spatial development of x-ray emission during the impulsive phase of a solar flare

S P A T I A L D E V E L O P M E N T OF X - R A Y E M I S S I O N D U R I N G

T H E I M P U L S I V E P H A S E OF A S O L A R F L A R E

C O R N E L I S D E J A G E R and A N D R I ~ B O E L E E *

Laboratoo, for Space Research, Utrecht, The Netherlands

and

D A V I D M. R U S T * *

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

(Received 18 October, 1983; in revised form 16 January, 1984)

Abstract. The flare of 11 November, 1980, 17:25 UT occurred in a magnetically complex region. It was preceded by some ten minutes by a gradual flare originating over the magnetic inversion line, close to a small sunspot. This seems to have triggered the main flare (at 70000 km distance) which originated between a large sunspot and the inversion line. The main flare started at 17 : 23 : 20 UT with a slight cnhancemcnl of hard X-rays (E > 30 keV) accompanied by the formation of a dark loop between two Ha bright ribbons. In 3-8 keV X-rays a southward expansion started at the same time, with v -~ 500 km s - '. At the same time a surge-like expansion started. It was observable slightly later in Ha, with southward velocities of >200 kna s ~. The dark II~ loop dissolved at ~ 17:24 UT at which time several ~Lmpulsive phenomena started such as a complex & h a r d X-ray bursts localized in a small area. At the end of the impulsive phase at 17 : 25 : 40 UT, a coronal explosion occurred directed southward with an initial expansion velocity of ~ 1800 km s - ~, decreasing in 40 s to ~500 km s - 1.

1. General Description; X-Ray Light Curves; HXIS Images

Starting at about 17 : 12 UT on 11 November, 1980, when the Solar Maximum Mission was pointing AR2779, the Hard X-Ray Imaging Spectrometer (HXIS) observed two successive flares, one being fairly weak, slow and gradual, mainly emitting at low energies, with a short impulsive hard burst from 17 : 12 : 00 to 17 : 12 : 30 UT and its maximum soft X-ray flux at about 17 : 15 UT, while the other was impulsive and much more intense. The time of maximum X-ray intensity of this second flare was about 17:25 UT. It was centered at S 13 E03, i.e. near the disk center.

The five parts of Figure 1 give the countrates in three HXIS channels (Figures la-c) , and Figures ld and le show the two impulsive phases preceeding the two flares, observed by the Hard X-Ray Burst Spectrometer (HXRBS). The hard X-ray bursts are already visible in Figure lc but they are shown much better (because of the higher time resolution of the diagram) in Figure le. But there is no large difference between these two figures. The bursts occurred between 17 : 24 and 17 : 26 UT, on the steep ascending branch of the soft X-ray response curve, as happens so often. In addition we note a weak

* Now at Fokker Aircraft Industries, Sehiphol, "['he Netherlands. ** Present address: Space Department , John Hopkins Applied Physics Laboratory, John Hopkins Road, Laurel, Md., U.S.A.

Solar Physics 92 (1984) 245-258. 0038-0938/84/0922-0245502.10. (c) 1984 by D. Reidel Publishing Company.

246 C . D E J A G E R E T A L .

~ q r r , l l , , , l l r ~ ' ] r [ l l l l , , i r , , l [ i r r

~ m . - i

p f

a . 3 . 5 - 8 . 0 keY

T T - m

b. 8 - 16 key

\ ] i

c. 1 6 - 3 2 keY

17:15 2 0 :25 :30 3 5 :40 17:45 UT

17:10 17:12

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~0

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c ts s -1

40

20

0 17:14 17:16 UT

/ e. 2 9 - 5 0 8 keV -] 3000

/ -12ooo

17:26 17:28 UT

Fig. 1. Integrated light curves of the flaring area. Figures (a)-(c) give count-rates observed in thc HXIS fine field-of-view for the energy channels 3.5-8.0 keY, 8.(I-16.0 keV, and 16-30 keV. Figures (d) and (e) give fnll-disk count-rates observed with HXRBS aboard SMM in energy channel 1 (E > 29 keV), and show the impulsive bursts at 17: 12:15 UT and at 17:25 UT. Both flares have apparently an impulsive phase. The

ordinates of (a)-(c) give countrates in an arbitrary scale.

hard X-ray plateau-like enhancement starting at 1 7 : 2 3 : 2 0 UT, which may indicate

some preflare heating, and which appears (cf. Section 3) to mark the onset & t h e flare.

H X I S takes X-ray pictures of a restricted area of the Sun using a coll imator and a matrix o f detectors. It thus obtains simultaneously pictures over a quasicircular area of

2'40" diameter (fine field of view, with 8" square resolution) and over a quasi-square area of 6' 24" diameter (coarse field of view, with 32" square resolution). The highest t ime resolution was 3 s in N o v e m b e r 1980. Observations are made simultaneously in six energy channels, quasi-logarithmically spaced between 3.5 and 32 keV (Van Beck et al., 1980).

SPATIAL DEVELOPMENT OF X-RAY EMISSION 247

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I- Fig. 2. The development of the X-ray flare in relation to the magnetic field pattern. (a) Magnetic field pattern derived from Kitt Peak magnetogram taken at 17 : 06 : 42 UT. (b)- (g) X-ray images overlaid over the magnetogram. The images were obtained in the energy band 3.5-8.0 keV; the exposure times are marked in every image. The isophotes are in a quasi-logarithmic scale with three or four steps per dex. For the mutual comparison of the various X-ray images some isophotes are specially marked: . . . . . : ~ 0.1 count per second; . . . . . . . : ~ 1; : ~ 10. (h)Over lay of the Kitt Peak magnetogram and the X-ray picture

obtained in the energy channel 11.5-22.0 keV at the same time as image (g).

248 c . D E J A G E R E T AL.

Figure2 shows HXIS images in the energy band 3.5-8.0keV at various times between 17:12 and 17:29 UT and this shows that two different flares are involved.

We decided to study this flare complex because good Ha images and magnetograms are available to aid in studying the flare origin and development, and to search for the sources of the hard X-ray bursts (Figure le). In addition we wanted to examine if this flare, too, had a 'coronal explosion', a phenomenon observed once before (De Jager and Boelee, 1984).

2. Magnetograms

By courtesy of Kitt Peak National Observatory, through Dr W. C. Livingston, we received a copy o f a magnetogram obtained at 17 : 06 : 42 UT that same day, hence less than 10 and 20 min before the maxima of the two flares. Figure 2a shows the outline of the magnetogram projected on the HXIS field-of-view. The areas of north and south polarity are indicated (dotted and white areas respectively); in the hatched area no magnetic field component was detected.

The mutual positioning of HXIS and the ground based instruments (magnetograph and filtergraph) was determined by involving observations with other SMM instruments in white light. Therefore the uncertainty in the positioning is about + 5" (for comparison: the three 'steps' in the four corners of the HXIS observations have sides of 16" length).

It is illustrative to compare this field pattern with X-ray flare images. To the end Figures 2b through 2g give overlays of HXIS images in the energy range 3.5-8.0 keV for a succession of images of the two flares obtained between 17 : 12 UT and 17 : 29 UT. Within the uncertainty of the positioning the succesion of overlays shows as general characteristics (a) that the flaring area as a whole is situated in the irregular part of the magnetic field and mainly over the magnetic 'islands' and the 'peninsulae', and (b) that the soft X-ray intensity maxima are mainly localized over the magnetic inversion lines, which suggests the existence of short loops, ~ 5" to 10" long, bridging the inversion line at several places, and being excited consecutively.

Note further: - The change of the X-ray emission pattern between 17 : 13 UT and 17 : 17 UT which

suggests excitation of a N - S oriented loop at 17:13 UT and of an E - W oriented loop at the later time.

- At 17 : 22 UT there are first weak indications of the start of the second flare, shown by an eastward extension and by the formation of a new shortlived maximum close to the eastern north-polarity field area. A comparison with the integrated light-curves of Figure 1 shows that at that time the first flare had nearly ended while we were still a minute before the impulsive start of the second flare, which occurred at ~ 17:23 UT (see in particular Figure le).

- At 17:23 UT the eastward extension of the X-ray flare became clearer. Twenty sec later the hard X-ray flux started to increase slightly (Figure le).

- At 17 : 24 UT the impulsive phase of the second flare started. The main X-ray


structure was a rather small, nearly circular (diameter ~ 25" ) flaring area between the easternmost spot and the inversion line. There is still a remnant of the first flare.

- At 17 : 28 UT, when the second flare was already in its decline, it had shifted till just over the inversion line. A very faint remnant of the first flare is still visible.

It is very illustrative to compare Figure 2g with the image obtained simultaneously in harder X-rays. Figure 2h gives the X-ray map in the energy range 11.5-22 keV. While the general position ofthe flare is virtually identical to that of the soft X-ray flare it seems significant that in this energy range the flare consists of several components, all showing their intensity maxima just over the inversion line, suggestive of three short loops. It is also remarkable that some of these bursts have no corresponding emission component in lower energy X-rays. Note that this happens in the declining (gradual) phase of the flare. This shows that even in the gradual phase relatively encrgetic )(-ray emission is still concentrated in separate small structures, in contrast to the less energetic X-ray emission.

An obvious and important question is whether the second flare was triggered by the first. Here the answer could be affirmative, as shown by Figures 2d, e, and f, but this issue is not analyzed further here in view of the rather uncertain statistical consideration needed to rule out a chance coincidence.

In the subsequent part of this paper we will restrict ourselves to a discussion of the second flare, which had its maximum at 17 : 25 UT.

3. Hard X-Ray Bursts and He Morphology

The flare at 17:24 UT occurred in a satellite sunspot region south-west of the large follower spot (the easternmost spot in Figure 2a) where small dark surges were seen almost continuously on the blue wing He images obtained at thc Holloman Solar Observatory. Figure 3 shows a sketch of the region, while Figure 4 gives the He photographs at three different times. The He flare seems to have occurred in two steps, each step associated with a different hard X-ray burst and a different magnetic field loop.

The first evidence of He activity that can definitely be associated with the flare was the appearance at 17 : 22 : 40 UT of a faint dark loop, clearly visible in Figure 4a. By 17 : 23 : 31 UT, when the HXRBS hard X-ray signal had risen to a low plateau (see Figure le), two faint flare ribbons were visible, one at each end of the dark loop (Figure 4a). These ribbons were slightly brighter by 17 : 23 : 41 UT and the dark loop was at that time clearly defined in the blue-wing pictures. The flare ribbons were in regions of opposite magnetic polarity (see sketch in Figure 3). The appearance of the dark loop just before the flare and then its sudden dissolution and slight southward displacement at 17 : 24 UT (cf. Figures 4b and 4c) suggest that it outlined an unstable magnetic flux loop.

As the loop dissolved, the first hard X-ray burst commenced with a sharp rise above the low pre-burst X-ray plateau that had been established 40 s before (Figure le). The next He image, at 17:24:11 UT (Figure 4b), showed such a sharp increase in He emission that the film was nearly saturated. Nevertheless, the outline of the two flare

2 5 0 C DE JAGER ET AL. "

N

l ~ W

_l a 1 7 : 2 4 : 0 0 UT b 17:25:11 UT

_J f ~

/ J

j~,,23:41

_~24:31 25:11

I /

\ ., / \ \

"x

I k \

(

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Fig. 3. Sketch of He ribbons, dark loop and He surge during the two hard X-ray bursts. Broken line: magnetic inversion linc (from Figure 2a); screened area: sunspot; dntted: dark He loop; horizontally hatched: l'Ict emission. The 'moving front' in the lower part of(a) refers to an He surge. The four front-profiles arc

labeled with time in minutes and seconds. See further for that topic: Section 7.

ribbons was clear, with one ribbon in the positive fields of the (positive) follower spot

and the other ribbon in the easternmost part of the area of negative (satellite) polarity.

The next filtergrams, at 17 :24 : 31 and 17 : 24 :41 , showed little change except in- creased local brightening. These pictures were taken in the interval between the two hard X-ray peaks (cf. Figure le). Thereafter the second hard X-ray burst started. Filtergrams at 17 : 25 : 11 and 17 : 25 : 30 U T show new flare ribbons in both positive and negative fields (see sketch in Figure 3b, see also Figure 4c). The ribbons we associate with the second burst were partly overlapping and partly south-east of the corresponding first-

burst ribbons. To summarize, we find a sharp rise in hard X-rays coinciding with the dissolution

of an Hc~ loop. The sequence of events is remarkably similar to that reported on by Rust et al. (1981) in which a gradually rising Ha loop (seen on the limb) broke up at the on set o f the hard X-ray impulsive emissions. We also find evidence for energy release during


Fig. 4. Series o f H ~ i m a g e s o f t he flare. (a) 17 : 23 : 4 1 U T ; (b) 17 : 24 : 11 UT; (c) 17 : 25 : 1 1 U T . By courtesy of U.S. Air W e a the r Service, Solar Observ ing Optical Ne twork (SOON) .

252 c . I)E JAGER ET AI_.

the two hard X-ray bursts (cf. Figure le; at 17 : 24 : 15 and 17 : 25 UT) in two distinct magnetic loops, the first loop with footpoints separated by about 30 arc sec, the second loop with footpoints separated by about 40 arc sec.

The next proble m to be investigated is that of the relation between the location of the H a emissions and of the X-ray emission during the two hard X-ray bursts.

4. Sources of Hard X-Ray Bursts

As shown by Figures lc and le there were two fairly complex hard X-ray bursts during the second (main) flare. These bursts had maximum intensity at 17 : 24 : 15 UT and at

17 : 25 UT. In between, at 17 : 24 : 40 UT, the hard X-ray count rate dropped to a small value. In the integrated low-energy intensity-time curves (Figures la and lb) {hese bursts are not visible, but we want to examine if they can be made visible in our X-ray images.

In order to determine the sources of these bursts our technique is to determine for each HXIS pixel a smooth curve representing the time variation of the count rate for the time when there were no hard bursts, to interpolate them for the times of the hard bursts,

and to define the excess count rates at these times. Thus we defined for each pixel count

rates c(l) for a number of times ( tR) at which there were no hard bursts.

TABLE I

Reference times ( t ) n , hard burst times (t)B and accumulation times At

Reference data Hard burst data

(~),, (UT) at (s) (~)~ (uT) At (s)

17 : 23 : 44.75 14

17:24:48.25 15

17:27:11.25 18 17 : 27 : 48.25 18 17 : 28 : 26.25 18 17:29:03.25 18

17:24:14.75 1.5 17:24:26.25 3

17:25:11.25 3 17: 25 : 2(I.25 15

These re/erence times are given in Table I. The c(t) function for these reference times was approximated (for each pixel) by a second degree polynomial. By interpolation the 'background count-rates' were thus deternained for the times of the hard bursts (given in the right-hand part of Table I), and by subtraction from the actual count-rates at these latter times the excess count-rates were determined for each burst and for each pixel. Thus we found the specific countrates and hence the location of the source regions for the hard bursts proper. This procedure was applied both to the count-rates in the low-energy channels I + II (= 3.5-8.0 keV) and the medium energy channels


17:24:15

UT.

17:24:26

17:25:11

17:25:20

3.5-8.0 keV

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Fig. 5, Left: Excess count-rates summed up over the two low-energy channels (3 .5-8 .0 keV) at four times during the occurrence o f the hard bursts. - . . . . : 2 a count rate ( 6 c t s s - t ) ; _ : 4a, 8a . . . etc.

count-rates (12, 24 , . . . cts s - ~ ). For comparison and reference purposes the magnetic inversion line has been drawn in. Right: The same for the H X I S channels IV + V (11.5-22.0 keV). - . . . . : 2tr + ( c ) count-rate

(0.9 cts s - l ) ; : 4tr+ ( c ) , 8tr+ ( c ) (1.7; 3.3).

254 c . DE JAGER ET At,.

IV + V ( = 11.5-22 keV). The results are shown in Figures 5a (for the low energy channels) and 5b (medium energies). As a reference the magnetic inversion line (Figure 3a) is also drawn. We note the following results:

- Sources of the hard X-ray bursts are clearly noticeable in hard as well as in soft channels. Hence, although the integrated X-ray light curve does not show any peak at

the time of the hard X-ray burst, it appears that small areas on the Sun do show an excess count-rate at these times. Hence, 'hard bursts' are not really 'hard', the low energy excess flux is just submerged in an intense gradual flux.

- The source regions largely coincide in the low and medium energy images.

- There is some fine structure in the source regions but its reality is marginal. - The sources are weakest during the first of the two hard X-ray bursts (17 : 24 : 5l)

especially at low energies, and become more and more important for later hard bursts.

Note that this sequence is the reverse of that at energies above 30 keV (see Figure le). - The source region coincides with that of the northernmost of the patches seen in

Ha; this is the area south-west of the easternmost sunspot, between it and the inversion

line. Overall, the images suggest that in the course of the two hard bursts of 17 : 24 : 15 and

17:25 UT (a period of about 70 s) one or more X-ray emitting loops developed, connecting the sunspot and the region beyond the inversion line. These results seem to

confirm a conclusion by Dwivedi et al. (1984) that peaks of hard X-rays can occur when excitation takes place in loops rooted with one end in a sunspot penumbra.

5. A C o r o n a l E x p l o s i o n

A phenomena which was called a 'coronal explosion' was observed in X-rays by De J ager and B oelee (1984) using HXIS observations of the flare of 12 November, 1980,

02:50 UT. Its characteristic is that the time of maximum X-ray brightness for a particular burst moves over the flaring area:

One or two pixels reach maximum first, and maximum occurs later in more and more

distant pixels. This suggests a wave motion. In the case of the 12 November flare the area at which pixels reached maximum brightness propagated with an average velocity of ~ 50 km s - J

Also in the 11 November flare there are clear indications for an X-ray coronal explosion but the speed of propagation appears to be more than ten times larger, a fact for which at present - with only two cases available - no explanation can be given.

To study the phenomenon in an objective way we used the count-rates c(t) in the low energy channels I + 1I (= 3.5-8.0 keV) for individual pixels at eleven different times between 17 : 23:45 and 17 : 28 : 26 UT. For each pixel the c(t) variation was then approximated by a second degrees polynomial, for which we determined t ..... and c(t, ..... ), the time of, and countrate at, maximum for each pixel. The t . . . . -values are shown in Figure 6a in which isochrones have been drawn. They suggest a south- to south-eastward expansion with decreasing velocity. To study the velocity behaviour in more detail we plotted distance-time diagrams along three directions, encompassing a


121

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Fig. 6. (a) The coronal explosion at 17 : 24 : 45 UT in X-rays of 3.5-8.0 keV. The lines, drawn over the magnetic field pattern (dashed; meant for reference) are isochrones, indicating the time (minutes and seconds after 17 UT) at which maximum intensity is reached in the various pixels. At the limits of the field studied the count-rate is small and so is the accuracy of the data. Therefore the 'counterwave' in the south-eastern part of the diagram may not be real. (b) There is no sign of a coronal explosion in higher energies (11.5-22.0 keV). Rather, the location of maximum intensity is distributed over the area south-west of the main spot. The lines are isophotes connecting pixels where the maximum intensity had the same value (but at different times). They are labeled with the count-rate in counts s - ~. Maximum intensity is reached

at 17:25:20 + 22 s UT, hence about 20 s before the start of the explosion in lower energies.

total angle of 90 ~ The results show slightly different velocities for the three directions. From the 'average' curve we derive the following velocities:

at 17:25:45 UT: 1840 km s- 1; 17:25:55 UT: 870 km s-~; 17 :26 :10UT: 500kms -~.

This shows the decrease of the velocity with distance from the 'source' and with time. The 'wave' behaves somewhat differently than those reported by Rust et al. (1984) and interpreted by them as thermal conduction fronts: these propagate along a well-defined loops, which is not the case with the wave studied by us.

In contrast to the low-energy observations, those at higher energies (we took the counts in channels IV + V; 11.5-22.0 keV) do not show such a systematic behaviour, a feature for which the explanation becomes apparent from a look at the X-ray 'light- curves' of Figure le: there is no question of a single peak but rather of a number of individual bursts, The time at which maximum intensity is reached in the various pixels for the radiation emitted in channels IV + V (i 1.5-22.0 keV) does not appear to vary systematically over the area considered. Rather, it seems, that various burst components have maxima in different pixels, but that is statistically not really significant. The average

256 C. DE JAGER ET AL.

time of maximum, determined in the same way as for the low energy channels, is for these channels 17 : 25 : 20 UT + 228, which is about 20 s before the start of the explosion in the low energy channels. Maximum countrate reached at that time is ,,~ 5 cts s - 1 cf. Figure 6b. The area of maximum count-rate is the same where the 'excess counts' reached maximum in the same energy range, cf. Figure 5b. It is noteworthy that the pixel of maximum hard X-ray brightness does not coincide with the pixel where the explosion started which we observed in lower energies.

6. Southward Motions over a Larger Area

In the foregoing section we described a wavelike motion presumably originating explo- sively, observed during one minute only, and over a fairly small area. Its velocity was large initially ( ~ 1800 km s - 1) but decreased to below 500 km s - 1 in less than a minute.

Here we will show that a motion with about 500 km s - 1 also was observable over a larger area, although this later motion started before the hard X-ray bursts, and hence before the fast explosion described in the preceding section, which followed the impulsive bursts.

In the coarse field of view (quasi-rectangular, diameter 6'; pixel size 32"), we define 32" x 32" pixels, which overlap the fine field, as those which confine the flare itself and ask whether there is any evidence for X-ray emitting material escaPing from the region. Our approach was to integrate the signal in energy band I (3.5-5.5 keV) over 20 s intervals from 17 : 23 to 17 : 30 UT and to draw 5-count contours around the region south of the flare. The resulting displacement of the contour level was plotted versus

K m

6 0 0 0 0

40 0 0 0

2 0 0 0 0

o

Est imated e r r o r ( one-ha l l pixel )

I I I ~ I 17:24:00 :20 :40

I I I m I J I , I 17:25:00 :20 :40 17:26:00 UT

Fig. 7. Displacement of 5 ct s- 1 X-ray contours southward. The distance scale has an arbitrary zero point.


time with the result shown in Figure 7. Although the approach is crude, a southward flow velocity of 520 + 50 km s - 1 at right angles to the line of sight is indicated. The

apparent start time was before 17 : 24 UT. It does not seem unreasonable to assume that the sourthward expansion started at 17 : 23 UT, or more precisely, at 17 : 23 : 20 UT when the flare had its first hard X-ray enhancement, when a dark loop became visible in Ha, and a southward propagating surge-like feature started.

The X-ray emitting material reached the edge of the HXIS coarse field of view at about 17 : 26 : 30 UT and the emission still in the field faded rapidly from about that time. By 17 : 29 UT there was no emitting material outside the six flare', pixels, indicating either rapid cooling or expansion outside the field of view, or both.

7. H~ Ejecta

Ha observations, too, show ejection of cool matter from the flare region, also in a southward direction, like the X-ray observations.

First, we mention the little bright surge-like feature observed between 17 : 23 : 41 and 17 : 25 : 11 UT moving southward from the southern component of the Ha emitting area. Figure 3a shows contours at four different times. From these we derive an average projected speed of 230 km s - t. Extrapolating this speed we note that the ejection would have started from the southern H~ flare ribbon at about 17 : 23 : 20 UT, which is the time at which the flare ribbons were formed (Section 3), and at which the first hard X-ray increase started - cf. Figure le. Apparently, this seemingly insignificant hard X- emission was therefore associated with, or led to gas motions of 230 km s - 1, and in addition (cf. Section 3) to the formation of the two faint flare ribbons.

At 17 : 26 : 30 faint dark strands were seen stretching toward the south from about 10 arc sec outside the flare ribbons. A large dark surge, composed of half a dozen fine strands, was clearly visible at 17:29:11 UT. Velocity of the surge material was ~ 200 km s - 1, but it must be borne in mind that the velocity was determined from successive positions of the leading edge of the dark strands as photographed through a 0.5 A filter in the blue wing of the Ha line. Thus, material is visible in the pictures only in an interval of ~ 30 km s- 1 in the line of sight. In brief, the deduced velocity of 200 km s - ' is a lower limit.

The clear indications of ejecta that escape the flare region show that some material, at least, was able to move from the small loops associated with the: hard X-ray burst into large-scale fields which fan out from the sunspot.

8. Scenario and Conclusions

The two flares that had maxima at ~ 17 : 15 and 17 : 25 UT on 11 Ne,vember, 1980 were related to a complex magnetic field pattern (Figure 2a), and seem related. There are a few crucial moments in the development of these flares:

17 : 12 UT: origin of the first flare emitting a weak impulsive burst (Figure ld) and appearing as a short loop bridging the inversion line (Figure 2b);

-~ 17 : 16 UT: eastward extension;

258 c. DE JAGER ET AL.

,,~ 17 : 22 UT: first soft X-ray loop appears at the inversion line in the eastern magnetic 'peninsula' (Figure 2d);

~ 17:23 UT: start of the eastern (impulsive) flare, between the eastern spot and the inversion line (Figure 3e). This time also marks: the first (gradual) hard X-ray enhancement at 17 : 23 : 20 UT (Figure le); the origin of a dark loop connecting two Ha flare patches (Figure 3a); a southward propagating surge-like feature with v "-~ 230 km s 1 (Figure 3a); the general southward expansion of the flare with v---520 km s-1 (Figure 7);

~ 17:24 UT: start of impulsive events: dissolution of dark surge (Figure 3b); two hard X-ray impulsive bursts with several spikes (Figure le); localized source regions of hard X-ray bursts (Figure 5; Figure 6b);

,,~ 17 : 24 : 40 UT: end of the impulsive phase. Start of a coronal explosion (Figure 6a).

Hence this flare complex started as a weak flare with a small impulsive hard X-ray spike and mainly a gradual character, which apparently triggered an impulsive flare at a distance of -,-70000 kin, in which the impulsive phenomena such as hard bursts and a coronal explosion closely followed the dissolution of a dark chromospheric loop. This may have been caused by circuit interruption or field line reconnection.

The coronal explosion was apparently related to the end of the impulsive phase, and must result from the heating of coronal or high chromospheric layers causing a conduction front or shock wave to propagate over the disk.

Acknowledgements

The development and construction of the HXIS was made possible by support from the Netherlands Ministry for Education and Science, and the Science and Engineering Research Council of the United Kingdom.

We are obliged to Dr Brian R. Dennis for providing us with HXRBS data on hard X-ray bursts and for stimulating discussions, to Dr W. C. Livingston at Kitt Peak National Observatory for the magnetogram, to the U.S. Air Weather Service for Ha photographs obtained by the SOON network. The work of one of us (D.M.R.) was supported by the US National Science Foundation under grant ATM 8115395. Thanks are due to Drs Peter Hoyng, Aert Schadee, and Zden~k Svestka for stimulating comments. An unknown referee made a number of constructive comments.

References

De Jager, C. and Boelee, A.: 1984, Solar Phys. 92, 227 (this volume). Dwivedi, B. N., Hudson, H. S., Kane, S. R., and Svestka, Z.: 1984, Solar Phys. 90, 331. Rust, D. M., Benz, A., Hurford, G. J., Nelson, G., Pick, M., and Ruzdjak,

Astrophys. J. 244, L179. Rust, D. M., Simnett, G., and Smith, D. F.: 1984, Astrophys. d., submitted. Van Beek, H. F., Iloyng, P., Lafleur, B., Simnett, G. M.: 1980, Solar Phys. 65, 39.

V.: 1981,

spatial development of x-ray emission during the impulsive phase of a solar flare

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