P R O M P T S O L A R P R O T O N E V E N T S A N D

C O R O N A L MASS E J E C T I O N S

S. W. K A H L E R

American Science and Engineering, Inc., Cambridge, Mass. 02139, U.S.A.

E. H I L D N E R

High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colo. 80303, U.S.A.

and

M. A. I. V A N H O L L E B E K E *

NASA/ Goddard Space Flight Center, Greenbelt, Md. 20771, U.S.A.

(Received 26 October, 1977; in revised form 5 January, 1978)

Abstract. We have used data from the HAO white light coronagraph and AS&E X-ray telescope on Skylab to investigate the coronal manifestations of 18 prompt solar proton events observed with the GSFC detectors on the IMP-7 spacecraft during the Skylab period. We find evidence that a mass ejection event is a necessary condition for the occurrence of a prompt proton event. Mass ejection events can be observed directly in the white light coronagraph when they occur near the limb and inferred from the presence of a long decay X-ray event when they occur on the disk. We suggest that: (1) the occurrence of mass ejection events facilitates the escape of protons- whether accelerated at low or high alt i tudes-to the interplanetary medium; and (2) there may exist a proton acceleration region above or around the outward moving ejecta far above the flare site.

1. Introduction

The physics of the acceleration and propagation of solar cosmic rays in the corona remains unclear after years of investigation (Svestka, 1976, Chapter V). This is due in part to the fact that spatially resolved observations of cosmic-ray flares have been limited mostly to photospheric and chromospheric regions with a distinct lack of good observations of the corona where the cosmic-ray production and propagation occur s .

The flight of Skylab from May 1973 to February 1974 has provided us with at least two kinds of coronal observations which could be relevant to cosmic ray problems. In this paper we use broad-band X-ray images of the inner corona recorded on film with the American Science and Engineering grazing incidence telescope (Vaiana et al., 1977), and broadband white-light (3700-7000 ~) images of the solar corona over the range 1.5 to 6R| from Sun center recorded on film with the High Altitude Observatory white light coronagraph (MacQueen et al., 1974). In addition, we have used data from the Solrad 9 satellite which measures the soft

* Also: Dept. of Physics and Astronomy, University of Maryland, College Park, Md. 20742, U.S.A.

Solar Physics 57 (1978) 429-443. All Rights Reserved Copyright (~) 1978 by D. Reidel Publishing Company, Dordrecht, Holland

430 s . w . K A H L E R E T AL.

X-ray flux of the visible solar disk. Finally, we have used data from the Goddard Space Flight Center cosmic ray experiment on the IMP-7 spacecraft, which moni- tored the cosmic ray fluxes at 1 AU, to identify and study prompt solar proton events during the Skylab flight. These prompt solar proton events signify particle acceleration in the solar corona.

Using these data, we show that most if not all prompt solar proton events observed at Earth (during a nine-month period near solar cycle minimum) were accompanied by coronal disturbances which apparently reconfigured the magnetic field lines above the near-surface solar events. From these observations we infer that the disruption of the coronal magnetic field is an important part of the mechanism whereby energetic solar protons are observed at 1 AU. First, the apparent reconfiguration of coronal field lines, with lines rising to great heights and

perhaps reconnecting to previously open interplanetary field lines, facilitates the escape of the protons from the vicinity of the Sun if they are accelerated somewhere near the rising and/or opening field lines. Second, the strong association of radio bursts with prompt protons found in this and other studies (Wild et al., 1963) leads us to speculate that the protons are accelerated in or near the shock front which apparently precedes the ascending ejected mass, rather t h a n - o r in addition t o - being accelerated at the flare site lower in the solar atmosphere.

2. Data Analysis

The GSFC cosmic-ray experiment on IMP-7 consists of two charged particle telescopes, one Low Energy Detector (LED) and one Medium Energy Detector (MED). Both operate in a standard dE/dx vs E mode which provides unam-

biguous element identification and energy spectrum resolution. In our study we have used data only from the low energy detector which is a two-element solid state telescope placed at the bottom of a plastic scintillator anticoincidence cup. In the double parameter analysis, the particle energy loss AE is determined in a 150/x thin silicon surface barrier detector and the residual energy E - AE is measured in a 3 mm thick lithium drift device. This mode provides measurements of proton and alpha particle spectra free of background contamination from 4 to 23 MeV nuc1-1. This energy interval can be extended down to ~1 MeV nuc1-1 by using the single parameter analysis for particles stopping in the front dE/dx element.

Typically the spacecraft onset time of a proton event whose intensity in the 4 -23MeV energy interval exceeds 3• -3 proton (scm2srMeV) -1 can be determined within • 15 min.

The onset times of proton flux increases in the 4-23 MeV energy range which showed a significant velocity dispersion during the rise phase are listed in Table I. In some cases the onset time is uncertain because the event is masked by the presence of an earlier event. The peak proton flux of each event is shown in the third column. The requirement for velocity dispersion was used to eliminate co- rotating and delayed storm proton events.

P R O M P T S O L A R P R O T O N EVENTS A N D C O R O N A L MASS E J E C T I O N S 431

In Table I we also list the following data for phenomena we associate with each event: the time intervals of the type II and IV radio bursts, the peak intensity (B = 10 -4, C = 10 -3, M = 10 -2, and X = 10 -1 erg cm -2 s -1) and time of peak of the

Solrad 1-8 ~ X-ray burst, the importance, location, and time of peak of the Ha flare, and the number of the McMath plage region (all from Solar-Geophysical Data, 1974). The column labeled 'WL tran' gives the time when the coronagraph was commanded on and first imaged the associated white light transient. It also gives the position angle, P A (measured in degrees from solar north through east) of

the event (Munro et al., 1976). The column labeled 'LDE' consists of those X-ray events with long-decay profiles during Skylab listed in Table I of Kahler (1977). Notes on particular events are listed in the last column and dashes in the Table indicate no observations during the appropriate times.

We find a high degree of correlation between the occurrence of prompt proton events and the occurrence of white-light coronal transient events. Of the 18 proton events, 14 have an observed associated white-light transient, two do not (at longi- tudes W 45 and E 19 from which a transient is not likely to be visible) and in the remaining two cases there were no coronagraph observations until at least 8 h after the Solrad 1-8 ~ X-ray peak. The visual appearance of each of the white-light

transients except possibly that of 25 January implies an obvious mass ejection from the Sun rather than a rearrangement of preexisting coronal material. Coronagraph observations of the mass ejection events of 10 June and 7 September in Table I have been discussed by Hildner et al. (1975) and Gosling et aL (1975), respectively. These events often appear as large magnetic loops expanding outward through the solar corona, but always appearing to retain their magnetic connection to the Sun (Gosling et al., 1974). An example can be seen in Figures 1 and 2 where we show respectively the proton and X-ray fluxes and the white light and X-ray images of the event of 15 January 1974.

We also find that 11 of the 15 events for which a correlated 1-8 ~ X-ray event could be found were included in the list of LDEs by Kahler (1977). We have examined the available AS&E X-ray images for the remaining four events to

determine whether the bar-like morphological structure characteristic of an LDE was present. No images were available for the event of 29 June until 12 h after the peak of the X-ray event. We found no evidence at that time of a characteristic LDE structure in McMath 398, but Kahler has pointed out that active regions tend to return to their pre-LDE morphology many hours after the event. The X-ray flare of 17 January was probably in McMath 686 which was about 20 deg over the west limb, and could only be partially observed. The morphology of that event is therefore uncertain. In the last two cases, 5 September and 4 October (see Figures 3 and 4), distinct bar structures inferred to be at high altitude with respect to the quiescent active region structures were present. This indicates that these two X-ray flares would have been listed as LDEs by Kahler had their X-ray fluxes been adequately intense in comparison to the X-ray flux from the rest of the Sun which establishes the Solrad background level. We therefore conclude that at least 13 of

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Fig. 1. The top panel shows 6-rain averages of the Solrad-9 1-8/~, X-ray fluxes. The M1 event at --11:00 UT is associated with the proton event of 15 January shown on an expanded time scale in the bottom panel with half-hour averages. The arrows in the top panel show the times of the white light or

X-ray images of Figure 2.

the 14 X - r a y even t s occur r ing on the vis ible d isk were the k ind of events d iscussed

by K a h l e r (1977). This resul t is no t surpr i s ing since whi te - l igh t t r ans ien t s and X - r a y

L D E s nea r the l imb are c losely associa ted , and the assoc ia t ion of one type of even t

wi th so lar p r o t o n events impl ies an assoc ia t ion b e t w e e n p r o t o n events and the o ther .

W e do no t feel tha t the high deg ree of co r r e l a t i on of p r o t o n even t s wi th X - r a y

L D E s and whi te l ight t r ans i en t events as shown in Tab le II can be due to chance

co inc idences wi th f r e q u e n t events . D u r i n g Sky lab the re were 227 days of obse r -

va t ions wi th the c o r o n a g r a p h and 77 conf i rmed mass e j ec t ion events , a f r equency

P R O M P T S O L A R P R O T O N E V E N T S A N D C O R O N A L M A S S E J E C T I O N S 435

WLC I I

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Fig. 2. The top panels show the white light coronagraph images before and during the 15 January mass ejection event. The lower panels show the X-ray images before and during the X-ray event. The X-ray image at 12:47 UT shows the characteristic LDE structure in McMath region 686 at a solar longitude of

W 85. Note the difference in spatial scales between the white light and X-ray images.

of about one every 3.0 days (Munro et al., 1976). Kahler's table of L D E events consisted of 31 events from a 250-day period, a frequency of only one every 8 days. Thus, white light ejections and X-ray LDEs were relatively infrequent. Actually, the criteria to establish event associations were far more stringent than simple ~ a m ~ - u ~ y u u u u H ~ n c ~ , v v ~ u ~ u , . u J u u u ~ t ~ cu ~ t ~ u n ~ n m ~ ~ p t ~ u x u n m ~ u m ~ u~

particle acceleration and the H a flare reports to establish the location of the proton flare. The LDEs were required to be in the appropriate active region and the white light transients to be at the correct limb of the Sun within a few hours following the Solrad 9 X-ray maximum.

It might also be suspected that the correlations shown in Table II arise from the selection of the most energetic events each of which has a high probability of

any causal associations among the correlated variables. Using peak Solrad 1-8 fluxes as a measure of flare energy, we find that there has not been a strong selection for extremely energetic flares. During the Skylab mission there were 15 events of intensity M1 (10 -2 erg cm -2 s -1) or greater which could be associated with western hemis ere ma nares ano mus consmerea n~e~y canalaates for associated energetic phenomena. Only 3 of these events were associated with proton events of Table II. Ten of the 15 Solrad events of Table II are smaller than M1 and several, including the event of 5 September, would have to be considered insignificant in comparison with typical Solrad events. Similarly, only 4 of 13 identified H a flares are of importance 2 or greater.

436 S . W . K A H L E 1 R E T AL.

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Fig. 3. The top panel shows the time history of Solrad-9 1-8/~. X-ray fluxes presented as 4-min averages for the event of 4 October. Most of the rise phase of the X-ray event at ~11:30 UT associated with the proton event of the lower panel occurred during a dark side passage of the satellite. The arrow indicates the time of one of the AS&E X-ray images of Figure 4. In the lower panel the time history of

the 4.0 to 22.3 MeV protons is shown on an expanded time scale with half-hour averages.

The data of Tables I and II indicate that within the limited statistics of 18 prompt

proton events the occurrence of a coronal mass ejection event is a necessary

condition for the observation of a prompt solar proton event. The mass ejection event is best observed directly with a coronagraph for flares near or over the solar limb and can be inferred for disk flares by its Solrad 1-8 ~ profile and the presence of typical X-ray L D E morphology. We have found no case in which a proton flare on the visible disk was not associated with either an X-ray structure characteristic

I P R O M P T S O L A R P R O T O N E V E N T S A N D C O R O N A L M A S S E J E C T I O N S 437

X-RAY

OCTOBER 4, 1973 Fig. 4. The top three panels show Ha images from NOAA patrol films of McMath region 540 before, during and after the Ha flare associated with the proton event of Figure 3. Arrows in the 12:10 UT ;m.,.,~ ~h,.,~, r,~,,4,.,,~ ,,f fl~ro hr~,,ht,~,,i.,, Tho *u,,. r.~t~,.m ~,..~.,.o compare the X~ray appearance of the active region before and after the flare. The arrow points to the presence of an X-ray 'bar' which is

characteristic of LDEs.

TABLE I!

Proton event statistics

White light transients

Solrad X-ray events

18 total proton events

associated not associated no data

14 2 2

associated LDE non-LDE no data

15 ( 13 0 2 /

I -->M1 Cl to M1 < C l

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of a n L D E o r a m a s s e j e c t i o n w h i t e l i g h t t r a n s i e n t . S i n c e t h e d e t e c t a b i l i t y of a n y

p r o t o n e v e n t d e p e n d s o n t h e d e t e c t o r s ens i t i v i t y , t h e d e t e c t o r p o s i t i o n r e l a t i v e to

t h e f l a re l o n g i t u d e , a n d t h e p r e s e n c e of p r e c e d i n g e v e n t s , i t d o e s n o t a p p e a r

p o s s i b l e t o d e c i d e a b o u t t h e c o r r e l a t i v e p o s s i b i l i t y t h a t a m a s s e j e c t i o n e v e n t is

su f f i c i en t f o r t h e o c c u r r e n c e of a s o l a r p r o t o n e v e n t .

438 s . w . K A H L E R E T AL.

3. Discussion

The association of prompt events with mass ejection events suggests that the mass ejection events must play an important role in the acceleration and propagation of protons in the corona. In this section we first discuss the escape of protons from the acceleration site and then consider the location of the proton acceleration site.

The basic problem of getting protons out of the acceleration region and onto magnetic field lines many tens of degrees of solar longitude removed from the flare site has prompted numerous theoretical ideas. Reid (1964) suggested a model in which particles diffuse along the solar surface away from the flare site and gradually escape the Sun via magnetic field lines extending into interplanetary space. He suggested that the diffusion path length might be on the order of the solar granula- tion scale size. McCracken and Rao (1970) suggested a similar idea based on the assumed presence of diffusive magnetic irregularities due to hydromagnetic waves. Fisk and Schatten (1972) suggested that energetic protons are transported in the corona by means of an enhanced gradient-B drift along thin current sheets in the corona. McKibben (1973) proposed that some protons would have access to open field lines immediately after acceleration while others would be trapped on closed field lines and diffuse in the corona, perhaps via the Fisk and Schatten mechanism, before escaping. Newkirk (1973) found some support for this idea by calculating the trajectories of particles introduced onto calculated potential field lines of 46 large flares. He found that 16 to 40% of the particles escaped immediately, but he assumed a large injection area of 36• 36 ~ at 1.3R| Palmer and Smerd (1972) suggested a model for a flare on 30 March, 1969 in which a blast wave travels away from the flare site and accelerates particles by interacting with a neutral sheet above a magnetic arcade.

Implicit in all these models except the last are two assumptions: (a) that the protons are accelerated in the lower corona near the observed flare site, and (b)othat the coronal magnetic field at the site of proton acceleration undergoes no large scale, cataclysmic change. If we assume that density structures follow magfletic field lines to trace out the magnetic field configurations, then the coronal magnetic configuration is substantially modified during the passage of a coronal mass ejection as shown in Figure 2. This result and the correlation between mass ejections and proton events suggests that assumption (b) is definitely wrong at least for prompt proton events. Schatten and Mullan (1977) recently arrived at the same conclusion by arguing that the particle propagation characteristics of a static field are energy dependent, which contradicts the observations. The importance of shock-induced interplanetary field changes for delayed low energy particle propagation has long been recognized (Haurwitz et al., 1965), but heretofore prompt protons have been assumed to propagate in a magnetic configuration basically unperturbed by the flare.

Many ejections leave behind them thin, raylike density structures which extend from the inner (-2R| to beyond the outer (6Ro) limits of the coronagraph's field

P R O M P T S O L A R P R O T O N EVENTS A N D C O R O N A L MASS E J E C T I O N S 439

of view and stand nearly radially for some hours after the leading edge of an

ejection has ascended beyond 6R| We infer that at least some magnetic field lines, which were closed prior to the ejection, are raised to near-Sun interplanetary space and may be opened by reconnection during the course of a mass ejection. These field lines will enable protons propagating along them to escape from the Sun more easily than if the field lines were not reconfiguring. The configuration of field lines with low density contrast is more difficult to discern, but it seems probable that the magnetic field lines just ahead of the body of a mass ejection are also raised and/or opened by the passage of the ejection. These field lines could also provide an escape route into the interplanetary medium for the energetic protons.

We will show later that protons may be accelerated near the shock front ahead of a coronal mass ejection. As the shock front moves away from the Sun, particles accelerated in or near the shock should have more immediate access to inter- planetary field lines for two reasons. First, the previously closed field lines are being raised and/or opened by the mass ejection, and second, the ambient field lines at higher altitudes are more likely to be open to the interplanetary medium. The open field lines of the ambient medium ahead of the mass ejection event not only provide paths for energetic protons to escape the corona quickly, but they may also allow for a substantial and variable amount of azimuthal spreading of the proton popu- lation. The angular extent of the white light mass ejections observed during Skylab ranged from 7 to 150 ~ as projected in the plane of the sky (Munro et al., 1976). While the azimuthal extent (i.e., along the line of sight) is not known, it is probably comparable to the values for the plane of the sky.

From the analysis of a large number of 10-60 MeV solar proton events Reinhard and Wibberenz (1974) deduced the existence of a 'fast azimuthal propagation region' in which protons propagate across ~<60 ~ in solar longitude in less than 1 hr in the initial phase of a flare. The nature of this mechanism was unknown. We can explain at least part of this region as due to the angular extent of the acceleration region associated with the shock front, as first suggested by Lin and Hudson (1976).

While it appears that mass ejection events play an important role in proton propagation mechanisms, protons may also escape the corona via various scattering mechanisms. We have no reason to think that field lines at Earth will in general be magnetically connected to the open field lines at the site of the mass ejection. Future studies of X-ray LDEs and white light mass ejection events should serve to clarify the propagation mechanisms. In particular, the High Altitude Observatory white light coronagraph on the Solar Maximum Mission should be useful for these studies.

Turning to the acceleration of prompt protons, we note that the ejections accompanying proton events tend to be much faster than the average speed of all ejection events. Gosling et al. (1976) give the average speed of coronal mass ejections as - 4 4 0 km s -1. The average speed of the 10 events of Table I for which measurements could be made was >775 km s -1, and only two events were slower than 450 km s -1. The high speeds may account for the fact that of the 13 ejections

440 S. W. K A H L E R E T AL.

accompanying proton events with radio coverage, type II and/or type IV radio bursts were reported for 10. We take the presence of these radio bursts to indicate the presence and probably the acceleration of energetic particles in or near the shocks preceding these ejections. Type II bursts have long been thought to arise from the interaction between electrons accelerated at a traveling shock and the surrounding plasma. High energy electrons can also be detected in the corona by means of type IV bursts attributable to gyro-synchrotron emission of the electrons (Wild and Smerd, 1972). Dulk et al. (1976) analyzed radio burst data for a transient observed with the Skylab coronagraph on 14-15 September, 1973. They found a 'compression region' ahead of the leading bright white light loop similar to those observed in other transients (Gosling et al., 1974), and found that most of the continuum radio emission occurred in the compression region. They attributed the continuum emission to nonthermal electrons and argued that rather than accelera- tion and injection onto loops external to the white light ioop, the electrons were accelerated locally in a shock front preceding the dense white light loops.

We suggest that MeV protons are accelerated ahead of the expanding loop structure along with the energetic electrons responsible for the radio bursts. This suggestion is consistent with the usual assumption that cosmic-ray protons are accelerated along with MeV electrons in the second phase of a two-phase accelera- tion mechanism (Bai and Ramaty, 1976). It is for the most part inconsistent with assumption (a) stated above, that the protons are accelerated near the observed flare site, since the shock fronts and radio sources are observed as far as several solar radii away from the photospheric flare site. Of course, it is possible that acceleration of energetic protons observed at Earth takes place both near the flare site and high in the corona.

We must emphasize that we do not have direct evidence that energetic protons are accelerated in shocks preceding white light loops, but this interpretation is suggested by the following considerations. First, the events listed in Table I show that a mass ejection event accompanied by an X-ray LDE is a necessary require- ment for the occurrence of a prompt proton event. Second, the acceleration of energetic protons is assumed to occur simultaneously and cospatially with that of the <~ 1 MeV electrons whose presence ahead of ascending ejecta can be detected through type II and IV radio bursts. Third, the most detailed study of type IV emission and a mass ejection event to date (Dulk et al., 1976) indicates that the energetic electrons are accelerated ahead of the expanding loop structure rather than accelerated elsewhere and trapped in or behind it.

Although our proposed view of proton acceleration at the shock front ahead of the mass ejection brings us into conflict with earlier theoretical ideas discussed above, there are other reasons to suggest the validity of our model. Lin and Hudson (1976) found that the interplanetary proton spectrum of the 4 August, 1972 flare was in good agreement with the thin target interpretation of the y-ray emission. They also found that only about 1% of the total proton energy was lost in collisions as most of the energetic protons escaped to the interplanetary medium or were

P R O M P T S O L A R P R O T O N EVENTS A N D C O R O N A L MASS E J E C T I O N S 44l

accelerated there. There is therefore no requirement from their observations for protons to be accelerated at a low altitude or to be stopped in a thick target. The problem of particle acceleration by fast shocks has been discussed by Sonnerup (1973). He concluded that single reflections at fast shock fronts could produce proton energies up to several MeV, but higher energies would require subsequent reflections resulting from an effective pitch-angle scattering mechanism. On the

I 0 -

x W E S T

_~- | EAST , I bd

7 ~ r Or)

T (D

!

d Z

h - a . ,01

I ,001~) 2

I I I I I

X~

X | x

X

I 4

I I I 6 8 I0

V (10 z KM S' l )

I 12 14

Fig. 5. The peak 4-23 M e V proton fluxes are plotted against the est imated speeds of the associated mass ejection events for 10 of the events of Table I in which the speeds could be measured. The three eastern hemisphere events are indicated with open circles. Speeds are typically measured to +50 km s -1. The data are in agreement with a possible correlation between proton intensity and mass ejection speed.

442 S. W. K A H L E R E T AL.

other hand, Sarris et al. (1976) observed protons of E > 25 MeV in close asso- ciation with 'almost' perpendicular interplanetary shock waves. They interpreted these observations in terms of particle acceleration at the shock fronts and point out that a maximum energy gain is achieved when the shock speed and ambient magnetic field strength are highest. Sturrock (1974), among others, has discussed a different acceleration mechanism, namely, stochastic acceleration in turbulent shocked gas associated with a collision free shock. Such acceleration typically leads to a power-law energy spectrum as is usually observed (van Hollebeke et al., 1975).

If particle acceleration does take place at a shock front, we should expect particle events to occur only when the mass ejection speed exceeds the estimated Alfven velocity of 400-500 km s -1 (Gosling et al., 1976) required for shock formation. In Figure 5 we have plotted the peak proton intensities against the speeds of the 10 associated mass ejection events for which the speeds could be measured. In only one case do we get a speed below 400 km s -1. We also find a rough correlation between the speed and the peak proton flux, but with considerable scatter of the data points. The longitude of injection may contribute to the scatter for the E > 4 MeV protons shown in the figure, but van Hollebeke et al. (1975) found the average intensity of 40 MeV events from 60-20 ~ E to be >~�89 the average intensity of events from 20-60 ~ W. The data of Figure 5 are consistent with the possibility that the accelerated particle intensities increase with the mass ejection speeds and consequently with the assumed shock speeds.

4. Conclusion

We find evidence that coronal mass ejection events were necessary requirements for the observation of prompt solar proton events during the May 1973 to January 1974 period near solar minimum. The mass ejections signify cataclysmic reconfigurations of magnetic field structures in the active region vicinity, unlike the conditions assumed to exist in previous ideas of proton propagation in the corona. We suggest that energetic protons are accelerated in the shock front just ahead of the expanding loop structures observed as mass ejections. These protons then probably have immediate access to magnetic field lines open or opening to the interplanetary medium. These results are qualitatively consistent with the obser- vational deduction of a 'fast azimuthal propagation region' by Reinhard and Wibberenz (1974). Particle acceleration may be due to stochastic processes in turbulent shocked gas or to reflection from 'almost' perpendicular shock waves.

Acknowledgements

We are grateful to K. Dere for providing the Solrad-9 X-ray data and to F. B. McDonald and T. von Rosenvinge for making available the IMP-7 data. This work was partly supported by NASA contracts NAS5-3950 (EH) and NAS8-27758 (SK). In carrying out this research, the authors have benefited considerably from

PROMPT SOLAR PROTON EVENTS AND CORONAL MASS EJECTIONS 443

t he i r p a r t i c i p a t i o n in t he Sky lab So l a r W o r k s h o p Ser i e s on So l a r F la res . T h e

w o r k s h o p s a r e s p o s o r e d by N A S A and N S F and m a n a g e d by t h e H i g h A l t i t u d e

O b s e r v a t o r y .

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