INTERPRETING THE LARGE LIMB ERUPTION OF JULY 9, 1982

INTERPRETING THE LARGE LIMB ERUPTION OF JULY 9, 1982

STUART JORDANLaboratory for Astronomy and Solar Physics Goddard Space Flight Center, Greenbelt, MD, U.S.A.

ADRIANA GARCIAObservatorio Astronomico da Universidade de Coimbra, Santa Clara, 3040 Coimbra, Portugal

VACLAV BUMBAAstronomical Institute, Czech Academy of Sciences, Ondrejov Observatory, CS 251 65 Ondrejov,

Czech Republic

(Received 28 October 1996; accepted 6 February 1997)

Abstract. A time series of K3 spectroheliograms taken at the Coimbra Observatory exhibits anerupting loop on the east limb on July 9, 1982 in active region NOAA 3804. The Goddard SMMHard X-Ray Burst Spectrometer (HXRBS) observations taken during this period reveal a hard X-rayflare occurring just before the loop eruption is observed, and SMS-GOES soft X-ray observationsreveal a strong long-duration event (LDE) following the impulsive phase of the flare. A Solwindcoronagram exhibits a powerful coronal mass ejection (CME) associated with the erupting loop. H�flare and prominence observations as well as centimeter and decimeter radio observations of theevent are also reviewed. A large, north–south-oriented quiescent prominence reported within theupper part of the CME expansion region may play a role in the eruption as well. The spatial andtemporal correlations among these observations are examined in the light of two different currentmodels for prominence eruption and CME activation: (1) The CME is triggered by the observed hardX-ray impulsive flare. (2) The CME is not triggered by a flare, and the observed soft X-ray flare isan LDE due to reconnection within the CME ‘bubble’. It is concluded that this event is probably of a‘mixed’ type that combines characteristics of models (1) and (2). The July 9 event is then comparedto three other energetic CME and flare eruptions associated with the same active-region complex, alloccurring in the period July 9 through September 4, 1982. It is noted that these four energetic eventscoincide with the final evolutionary phase of a long-lasting active-region complex, which is discussedin a companion paper (Bumba, Garcia, and Jordan, 1997). The paper concludes by addressing ‘thesolar flare myth’ controversy in the light of this work.

1. Introduction

This paper addresses one problem, and introduces another. The first problem wasto assess the nature of a highly energetic eruptive event that occurred on July 9,1982 near the Sun’s East limb. The second problem was to assess the more globalprocess leading to the destruction of the large, long-duration active-region complexassociated with this and three other energetic eruptions that occurred between July 9and September 4. This second problem is addressed in a companion paper (Bumba,Garcia, and Jordan, 1997).

To assess the complex event of July 9, 1982, we first consider observations ofan east-limb active-region prominence eruption that occurred in NOAA 3804 onJuly 9, 1982 and was almost coincident with an impulsive flare seen in H� andin both hard and soft X-rays. The start of the impulsive flare was followed aboutan hour later by a characteristic long-duration event (LDE) that is assumed to be

Solar Physics 173: 359–376, 1997.c 1997 Kluwer Academic Publishers. Printed in Belgium.

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360 S. JORDAN, A. GARCIA, AND V. BUMBA

approximately cotemporal with an energetic CME (Cane et al., 1987)�. Also presentduring this event was a large quiescent north–south prominence that appeared topenetrate into active region 3804.

From these and other observations reviewed here, we propose a possible modelfor this event. The eruption has some features in common with a model for energeticCMEs described by Hundhausen (1997). However, there are also important differ-ences, which are described below. These differences lead us to propose a modelin which an impulsive flare like the one observed as a prelude to this complexevent may act as the ‘trigger’ to the event, but in which the subsequent evolu-tion follows a pattern broadly similar to that described in the Hundhausen model.Our proposed model lies on a intermediate ‘spectrum’ of models for flare/CMEphenomena discussed in recent reviews by Dryer (1994) and Sakai and de Jager(1996).

Three other energetic CMEs with associated flares also erupted from the sameactive-region complex during the period July 9 through September 4 and are com-pared to the better observed event of July 9. The four energetic events of July 9,12 and 22, and of September 4 represent an unusually high level of activity whenthe SMM HXRBS record for the entire period 1980–1988 is reviewed. Togetherthese events signal the destruction of a large, long-duration active-region complexand its replacement by a coronal hole, similar to what has also been observed forthe following solar cycle (Bumba, Klvana, and Sykora, 1995).

In discussing our conclusions, especially on the model for the July 9 event,we will comment on what has been called ‘the solar flare myth’ controversy. Oneposition argues that solar flares either cause CMEs or that the CME itself is onemanifestation of a flare. A recent statement of this position appears in Svestka(1995). An alternative viewpoint argues that many CMEs occur without evidencefor flare triggering and can be triggered instead by instabilities in the associatedmagnetic field, and that the frequently observed, almost cotemporal soft X-rayLDEs result from reconnection of the magnetic field blown open by the CMEeruption. This position is argued by Gosling (1993) and Gosling and Hundhausen(1995).

Section 2 reports on our, and other observations of the July 9, 1982 event.Section 3 argues for our proposed model for the event, making use of some recentwork on MHD modelling. Section 4. reviews the other three energetic eruptionsmentioned above and compares them to the July 9 event. In Section 5 we discussthe significance of our conclusions.

� The question of relative start-times for CMEs and LDEs is discussed extensively in Hundhausen(1997) and Harrison (1991, 1995), and remains an open question subject to further research.

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INTERPRETING THE LARGE LIMB ERUPTION OF JULY 9, 1982 361

Table IMean velocities of July 9 east limb erupting prominence

Time interval Height change above limb Radial velocity(UT, July 9, 1982) (in km) (in km s�1)

09:40–09:56 1:3� 104 13.509:56–10:28 8:0� 103 4.210:28–10:38 below detection small10:38–10:54 4:8� 103 5.010.54–11.06 3:2� 103 4.411:06–12:35 below detection small12:35–12:56 1:6� 104 12.712:56–13:26 8:0� 103 4.413:26–14:02 below detection small14:02–14:30 1:1� 104 6.614:30–14:59 below detection small

Note evidence for several impulsive accelerations of the prominencematerial that was rising within the CME, not one constant velocity orconstant acceleration more typical of CME acceleration. This suggestsdifferent forces at work in the two situations.

2. Observations of the July 9, 1982 Event

The active region NOAA AR 3804 was one of the most complex of solar cycle 21.As NOAA 3804 rotated around the east limb, a small prominence was observedat N10 in K3 spectroheliograms taken at the Coimbra Solar Observatory between09:47 and 17:25 UT on July 8, 1982 (Figure 1). During this period the prominenceappeared stable; there was little measurable change in its height or appearance.Subsequently, an eruptive loop prominence was observed at the same latitudebetween 08:54 and 14:59 UT on July 9, 1982, with remnants of the eruption stillfaintly visible at 15:31 (Figure 2). Measurements of the velocity of the risingloop between 09:40 and 14:59 UT yield values shown in Table I. The prominencereached a maximum measured velocity of 13.3 km s�1 during this period, and theacceleration was clearly not steady, but episodic, suggesting complex changes overseveral hours in the local magnetic field. We cannot determine the start-time of theeruption from these observations, except to note that it preceded 09:40 UT, andwas closer to the time of the first observation (08:54 UT) if the accelerating forcedid not change dramatically during the interval between 08:54 and 09:40 UT. It isquite possible that the eruption commenced before 08:54 UT, as we might expectfor a flare loop from a flare whose impulsive phase began more than an hour earlier(see below).

Whether we are seeing the same feature that erupted on July 9 in the spectrohe-liograms of July 8 is hard to say in spite of their common latitudes, though it seemslikely. Figure 2(a) reveals a hint of the erupting feature just at the limb at N10. If

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Figure 1. Series of K3 spectroheliograms obtained on July 8, 1982 at the following times: (a) 09:47 UT,(b) 14:08 UT, (c) 14:54 UT, (d) 15:04 UT, (e) 15:15 UT, (f) 15:33 UT, (g) 15:42 UT, (h) 15:47 UT,(i) 17:15 UT, (i) 17:20 UT, (k) 17:25 UT.

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Figure 2a–l.

Figure 2. (a) Series of K3 spectroheliograms obtained on July 9, 1982 at the following times:(a) 08:54 UT, (b) 09:40 UT, (c) 09:56 UT, (d) 10:28 UT, (e) 10:33 UT, (f) 10:38 UT, (g) 10:54 UT,(h) 11:06UT, (i) 11:15 UT, (j) 11:32 UT, (k) 11:46 UT, (l) 12:35 UT; (m) 12:56 UT, (n) 13:26 UT,(o) 14:02 UT, (p) 14:30 UT, (q) 14:59 UT, (r) 15:31 UT, (s) 15:46 UT, (t) 16:00 UT.

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Figure 2m–t.

this is the same feature observed the day before, it is easy to show that the visibletop of that July 8 feature must have descended over 30 000 km radially to not bevisible above the limb 17 hours later. Either there was significant restructuring ofthe local magnetic field, as seems likely in view of subsequent events, or the twofeatures were different but almost cospatial.

One also sees in both sets of spectroheliograms a large north–south quiescentprominence at the east limb to the north of the erupting feature. A K1v spectro-heliogram taken at Coimbra at 09:01 UT on July 9 (Figure 3) suggests that thesouthern extension of this large prominence terminates in a sunspot that extendsdown from N20 to N10, with the southern end of the sunspot terminating at thelatitude of the erupting prominence. This large prominence was first observed atthe limb on July 7 (Solar Geophysical Data (SGD) Prompt Reports, July 1982).

Somewhat earlier than the observed rise of the erupting prominence, at approx-imately 07:31 UT on July 9 an H� flare was observed in AR 3804 with centerlocated near N18 E76 (SGD Prompt Reports, July 1982). Powerful radio emissionat 8800 MHz and 2695 MHz was detected at three different radio observatoriesin time interval 07:29.3–07:33.3 UT and peaking, typically, a few minutes later(see Table II). The SMM HXRBS instrument observed the sharp rise of this flare,

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Figure 3. K1v spectroheliograms obtained (a) on July 8, 1982 at 14:13 UT and (b) July 9, 1982 at09:01 UT.

Table IIRadio observations during impulsive flare and early period of subsequent LDE of July 9,1982 (SGD Prompt Reports)

Station Frequency Start Max Duration Peak-flux density(MHz) (UT) (UT) (min) (10�22 W m�2 Hz�1)

Athens 8800 07:29.3 07:37.1 39.2 2399Athens 2695 07:29.6 07:37.3 34.9 2000Learmouth 8800 07:32.0 07:32.3 29.3 3100Learmouth 2695 07:33.3 07:37.1 25.5 1800Manila 8800 07:32.0 07:36.C 11.5 5700Manila 2695 07:33.1 07:37.0 13.4 2399

Athens 8800 08:16.8 08:34.3 65.2 230Athens 2695 08:20.8 08:38.3 61.2 63Learmouth 8800 08:22.1 08:27.8 14.7 96Learmouth 8800 08:36.8 08:39.3 31.2 169Learmouth 2659 08:24.8 08:28.1 12.0 27Learmouth 2695 08:36.8 08:38.3 31.2 70

Note that a distinct break occurs between the two sequential series, with the second seriesof somewhat longer duration and greatly reduced intensity.

beginning at 07:25 UT, and GOES soft X-ray observations reveal an impulsiveburst at approximately the same time (Figure 4).

Observations made during the impulsive phase of the flare were followed by asecond series that began about one hour later. Observations in this second seriesinclude the following:

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Figure 4. (a) Hard X-ray spectra for the impulsive flare of July 9, 1982, from the SMM HXRBS DataBase for the energy ranges indicated in the 15 channels. Data gaps are due either to satellite night (N)or, during solar visibility (D), to interference from the South Atlantic Anomaly. The impulsive hardX-ray spike is already beginning to decline when satellite night intervened. (b) Soft X-ray spectrafor the same flare of July 9. Following the impulsive spike, the spectrum assumes the character of anLDE. (c) Soft X-ray spectra for the event of July 12. While this was a very energetic event, there isno evidence of an impulsive spike in these spectra. The July 22 event exhibited soft X-ray spectrasimilar to (c), while the September 4 event exhibited soft X-ray spectra like that of July 9. The spectraof (b) and (c) from SMS-GEOS appear in the SGD Prompt Reports, July 1982, but a printing errorgives the wrong time-line for the July 9 event (see Acknowledgement).

(1) An erupting prominence seen in H� at the Wendelstein Observatory during08:48–09:47 UT at N12 E79, and for over 5 hours after 11:17 UT from the RameyObservatory at N12 E73. The Wendelstein Observatory noted the base of the

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observed prominence was less than 90 deg from the central meridian (SGD PromptReports, July 1982). This suggests, but does not prove, that the eruption may havestarted from below the limb, and could have begun at the time of the earlier seriesof observations associated with the impulsive flare.

(2) Further radio emission at 8800 MHz and 2695 MHz, less powerful and oflonger duration than that observed earlier, with a definite time-lapse between thetwo sequences (Table II).

(3) No indication in the SMM HXRBS observations of a further impulsive flare(see Figure 4(a) and discussion below);

(4) A GOES soft X-ray spectrum typical of an LED, beginning at about 08:25 UT(Figure 4(b)).

The erupting prominence reported by the Wendelstein and Ramey Observatoriesis clearly the same one observed at the Coimbra Observatory. The SMM HXRBSwas in the South Atlantic Anomaly at the start time of the other observations inthis second series, so a small brief impulsive event cannot be ruled out. However,combining the HXRBS data after 08:42 UT with the lack of evidence for animpulsive flare around 08:30 UT in the other observations, one can conclude therewas no significant impulsive flare during this second series of observations.

Examination of the Solwind record reveals that an energetic coronal mass ejec-tion (CME) occurred during this eruption. The coronagraph on the Solwind satellitedetected the remnants of an extended CME at 12:52 UT on July 9 (Cane, Sheeley,and Howard, 1987). Cane, Sheeley, and Howard estimate a minimum mean velocityfor this CME of 490 km s�1, by assuming the start-time of the CME coincidedwith the rise-time of the observed GOES soft X-ray LDE. The projected latitude atwhich the center of the CME remnants would have crossed the solar disk is N20,which lies very close to the latitude of the impulsive flare, and is about 10 degnorth of the erupting prominence. It is also the approximate southern footpoint ofthe large north–south quiescent prominence. (Since these are near-limb events, theeffects of projection are negligible for this study.) This large quiescent prominencesurvived the eruption of the active-region prominence observed at Coimbra largelyintact (SGD Prompt Reports, July 1982).

3. Interpreting the July 9 Event

We see from the observations that the impulsive flare clearly preceded the LDE andprobably preceded the erupting prominence. If the CME and the LDE have approx-imately the same start-times, then the CME initiation also follows the impulsiveflare. From this and the episodic acceleration of the erupting prominence, we con-clude that the observed prominence eruption was a consequence of the magneticdissipation and restructuring processes that produced the impulsive flare, and mayhave been a direct consequence of the flare itself.

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We now ask if the implusive flare could have triggered the observed CME.Rust and Svestka (1979) studied slowly moving X-ray disturbances in the coronaapparently triggered by filament eruptions that were in some cases flare-relatedand in other cases occurred in the absence of any reported flare. They noted thatthese disturbances travelled with an initial velocity of about 400 km s�1, whichdecreased to less than 20 km s�1 in about four hours. Of greatest significance,these disturbances were often observed to (apparently) provoke second filamentdisappearances of the kind now known to be often associated with CMEs. Fromthe numbers in Section 2, it is easy to show that the H� image of the July 9 flarefirst appeared at a point on the order of 105 km from the ‘center’ of the CMEexpansion. Again assuming the CME erupted at approximately the same time asthe observed LDE, i.e., about one hour after the first H� appeared, an averagepropagation speed of some 30 km s�1 would cover that distance, well within thevalue determined by Rust and Svestka. There is one caveate to this picture. Thelarge quiescent prominence of July 9 did not disappear. However, since the detailedmagnetic structure preceding these complex events differs in detail in each case,and we clearly lack the observations to model our event in this much detail, wemust be content to note the possiblity of flare triggering of the CME, perhaps bythe process just described.

Further support for flare triggering comes from the large area of the H� impuls-ive flare (SGD Prompt Report, July 1982) and the probably earlier start-time for theflare than for the erupting prominence. The large area of the flare almost impingesonto the projected center of the CME. It is also highly probable that the flaretriggered this prominence eruption, which emerged from NOAA 3804, possiblyout of the large sunspot observed in Figure 3. At the least, the prominence eruptionis almost surely due to either the flare itself, or to disruptions in the local magneticfield that produced both the flare and the prominence eruption. We note that thisprominence does not conform to the type of erupting prominence described withinmany CMEs by Hundhausen (1997), on many different grounds.

While the available observations suggest that the impulsive flare might havetriggered the CME, in contrast to the model provided by Hundhausen (1997), thesubsequent evolution exhibits one strong similarity to this model and one significantdifference. The LDE observed on July 9 lasted at least 6 hours. This LDE couldwell result from magnetic reconnection as a new arcade of loops establishes itselfacross the neutral line of the large quiescent prominence that persists during theevent, a prominent feature of Hundhausen’s model, and one that has been modeledwith some success since the work of Kopp and Pneuman (1976). Detailed studiesof a comparatively low-energy CME observed with high spatial resolution byGopalswamy et al. (1996), exhibit a moving source of the soft X-ray emission thatconforms to the above reconnection scenario.

Nevertheless, this interpretation of the soft X-ray LDE as a byproduct of theCME depends on the assumption that the CME erupts before the rise of the LDE.Harrison (1991, 1995) has worked extensively on this problem and has moved

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from a position supporting this assumption, based on the analysis of SMM datain the first-referenced paper, to a more tentative position expressed in the second-referenced paper. We return to this point in Section 5.

If the CME was, in the above sense, the source of the LDE, could it have eruptedindependent of the observed impulsive flare due to a large-scale restructuring ofthe magnetic field throughout the entire active-region complex, a restructuringthat triggered both the impulsive flare and the CME as essentially independentevents? A number of theoretical models suggest this is possible. A review of someof these models appears in Sakai and de Jager (1996). In a typical model, theprocess begins as energy is added to a helmet-streamer-like magnetic configurationby shearing motions in the photosphere that increase the magnetic free energy ofthe configuration until the magnetic field becomes unstable. Wu, Guo, and Wang(1995) have recently introduced a magnetic bubble structure into the closed fieldregion of such a helmet streamer and, using a time-dependent 2-D MHD code, havecomputed the evolution of the streamer-bubble (a simulated CME). They concludethat for bubble radii larger than a certain value, 0.25 of the streamer’s length in theircomputation (and effectively the solar radius in their model), the bubble streamerwill slowly swell until it breaks up, simulating some of the observed features ofCMEs.

There remain theoretical problems with these ‘non-flare’ scenarios, but workunderway suggests that the major problems are solvable. Low (1993) reviewsprogress in overcoming what might be the major obstacle to a simple theoreticalmodel. A conjecture by Aly (1984) asserts, though not yet rigorously, that the energyof a completely open force-free magnetic field equals or exceeds the energy of thatfield in a closed configuration, thus requiring other sources of energy to go from theclosed (helmet streamer) state to the open (CME) state. Yet other arguments assertthat photospheric motions are too slow to pump adequate magnetic energy into aCME following the eruption. Low offers a number of ways around this apparentimpasse. There exist partially open field configurations that contain less energy thanthe closed configuration of the same field, and the pressure and mass motion withinthe expansion, once started (but not completely ‘open’) can draw additional energyand mass motion from the low corona into the ‘CME’. Cross-field currents, omittedfrom many earlier theories, are an additional mode of energy storage. A definitive2.5D or 3D MHD model simulating a relatively fast CME, with the effects of themagnetic field, gravity, and dynamic pressure does not yet exist, but there is reasonto believe one can be developed that will satisfy known constraints. We concludethat it is entirely possible that both the observed July 9 impulsive flare and eruptingprominence, on one had, and the CME/LDE, on the other, could have both resultedfrom a large-scale restructuring of the magnetic field within and around the largeactive-region complex containing NOAA 3804, as essentially independent events.

However, given the proximity in space and time and the existence of a clearlyhighly stressed magnetic configuration in the helmet streamer that erupted, we

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believe it likely that the impulsive flare was the ‘proximate cause’ or trigger of thisparticular CME.

We stated above that there was one significant difference between the CMEobserved on July 9 and the scenario for CME evolution put forward by Hundhausen(1997). In both cases a large quiescent prominence is present well within the regionof the CME expansion. In the Hundhausen model, this prominence erupts anddisappears (a disparition brusque), sometimes returning later in the same positionand sometimes not. In our case the large quiescent prominence remains in place,as Figure 2 reveals. Also in the model the erupting quiescent prominence riseswith either a nearly constant velocity or a nearly constant acceleration, drawn outby the expanding magnetic field within the CME cavity. The July 9 prominenceeruption fails to conform to this picture. Especially in view of the episodic velocityand acceleration of the observed erupting prominence, it is difficult to imaginea scenario in which the large-scale CME expansion alone is responsible for theobserved prominence eruption of July 9.

However, we conclude that the magnetic field associated with this large prom-inence, and certainly the field of the associated helmet streamer played a role in theCME. Indeed, the centroid of the CME remnants on the east limb at 20N appears tocoincide with the southern terminus of this large prominence, so this prominencelies well within the 140 degree sector centered on 20N described earlier.

The July 9 event could well be of a ‘mixed’ type, incorporating features from anumber of extant models, in which both the local magnetic disruption associatedwith an impulsive flare and a larger-scale magnetic disruption associated with thefields around the large prominence both play a role. In this case, the impulsive flareseems a likely ‘trigger’ for the observed erupting prominence. Thereafter the picturebecomes more complex. Our observations do not rule out ‘flare triggering’ of theCME as well, but they by no means demand it. A larger-scale restructuring of themagnetic-field complex could be responsible for both flare-erupting prominenceand CME-LDE, with the large quiescent prominence playing some role in thelatter, but not sufficient to lead to disappearance of this prominence (i,e., with nodisparition brusque). If the impulsive flare is responsible for the CME, and if thesoft X-ray LDE which follows immediately the hard and soft X-ray impulsive flareactually begins before the rise of the CME, then we have probably observed a‘flare driven’ event as opposed to a mixed event of the type described above. Theseobservations cannot completely resolve this question. However, they do shed somelight on the current ‘solar flare myth’ discussion. Our final section commentsaccordingly, and emphasizes one observation critical toward clarifying this issue.

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Table IIIProperties of July 9, 1982 coronal mass ejection from Solwind satellite observatons andsupporting data�

Date X-ray time H� Emerged Speed Mass K.E.(UT) location (center) (km s�1) (1015 g) (1030 erg)

July 9 08:25 N18 E76 N20 E >490 29.4 >8.8

Additional CME observations during July-September

July 12 09:16 N11 E36 S10 E >730 37.8 >25.2July 22 16:48 N16 W89 N25 W 1820 18.9 78.3Sept. 4 02:00 N22 E29 360 990 37.8 46.3

�Taken from Cane, Sheeley, and Howard (1987).

4. Three Further Energetic Solar Eruptions Arising from ThisActive-Region Complex before It Was Replaced by a Magnetically Open

Region

Following the July 9, 1982 eruption, three other energetic CMEs occurred in thissame active-region complex. These eruptions, which occurred on July 12 and 22,and on September 4, were unusually energetic, and two solar rotations after theSeptember 4 eruption this active-region complex disappeared (Bumba, Garcia, andJordan, 1997). Table III summarizes the properties of these three CMEs alongwith the one associated with the July 9 prominence eruption, all taken from Cane,Sheeley, and Howard (1987).

The second energetic event in this series occurred on July 12. This event exhibitsan LDE that lasted more than 12 hours. The soft X-ray data for this event yieldthe profile of a less impulsive, longer-duration flare, but we cannot be sure of thehard X-ray start-time because the HXRBS was in the South Atlantic Anomalyat the crucial time. Figure 4(c) exhibits the strong soft X-ray LDE in the GOESdata for this event, but gives no hint of a preceding impulsive flare, in contrast toFigure 4(b) for the July 9 event. The soft X-ray profile conforms closely to that ofseveral of the CMEs upon which the Hundhausen (1997) model was based. In theabsence of evidence for an impulsive-flare trigger for the CME, and especially ifthe observed LDE occurred after the rise of the CME (which cannot be verifiedfrom the available data), this event could have conformed more closely with thatmodel than the July 9 eruption.

The third, or July 22, CME was the most powerful of all, and occurred asthe active-region complex was at the west limb. With an estimated velocity of1820 km s�1, the kinetic energy of this CME was almost 1032 ergs. Indeed, thisCME had the highest measured velocity and the largest estimated kinetic energyof the 29 events studied by Cane, Sheeley, and Howard (1987). Here, the GOESsoft X-ray observations reveal a spectrum that resembles the one of the July 12

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eruption, which was itself the fifth most energetic among the 29 studied. There isno evidence in this spectrum for a preceding impulsive flare.

The final highly energetic event in this series occurred on September 4. Thisevent shows many similarities to the one of July 9, including an initial series of(presumably pre-CME) GOES soft X-ray bursts that were absent in the July 12and 22 events, followed by a soft X-ray LDE that lasted, in this case, on the orderof a day. Unfortunately we have no hard X-ray observations for this event that canestablish the time of any associated impulsive hard X-ray flare, as the SMM satellitewas either in Earth occultation or the South Atlantic Anomaly during most of thepre-CME period. However, the GOES soft X-ray spectra show that an energeticimpulsive flare began about two hours before the rise of the LDE, and so like theJuly 9 event probably preceded the CME. Thus flare-triggering of the CME couldhave occurred in this event. This fourth CME was the third most energetic of the29 studied by Cane, Sheeley, and Howard (1987).

The study of Cane, Sheeley, and Howard (1987) spanned the period of March1979, when the Solwind coronagraph began observations, through February 1983.Event selection was based on looking for CMEs associated with interplanetarytype-II radio events in the coronagrams and including those that provided adequatecoronal coverage to characterize the CME. Of the 29 events described in this way,we have seen that three of the four described here were among the most energeticCMEs observed. (The fourth, our July 9 event, was in the mid-range of kineticenergies for CMEs.) If we ask what the solar flare record reveals over a longertime interval, the HXRBS hard X-ray burst record for 1980–1989 confirms that allfour events were highly energetic. The HXRBS record (now available on the WorldWide Web at URL http://umbra.nascom.nasa.gov/smm/hxrbs.html) of over 12 000events during this period reveals that in both total counts and peak counts the flaresof July 9 and July 12 were among the most powerful hard X-ray flares of cycle 21.The exact ranking by total counts is July 9, tenth; July 12, ninth. If we excludeyear 1989 with its dramatic rise toward the peak of solar cycle 22, the respectiveranking is July 9, fourth: July 12, third! That is among over 10 000 hard X-rayflares observed during 1980–1988. During this same, latter period the July 9 flarealso exhibited the fourth highest peak count rate in the four X-ray detectors carriedby the HXRBS experiment. It is unfortunate that we do not have equally completehard X-ray data for the events of July 22 and September 4, which produced themost energetic CMEs of the four described.

One conclusion can be asserted with confidence. The four eruptive events thatoccurred between July 9 and September 4, 1982 were unusually energetic. Thisassumes particular significance in a companion study (Bumba, Garcia, and Jordan,1997), where it is shown that these energetic eruptions signal the destruction ofa large, long-lasting active-region complex, a phenomenon also observed in thedeclining phase of cycle 22 (Bumba, Klvana, and Sykora, 1995) and one thatseems to be characteristic of the way the global magnetic field is restructuredduring this phase of the solar cycle.

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5. Conclusions and Discussion

Examination of the four most energetic events occurring in the active-region com-plex containing NOAA 3804 during the period July 9–September 4, 1982 showsthat this was a time of maximum coronal disruption during the declining phase ofsolar cycle 21. Of the four CMEs described above, the well observed event of July 9and possibly also the event of September 4 appear to be either (1) a CME triggeredby the impulsive flare, where the LDE results from reconnection within the CMEas a new magnetic arcade is established over the persistent quiescent prominence,or (2) of a ‘mixed’ type characterized by an impulsive flare and a CME/LDE bothindependently driven by a large-scale restructuring of the magnetic field in the largeactive-region complex. The other two eruptions, of July 12 and 22, could conformto an alternative model, in which a more gradual process of photospheric motionsfeeds energy into the magnetic field of a large coronal streamer that subsequentlygoes unstable and erupts in a CME, with the observed LDE a consequence of theCME in the manner already described. There is no evidence for flare triggering inthe latter two cases.

These conclusions can be discussed in the context of ‘the solar flare myth’controversy. However, to do so it is necessary to first establish exactly what thiscontroversy is. Since the issue is partly one of definition, the following definitionsare offered, quoted directly from articles in the literature.

Perhaps the best statement of what is meant by the expression ‘the solar flaremyth’ appears in Gosling (1993). “In its most elementary form the paradigm mightbe stated: large solar flares are the prime cause of large nonrecurrent geomagneticstorms, transient shock wave disturbances in the solar wind, and major energeticparticle events. This paradigm is what I call ‘The Solar Flare Myth’.” Svestka (1995)states “The ‘Solar Flare Myth’ postulated by Gosling (1993) is a misunderstanding.”He goes on to say “It is true that most sources of coronal mass ejections (CMEs)cannot be classified as flares in the common old sense of the word. However, justfor this reason the term ‘eruptive flare’ has been introduced for all solar activephenomena in which an opening of the field lines is involved and which lead tomagnetic-field and mass ejections resulting from a CME.”

The Svestka (1995) article is followed immediately by a reply by Gosling andHundhausen (1995). They begin by saying “In the accompanying Letter Svestka(1995) does not accurately state what we consider the solar flare myth to be.Stated succinctly, the myth is that solar flares are the fundamental physical causeof large shock disturbances in the solar wind, major energetic particle eventsin interplanetary space, and large, non-recurrent geomagnetic storms (Gosling,1993).” In reply to Svestka’s discussion of how to define different kinds of flares,they go on to say “We take the point of view that use of a word with such a clearhistorical connection to a radiative phenomenon to describe dynamical phenomenaand processes, and in particular CMEs, confuses and obscures physical differencescrucial to understanding the latter.”

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Clearly a discussion of ‘the solar flare myth’ controversy must be addressedon two levels: the semantic and the level of physical processes involved in flares,CMEs and associated phenomena. We believe the above quotations offer the readera fair presentation of the issue. If we accept as the criterion for a good definitionthat it should reflect as clearly and unambiguously as possible the event to which itrefers, and that definitions, once agreed to and established, should only be changedfor compelling reasons, the question becomes one of how strong are the argumentsthat flares, once defined by their radiative outputs, should now also be viewed ascomprising coupled dynamical phenomena leading to, or possibly even including,CMEs. This would seem to depend on how strong is the coupling between theradiative and dynamical aspects, both statistically and energetically. We must leaveto the broad flare and CME community the answer to this question. In the interest ofclarity, we adopt the following position. We consider flare triggering of a CME if anobserved impulsive flare occurs in reasonable spatial and temporal proximity to theCME, especially if, as in the July 9 case, the observed prominence eruption is clearlya flare-associated eruptive prominence and not a large quiescent prominence lyingalong the neutral line of the soon-to-erupt helmet streamer, and more especiallyif the latter prominence persists during the eruption. If there is no such evidencefor an impulsive flare, given reason to believe on the basis of extensive modellingthat energetic restructuring and destabilization of the large-scale magnetic fieldassociated with the soon-to-erupt streamer is a viable alternative process, we regardsuch CMEs not to be flare triggered.

In the light of the definitions we use here, two of the four events in our sampleshow evidence of possible flare triggering and the other two do not. While fourevents do not constitute a statistical sample, it is interesting to note that eventhis small number of events supports Dryer’s (1994) claim that CMEs fall on a‘spectrum’ of events, ranging from those that are clearly flare-related and possiblyflare triggered to those which show no association with impulsive flares at all.However, we note that our events do not conform to his picture of the weakerevents being of the latter type and the stronger of the former. This point should notbe weighted heavily, in view of the small number we studied. A more significantobservation on Dryer’s latter conjecture comes from the work of St. Cyr and Webb(1993), who did the most extensive statistical study to date of CMEs that occurredduring solar minimum (1984–1986). St. Cyr and Webb do note that these ‘solarminimum’ CMEs are generally much less energetic than those observed during theprevious two maxima, and this would seem to support Dryer’s conjecture that themore powerful CMEs are more likely to be the ones triggered by flares. However,St. Cyr and Webb also found “no significant morphological characteristics inthe slow mass ejections that distinguished them from the general population ofCMEs...” They then go on to suggest that, if allowance is made for the fact thatsiqnatures of associated events (like, in this case, weak subflares) will sometimesfall below the threshold of routine observations, there is probably no differencein the frequency of these associated events that are candidates for CME causation

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over the range of CME energies. This does not disprove Dryer’s conjecture, but itmeans that it remains a conjecture requiring further study at this stage, as suggestedby our results here.

Another reasonable, but unproved, conjecture requiring further investigation,and bearing on our interpretation of the observed LDEs described here, is that theseLDEs followed the CME eruption in time. One encounters this as an assumptionin many discussions of CME modelling, including the studies by Low (1993) andWu, Guo, and Wang (1995) referenced here. It is instructive to read two papers bythe scientist whose work is usually quoted in support of this assumption. Harrison(1991) reviews the perfectly reasonable, but still somewhat model-dependent workwith SMM and coronal polarimeter observations that support this assumption. In amore recent paper, Harrison (1995) notes that this work must be rechecked carefully.The crux of the matter is that one seldom could follow the CME observed with SMMfrom the solar limb on out, but was dependent on matching the CME with coronalpolarimeter observations that were not always available or were not definitive forestablishing temporal priority between events that projections suggested startedwithin minutes or a few tens of minutes of each other. Fortunately Harrison iscurrently involved in a campaign with the SOHO, which has superior capabilityfor settling this question. The assumption that the CME precedes the LDE maybe correct, but it has yet to be proved, and is critical to interpreting the LDE as apost-CME reconnection event.

Finally, these four large-scale, highly energetic events reflect a large-scale dis-ruption of the surface and subsurface magnetic field during the two month intervalinvolved. Since the ultimate source of this disruption must be subphotospheric, itis interesting to note a recent study by Sudan and Spicer (1996), who concludefrom energy considerations that continuously emerging flux is necessary to supplythe energy for large solar flares; i.e., that the energy cannot reside in the preflaringatmosphere itself. While their study was applied to large flares, powerful CMEshave equal energy requirements, as Table III demonstrates. In fact one can doublethe values in the last column of Table III, to include the energy required for theCME to overcome solar gravity. Indeed, the Sudan and Spicer constraint applieseven more to a CME, moving rapidly away from the Sun, than to a flare that remainsin its vicinity. This emphasizes even more the importance noted by Low (1993) ofincluding gravity and pressure forces in MHD modelling of CME expansion, inorder to generate models that extract energy from sources other than the pre-CMEmagnetic field.

Under these conditions, a sequence of major surface changes of the typeobserved must reflect changes in the source region that has produced the observedlarge long-duration active-region complex containing NOAA 3804. These changesoccurred near the end of the complex’s lifetime, and led to the maximal phase ofactivity before the field in the complex was completely restructured. We show inour companion study that this resulted in the progressive penetration of a largenegative-polarity coronal hole into the same area of the solar surface during two

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further rotations (Bumba, Garcia, and Jordan, 1997). Such an effect was also notedduring the same phase of the following solar cycle (Bumba, Klvana, and Sykora,1995). The main significance of the four events of July 9 through September 4,1982 appears to be their direct manifestation of this maximal phase of activity inthis active-region complex. The well observed eruption of July 9 can be viewed asthe event that heralded this process.

Acknowledgements

The authors wish to thank Dr Neil Sheeley for bringing to our attention an errorin the Solar Geophysical Data Prompt Reports (see caption to Figure 4) thatcleared up considerable confusion over the July 9, 1982 event. We also thankHXRBS Principle Investigator Dr Brian Dennis for reading an earlier versionof the manuscript carefully and correcting an overinterpretation of the hard X-ray observations from that experiment. Finally, we thank the referee for valuablesuggestions that helped clarify the significance of this work in the context of thecurrent debate over ‘the solar flare myth’.

References

Bumba, V., Garcia, A., and Jordan, S.: 1997, Astron. Astrophys., submitted.Bumba, V., Klvana, M., and Sykora, J.: 1995, Astron. Astrophys. 298, 923.Cane, H. V., Sheeley, N. R., Jr., and Howard, R. A.: 1987, J. Geophys. Res. 92 (A9), 9869.Dryer, M.: 1994, Space Sci. Rev. 67, 363.Gopalswamy, N., Hanaoka, Y., Kundu, M. R., Enome, S., Lemen, J. R., Akioka, M., and Lara, A.:

1996, Astrophys. J. 475, 348.Gosling, J. T.: 1993, J. Geophys. Res. 98 (A11), 18 937.Gosling, J. T. and Hundhausen, A. J.: 1995, Solar Phys. 160, 57.Harrison, R. A.: 1991, Adv. Space Res. 11 (1), 25.Harrison, R. A.: 1995, Astron. Astrophys. 304, 585.Hundhausen, A.: 1997, in K. Strong, J. Saba, and B. Haisch (eds.), Coronal Mass Ejections, The

Many Faces of the Sun, Springer-Verlag, Berlin, in press.Kopp, R. A. and Pneuman, G. W.: 1976, Solar Phys. 50, 85.Low, B. C.: 1993, Adv. Space Res. 13 (9), 93.Rust, D. M. and Svestka, Z.: 1979, Solar Phys. 63, 279.Sakai, J.-I., and De Jager, C.: 1996, Space Sci. Rev. 77, 1.Sheeley, N. R.: 1995, private communication.St. Cyr, O. C. and Webb, D. F.: 1993, Solar Phys. 136, 379.Sudan, R. N. and Spicer, D. S.: 1996, Comm. Plasma Phys. Controlled Fusion 17, 77.Svestka, Z.: 1995, Solar Phys. 160, 53.Wu, S. T., Guo, W. P., and Wang, J. F.: 1995, Solar Phys. 157, 325.

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INTERPRETING THE LARGE LIMB ERUPTION OF JULY 9, 1982

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