Study of combined soft and hard X-ray images of solar flares

Adv. Space Ses. Vol.4, No.7, pp.91—94, 1984 Printed in Great Britain.

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0273—1177/84 $0.00 + .50 Copyright C COSPAR

STUDY OF COMBINED SOFT AND HARD X-RAY IMAGES OF SOLAR FLARES M. E. Machado,* A. M. Hernández,** M. G. Rovira** and C. V. Sneibrun* *Observatorio de Fisica COsmica, CN!E, 1663 San Miguel, Argentina *

*IA F. E., Buenos Aires, Argentina

AHSTRACT U. hay. studied soft and hard X-ray images of 13 solar’ flaxes from six active regions observed by the Hard I—ray Imaging Spectrometer (HXIS). Our results indicate the praline. of pm—hard X-ray burst excesses In the il.5—30.0 keV range, indicating a slow buildup of the acc.leration proc.ss or a strong preh.ating. During the impulsive phase, all of the events show the simultan.ous energization of neighboring field structures, which, in the cass we show in some detail, shar. about equal ameunts of the released energy. This association seems to be indicative of strong acceleration and energy release triggered by the interaction between magnetic loops. PBEFLA~ACTIVITY U. have studied the characteristic, of 13 solar flares obs.rv.d by the HXIS from April to November, 1980. Out of this sample, we studied the preflare of 10 events, th, cases in which the data was found to be appropriate in terms of temporal coverage. We find, as in de Jsger et al. /1/, n1~eroussoft X—ray brightenings which may or may not be related to the subsequent flare. However, studying the hard X-ray (11.5—30.0 ccv) records within a five mmutse period preceding the hard I—ray burst (as defined by the HIBBS records), we find significant high energy excess in 9 cases. The excess is found after substraction of the thermal contribution of the background soft X—ray emitting plaama, and appears within a timescale ~ 2 minute. before the burst, but considerations of counting rates and HXIS .ffiei.ncy reveals that this limit may be due to sensitivity effects. The high energy excesses appear to be highly localized (eneil size, sometimes of the order of the HZIS fine resolution ~ 8”) and associated with magn.tic features which subsequently develop into flaring loops. In three cases, the hard X—ray emission appear. to be correlated with preburst type III radio emission. This correlation may be larger, since good radio data was only available for five events. The spectral index of the hard X—ray emission varies within the range 4.5 ~ 2,5, In a 1ope corresponds to temperatures 7 x 107S T ~ 2.5 x 108 K. thermal interpretation, this s emission, the rang. of particle fluxes above 16 kaV is If 4nterpreted as th~,cktarget lO~~ ~l6 ~ 5 x 103’ s’ , while the thermal interpretation emission measures are within F~ l0~ cm3. These low values indicate the possibility of a sensitivity threshold problem in the temporal association, as mentioned above. In the thick target j.nterpretation, the power in the ~ 16 c.V electrons turns out to be P 16 ~ 3 x 1026 erg s , in the events we hay, studied. The 21 May 1980 two ribbon event, described by Hoyng et .1. /2/, shows preburst high energy 1l~a~nt. It later develops the brightest long enduring footpoint A, as defined by excess clearly localised at into the plac. of the emerging flux region, underneath the erupting fDuijveman et al. /3/. This behavior is d.piet.d In Figure 1. Similar phenomena are observed in other cases where there are evidences of interacting field structures. We also studied periods in which weak soft X—ray bright.nings did not develop into class C4 or brighter I—ray flams, and found no statistically significant evidence of high en.rgy exc.ssee. This may, again, be a sensitivity problem. As for the interpretation of this phsncmsnon, we have no clear means of discriminating between the nonthemal and thermal hypotheses. The possible association with type III radio emission may favor the former, and we note that the electron fluxes we derive are close to Kane’s /4/ estimate that th. total nunb.r of ‘ Z) keV electrons responsible for an intense type III group is ~ l0’4.

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M.E. Machado at al.

(~J~ Fig. 1. Soft (3.5—8.0 lcaV, dashed) and hard (16—30 keV, full) X—ray emission contours of the 21 May 3.980 flare. The image on the left corresponds to the preburst phase, where we see the outline of the erupting filament in soft X—rays and th. site of the early hard X-ray emission, located above the emerging flux region (see discussion in /2/~. The right image corresponds to the first peak in the bard I-ray burst, where we see the two separated footpoints in 16-30 keV emission /2,3/. Note that the early hard X—ray source develops into the brighter footpoint. MAGNETIC INTERACTIONS In all cases, without exception, we have found simultaneous strong (large dI/dt) brightening of at least two distinct magnetic structures, in close association with the hard X—ray burst. This confirms earlier results /5/, and extends the analysis to flares occurring in six different active regions. In Figure 2 we show the particular case of the 14 July 1980 flare at O8h 24m UT, which is one of the best examples of what could be a “concentrated” flare /6/. The first contour diagram (from HXIS fine field of view records in the 3.5-8.0 key range) shows the early soft X—ray brightening concentrated over a restricted region. The subsequent contours show the X-ray development as seen in the HIIS coarse field (32” resolution), from the peak in the hard X—rays (Figure 2b) to the decay, In which only the long lasting large structure is seen. H—alpha pictures from !uxman Observatory /7/ reveal the appearance of a large system of cool loops in the vel7 lat. phases of th, flare.

QF

Fig. 2, Soft X—ray development of the 14 July flare. (a) Preburst image from the fine field, showing the extant of the early brightening. (b) Coarse field image obtained at the time of the hard X-ray burst. Note the larger appearance of the early flaring region, F, under coarser resolution, and the early brightening of a larger X—ray structure connecting the main flaring region with distant H—alpha

Soft and Hard X—Ray Images of Flares

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bright regions (showa as dashed contours here). (c—d) Subsequent developeent of the soft X-ray sources. Note the decay of F, indicative of a rapid cooling of this discrete magnetic structure, and the long enduring character of the large source which corresponds to a different set of large loops. The tines of the different contour plots are: O~s24z52,08:2th00, 08i32*03 and 0th35:27 UT. We also studied the energetic relationship between the different structures, from the temporal evolution of the thermal energy content of each system, given by ~th

3 ~4(t)

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,

where EM(t) and T(t) are knowa quantities, and n(t) relies on an estimat. of the emitting volixe. The latter is the larger source of error in th, analysis, particularly when coarse field data is concermed. Still, we should bear in mind that it mainly affects the absolute magnitude of ~ but not its temporal behavior (shape of E~hvs t curves). We show in Figure 3 one of the best examples of this type of study, from the observations of the well knowo flare of 5 November 1980 /3/, which was completely cover.d by the HX~Sfin. field of view. As noted In /3,8/, two ioop systems participated in the flare, which we label AB and BC in Figure 3. Two hard X-ray peaks were observed, P3. and P2 as labelled in the figure, where we also note the tOtal temporal extent of the hard X-ray burst. During P1, three discrete hard X—ray footpoints were observed in A, B and C /3/, while P2 showed single source appearance, embedded within the AB loop. The Eth( t) curves show the close association of a r.pid variation, dE/dt, In both structures in coincidence with the first peak (P1). The thermal energy content continues to rise during P2, when the high time resolution curve of LB shows a distinct change in it. slope. The Eth(t) behavior or both structures indicate continuing energy input through the duration of the burst, in agreement with the results of Duijveman et al. /8/, who showed that a single injection of particles Into BC, during P1, cannot explain its behavior. Thes. observations thus provide evidence of continuous inter’connection in the activity of both loops throughout the duration of the burst.

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3), right, with time Pig. 3. Bun thermal Et (erg), left, density for the two of loops of theenergy 5 Novem~erflare. Alsoand noted (topn(cnr left) are the times of the two hard X-ray peaks and the total temporal extent of the burst. Note the strong response of E~h(LB) and n(&B) to the two peaks, and the slower behavior of the large loop BC, The Insert shows a fills soft X—ray Image of the event and the magnetic neutral line (see /3/ for further details) Two additional casements should be made about the graphs of Figure 3. We first note that E in BC seems to be larger than in AB; this may be due to en overestimat, of the volume of large 1oop. By trial and error we have found that the existence of a 16—30 keV footpoint in C is only compatible with a maximum density n(BC) (cf. mean free path arguments in /3/) that gives less than a factor of two (1.8) lower E,~In BC as compared with AB. Thus, the two loop systems share about equal amounts of the released energy. Secondly, the slower behavior of the response of BC can be explained by it. large dImensions

94

M.E. Machado at a~.

noting that its hydrodynamical response time, t~~ L(BC)/v

5 (v~beIng the sound speed), is of the order of 2.5 minutes. Th. slow Increase of n(BC), as showa in Figure 3, gives support to this interpretation. As a final point, we would lik, to mention that Macbade et al. /9/ have computed the expected spatial distribution of thick target bard X-ray. in LB, both durIng P1 and P2. Convolving their results with the HXIS instrumental response, they predict footpointe during the first peak, and a single source appearance during P2. This behavior is due to a combination of the HXIS characteristics and the Increased stopping of beam electrons, due to the density increase driven by evaporation (of. the n(LB) plot of Figure 3), Thus, both peaks ~ be associated with acceleration processes which, as far as our results show, take place during the interaction of the two distinct magnetic structures. A short summary of our results has been given here, while an extensive discussion will be published elsewhere. ACKN0WLEDG~4ENTS The developoent end construction of the EllS was made possible by support from the Nether. lands Ministry of Education end Science and the Science Research Council of the United Kingdom. MB~4acknoledges financial support from SCOSTEP and COSPAR BEFEBENCES

1. C. de Jager, H. E. Macbade, A. Schadee, K. T. Strong, 1. Svestka, B. E. Woodgate, and W. van Tend, Solar Phys. 84, 205 (1983) 2. P. Hoyng, A. Duijyeman, H. H. Machado, D. M. Bust, Z. Svestka, A. Bodes, C. de Jager, K. 3. Frost, H. La Fleur, G. H. Simnett, H. F. van Beak, and B. H. Woodgate, Astrouhva. 3. 246, Ll55 (1981)

3. A. Duijveman, P. Hoyng, and H. B. Maohado, Solar Phvs. 81, 137 (1982) 4. S. R. Kane, Solar PhTB. 27, 174 (1972)

5. H. B. Macbade, B. V. Somov, H. G. Bovira, and C. de Jager, Solar Phvs. 85, 157 (1983) 6. A. Duijveman, and P. Hoyng, Solar Phvs. 86, 279 (1983) 7. 3. Xuan, Z. Li, X. Gu, W. Li, A, lu, and I. Tang, Adv. Suace Res. 2 No. U, 157 (1983) 8. 4. Duijveman, B. V. Sccov, and A. H. Spektor, Solar Phva. 88, 257 (1983) 9. H. B. Macbade, H. G. Rovira, end C. V. &~eibr~m, Solar Phvs. (subeitted)