Eruption and acceleration of flare-associated coronal mass ejection loops in the low corona

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. All, PAGES 25,215-25,225, NOVEMBER 1, 2001 Eruption and acceleration of flare-associated coronal mass ejection loops in the low corona W. M. Neupert Raytheon STX Corporation, Lanham, Maryland B. J. Thompson and J. B. Gurman Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland S. P. Plunkett Solar Physics Branch, Space Science Division, Naval Research Laboratory, Washington, D.C. Abstract. Observations made by the EUV imaging telescope (EIT) and the Large-Angle Spectrometric Coronagraph (LASCO) on board the Solar Heliospheric Observatory (SOHO) have been used to characterize the eruption and acceleration of flare-associated coronal mass ejections (CMEs) in the low corona. For three well-observed limb events we tracked CME loops back to preexisting but faint EUV-emitting loops at heights of too- 250 Mm that initially brightened slowly and possibly increased slowly in height, apparently in response to filament activity and eruption in the associated active regions. Subsequent CME acceleration coincided with a rapid rise of the soft X-ray flux, occurred between too and 350 Mm above the surface, and may have been as high as 0.5 km s - s -, consistent with an impulsive acceleration of the CME to the speeds observed in subsequent whitelight observations. The existence of a delay of up to 30 min observed between initial filament eruption in H alpha and subsequent high acceleration of the CME in one event implies that there may have been two separate phases of magnetic reconnection, with the initial filament activity acting as a trigger for subsequent CME and energetic particle acceleration in the impulsive stage of the flare. The presence or absence of this impulsive phase may provide a basis for the two types of CMEs that have been discussed in the literature. 1. Introduction To describe fully the dynamic properties of coronal mass ejections (CMEs), one must explore the full range of both coronal heights and temperatures over which they originate. Visible light observations, using satellite-borne and groundbased coronagraphs, initially characterized their dynamic characteristics to as low as 0.2 Rs (Rs being the solar radius) above the solar surface [Tousey, 1973; MacQueen et al., 1974; Hansen et al., 1974; MacQueen and Fisher, 1983]. Observations by the soft X-ray telescope (SXT) on Yohkoh have identified coronal dimming [Hudson et al., 1996] and the evolution of sigmoidal emission features [Rust and Kumar, 1996; Canfield et al.,!999; Aurass et al., 1999; Sterling et al., 2000] as critical to the origins of CMEs. Additional results include detection of soft X-ray (SXR) jets during the impulsive phase of many flares [Ohyama and Shibata, 1998] and the possible presence of shock waves from flares leading to eruption of transequatorial loops [Khan and Hudson, 2000]. The extensive field of flare-cme relationships has been frequently reviewed [see, e.g., Kahler, 1992; Dryer, 1994; Gosling, 1997; Hudson and Webb, 1997; Hundhausen, 1998] and need not be summarized here. Now at NOAA Space Environment Center, Boulder, Colorado. Copyright 2001 by the American Geophysical Union. Paper number 2000JA /01/2000J A ,215 The launch of the extreme ultraviolet (EUV) imaging telescope (EIT) [Delaboudinieret al., 1995] on board SOHO in December 1995 opened up the opportunity to examine the transient corona in the temperature range of 1-2 MK associ- ated with CMEs. EUV observations by EIT have characterized coronal dimming [Zarro et al., 1999] and waves [Thompson et al., 1999; Klassen et al., 2000] against the solar disk that are frequently associated with CMEs. Dere et al. [1997] presented the first on-disk observations of the initiation of a CME in association with the eruption of a prominence. Possibly the most comprehensive description of CME initiation to date has been given by Innes et al. [1999], who combined ground-based and space observations to relate high-temperature jets, EUV emission, and H alpha filament eruption. EUV data during a critical portion of the event were missing, however, and a relationship between the activity in the low corona and the beginning of the white-light CME could not be made. In this paper we extend the results of Dere et al. [1997] and Innes et al. [1999] by tracking CME loops (or, possibly, threedimensional shells) from their observation with the Large- Angle Spectrometric Coronagraph (LASCO) back through the field of view (FOV) of the EIT to preflare features in the 1.5-MK corona and relating their launch as CMEs to preceding activity in the low corona and chromosphere. Although an association between erupting filaments and CMEs has long been known [Joselyn and Mcintosh, 1981], these EIT observations are the first to characterize the dynamic behavior of a CME source region at CME initiation and to relate that initi-

2 25,216 NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS ation to underlying filament activity. The observations are consistent with coronal loop destabilization as a reaction to filament eruption (and by implication, local magnetic field changes) rather than to changes in the large-scale fields overlying the active region. We locate the range of heights over which acceleration of CME loops takes place and estimate the polarity inversion line. Indeed, a diffuse array of coronal loops (indicated as C in Figure l e) connected the leading bright area with the extreme eastern end of the trailing bright ribbon. In Yohkoh SXT observations on prior days the region appeared to have a structure that could be interpreted as a sigmoid, but its soft X-ray morphology on September 2-3 was uncertain due magnitude of that acceleration, which seems to be of short to its nearness to the solar limb. duration and may be impulsive. We also compare the EIT observations of National Oceanic and Atmospheric Administration (NOAA) region 8210 with published soft X-ray analyses and conclude that EUV and soft X-ray observations, cov- The first evidence of activity leading to this CME was recorded in on-band H alpha images [Denker et at., 1999] taken at Big Bear Observatory (Figure la), when the active region filament (feature A) began to darken at 2258 UT and fade at ering different ranges of temperature, provide complementary 2305 UT (note that H alpha frames have been selected to information on flare-associated CME eruptions in the low corona. coincide with EIT observations). This activity occurred in the vicinity of the coronal loop foot points rooted in the leading polarity of the region. Subsequently, another filament segment 2. Instrumentation (Figure lb, feature B), along the same neutral line but approximately 150 Mm to the southeast of the region, began to fade at 2307 UT and was replaced by a weak H alpha emission at The design of the EIT has been discussed in several papers [Detaboudinieret at., 1995; Moses et at., 1997]. It has frequently been operated in a full-disk mode to record CME activity for correlation with the LASCO images, and that was the case for the three events reported here. The typical 12-min cadence (with some interruptions) was being used with the Fe XII (195- ) passband. In the absence of flare activity, the 10- (full width at half maximum (FWHM)) wide passband cen- tered at 195 represents plasma in the 1.3- to 1.8-MK temperature range. At times of flares the 192-, emission line of Fe XXIV produced at MK [Neupert, 1971] may contribute strongly to the signal from postflare loops. The LASCO has also been described elsewhere [Brueckner et al., 1995]. It was operating in a standard synoptic mode in which C2 and C3 images were taken at 24-min intervals. A limiting factor in observing pre-cme changes in the EUV corona against the solar disk and off the limb is the low contrast of the erupting features as they move to the edge of the EIT FOV. Small active regions typically have processed signal levels of data numbers (DN) per pixel, where one DN unit nominally corresponds to 18 electrons per pixel detected in the EIT's CCD, and bright, stable off-limb loops have levels of DN. The erupting off-limb coronal loop on September 2 that we will be discussing (letters C and E in Figure 1) had an average DN of only 1.3. However, the total signal attributed to that rising feature was 9300 DN, so that reliable estimates of total emission measure were still possible. 3. Observations and Analysis 3.1. A CME From NOAA Region 8679 on September 2-3, 1999 A prototypical three-component CME (a bright loop or shell encompassing a dark cavity and a bright filament eruption) was observed by LASCO early on September 3, 1999, and was attributed to activity in NOAA region 8679, then at 36øS, 41øW on the solar disk. The surface activity and subsequent EUV coronal eruption leading up to the LASCO observations are shown in Figures 1 and 2. Prior to this time the region had been quiet during its disk passage, producing no reported flares or CMEs. It was last reported as a sunspot group on September 2, when it was classified as spot class AX: unipolar with no penumbra. Kitt Peak magnetogramshowed no evidence of parasitic polarity in its vicinity. In EIT observations preceding the CME the region appeared as a pair of stable bright ribbons, presumably representing foot points of loops over the region's 2334 UT, by which time the two-ribbon flare emission was also visible in H alpha. Type III emission was reported at UT and UT by several observatories, and type IV emission was recorded at Learmonth at UT on September 3. Major components of the associated EUV transient, which began after filament activation, are shown in Figure 1 and Figure 2, where running differences are used to emphasize subtle coronal changes [Thompson et at., 1999; Klassen et at., 2000]. LASCO images of the subsequent 570 km s - CME are also shown. Elements of the transient are designated, in order of their appearance, as D, an EUV emission feature at the site of filament activation and near the leading foot points of the overlying coronal loops; E, increase in height and lateral extent with time of the preexisting coronal loops; F, decreasing brightness of the on-disk corona, which we identify as "coronal dimming;" G, an eruption of coronal emission from the trailing foot points of the rising loops at the site of the second filament activation; H, enhanced emission directly over the active region, identified as postflare loops; and I, a slowly rising dark feature or cavity over the postflare loops. This event is a clear example of a CME offset from a flaring region [Harrison, 1986; Harrison et at., 1990]. By 2346 UT the foot points of the loop system were sepa- rating at a rate of about 100 km s -, consistent with coronagraph observations above the solar limb by MacQueen and Fisher [1983]. The rate of radially outward expansion was then so high that the apex of the EUV loop system was already outside of the FOV of EIT, and so we assumed a geometric expansion of the loops, fitted to their lower portions observed at 2334 UT, to estimate their height. Outward displacements of the expanding EUV loops were measured from the apexes of preexisting loops to the center of the bright (positive) expanding features in the running difference images. The timing of loop expansion relative to the concurrent GOES soft X-ray event and inferred eruption speeds in the plane of the sky as a function of time are discussed in section 4.2. Had the CME erupted at 2334 UT, a constant acceleration of 0.4 km s - s - in the plane of the sky would have been required to account for its position at 2346 UT. Estimates of masses of the outward moving EUV loops, the on-disk coronal dimming after loop eruption, and the subsequent white-light loop were comparable. For the rising EUV loops at 2334 UT we assumed a half-toroid volume with a loop thickness of 30 Mm and toroid semidiameter of 120 Mm, a

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5 NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS 25,219 temperature of 1.5 MK (the temperature for maximum emis- and 3b) and overlying unresolved loops (feature B) at sivity of the 195- line of Fe XII), and a fill factor of 1 to Mm that were becoming slightly brighter or were slowly inestimate an emission measure of 1.6 x 1029 cm 3 and a mass of x 10 TM g (see the SOHO EIT Users Guide 1999, by J. S. creasing in height. Dark eruptive EUV features at CME initiation have been documented by Dere et al. [1997] and Innes et Newmark et al., at al. [1999]. A comparison of SXT and EIT images at 0157:03 de.html). and 0156:59 UT, respectively, at the very beginning of the The mass deficit in the region of coronal dimming (assumed impulsive phase, reveals considerably differing morphologies volume of 90 x 75 x 20 Mm) was estimated 8 x 1013 g. The just above the limb: The SXR images show a well-defined loop EUV estimates are lower limits, as the 195- EIT channel system with loop tops at 35 Mm, whereas the EUV has a samples primarily plasma at about 1.5 MK, and as Khan and Hudson [2000] have shown, erupting loops at 3-4 MK can also be present in at least some CME events. A deviation of the temperature from the assumed 1.5 MK would result in a higher mass estimate, as would the use of a fill factor of less than 1. SXR jets may also be present at the foot points of flare loops [Ohyama and Shibata, 1998] and may contribute additional mass. Mass estimates for the LASCO CME loop made by G. Lawrence (unpublished data, 2000) range from 1 to 4 x 10 TM g, in excellent agreement with the EUV estimates. We can therefore be confidenthat in this event the EUV loops are properly jumble of weak low-lying loops with the strongest enhancement at h _< 10 Mm (i.e., below the enhanced loops at Mm that have the morphology of the subsequent CME). An impulsive event began at 0157 UT, with the Nobeya radio heliograph recording a fast rising 17-GHz burst beginning at 0156:51 UT and peaking at 0157:55 UT and GOES registering a sharp SXR flux increase. Type III bursts of intensity 2 from 0156 to 0200 UT and centimetric bursts from 0155 to 0159 UT were recorded by numerous observatories. By 0209 UT the EUV loop system (feature C) was in a phase of rapid expansion, with the apexes of erupting loops barely outside of identified with the subsequent CME loop. Hildner et al. [1975] the EIT field of view. We estimated a lower limit on their made a mass comparison for a SXR loop and subsequent height at 0209 UT of 350 Mm and a lower limit on the average white-light CME using Skylab observations and obtained a similar result. More recently, Nitta and Akiyama [1999] reported a three-part SXR structure reminiscent of a CME. That event was reported in the LASCO CME catalog as a cloud of prominence material moving at 1256 km s -. Comparison of EIT and LASCO images shows that the irregular emission feature observed within the dark CME cavity in the LASCO image (feature G in Figure 2e) arose not speed between 0157 and 0209 UT of 180 km s -1. Loop displacements and estimated speeds are discussed in section 4.2. Had the CME launched at 0157 UT, an acceleration of 0.5 km s-1 s-1 would have been required to reproduce its position at the next EIT observation at 0209 UT. Nitta andakiyama [1999] reported an SXR ejection (their ejection "B") at a constant outward speed of 264 km s-1 beginning at 0204 UT, well after the EUV filament disappearance. At that speed the SXR ejec- from the eruption of a filament from the core of the active region but from the filament eruption (B) to the southeast of the active region between 2311 and 2322 UT (Figures lb and lc). Where then in the LASCO FOV was the filament eruption from the core of the active region? We propose that that filament erupted radially outward and can be identified with feature J in Figure 2f, moving at about 400 km s - in the LASCO C3 FOV. It may be present, but is not resolved from emission from the western foot point of the erupting CME loop, in Figures 2c and 2e. The continuing EUV dimming (feature I) of the corona directly over the postflare loops is less easily explained. It may also be present at the base of the western CME foot point in the subsequent LASCO C2 image. If that association is correct, then the dimming would represent a mass depletion rather than a temperature shift from 1.5 MK. The LASCO C3 image is a direct (not differenced) image and showed no evidence of a preexisting helmet streamer and no white-light activity prior to the CME. tion would have reached a height above the limb of 79 Mm by 0209 UT, well below the 350-Mm height of the EUV loop at that time, so it cannot be identified with the outward moving EUV loop. On the other hand, the larger, tangentially expanding SXR feature (labeled C in Nitta and Akiyama's Figure 1), which seems to have the same position as an emission feature identified as a soft X-ray wave by Khan and Hudson [2000], had dynamic characteristicsimilar to the most distant EIT feature (feature D) in our Figure 3e. Its average tangential speed between 0209 and 0157 UT was 503 km s-1 as compared with Nitta andakiyama's [1999] reported SXR speed of 678 km s -1 at 0200 UT. It is unlikely that this expanding feature represented the northern boundary of the LASCO CME, for in LASCO images the northern extent of the CME was moving outward at only 21 km s -1 between 0327 and 0426 UT (the first complete LASCO images of the CME). One possibility is that the EUV feature (feature D) was an EIT wave seen side on and that the diffuse EUV emission on the disk (feature E) represented the on-disk manifestation of that wave. In this scenario the EIT recorded the more distant characteristics of 3.2. CMEs From NOAA Region 8210, May 1998 the SXR shock wave proposed by Khan and Hudson [2000], Two CME events that occurred on May 8 were associated and both of these may have been manifestations of a fast mode with region 8210, which had passed over the west limb on May 7-8. We examined earlier CME events that originated while the region was on the disk and could detect no erupting EUV loops that corresponded to erupting or expanding CMEs. That shock, as originally proposed by Uchida [1968]. The LASCO C2 observation showed that a preexisting narrow streamer to the south of the CME (Figure 3g) was temporarily pushed southward during passage of the CME. is understandable in light of the weak emission from such May 8, 1998: UT. A CME loop eruption features discussed earlier. We have therefore turned to two that reached a speed of 405 km s -1 was first detected by limb events for further evidence of CME initiation in the EUV. LASCO at 1300 UT and was also preceded by transient activity May 8, 1998:0200 UT. EUV activity prior to a 405 in the EUV (Figure 4). Contrary to the significant radio sigkm s - CME first observed by LASCO at 0227 UT on May 8 consisted of a fluctuating dark, filament-like feature that disappeared between 0137 and 0157 UT (feature A in Figures 3a nature of the event at 0200 UT, only a single observation of type III emission was reported at the appearance of the filament, at 1224 UT, and then nothing further until 1325 UT,

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8 25,222 NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS when both type III and centimetric radio emissions were observed at intervals between 1325 and 1424 UT. An irregular filament (feature A) first appeared in EUV emission between the foot points of bright loops at 1225 UT and was clearly erupting as a radially oriented feature at 1237 UT. The direct images give little indication of expansion of overlying loops (feature B) during this initial filament activity, but difference images (Figures 4d and 4e) show either that higher loops were brightening as lower loops faded or that the loop system was expanding slowly. Foot points of the expanding loops (feature C) at 1237 UT coincided with the foot points of the preevent loops, and the CME eruption process was certainly one of outward expansion. A transition to rapid outward acceleration of the loop system (feature C) again occurred in coincidence with a sharp increase (but not to an immediate maximum) in SXR emission. An irregularly increasing SXR event had begun about 30 min before filament activation. The position of the active region behind the west limb suggests that the SXR emission may have been partially occulted by the limb and only maximized as postflare loops later rose above the limb. No disruption of the overlying coronal field was observed in the EIT FOV. Although the EIT observations are ambiguous as to whether the initial loop was eventually blown open or whether it maintained its structure as it expanded beyond the EIT FOV, the subsequent LASCO C2 images resolved that question by showing clearly that the loop structure was maintained to 6 Rs. The existing streamer to the south was again temporarily displaced but not disrupted by passage of the CME Other events associated with region As discussed earlier, the relatively long interval between EIT obser- vations (typically 12 min) and its limited off-limb FOV means that limb eruptions of more than a few hundred kilometers per second are likely to be missed altogether. Two events discussed by Khan and Hudson [2000] fall into this category: a CME originating in region 8210 on May 6, 1999, which reached a speed of 1053 km s -, and another CME on May 9, 1999, that neously with sharp increase in the soft X-ray emission and had a speed of 1726 km s -. In each instance, EIT observed concurrently with other signatures of particle acceleration such changes from the pre-cme morphology but no outward moving features Transequatorial loop disappearances. Khan and Hudson [2000] reported three CME events between May 6 and as radio bursts and hard X-ray emission. Displacements of the erupting loops are plotted as a function of time for each event against the GOES to 0.4-nm SXR flux in the left-hand panels of Figure 5. Estimates of eruption speed as a function of May 9, 1998, during which SXR loops between the northern time were derived from these measurements and are shown in and southern active regions shown in Figure 3 disappeared. the right-hand panels of Figure 5. Vertical bars are estimates We have examined the corresponding EIT regions for all three events and found no obvious EUV counterparts for the May 6 and early May 8 CMEs. Specifically, for the May 6 event, the major SXR loop disappearance region lying just within the solar limb remained unchanged in the EUV to within 0.5 _+ 1% between 0603 and 1112 UT except for a 2.3 _+ 1% increase at 0856 UT. For the early May 8 CME, the average EUV signal outside the limb at the equator increased by 2-3% at 0209 and 0221 UT when the SXR decreased. An EUV decrease of 5-15% was indeed found, but at 0236 UT, with a recovery to above pre-event values by 0320 UT. Only for the May 9 event did we find a gradual off-limb decrease of 35% between 0312 and 0403 UT, consistent with the SXR observations. For the CME at 1200 UT on May 8 (not analyzed by Khan and Hudson) the region that might have been the site of transequatorial soft X-ray loops brightened by 2.5% at 1237 UT and then diminished slowly to about 90% of preevent values by 1441 UT. These results, both increases and decreases of EUV emission in regions of SXR depletions, are likely due to the fact that the EIT and SXT imagers have different temperature sensitivities and registered changes in different coronal features. The combined data sets for the May UT CME might be interpreted, for instance, as the eruption of loops at about 1.5 MK over the active region and, concurrently, eruption of a transequatorialoop at about 4 MK. 4. Results The EIT observations of activity prior to and during CME launch can be summarized as follows Pre-CME Activity 1. CME initiation began with the brightening and possibly slow rise of already existing coronal loops at Mm that eventually formed the leading bright loop of the outgoing CME. This presence of EUV loops that evolve into CMEs differs from published reports either of the appearance of an EUV loop only after the beginning of filament activity [Dere et al., 1997] or of the disappearance of a portion of an EUV loop structure during a half-hour interval that encompassed an estimated CME onset time [Innes et al., 1999]. The initial changes in EUV loops that we observed appeared to be in response to filament activity that began, in one instance, as much as 30 min before CME eruption. Slow increases in the soft X-ray flux observed by GOES coincided with this filament activity. 2. Filament activity occurred with no detectable change in the morphology of the large-scale structure of the corona as observed by EIT or LASCO, suggesting that CME initiation was likely not due to magnetic reconnections in the overlying corona CME Acceleration 1. A change from a slow evolution during filament activation to a rapid acceleration of the loops occurred simulta- of error (_+30% of loop widths) that were propagated into the estimated speeds. The apparently impulsive acceleration of the CMEs was such that they attained 50% or more of their eventual speeds by 0.5 R s, implying acceleration levels as high as 0.5 km s - s -. Such accelerations are far greater than the accelerations of km s- s- over large distances in the high corona that are associated with large erupting prominences [Hildner et al., 1975; Rust and Hildner, 1976]. 2. CME acceleration did not always follow immediately upon filament activation but in one instance was delayed by 30 min. In that instance, magnetic reconnections associated with the filament eruption may have acted as a trigger for the CME eruption rather than as the primary driver of the CME. 3. The morphology of the outward moving CME loops reflected that of the coronal loops that erupted. A mass anal- ysis of one event demonstrated (as has been shown before) that the outward moving CME loop represented material originally in the corona. 4. The loops that erupted were in two instances located

9 - NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS 25, /09/ O /09/ ) OO OO oo Ol UT ( hours ) 1(:78 23 oo Ol UT (hours) /05/08 ', 1( E 400 >, 10-? i x O00 2O U T (hours) I U T ( hour s ) U T ( hours ) io s 98/05/08! / z _ I Figure 5. CME loop displacements (left-hand graphs) and speeds in the plane of the sky (right-hand graphs) derived from the measure displacements plotted as a function of time for three events. Solid circles represent EIT observations, and open circles are LASCO data. EIT loop displacements have been multiplied by a factor of 10 for clarity. The results are superimposed on the integrated to 0.4-nm soft X-ray flux measured by GOES 10. Vertical bars represent estimates of position uncertainties in loop heights (_+30% of loop half widths) and are propagated into inferred CME speeds. Dashed lines, although consistent with the data, are inserted only to guide the eye (n 6O0 4OO 2O0 õ directly over an active region (with foot points in magnetic fields of the active region) and in one case adjacent (with only one foot point directly associated with an active region). Both of these initial conditions are consistent with reconnection near the foot point of a complex coronal arcade [Innes et al., 1999, and references therein]. 5. It was shown in one instance that a bright prominence within the CME cavity originated far from the active region site of first filament activity, whereas the filament from the active region appeared along one leg of the departing CME loop. This observation suggests that prominence material observed within the dark cavity of white-light CMEs need not have a fundamental morphological relationship (e.g., an erupting flux rope) to the ascending white-light loop Magnetic Configurations at the Base of the Corona Antiochos [1998] has emphasized that two elements must be present in the corona to precipitate a CME. These are magnetic complexity and shear of the magnetic field. These two attributes were very likely present in the complex NOAA region 8210 on May 8-9, 1998, although its location behind the solar limb made it impossible to describe the magnetic configuration at the base of the interacting loops. On the other hand, we do not find any complexity in the simple, declining active region, NOAA region 8679, of September 2-3, 1999, which had decayed to a unipolar spot region with no parasitic polarities and no evidence of newly emerging flux when it produced a CME. Perhaps magnetic complexity may not always be a requirement for a CME, as the many polar crown filament eruptions and subsequent CMEs from spotless regions with superficially stable magnetic fields amply demonstrate. However, the oblique orientation of the first EUV emission relative to the neutral line in the September event suggests that shear was to some extent present in the September region and that free energy could have been stored in a closed sheared arcade. CME initiation based on transition from stable to unstable loop configurations, driven by photospheric foot point motions, has been discussed by Low [1981] and Mikic and Linker [1997] and may be an appropriate process for the CME of September 2, Discussion Past studies [see, e.g., Hundhausen, 1998], and references therein] have frequently fitted the displacement of eruptive prominences and subsequent CMEs with a curve representing constant acceleration. That approach may be appropriate for simple filament eruptions but may be too simple for flareassociated CMEs. The observations presented here suggest that a flare-associated CME launch may be composed of two distinct phases, the first involving a disruption of low-lying magnetic fields that is manifest in filament activation and a slow response in overlying coronal loops. The second phase, the rapid outward acceleration of these loops, coincides with

10 25,224 NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS an impulsive release of energy that also produces nonthermal radio and hard X-ray emission and the eruption of X-ray jets. This impulsive stage may also produce type II radio emission [Klein et al., 1999], soft X-ray waves [Khan and Hudson, 2000], and EIT waves [Thompson et al., 1999]. Our basis for a twostage hypothesis is that CME acceleration does not always follow immediately upon prior filament activity. The first phase may be short (at least in terms of the current EIT image cadence) or as long as 30 min, as in the September 1999 event. A further implication of our two-stage hypothesis is that when there is no impulsive energy release, there may also be no impulsive acceleration of the CME. That would be the case, for instance, in nonactive region prominence eruptions, with the consequence that only a slower acceleration, perhaps extending over a greater range in heights, would be present. We are currently searching the EIT database for appropriately posi- tioned simple prominence eruptions and will discuss their eruptive properties in a future paper. Our hypothesis would account for two types of CME velocity profiles associated with flares and with simple prominence eruptions, respectively [MacQueen and Fisher, 1983; Sheeley et al., 1999]. It also provides support for the conjecture by MacQueen and Fisher [1983] that "there is a qualitative difference between the nature of initial transient input" for flare- and prominence- associated events. The evidence we have presented implies that filament eruption is a predecessor, not a consequence, of magnetic reconfigurations in the corona that propel the CME. Observations of filament eruption relative to the sigmoid to arcade transition during flares and further comparisons of concurrent EUV and SXR images during eruptive events would be additional approaches to establishing the sequence of energy release in the early stages of CMEs. Acknowledgments. We thank C. Denker, of the Big Bear Solar Observatory, New Jersey Institute of Technology, for supplying the H alpha patrol observations for the September 2-3, 1999, event. One of us (W.M.N.) wishes to express his thanks to E. Hildner and the staff of the NOAA Space Environment Center for their hospitality and technical support. We also thank two referees for their constructive comments, which improved the paper. W.M.N. was supported by NASA contract NAS with the Raytheon STX Corp. SOHO is a project of international cooperation between ESA and NASA. Janet G. Luhmann thanks Yoichiro Hanaoka and another referee for their assistance in evaluating this paper. References Antiochos, S. 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11 NEUPERT ET AL.: ERUPTION AND ACCELERATION OF CME LOOPS 25,225 Delaboudiniere, O. C. St. Cyr, S. Stezelberger, K. P. Dere, R. A. Howard, and D. J. Michels, SOHO/EIT observations of the 1997 April 7 coronal transient: Possible evidence of coronal Moreton waves, Astrophys. J., 517, L151-L154, Tousey, R., The solar corona, Space Res., XIII, , Uchida, Y., Propagation of hydromagnetic disturbances in the solar corona and Moreton's wave phenomenon, Sol. Phys., 4, 30-44, Zarro, D. M., A. S. Sterling, B. J. Thompson, H. S. Hudson, and N. Nitta, SOHO EIT observations of extreme ultraviolet "dimming" associated with a halo coronal mass ejection, Astrophys. J., 520, L139-L142, J. B. Gurman and B. J. Thompson, Laboratory for Astronomy and Solar Physics, Code 682, NASA Goddard Space Flight Center, Greenbelt, MD W. M. Neupert, NOAA Space Environment Center, Code R/E/SE, 325 Broadway, Boulder, CO (wneupert@sec.noaa.gov) S. P. Plunkett, Solar Physics Branch, E. O. Hulbert Center for Space Research, Naval Research Laboratory, Washington, DC (Received May 16, 2000; revised November 13, 2000; accepted November 13, 2000.)