a barrier and was stopped. The whole
prominence was also seen in absorption by EIT and showed some
activation, similar to what was seen in microwaves. A large coronal
cavity was observed in the pre-eruption phase, which might have acted as
the barrier (Plunkett et al., 2000). The EUV ejecta was seen in emission
which means that part of the prominence must have been heated to coronal
temperatures even in this small-scale eruption. Figure 4 shows a
superposition of microwave contours on an EIT difference image when the
eruption reached its maximum height (darkest features corresponds to
brightest EUV emission in the reverse color table). Note that there were
two regions with enhanced EUV emission: one is the ejecta and the other
near the southern leg of the prominence in projection. This suggests the
overall participation of the prominence as is also evident from the
microwave prominence. The EUV ejecta corresponds to the thin upper part
of the microwave prominence.

[Carrington Maps]{.underline}

In order to look at the long-term evolution of the 1998 June 01 streamer
event, we examined the Carrington maps of the corona obtained from the
LASCO C2 images (Rich et al., 2000). Figure 5 is a Carrington map
constructed from the west limb images at three different radial
distances (2.5, 3.0 and 4.5 Rs) for the rotation 1936. The feature we
are interested in is the bugle roughly at the center of the map (pointed
by arrow). The feature can be on maps of all heights with a narrower
width at larger heights. Note that this streamer was the brightest in
the Carrington map. The streamer first appeared on 1998 May 28 and
slowly evolved as a bugle until

![](media/image8.png){width="3.120138888888889in"
height="3.120138888888889in"}

Fig.4. Overlay of the microwave (17 GHz) Contours on an EIT difference
image.

the beginning of June 2, when it erupted as a spectacular CME with a
heavily structured prominence core (see Plunkett et al 2000 for
details). The vertical feature at the wider end of the bugle corresponds
to the CME. The measurements were made starting from 08:08 UT on 1998
June 02 from a height of about 3.3 Rs. Note that the

![](media/image9.png){width="3.5729166666666665in" height="4.5in"}

Fig. 5. Carrington maps for the period 1998 May 19 to June 14 showing
the streamer event of 1998 June 1. The maps were constructed by
assembling strips of the coronal images at a given height (marked at the
right side of the maps). Typical resolution on the maps is \~0.5 degrees
in longitude and 1 degree in latitude. At the bottom, the Carrington
longitudes are given.

streamer was completely destroyed due to the CME eruption. The
Carrington map shows many CMEs (indicated by the sharp vertical
features), but there are not many bugle-type events. The CME had a
significant acceleration and CME took almost eight hours to cross the
field of view of LASCO. It attained a speed of 1278 km/s when it left
the LASCO field of view, though the average speed was only 750 km/s. The
prominence observed in microwaves and EUV became the highly-structured
core of the CME.

**Table 1. List of prominence eruption events**

**STATISTICAL RESULTS**

Table 1 gives list of streamer-related prominence events we studied. The
position angle of occurrence, the speed and the largest height attained
(in units of solar radii, Rs) are listed for each prominence event. The
negative speeds correspond to events in which only the descending phase
could be measured. In the last column we have also noted the direction
of the trajectory (R and T). Gopalswamy et al. (2003) found that the
prominence trajectories could be classified as radial (R) or transverse
(T) to denote the predominant direction of motion of the prominence. The
T events generally correspond to prominence activation events while the
R events correspond to prominence eruption events. The R events
generally acquired larger final heights and moved faster on the average.
On the other hand the T events were confined to lower heights and never
left the field of view of the radioheliograph. Most of the R events were
associated with white-light CMEs while only a small fraction of the T
events had CME association. In the preceding sections, we described two
events: the T event of 2000 January 18 and the R event of 1998 June 01.
Note, however, the later eruption of the 2000 January 18 event was
radial because the prominence had erupted. Table 2 compares the speed
and final heights of the streamer-related events with those of the R and
T events in general. It is clear that the streamer-related prominence
events had properties intermediate between the R and T events, but
closer to the T events because a majority of the streamer events (7/11 =
64%) had transverse trajectories. Even those with the radial
trajectories had a final height much smaller than the average final
height of all the R events (see Table 2).

**Table 2. Comparison of the streamer-related prominence**

**events with the R and T events (Gopalswamy et al. 2003)**

We tracked the streamers beyond the prominence event and found that more
than half of them subsequently erupted along with an associated CME. The
times of the subsequent CMEs are given in the last column of Table 1
(with the suffix 'n' denoting that the time corresponds to the next
day). Note that the subsequent CMEs occurred 4 -- 42 hr later, with an
average delay time of \~17.3 hr. In some events the streamers distended
and faded out gradually, so it was difficult to say whether there was a
CME. These events are marked by a '?' in the last column of Table 1.
With the small number of CMEs it is difficult to say whether the
prominence events discussed in this paper can be treated as a precursor
to the subsequent eruption. It is possible that these streamer events
represent an intermediate level of activity between the prominence
eruptions and very slow evolution.

# **SUMMARY**

We performed case studies of two prominence events and the associated
streamer events to show that the two phenomena are related similarly to
the CME-prominence core. We also performed a statistical analysis of a
set of 11 such events and found that they correspond to eruptions less
energetic than typical prominence eruption events, which invariably end
up in the cores of white light CMEs. The prominence motion is horizontal
for a majority of the cases, which will be normally classified as
prominence activation events. There was also a sizable number of radial
events, in which the prominence seems to be stalled after an initial
radial motion. In the radial event we discussed, we know that there was
a huge cavity (flux rope) overlying the filament. It is possible that
the flux rope prevented the prominence from escaping. Since streamer
changes are closely associated with prominence activations or partial
eruptions, this may have important implications for understanding the
CME initiation process. Very slow evolution of streamers in brightness
and size has been linked to changes in photospheric magnetic flux. In
particular, increase in brightness is known to be correlated with
decrease in photospheric flux, which was interpreted as opening of flux
tubes under the streamer (Poland and MacQueen, 1981). In this sense, we
think that the prominence activations produce a similar effect of
opening some field lines. Unfortunately, we cannot verify this because
it is difficult to observe the photospheric magnetic field changes for
limb events. Prolonged coronal dimming associated with some eruptions
may also correspond to these field openings (Gopalswamy et al., 1999).
We need a reverse study to look at a large number of streamers as they
evolve to arrive at firm conclusions regarding the connection between
the small-scale prominence eruptions and activations on the one hand and
the stability of the streamers on the other.

**ACKNOWLEDGMENTS**

This work was supported by United States NASA (Living with a Star), Air
Force Office of Scientific Research, and NSF/SHINE (ATM 02045880).

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