The Astrophysical Journal, 561:L219–L222, 2001 November 10
䉷 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
EIT AND SXT OBSERVATIONS OF A QUIET-REGION FILAMENT EJECTION:
FIRST ERUPTION, THEN RECONNECTION
Alphonse C. Sterling1,2 and Ronald L. Moore
NASA Marshall Space Flight Center, SD50/Space Science Department, Huntsville, AL 35812;
[email protected], [email protected]
and
Barbara J. Thompson
NASA Goddard Space Flight Center, Code 682, Greenbelt, MD 20771; [email protected]
Received 2001 September 5; accepted 2001 October 3; published 2001 October 25
ABSTRACT
We observe a slow-onset quiet-region filament eruption with the EUV Imaging Telescope (EIT) on the Solar
Heliospheric Observatory (SOHO) and the soft X-ray telescope (SXT) on Yohkoh. This event occurred on 1999
April 18 and was likely the origin of a coronal mass ejection detected by SOHO at 08:30 UT on that day. In
the EIT observation, one-half of the filament shows two stages of evolution: stage 1 is a slow, roughly constant
upward movement at ≈1 km s⫺1 lasting ≈6.5 hr, and stage 2 is a rapid upward eruption at ≈16 km s⫺1 occurring
just before the filament disappears into interplanetary space. The other half of the filament shows little motion
along the line of sight during the time of stage 1 but erupts along with the rest of the filament during stage 2.
There is no obvious emission from the filament in the SXT observation until stage 2; at that time, an arcade of
EUV and soft X-ray loops forms first at the central location of the filament and then expands outward along the
length of the filament channel. A plot of EUV intensity versus time of the central portion of the filament (where
the postflare loops initially form) shows a flat profile during stage 1 and a rapid upturn after the start of stage
2. This light curve is delayed from what would be expected if “tether-cutting” reconnection in the core of the
erupting region were responsible for the initiation of the eruption. Rather, these observations suggest that a loss
of stability of the magnetic field holding the filament initiates the eruption, with reconnection in the core region
occurring only as a by-product.
Subject headings: Sun: chromosphere — Sun: corona — Sun: flares — Sun: UV radiation
concurrent examination at less energetic wavelengths in studying solar eruptions.
1. INTRODUCTION
In order to understand the mechanism for solar eruptions
that lead to soft X-ray flares or coronal mass ejections (CMEs),
it is vital to examine data at the very start of eruption onset.
While higher energy aspects, e.g., the soft and hard X-ray
regimes, of these eruptions can tell us much about the eruption
process, it is also important to remember that there can be
substantial “preflare” activity at longer wavelengths. For example, several workers have recorded the activation of Ha
filaments during the preflare phase (e.g., Martin & Ramsey
1972; Rust 1976). This activity can precede the main flare
outburst by several minutes or even hours (e.g., De Jager &
Svestka 1985). We can use observations of such pre-eruption
activity to help set constraints on different theoretical concepts
for solar eruptions.
In this Letter, we present observations of a well-observed
quiet-region filament eruption, seen in absorption in EUV coronal emission with the EUV Imaging Telescope (EIT) on the
Solar and Heliospheric Observatory (SOHO) spacecraft. The
Large Angle and Spectrometric Coronagraphs (LASCOs) on
SOHO detected a CME that is likely associated with this event.
We trace the pre-eruption evolution of the filament and compare
the timing of its eruption with the time of the first coronal
brightenings of an associated weak soft X-ray flare. Our findings place severe limitations on the “tether-cutting” magnetic
reconnection model of solar eruptions. We also show that our
conclusions may have been different if we had only considered
soft X-ray data, and thereby we point out the advantages of a
1
2
2. INSTRUMENTATION AND DATA
Our primary observations are from SOHO’s EIT instrument
(Delaboudiniere et al. 1995), which observes the full solar disk
with a pixel resolution of 2⬙. 6 using four EUV filters. For this
study, we primarily used the 195 Å Fe xii filter, which is most
sensitive to emission at 1.5 MK. We also examine data from
EIT’s 304 Å filter, which contains He ii emission at
2 # 10 4–8 # 10 4 K and Si xi emission at 1 MK; for our observations, the He ii line dominates the 304 Å emission. Our
EIT 195 Å data have a time cadence of approximately
12 minutes, but EIT took 304 Å images only once in 6 hr.
We also use image data from the soft X-ray telescope (SXT)
on the Yohkoh spacecraft (Tsuneta et al. 1991), which detects
coronal emissions of temperatures ⲏ2–3 MK. We only used
full-disk SXT images, which have a pixel resolution of 4⬙. 9 or
9⬙. 8 and a time cadence of about 10 minutes, periodically interrupted by Yohkoh’s satellite night.
We observed an eruption that occurred on 1999 April 18,
with the first soft X-ray emission beginning between SXT images at 06:33 and 07:21 UT; this event occurred in a quiet
region and did not produce a soft X-ray signal substantially
above the mid-B class background in the detectors on the
GOES-8 satellite. Nonetheless, as the images from EIT and
SXT show (Fig. 1, discussed below), this eruption generated
an arcade of evolving postflare loops that is a fundamental
feature of two-ribbon flares. The LASCOs on SOHO detected
a partial-halo CME at about 08:30 UT on 1999 April 18 that
likely came from this event.
NRC-NASA/MSFC Research Associate.
Also with United Applied Technologies, Huntsville, AL.
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Fig. 1.—(a–c) SXT images of the eruption at three different times. (d–f) EIT 195 Å (Fe xii) images at approximately the same three times as (a), (b), and (c),
respectively. In the images, north is up and west is to the right. The arrows labeled “f” in (d) point to a filament, and the arrow labeled “bp” points to a bright
point, relative to which the filament moves between (d) and (e). A box marks the central part of the filament in (d); Fig. 2 plots a light curve from this box.
Lines a and b in (e) are used to trace the filament’s motion (Fig. 2).
3. OBSERVATIONS
Figures 1a–1c show the evolution of the erupting region in
SXT. SXT images on 1999 April 18 between 04:22 (Fig. 1a)
and 06:33 UT (the last one before Yohkoh night) show no essential difference from the 04:22 UT image; that is, the first clear
indication of flare activity in SXT is in the first image following
Yohkoh satellite night, the image at 07:21 UT (Fig. 1b).
Figures 1d–1f show corresponding EIT 195 Å images. Different from the preflare SXT images, the EIT images show a
distinct filament in absorption in the preflare phase (Fig. 1d).
This filament moves northward in the preflare phase and is noticeably displaced in Figure 1e relative to its position in Fig-
Fig. 2.—Paths a and b trace the height of the filament along their respective
paths in Fig. 1e, relative to the location at 00:00 UT on 1999 April 18. Two
thin lines overlying these paths are best-fit lines with slopes of 1.2 and 16.5
km s⫺1. Two vertical lines, at 06:36 and 06:48 UT, show the times bracketing
the start of the filament ejection. The curve labeled “box” shows the intensity
(total counts per second) as a function of time summed over the box at the
center of the filament in Fig. 1d.
ure 1d (compare with the position of the filament relative to the
bright point “bp” in Figs. 1d and 1e). By the time of Figure 1f,
the filament is no longer visible; from a movie formed from the
complete sequence of EIT images, it is obvious that the filament
erupts outward in between the times of Figures 1e and 1f. In its
wake is a system of EUV postflare loops (Fig. 1f), corresponding
to that visible in soft X-rays in Figure 1c. A box in Figure 1d
marks the location from which these postflare loops originally
brighten, before spreading out in both directions along the channel where the filament resided prior to eruption.
Using EIT 304 Å images, we have verified that the absorption feature we observe in EUV is a chromospheric filament
that erupted. It is visible in 304 Å images prior to 07:19 UT
on 1999 April 18 but is not apparent in a 304 Å image at
13:19 UT on the same day.
Figure 2 shows a plot, derived from the EIT 195 Å images,
of the height of the filament (i.e., its northward displacement)
as a function of time, relative to its location at 00:00 UT. We
follow the filament’s movement along two paths, labeled “a”
and “b” in Figure 1e; paths a and b run through the northeast
and southwest halves of the filament, respectively. Random
uncertainties in determining the filament’s position along the
paths are similar to the size of the fluctuations of the respective
curves in Figure 2.
Along path a, the filament begins to show upward motion
near 00:00 UT; at later times, the filament’s evolution follows
two distinct slopes: for 29 data points between 00:00 and
06:24 UT, a linear least-squares best fit gives a slope of 1.2
km s⫺1; and for the four measured times between 06:36 and
07:25 UT, the slope is 16.5 km s⫺1. We call these slow and
fast phases of the filament’s evolution stage 1 and stage 2,
respectively. Although a linear fit appears to be an appropriate
approximation for stage 1, we have too few data for stage 2
No. 2, 2001
STERLING, MOORE, & THOMPSON
Fig. 3.—Schematic of magnetic tether cutting, where (a) is before eruption
onset and (b) is from shortly after eruption onset. This interpretation of (b) is
based on the results of this Letter; it differs from the interpretation of Moore
& LaBonte (1980) and Moore et al. (2001), which suggested that the reconnection represented in (b) occurred at the very start of eruption onset. Solid
lines indicate magnetic fields; dashed lines show the projected neutral line;
and the shaded area in (a) indicates filament material, omitted in (b) for clarity.
to know whether a power-law or exponential fit may be more
appropriate than the linear fit. The two vertical lines in Figure 2, at 06:36 and 06:48 UT, show the time interval in which
the filament eruption begins (the start of stage 2).
In contrast to path a, the filament is virtually stationary along
path b; that is, this portion of the filament appears to be nearly
fixed in location during the time it is visible. Inspection of the
EIT movie of the images also gives the impression that the
filament is stationary along path b until shortly after 06:00 UT.
At that time, the filament appears either to undulate slightly or
to move primarily along the line of sight of observation. After
07:13 UT, this portion of the filament becomes totally invisible
in the EIT images, appearing to have violently erupted outward
after this time. The change in slope in the path b curve beginning with the leftmost of the two vertical lines in Figure 2
is coincident with the start of the violent eruption of the southwest portion of the filament toward the observer.
Also in Figure 2, the curve labeled “box” shows the summed
intensity of emission in the box covering the central portion
of the filament in Figure 1d. It is from this central region that
the Fe xii postflare loops develop. Although there are fluctuations in the box intensity early on, there is no substantial
increase in the intensity until after the two vertical lines in
Figure 2. That is, the intensity slowly begins to increase concurrent with or slightly after the onset of the filament ejection
and shows a marked increase only after the time of the onset
of the filament ejection.
4. DISCUSSION
Filaments commonly undergo two stages of evolution. Zirin
(1988) points out that virtually all prominences that rise above
50,000 km erupt, implying a slow early rise followed by a fast
eruption. There have been detailed studies of active-region
prominence eruptions. For example, Tandberg-Hanssen, Martin, & Hansen (1980) identify flare sprays as filament eruptions,
and the height-time curves of several of their sprays resemble
the two stages in the evolution of our filament in Figure 2.
Being active-region events, the velocities of the TandbergHanssen et al. (1980) eruptions are much higher (reaching several hundred kilometers per second) than those that we find.
Possibly related, if not identical, to these flare-spray erupting
filaments are the soft X-ray plasmoids identified in SXT images
(e.g., Ohyama & Shibata 1997; Nitta & Akiyama 1999), which
also show relatively low velocities (∼10 km s⫺1) prior to the
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onset of a flare-associated hard X-ray burst and higher velocities (up to several hundred kilometers per second) after the
burst; once again, these numbers correspond to active-region
eruptions and not to quiet-region eruptions such as those that
we report here.
The initial rise of the filament early on (i.e., stage 1 of the
evolution reported here) is apparently part of a gradual evolution of the initial magnetic configuration containing the filament, an evolution toward global instability of the field (e.g.,
Moore & Roumeliotis 1992). This might be due to, e.g., interaction of intruding, newly emerging flux with the preexisting
fields. The actual eruption of the filament occurs during stage
2, and this is the period of the filament’s evolution that we are
trying to understand here.
Under the assumption that quiet-region eruptions are basically
the same as active-region eruptions, except that the quiet-region
eruptions are typically larger scale, more slowly evolving, and
generate less intense coronal emissions (all consequences of
weaker magnetic fields in quiet regions), our results in Figure 2
can have key implications regarding the mechanism for eruptions. Based on soft X-ray observations from Skylab, together
with ground-based Ha observations, of a filament eruption,
Moore & LaBonte (1980) suggested that eruptions result when
magnetic field lines in the “core” (i.e., the most highly sheared
portion) of a magnetic bipole-like region reconnect, initiating
the release of the energy stored in the magnetic fields. That is,
the magnetic lines are like tethers holding back the ejection, and
the reconnection “cuts” the tethers (“tether cutting”). Based on
SXT images, Moore et al. (2001) argued in favor of the Moore
& LaBonte viewpoint, noting that the earliest location of soft
X-ray emission in several eruptions occurred in the core of apparently bipolar regions. In our study here, the first soft X-ray
brightenings also occur in what appears to be the core of the
magnetic region (Fig. 1b, and also based on magnetograms of
the region that we have inspected). Nonetheless, our findings do
not suggest that tether cutting initiated the eruption, since the
rapid rise of the filament (stage 2) began at or before the slow,
faint onset of the EUV emission and well before the steep rise
of EUV and soft X-ray emissions.
Our reasoning is as follows. If tether-cutting reconnection
initiates the eruption, then we expect high temperatures (such
as those observed in flares) to accompany this reconnection.
Due to thermal conduction or particle acceleration, this would
lead to excitation of the material in the chromosphere connected
to the reconnecting magnetic field lines. This material should
radiate in the EUV as footpoints of the newly reconnected
magnetic loops, and we expect to see this as an increase in
intensity in EIT 195 Å images. According to the tether-cutting
idea, only then should the magnetic fields associated with the
filament begin to erupt away from the Sun. Since we do not
see the increase in brightness in EIT or SXT images prior to
the eruption, it appears that the eruption of the filament began
first and that this was followed by reconnection, which in turn
resulted in the formation of postflare loops as described by,
e.g., Hirayama (1974).
Figure 3 depicts the setup for the tether-cutting idea. Prior
to eruption, highly sheared field lines are poised to reconnect.
According to the original idea espoused by Moore & LaBonte
(1980) and Moore et al. (2001), reconnection between these
field lines initiates the eruption itself. Our observations here
suggest that this is not the case. Rather, we find that the tethercutting reconnection may still be occurring, but only after the
eruption onset was triggered by some other process.
One way in which the tether-cutting-at-eruption-onset idea
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could still fit with our observations is if the reconnection occurs
at low temperatures early on. In this view, rapid, but cool,
tether-cutting reconnection would lead to the start of the eruption of the filament. At some point, the tether cutting either
changes form or becomes intense enough to start generating
higher temperatures, finally leading to an increase in EUV intensity. Our current data set does not allow us to check this
possibility of an initial period of “cool reconnection,” but our
observations do tell us that some such cool reconnection is
likely if it is tether cutting that initiates the filament eruption.
It is still conceivable that hot reconnection does occur from
the start, but if so, it has only a tiny emission measure below
our observational threshold. In this way, our work sets new
constraints on the idea that tether cutting initiates the eruption.
This work only considers a single event, but, using similar
timing arguments, Sterling & Moore (2001) and Sterling et al.
(2001) present evidence for the eruption beginning prior to the
earliest indications of the flare (reconnection) heating in the
core region for different events.
From our findings, we suggest that tether cutting is not responsible for the very start of the eruption reported here; rather,
first the core field carrying the filament begins to escape from
the Sun, and then runaway tether cutting ensues. As noted
above, SXT data alone would have suggested that the earliest
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activity begins in the core field region (Figs. 1a–1c). Thus, our
current work points out the importance of considering possible
activity prior to the first high-energy emissions in searching
for the eruption trigger.
In the absence of cool-temperature reconnection (discussed
above), other suggested explanations for CMEs may be in better
agreement with our observations than tether cutting. For example, Forbes & Isenberg (1991), Rust & Kumar (1996), and
Antiochos, DeVore, & Klimchuk (1999) suggest that an eruption can take place without early internal tether cutting as a
requirement. Forbes (2000) reviews various models for solar
eruptions.
A portion of this work was completed while A. C. S. held
a National Research Council–NASA/MSFC Research Associateship. A. C. S. and R. L. M. were supported by funding
from NASA’s Office of Space Science through the Solar Physics Supporting Research and Technology Program and the SunEarth Connection Guest Investigator Program. Yohkoh is a mission of the Institute of Space and Astronautical Sciences
(Japan), with participation from the US and UK, and SOHO
is a project of international cooperation between ESA and
NASA.
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