1. INTRODUCTION - Solar Physics Dpt

THE ASTROPHYSICAL JOURNAL, 534 : 468È481, 2000 May 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

FACTORS RELATED TO THE ORIGIN OF A GRADUAL CORONAL MASS EJECTION ASSOCIATED WITH AN ERUPTIVE PROMINENCE ON 1998 JUNE 21È22 NANDITA SRIVASTAVA, RAINER SCHWENN, AND BERND INHESTER Max-Planck-Institut fur Aeronomie, 37191, Katlenburg-Lindau, Germany

SARA F. MARTIN Helio-Research, 5212 Maryland Avenue, La Crescenta, CA 91214

AND YOICHIRO HANAOKA Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan Received 1999 September 11 ; accepted 1999 December 18

ABSTRACT We present observations of a coronal mass ejection (CME) associated with an eruptive prominence during 1998 June 21È22 by LASCO (Large Angle Spectroscopic Coronagraph) aboard SOHO (Solar and Heliospheric Observatory). Various features in the three-part structured, white-light CME as observed by LASCO-C2 and C3 coronagraphs were compared with features in the other wavelengths, for example, in Fe XIV and Fe X emission lines obtained from LASCO C1, in Ha from Helio-Research and at 17 GHz obtained from Nobeyama Radioheliograph. We have investigated conditions in several data sets to understand the eruptive and the pre-eruptive scenario of the CME. The CME and the eruptive prominence accelerate up to D20 R and then decelerate to the velocity of the ambient slow solar wind. The _ particular CME is a typical case of a very slow or gradual CME for analysis clearly shows that this which it is difficult to deÐne an exact onset time. The CME could be tracked for about 30 hours until it crossed a distance of 30 R and disappeared from the Ðeld of view of the LASCO-C3 coronagraph. The _ features of this CME suggest that the leading edge of the CME and the height-time proÐles of various top of the prominence or the core follow similar pattern, implying a common driver for both the CME and the eruptive prominence. The observations provide strong evidence that the CME and the prominence eruption resulted from a common cause which is the global restructuring of the magnetic Ðeld in the corona in an extensive volume of space near and including the CME. The restructuring in turn was a consequence of newly emerging Ñux regions near and within the neighboring active regions close to the base of the CME. Subject headings : Sun : activity È Sun : corona È Sun : prominences 1.

INTRODUCTION

Earlier attempts to study the initiation of CMEs i.e., in the inner corona were made using ground-based coronagraphs (Fisher et. al. 1980 ; Fisher & Poland 1981 ; MacQueen & Fisher 1983). Although studies have been made in recent years to detect CMEs in the lower corona based on soft X-ray images obtained by Y ohkoh (Hudson & Webb 1997), only a few studies have been reported in the past on the relationship of green-line transients with white-light CMEs (Plunkett et. al. 1997 and references therein). Prior to the launch of SOHO with its internally occulted LASCO-C1 coronagraph on board, the observations from space coronagraphs were constrained in the inner limits of the Ðeld of view as they had to be occulted externally. However, LASCO-C1 has been able to overcome this limitation and is capable of imaging the inner corona in Fe X and Fe XIV lines, suitable to record the CME in initial and eruptive phase. As most CMEs are associated with prominence eruptions (Gosling et. al. 1974 ; Munro et al. 1979), detailed studies of eruptive prominences give additional information relevant to CME launch and propagation. Erupting prominences have been best observed for a long time in the Ha line and have been well documented as disparition brusques in the literature. However, until now, no satisfactory explanation to their relation with CMEs has been given. Hu (1983a, 1983b) suggested that it is the prominence that acts as a

Coronal mass ejections, or CMEs are large-scale eruptions of material from the sun propagating outward through the heliosphere. In white-light observations of the sun, which are due to Thomson scattering by coronal electrons, they are known to exhibit a classic three-part structure, namely, a bright leading edge, followed by a dark cavity devoid of material, and an embedded prominence, as shown earlier by Hundhausen (1984). The initiation or trigger of an individual CME, and the nature of the acceleration as it moves outward in the corona, are among the basic questions that remain unsolved. The LASCO coronagraphs aboard SOHO are able to track a CME to a larger distance from the sun than ever before, i.e., from 1.7 to 32 solar radii in white light using C2 and C3 coronagraphs. In addition, LASCO-C1 o†ers a unique opportunity to image nearly simultaneously the inner solar corona from 1.1 to 3.0 R in Fe XIV (5303 A , _ (Brueckner et. al. green) and Fe X (6374 A red) emission lines 1995). It enables us to identify at early stages of a CME, in the inner corona, the emission line counterparts of the three-part structure if there exist any, as seen in white light in the outer corona. This comparison could provide an important clue to the understanding of the origin or the initiation of the CME and its acceleration in the observed Ðeld of view. 468

ORIGIN OF A GRADUAL CORONAL MASS EJECTION driver for the CME. On the other hand, Harrison (1995) argued that CMEs and prominences are consequences of the same magnetic ““ disease ÏÏ and that one does not trigger the other. Others have advocated the idea that CMEs appear to drive the eruption of prominences as they occur prior in time to the prominence eruption onset (Wilk et. al. 1997). Yet another idea is that the magnetic Ðeld of the prominence cavity is responsible for driving a CME (Low 1997 ; Gibson & Low 1998). According to their view, the dark cavity, interpreted as a region of enhanced magnetic Ðelds, is the most important CME precursor. Various theoretical models have been proposed for a magnetically coupled eruptive prominence and an associated CME (e.g., Mouschovias & Poland 1978 ; Anzer & Pneuman 1982). The latter suggested that magnetic reconnection of the type associated with Ñares could drive or push a CME, although observations indicate that this is unlikely to occur as a Ñare follows a CME after an average delay of 17 minutes (Harrison 1986). Steele & Priest (1989) proposed another model of the CME and eruptive prominence developed on the assumption that reconnection of the adjacent coronal Ðeld below the prominence is driven by the CME as magnetic Ðeld lines are stretched out. This causes the ascending arcade to be more rapidly driven outward and manifested as the CME. A comparison of speeds and acceleration of di†erent features of the CME provides an additional important tool to understand the mechanism of the initiation and the driver of the CME. For instance, Gopalswamy et. al. (1996, 1997) determined the speed and acceleration in a rising prominence and the associated coronal arcade using the soft X-ray Y ohkoh images and the radio data obtained from Nobeyama Radioheliograph at 17 GHz. They found that the speed of the prominence beneath the arcade was at all times lower than the leading edge of the CME, thus ruling out that a prominence could drive a CME. Some important clues for obtaining an in-depth insight into the mechanism of the CME could also be gained by studying the preeruptive phase of quiescent solar prominences during their entire disk passage to learn the conditions that lead to their instability from a seemingly stable conÐguration. The signatures of chirality of the Ðlament before eruption, being sinistral or dextral (Martin, Bilimoria, & Tracadas 1994), and their relation with the helicity of overlying coronal arcades provide a means to predict the changes in magnetic Ðeld that take place during the ascension of the magnetic Ðeld of the CME and erupting Ðlament as shown by Martin & McAllister (1997). Their model depicts the formation of separate helical Ñux ropes for CME and erupting Ðlament with opposite helicity ; the Ñux rope of the erupting Ðlament is embedded within the Ñux rope of CME of opposite helicity. Further, the works of Rust & Kumar (1994) and Marubashi (1997) indicate that the signs of helicity developed before or during the eruptive stage are preserved and are observable in interplanetary space. The preservation of the sign of helicity is found to be consistent with the conservation of helicity (Ruzmaikin 1996). In this paper, we report the results of our study of the eruptive and preeruptive evolution of a huge CME observed by LASCO coronagraphs aboard SOHO. This CME was associated with an eruptive prominence observed in Ha at Helio-Research during 1998 June 21È22 at the northwest limb. We have compared the three-part structure of the CME in white light (LASCO-C2, C3 coronagraphs)

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with the features observed in Fe XIV, Fe X lines (LASCO-C1), in Ha line (Helio-Research), and at 17 GHz (Nobeyama Radioheliograph). We have determined the height-time and speed proÐles for the leading edge of the CME and also for the underlying eruptive prominence. We have made a comprehensive e†ort to understand the preeruptive scenario of the CME based on data sets in di†erent wavelengths corresponding to di†erent layers of the solar atmosphere. 2.

OBSERVATIONS1

Observations by LASCO : The three coronagraphs of LASCO namely C1, C2, C3 are designed to observe the solar corona from 1.1 to 3.0 R , 1.7 to 6 R , and 3.7 to 32 _ _ R , respectively (Brueckner et. al. 1995). They were able to _ track the coronal transient during the interval 1998 June 21È22. Observations were obtained in coronal emission lines, Fe XIV green line (5303 A ) and Fe X red line (6374 A ) by LASCO-C1 with an on-line image, on average every 20 minutes and 120 minutes, respectively. The details of the analysis and processing of the images obtained by LASCOC1 has been discussed by Schwenn et. al. (1997). The whitelight observations were made by the C2 coronagraph at an average rate of 20 minutes, while the images by LASCO-C3 coronagraph were taken at irregular intervals with a larger time gap between individual frames in the beginning than toward the end of this event. The initial phase of the large CME of 1998 June 21È22 was recorded by LASCO-C1 aboard SOHO as a slowly rising loop from 10 : 47 UT on June 21 through 01 : 31 UT on June 22. LASCO-C1 recorded images in the lower corona in Fe XIV and Fe X emission lines. This event could be further tracked out in the outer corona by LASCO-C2 and C3 until 17 : 35 UT on June 22. The CME was found to be associated with a large prominence eruption recorded in high-resolution Ha images from 18 : 47 until 22 : 14 UT on 1998 June 21 at Helio-Research at La Crescenta, California. The prominence was slowly ascending throughout this interval. The Ðeld of view of each frame was approximately 8@ ] 11@, and the images were obtained through a 2.5 A passband Ha Ðlter. Radio observations of the erupting Ðlament were recorded by the Nobeyama Radioheliograph (NRH), which images the sun at 17 GHz. NRH is an 84 element radio interferometer with a high dynamic range of about 25 dB. It has a high time resolution of 1 second and spatial resolution of about 13A (Nakajima et. al. 1994). At this radio frequency, the corona is optically thin and the chromosphere is optically thick, thus allowing the Ðlaments or prominences to be observed. The observations by NRH for this event started from 22 : 45 UT on June 21, and continued until the prominence disappeared from the Ðeld of view at 02 : 45 UT on June 22. Close to the timings of the CME, GOES soft X-ray (1È8 A ) satellite recorded two C-class Ñares, (C1.0 and C1.7), the maxima of which occurred at 18 : 08 and 21 : 09 UT, respectively, in NOAA AR 8243. The latter Ñare seems to have occurred just before the initial rapid acceleration of the CME took place.

1 The mpeg movies of the data analyzed in this study can be accessed from fttp ://star.de/pub/nandita/980621.

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FIG. 1.ÈEvolution of the Ðlament shown as a composite of daily Ha images recorded by the Big Bear Solar Observatory. On June 20, the southern section of the Ðlament (marked by an arrow in the June 19 frame) disappeared.

2.1. Pre-eruptive Phase of the Associated Prominence The prominence associated with the CME observed on 1998 June 21È22 had been stable before for quite some time ; it had already survived one solar rotation as it was observed during 1998 May 14È26 on the disk with a similar shape and structure to that observed in 1998 June. It is interesting to note that no CMEs were reported by LASCO during this period anywhere in the vicinity of this prominence/Ðlament when observed either above the limb or during its transit across the disk in May or June. (We hereafter refer to this prominence as a Ðlament in observations of it against the disk). Thus, there is no evidence of any prior eruption or rebuilding of the prominence/Ðlament in May or June. This is remarkable in contrast to the majority of prominences on the sun in this phase of the solar cycle. Within a duration of 12 days, i.e., from 11 through 22 June, spanning the disk passage of the associated Ðlament under study in this paper, there were at least 14 disappearing (or erupting) Ðlaments/ prominences, implying one or more eruptions per day on average. Therefore eruption seems to be a typical characteristic of the fate of most prominences/Ðlaments observed during this phase in the solar cycle. In the next rotation, the Ðlament appeared on the east limb of the sun on 1998 June 12 as low-lying long feature with a latitudinal extent of approximately 25 heliographic degrees as seen in Ha images obtained from Big Bear Solar Observatory (BBSO) shown in Figure 1. From these daily images in Ha, one could also determine the chirality of the Ðlament. The images of June 15 and 16 clearly show that the southern end of the Ðlament on the disk was dextral according to the classiÐcation of the helicity by Martin et. al.

(1994). This leads to the conclusion that the entire Ðlament is dextral, as there is no evidence that a single Ðlament can change its chirality along its length without Ðrst erupting or breaking into separate segments. Even though the Ha observations on June 15 and 16 seem to suggest that the Ðlament is partially fragmented, full-disk Ca K3 images obtained from Observatoire de Paris (not presented here) show a continuous Ðlament, thus revealing more mass that can be seen in the best Ha images such as from BBSO. The Ha and Ca K3 daily images also show that the prominence remained stable during its entire disk passage from June 12 until June 19. However, by June 20, the Ðlament lost its southern section. The southern end of the Ðlament on this day was approximately 18 heliographic degrees north of the emerging Ñux to the west of the NOAA AR 8243 just west of central meridian. The odd shape of the rest of the Ðlament after the sudden loss of the southern part indicates that it became unusually tall in the middle and upper sections. By early June 21, most of the Ha Ðlament had risen over the northwest limb with a portion still to be seen on the disk. Both Ha and Ca K3 images are found to display similar structures with the sudden loss of its southern section by June 20, thereby conÐrming that the disappearance in Ha was not just a temporary disappearance due to Doppler-shifted material. This further suggests that the Ðlament was already activated a few days prior to its Ðnal eruption on 1998 June 21. 2.2. Eruptive Phase of the Prominence and the Associated CME LASCO-C1 observations in Fe XIV green line, which are

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ORIGIN OF A GRADUAL CORONAL MASS EJECTION

presented in Figure 2 show the transient activity recorded during June 21È22. From an examination of these images, it is found that a bright stable loop (L ) is present since 08 : 45 G UT on 1998 June 21 on the northwest limb with a welldeÐned shape adjacent to a bright loop system (marked by ““ S ÏÏ in Fig. 2, 10 : 47 UT frame). Although this loop seems to be gradually rising, a rather conspicuous rise begins around 17 : 13 UT. The loop is followed by a dark region similar to the dark cavity observed in white-light CME, and then, as the outer loop ascends, a rising core of bright material (P ) G is observed. The feature we see in LASCO-C1 images looks more complicated than what would be expected from a rising simple arcade system. We observe the bright core material to take a curly shape in Fe XIV emission. This feature Ðrst becomes visible at 21 : 54 UT and has been displayed in Figure 2 (23 : 24 UT frame). The observations of the event as recorded in Fe X line by LASCO-C1 are pre-

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sented in Figure 3. In Fe X observations, the Ðrst appearance of a low-lying bright feature (P ) occurred on 1998 June 21 at 15 : 30 UT with only a veryRfaint impression of an overlying loop. The consecutive frames taken later on this day show that this feature ascends gradually. Between 21 : 30 UT and 01 : 31 UT (June 22) the feature rose dramatically and evolved into a complex structure which is similar to that seen in Fe XIV images, marked by an arrow (Fig. 3, 01 : 31 UT frame). We believe that these observations provide evidence for additional twist of the inner Ðeld lines of the rising loop-system. The next image taken at 04 : 05 UT showed that this structure had disappeared in Fe X images, indicating that it had left the Ðeld of view of LASCO-C1 between 01 : 31 and 04 : 05 UT. It must be noted that the images shown in Figures 2 and 3 are on-line images subtracted from a base (on-line) image taken prior to the activity. Thus, only the changes with respect to the reference

FIG. 2.ÈTime-lapse images taken by LASCO-C1 coronagraph in Fe XIV emission line. The Ðeld of view is 1.1È3 R . All the images shown here have been _ UT frame is an on-line image with a subtracted from a reference image taken before the occurrence of the CME, except the one at 10 : 47 UT. The 10 : 47 nearby continuum subtracted from it in order to show the bright streamer adjacent to the CME.

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FIG. 3.ÈComposite of Fe X (red line) images recorded by LASCO-C1. These images have been subtracted from a base image taken at a time prior to the CME.

image become visible. This signal may contain contributions from both the continuum emission (electron-density) and green (red) line emission due to Fe XIV (Fe X) ions. The Ha observations as recorded by Helio-Research are presented in Figure 4. A movie made from individual frames recorded once per minute in Ha during the eruption phase clearly shows that the prominence is activated in addition to ascending. There is streaming in the prominence along the imaginary Ðeld lines. The dominant streaming appears to be upward in the more equatorial leg, horizontal in the middle-upper parts and downward in the other leg toward north. However, some coarse counter-streaming can be seen in the equatorward leg and in the horizontal section in the middle of this event. Spatially and morphologically, the bright loop seen in Fe X line appears to coincide well with the northern end of the Ha prominence over the limb. The images of the radio prominence at 17 GHz obtained by NRH is shown in a mosaic in Figure 5. The time-lapse images clearly exhibit the slow eruption of the radio prominence from 22 : 45 UT until 02 : 45 UT on 1998 June 22. One also Ðnds that there is a slight lateral expansion of the radio prominence in the early phase of the eruption which is consistent with Ha observations. This is then followed by a slow rise and Ðnal disappearance of the prominence from the Ðeld of view at 02 : 45 UT on June 22. Although in principle the chromospheric features that can be seen at 17 GHz are similar to those seen in Ha, these observations are complementary to each other and therefore provide important information. This stems from the fact that the observed emission in Ha can be reduced by large Doppler shifts out of the Ðlter passbands whereas the observations at 17 GHz are not a†ected at all (Hanaoka & Shinkawa 1999). Note

that LASCO-C1 observations cannot detect features moving with line-of-sight velocities larger than D30 km s~1, as the passband of the Ðlter is very narrow. In the outer corona, the CME and the associated prominence could then be tracked in white light by LASCO-C2 and C3 coronagraphs, as shown in Figures 6 and 7, respectively. The CME in LASCO-C2 (white-light) is observed to rise below a bright preexisting large-scale streamer. Between 01 : 19 and 01 : 38 UT the leading edge (LE) or the outer bright rim was seen to emerge from behind the occulter in LASCO-C2. The leading edge of the CME could be tracked further until 04 : 31 UT when it left the Ðeld of view. The CME in white light was observed as a series of bright arcades with bright emission core or knot following it. The front end (P ) of this knot left the Ðeld of view at W its other end (P ) at around 07 : 34 around 05 : 44 UT and Tl UT (as shown by arrow). Because the large-scale streamer is no longer visible after the CME has gone, the event can be considered a ““ streamer blow-out ÏÏ as classiÐed by Howard et. al. (1985). In this structural class of CMEs the streamer is believed to be a simple arcade like structure before the eruption, which swells and rises slowly. We do observe such a single streamer in white-light (LASCO-C2 and C3). However, as discussed above, in the lower corona, i.e., in Fe XIV green-line observations the CME leading edge (L ) is G 2, seen to lie adjacent to the bright loop-system ““ S ÏÏ (Fig. 10 : 47 UT frame). The observations suggest that both the CME and the loop-system in green line merge into a single large-scale streamer as observed in white light, at the northwest limb. In the Ðeld of view of LASCO-C3, which recorded whitelight images at variable intervals ranging from about more

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FIG. 4.ÈHigh- resolution Ha images obtained by an Ha telescope with a 2.5 A passband Ðlter at Helio-Research. These images show the slowly ascending prominence over the northwest limb on 1998 June 21 between 19 : 50 and 22 : 50 UT.

than an hour to about 20 minutes, the CME on the northwest limb, appeared between 01 : 48 and 03 : 40 UT. From this time onward the CME is observed to rise and develop in the shape of a huge ““ ear-shaped ÏÏ balloon, extending from the northern hemisphere to south of the solar equator centered at a position angle of 290¡ and with an average angular width of 95¡. Another interesting feature is the slightly curved shape of the leading edge in the middle. The CME thus exhibits the classical case of a three-part structure CME with an outer bright leading edge (LE) followed by a dark cavity and then a bright core of material (P ), which is due to the prominence at one end (northern end)Wof the CME in LASCO-C3 images (Fig. 7). The CME is observed to rise continuously, while expanding and remaining attached to the solar surface. The leading edge of the CME left the Ðeld of view between 12 : 00 UT and 13 : 20 UT. It is then followed by the elongated prominence/core material, the top of the prominence leaving the Ðeld of view at 15 : 06 UT as shown by an arrow. The bright core bears a hooklike tail (P ) which remains undistorted until the last Tl

frame (17 : 35 UT, Figure 7). It disappeared from the Ðeld of view at around 19 : 00 UT on June 22. If this feature would have been massively accelerated between 4.5 and 24 solar radii, where it is observed, it would likely be stretched out radially. Since this is not the case, there is only little acceleration of the tail of the prominence in this region. This indirect conclusion is consistent with the computed acceleration values derived from the time-height proÐle as discussed in the next section. 3.

DISCUSSION

Here we describe in detail the major inferences and conclusions that can be drawn from observations of the erupting prominence and the CME as obtained in various wavelengths : coronal green (Fe XIV), the coronal red (Fe X) line, white light, the chromospheric Ha line and 17 GHz radio emission ; all were obtained by di†erent instruments. In the past, very few attempts have been made to understand the relationship of green-line transients with whitelight CMEs (Plunkett et. al. 1997 and references therein).

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FIG. 5.ÈDisappearance of the prominence as seen in the time-lapse images of the sun in radio wavelengths (17 GHz) over the limb between 23 : 00 UT on June 21 and 02 : 45 UT on June 22.

The white-light coronagraphs detect Thomson-scattering of photospheric light by the free electrons, the white-light intensity being a measure of density rather than temperature. On the other hand, the Fe XIV green and Fe X red line observations respectively reÑect the plasma ions that are at a temperature of 1.8 ] 106 K and 1.0 ] 106 K. For the present case, Fe XIV images were overlaid with isointensity contours of the Fe X images obtained by LASCO-C1 such that the times of the two types of images were as close as possible. These are shown as a composite in Figure 8. Examination of this Ðgure shows that most of the bright loop (L , as marked in Fig. 2) in the Fe XIV (green G line) has no emission counterpart in Fe X (red line) images. In contrast, the dense bright knot below the loop in the Fe XIV images does emit strong emission in Fe X line although it is slightly lower in height in Fe X in the early phase. However, as time evolves, these features match very well spatially. The absence of as bright a frontal loop ahead of the twisted structure in Fe X indicates that the temperature of the leading edge is much higher. It is probably close to 2 ] 106 K, and while most of the prominence material or the knot is comparatively cooler, some fraction of its mass also is heated to at least 2 ] 106 K. This is consistent with the absence of any evidence of the overlying loop in both the Ha and 17 GHz radio images. The leading edge or the frontal loop (LE) visible in LASCO-C2 and C3 images matches well with the well-

deÐned outer loop (marked as L in the Fe XIV green-line G a few hours prior to its images). This loop was seen as stable gradual rise and subsequent eruption. In addition, the top of the bright core (P ) within the huge CME observed in W well with the top of the bright knot the white-light matches P in Fe XIV images and with the top of the twisted loop G structure P observed in the Fe X red line. A qualitative comparisonR of the structure of the northward leg of the prominence in Fe X images and Ha suggests that they outline the same feature in the form of helical structure. The simultaneous disappearance of the prominence in both Ha and 17 GHz images as discussed in the previous section could be explained as partly due to heating of the prominence and partly due to dilution. This is evident from the long duration of the bright core. This bright core corresponds to the prominence material in the lower corona in Fe X and Fe XIV emission lines. The bright core is still visible at greater heights in the outer corona as observed by the LASCO-C2 and C3 coronagraphs. 3.1. Kinematics of the CME Features The pre-eruptive phase of this CME is characterized in the Fe XIV line images by a well-deÐned bright structure of a hot loop overlying the prominence material. The height of this overlying loop is approximately 1.5 R from the sunÏs _ center. This implies that the height of the CME onset is 1.5 R . A comparable starting height of 1.3 solar radii was _

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FIG. 6.ÈLASCO-C2 observations of the CME in the white light on 1998 June 22 in a range of 2È6 R from the center of the sun _

found by Gopalswamy et. al. (1997) from a study of a CME in soft X-ray Y ohkoh images and an associated prominence in Nobeyama radioheliograph images. A comparison of the height-time diagrams of various features observed in di†erent wavelengths is shown in Figure 9. The leading edge of the CME and the prominence underneath can be seen to evolve for several hours before the accelerated onset of the CME around 23 : 00 UT on June 21. The height versus time plot for the leading edge of the CME, as well as for the top and tail of the prominence, as identiÐed in various wavelengths earlier, indicates a similar rising behavior for both features in the outer corona. An enlarged height-time plot for the CME features covering the observations only up to 4 R is shown in Figure 10. The rise of the CME, the leading _edge and the prominence features are so gradual that it is rather difficult to deÐne an exact onset time of the CME. The leading edge in Fe XIV line was deÐnitely rising continuously from 12 UT on June 21. After 12 UT the ascension of the CME is real and cannot be attributed solely to a projection e†ect caused by

solar rotation. A more obvious rise in the outward speed of the CME-leading edge occurs at approximately 23 : 00 UT (June 21) and for the prominence one hour later. Thus, the duration of the slowly ascending phase of the CME was 11 hours if we suppose the rapid acceleration phase as beginning at 23 : 00 UT on June 21. The relative speed between the leading edge or the frontal loop and the embedded eruptive prominence could provide important clues to the understanding of the CME driver. We have measured the projected speeds of the CME and the eruptive prominence against the sky-plane. Figure 11a shows the variation of the speeds with time for both features in the lower corona. It is seen that the prominence at any instant of time is slower than the CME. This implies that the prominence could not have served as a driver, solely in the sense of a piston pushing the CME as already pointed out by Hundhausen (1987). However, upward pressure from the magnetic Ðeld of the prominence exerted on the coronal Ðeld of opposite helicity can be an important contributing factor to the triggering of the complete erupting event.

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FIG. 7.ÈTime-lapse images of white-light CME in the outer corona from 3.7È30.0 R as recorded by LASCO-C3. These images clearly display the _ propagation of a gradual balloon type CME and its three-part structure.

Figure 11b shows a plot of speed versus distance for this event. The variation of speeds with radial distance in the di†erent parts of the CME is similar to that of the slow solar wind proÐle obtained by Sheeley et al. (1997) for slowly moving blobs observed as density enhancements in LASCO-C2 and C3 images. From Figure 11b, one notices that the leading edge of the CME and the prominence top and tail accelerate from almost zero to 300 km s~1 within 5 R of the sun ; the value of the acceleration rose from 0 to 20_m s~2 (Figure 12). This acceleration is within the range of 0È50 m s~2 reported for eruptive associated events by MacQueen & Fisher (1983). The CME leading edge acceleration slows from 20 to 5 m s~2 as the CME traverses the distance from 5 until 20 R . The corresponding speeds in this distance are between _ 300È500 km s~1. The estimated errors for the values of the speed in the Ðeld of view of di†erent coronagraphs are approximately ^5 km s~1 (C1), ^10 km s~1 (C2), and ^50 km s~1 (C3). The value of the acceleration toward the end is 5 m s~2, which is consistent with the acceleration value of 4 m s~2 obtained for slowly moving blobs (Sheeley et. al. 1997). In the present case, the acceleration is followed by deceleration beyond D 20 R . The _

kinematics of this event show that it is a typical example of a distinct class of slowly evolving or gradual CMEs as recently reported by Srivastava et. al. (1999a, 1999b). The magnitude of the speeds of these gradual CMEs have been found to be comparable and evolving as that of slow solar wind. Therefore, they appear to drift away in the slow solar wind. We were able to compute the total ejected mass during the June 21/22 event using LASCO-C3 images obtained during the propagation of CME from 3 to 30 R following _ Howard et. al. (1997). The calculation of mass involves the measurement of the electron density, which is computed from the excessive brightness after removing the pre-CME brightness. The method is based on the assumption that a single electron at a certain point in the atmosphere will scatter a known amount of solar disk intensity. Thus, by measuring the intensity and assuming that all of the mass is in a single volume element, the number of electrons can be computed. Then, the mass of the CME is calculated assuming charge neutrality. We Ðnd that the mass of the CME increases from a value of 2.0 ] 1015 gm at a distance of 3 R up to 2.25 ] 1016 gm at 25 R , as shown in Figure _ _

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FIG. 8.ÈOverlay of Fe X intensity contours on Fe XIV images. These images show coronal material at formation temperatures of Fe X and Fe XIV lines with time.

13. As the leading edge propagates outward, the total mass of the CME increases because more of CME material begins to appear in the Ðeld of view of coronagraph from behind the occulter. Using a large sample of Solwind CMEs, which were observed up to a distance of 10 R , Howard et. al. (1985) reported that the total mass of a_CME ranges from 2 ] 1014È4 ] 1016 gm with an average value of 4.1 ] 1015 gm. The estimated mass of the present CME is in good agreement with the values obtained for Solwind CMEs and those obtained for LASCO CMEs (Howard et. al. 1997). 3.2. Factors Related to the Origin of the CME We suggest that it is quite signiÐcant that the eruption on the northwest limb of the Ðlament on 1998 June 21È22 is one of the Ðve Ðlaments that erupted between midÈ21 June and early 22 June. The other four Ðlaments were located in the southern hemisphere and were close to the central

meridian. All of them erupted between 16 : 38 UT on June 21 and 07 : 30 UT on June 22 ; the LASCO coronagraphs recorded at least four major CMEs during this time period which may have been associated with these eruptive Ðlaments. This enhanced activity of the sun is suggestive of a major global restructuring of the magnetic Ðeld conÐguration that might have taken place shortly before or concurrent with the erupting prominence at the northwest limb, under study here. There was another erupting Ðlament and Ñare in the northern hemisphere on the north side of NOAA AR 8243, to the south-west of the eruption we describe on the northwest limb. These two events appear to be concurrent. However, one needs to address what initiated all of this activity on the sun, including the gradual CME under study. In seeking evidence for reconnection as a possible cause of the CME, we examined soft X-ray images obtained by Y ohkoh (not shown here) during the launch of the CME.

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Distance in solar radii

4 Leading Edge (C2+C3) Fe XIV Prominence Top (C2+C3) Fe XIV (Prom) Fe X (Top) H-alpha (Top) Prominence Tail (C2+C3) Radio H-alpha (Lower) Fe X (Lower)

3

c b a

2

1

0 Jun 21

6

12

18

0 Jun 22

6

Time in UT

FIG. 9.ÈPlot of height (on log scale) against time for di†erent features of the CME, viz., the leading edge, the prominence top, and the tail, as measured from the images obtained in di†erent wavelengths by various instruments.

There was no obvious feature in SXT images that could be related directly to any of the CME features, namely the leading edge, the prominence top, and the tail. However, another small feature was observed which brightened up in SXT images at around 20 : 30 UT on June 21, concurrent with the C-class, GOES Ñare, the peak of which occurred at 21 : 00 UT. This Ñare was reported to have occurred in the NOAA AR 8243, to the east of the Ðlament. The active region NOAA AR 8243 was also the source location of several other Ñares prior to this Ñare during 1998 June

FIG. 10.ÈEnlarged height-time plot (up to 4 R ) showing the evolution _ of di†erent CME features in lower corona during the initial phase. The arrows ( from left to right) mark the timings of actual ascension of the CME (a), the initiation of the brightening in Y ohkoh images (b), and the occurrence of a C1.7 Ñare in the NOAA AR 8243 (c).

21È22. A close investigation of various data sets rules out the possibility that this Ñare could have triggered the CME and the Ðlament eruption since the Ðlament was observed rising much earlier. From an examination of daily coronal hole maps obtained from Kitt Peak, National Solar Observatory, we found that a transient coronal hole was formed between the southern section of the Ðlament and the active region NOAA AR 8243 to the east of the southern section on June 20, as seen in Figure 14. This coronal hole was not present a day before. It is quite probable that the formation of this transient coronal hole resulted from two large CMEs on

FIG. 11.ÈTime variation of speeds of the CME features, viz., the leading edge of the CME, the prominence top, and the tail. Fig. 11b shows the variation of the CME speeds with the radial distance. The speed-distance proÐles are almost similar for the leading edge of the CME and the prominence top, while the tail end of the prominence follows a similar curve but with comparatively lower values of speed.

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FIG. 12.ÈAcceleration vs. distance proÐles for di†erent features of the CME.

June 20, as the Ðeld lines would have been blown open (extended vertically such that the Ðeld conÐguration was e†ectively ““ open ÏÏ at the solar surface). The CMEs on June 20 were observed by the LASCO-C2 coronagraph at the northwest limb at 07 : 57 and 15 : 37 UT. The high timeresolution longitudinal magnetograms obtained from Mt.

FIG. 13.ÈThe variation of total mass (on log scale) of the CME with distance as obtained from LASCO-C3 images.

Wilson show a few newly emerging Ñux regions which appeared just west to this AR NOAA 8243 and were named NOAA 8251. It therefore appears plausible that, owing to the sudden emergence of these new magnetic Ñuxes, major

FIG. 14.ÈKitt Peak coronal hole maps as obtained from observations in He 10830 A line. Appearance (marked by the arrow) and disappearance of the coronal hole in the northern hemisphere on June 20 and 22, respectively, is clearly seen.

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CMEs occurred in this region on June 20, which led to formation of a transient coronal hole. This may in turn have also led to subsequent reconnection between the newly opened Ðeld lines and Ðeld overlying the southern section of the Ðlament. Continued reconnection could then have resulted in the sudden disappearance (eruption) of the southern section of the Ðlament. The new Ñuxes continued to emerge after June 20 ; thus, continued large-scale restructuring of the magnetic Ðeld conÐguration in the northwest region might explain the slow rise of the remaining larger northern section of prominence over the limb. The Ðnal eruption of the Ðlament and the CME therefore could have been a secondary response to these changes in the Ðeld-line orientation at the photospheric level in the neighboring active regions, NOAA 8243 and NOAA 8251. Further evidence of this large-scale restructuring is the disappearance of the transient coronal hole of positive polarity that appeared on the solar surface at N35W15 location as seen in Figure 14. The coronal hole had disappeared by June 22 (14 : 58 UT frame). Hence, the disappearance of the coronal hole took place either during or after the launch of this CME. In summary, we suggest, it is the major restructuring of the global magnetic Ðeld that caused the Ñare and the CME in the nearby active region NOAA 8243. The restructuring of the magnetic Ðeld also acted as a trigger for the prior eruption of the southern part of the Ðlament and the subsequent slowly ascending loop system overlying the remaining longer northern part of the prominence. This overall scenario provides an important clue to the CME initiation. Rapid magnetic reconnection or impulsive energy release does not seem to be the cause or the origin of this CME. A possible model to explain the launch for this CME in particular is that such a CME is a consequence of the nonequilibrium of preexisting stable structure as deÐned by Low (1981). The magnetic Ðeld stress builds in such a stable structure and Ðnally attains a critical stage where the preexisting arcade structure just moves outward gradually like a balloon. The prominence eruption in this event occurs because of loss of equilibrium as the CME rises above. The magnetic Ðeld of the prominence and CME are separate because their Ðelds have opposite chirality and hence opposite helicity. This assumption is based on the results of Martin & McAllister (1996, 1997). First they found that the respective chirality of Ðlaments and their overlying arcades is, without exception an inverse relationship, for quiescent prominences. Second, they demonstrated via an empirical model how the chirality structure of both Ðlaments and their overlying arcades are transformed, by magnetic reconnection in the eruptive phase, into helical structure of the same sign as the previously observed chirality. The prominence magnetic Ðeld is initially in balance with the overlying arcade such that upward force in the prominence magnetic Ðeld is balanced by the downward force of the overlying arcade of opposite helicity. The height versus time curves also indicate that this system balance is maintained throughout the eruption of both. This means that neither the erupting prominence nor the CME is acting like a driver for the other. However, the rising CME magnetic Ðeld leads to the rise of the spine or axis of the prominence magnetic Ðeld. This rise, in turn sets the stage for magnetic reconnection between the barbs on each side of the prominence. The magnetic reconnection creates helical structure in the rising

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prominence and enables further outward expansion. The entire system including the prominence is then in runaway loss of equilibrium in the later phase. 4.

CONCLUSIONS

The present work based on the analysis of the multiwavelength observations of the CME and the associated prominence by LASCO and other instruments, shows that this CME is a typical example of a very slowly evolving or gradual CME. To our knowledge, no detailed analysis has previously been made of such a gradual event tracked continuously from the inner to the outer corona up to a distance of 30 R . One of the best reported CMEs is the one _ associated with an eruptive prominence observed by Skylab, covering data in various wavelengths (Schmahl & Hildner 1977). Fisher & Garcia (1984) had reported such a slow CME from the observations made by MK-III K coronameter from 1.24È2.31 R . More recently, Plunkett _ study of two fast and one et. al. (1997) made a comparative slow green-line and white-light transients observed by LASCO in order to understand their interrelation. Based on the present analysis, we conclude the following : 1. The CME exhibits the classical three-part structure of an outer expanding envelope of high-temperature coronal mass, a cavity with low mass, and a bright core consisting of denser, cooler prominence mass. 2. The pre-eruptive conÐguration is well represented by a bright loop-shaped structure (leading edge) at a height of 1.5 R from the SunÏs center, visible only in Fe XIV images, which_overlies a bright knot (prominence) ; the temperature of the leading edge is approximately 2 ] 106 K. This loop lies adjacent to a streamer in the inner corona. Thus, there is clear observational evidence from LASCO-C1 data, that the lower corona already resembles or assumes the threepart structure which is later on observed in white light after the CME erupts. 3. The CME studied here is a typical example of a gradual or slowly rising event for which a precise onset time is difficult to deÐne. The CME is seen to ascend for many hours with low speeds of less than 50 km s~1 in the lower corona, i.e., below a distance of 3 R . The acceleration in _ and a clear decrease the outer corona is also quite gradual, in acceleration follows the initial acceleration. The gradual nature of the CME under study here is typical of the class of gradually evolving CMEs. These CMEs tend to reach a terminal speed of 300È500 km s~1 at 20 R (Srivastava et. _ wind proÐle al. 1999a, 1999b) and trace out the slow solar given by Sheeley et. al. (1997). 4. The estimated speeds range between 0È500 km s~1 and the values of acceleration range between 0È20 m s~2. These values are consistent with those obtained by MacQueen & Fisher (1983) for eruptive-associated events based on their study of the coronal transients. The minimum value of the acceleration coincides with the constant acceleration value of 4 m s~2 obtained by slowly moving blobs (Sheeley et. al. 1997). 5. There is no observational evidence of magnetic reconnection or impulsive energy release to be the cause or the origin of this CME. Instead, the observations clearly show that the major restructuring of the global magnetic Ðeld caused the Ñare and the CME in the nearby active region NOAA 8243. It was also responsible for the prior eruption of the southern part of the Ðlament and the subsequent

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slowly ascending loop system overlying the remaining longer northern part of the prominence. 6. The speed versus distance plots are similar for both the leading edge of the CME and the eruptive prominence. This suggests that the same mechanism drives both the leading edge of the CME and the prominence and that neither of these is necessarily a cause or consequence of the other. The simultaneous motion of the two suggests that acceleration has a more global cause, such as loss of MHD stability and the restructuring of the magnetic Ðeld. The event might be initiated by the accumulation of sufficient free energy in the form of sheared, twisted Ðeld lines which eventually cannot be contained. During the ascension phase, the energy is gradually released. 7. Our preferred scenario for the erupting prominence part of the overall event is its loss of equilibrium. As the pre-CME corona slowly expands outward, the prominence also rises but the distance between the prominence Ðeld remains the same or increases. This implies that the promi-

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nence magnetic Ðeld exerts as much pressure on the overlying arcade as the coronal Ðeld exerts upon the prominence magnetic Ðeld. Loss of equilibrium in the coronal Ðeld then results in the loss of equilibrium for the prominence magnetic Ðeld, and both are expelled.

We thank the LASCO/EIT Consortium and Operations team at GSFC. Work at NRL and USRA was supported under NASA contract NDPR S[92385[D. The German e†ort was supported by DLR, and the French participation was Ðnancially supported by CNES. The British contribution was supported by the Particle Physics and Astronomy Research Council in the United Kingdom. SOHO is a mission of international cooperation between ESA and NASA. The Helio-Research observations were possible because of NASA grant NAG 5-3220, and contributions to the text were supported by NASA grant NAG 5-4180.

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