3 Investigation of images of loop-top hard X-ray emission sources

The HXR light curves have suggested a non-uniform structure of loop-top HXR emission sources. Thanks to the imaging capability of the HXT we can study the origin of the smooth component and the impulses, and find the reasons for the complex character of the smooth component in some events.

The Yohkoh/HXT images were synthesized using the Maximum Entropy Method adapted by Sakao (1994). The recent HXT instrument response function derived from in-orbit calibrations (Sato 1997) were used. In the channel L all investigated flares had enough counts to synthesize at least five images. In the channel M1 this has been achieved for a half of events while in the channel M2 only a few images have been obtained for two flares.

The obtained HXR images were compared to the closest in time SXR image derived by the SXT. The accuracy of the coalignment was about 1 arcsec. If we consider the position of HXR emission sources in SXR images, the obtained loop-top sources can be divided into two main types, A and B. Sources of type A are co-spatial with the SXR bright loop-top kernels or "bright knots'' (Acton et al. 1992). The SXR counterparts of sources of type B are extended and weak structures situated distinctly higher than bright loop-top kernels. Examples of both types of loop-top HXR emission sources are presented in Fig. 5.

Figure 5: Examples of the loop-top HXR emission sources. In the upper images of the event No. 7 the source of type A is seen. In the bottom images of the event No. 11 both types of sources, A and B, are shown. In all images solar north is up, east to the left and the size of each pixel is 2.45 arcsec. The SXT(Be119) emission distribution is represented by a grey scale. Contours of the HXR emission (75, 50, 30 and 15% of the maximum intensity) are overplotted in the left images for the channel L and in the right images for the channel M1. The times of the SXT(Be119) images are showed. The solar limb is marked by the straight line as a rough guide only

 

 

Table 4: Morphological characteristics of loop-top HXR emission sources in the channel L

No.	Type of	FWHM	HXR-SXR
	the source	size$^{\rm a}$	shift$^{\rm a}$
1	A	2.5 $\times$ 3.5	1-1.5
	A	3 $\times$ 3.5	< 1
2	A	3 $\times$ 4	< 1
3	B	3 $\times$ 12	N/A
	A	3 $\times$ 3.5	< 1
4	A	4 $\times$ 6.5	1-1.5
	B	4.5 $\times$ 9	N/A
5	A	3.5 $\times$ 8.5$^{\rm b}$	2-2.5
6	A	1 $\times$ 3$^{\rm b}$	1-1.5
7	A	3 $\times$ 3.5	< 1
8	A	3 $\times$ 4	1.5-2
9	A	3 $\times$ 3	< 1
	A	2 $\times$ 3	1.5-2
10	A	2.5 $\times$ 3.5	1-1.5
11	B	3 $\times$ 10	N/A
	A	3.5 $\times$ 4	< 1
	A	2.5 $\times$ 5	< 1
12	A	2 $\times$ 3	< 1
	A	3 $\times$ 3.5	< 1
13	A	5.5 $\times$ 5.5	2-2.5
	B	4 $\times$ 7.5	N/A
14	A	3 $\times$ 4	1.5-2

$^{\rm a}$ In SXT pixels (1 pixel = 2.45 arcsec $\simeq$ 1.8 10³ km).

$^{\rm b}$ Actual shape was cut by the solar limb.

The majority of analyzed HXR emission sources were of type A - every flare showed at least one such a source. Sources of type B never existed separately, they always had a companion of type A. The events in which the source of type B occurred always had a complicated magnetic structure and their SXR evolution was prolonged (Table 1). The occurrence of the individual types of sources in the investigated flares is shown in Table 4.

The FWHM shape of the reconstructed HXR emission sources in the channel L was usually constant during the evolution in spite of continuous increase of its height. As a rule, the sources of type A were circular or slightly elongated with the maximum in the middle. A typical diameter was about 5-7 10³ km (Table 4). A shift between the brightest part of the source of type A and the maximum of the SXR bright loop-top kernel only sporadically was above 2.5 10³ km. The sources of type B had strongly elongated arch-shape with at least one end reaching the solar limb. The location of the brightest part of the sources of type B changed dynamically from image to image. For this reason and because of the very weak emission of the SXR counterparts, no reliable values of the HXR/SXR shift for the sources of type B have been obtained.

Figure 6: a,b) Two images of the 6 February 1992 flare (event No. 4) in which two different HXR loop-top emission sources are seen, the first - of type A, and the second - of type B. Contours of the HXR intensity are marked with the solid and dotted lines for the channels M1 and L, respectively. For other details - see the caption of Fig. 5. c) Regions of interest for which the individual light curves are presented in Fig. 7

In the channel M1 images only minor differences were seen in comparison with the images in the channel L. For example, the FWHM sizes of type A sources were sometimes 10-20 percent lower and the SXR/HXR shift were sometimes slightly greater than for the channel L images. In the case of type B sources, a temporal fragmentation occurred (Figs. 5 and 6b) inside the source area seen in the channel L.

The main details of the HXR light curves can be easily identified in the HXR images, i.e., each smooth component has a distinct spatial counterpart in the form of individual HXR emission source. As long as the same source is observed in the HXR images, the HXR light curve shows a single smooth component. If an additional HXR source becomes visible, a separate smooth component in the light curve can be detected. For example, Fig. 6 shows two X-ray images for the 6 February 1992 flare (event No. 4) in which two different HXR emission sources are seen. Individual light curves obtained from the HXR images for the detected emission sources are shown in Fig. 7. We see that each source provides some contribution to the total light curve which can be resolved as consisting of two individual smooth components. The two sources had similar temperatures, about 35 MK, at their peaks of intensity.

Figure 7: The HXR light curves of the 6 February 1992 flare (event No. 4) for the total flux and for the individual region of interest which are marked in Fig. 6c. The plots for the channel L (the upper panel) and for the channel M1 (the middle panel) are presented. In the bottom panel temperatures estimated from the M1/L hardness ratio are showed. Relative error of the temperature was about 10-15%

Also in other flares, several emission sources in the HXR images were seen when the HXR light curves had several smooth components. However, it was sometimes difficult to separate the sources when they were situated partially along the line-of-sight (events Nos. 1, 3 and 13). For flares with a low number of counts, fewer HXR images were available. Consequently, the light curves for the individual emission sources had less detail than in Fig. 7 and their M1/L diagnostics was more problematic.

Although we can deduce the number of emission sources from the multiple smooth components in the HXR light curve, we cannot obtain more detailed information without imaging. In particular, it is impossible to distinguish between the emission sources of type A and type B on the basis of HXR light curves alone. Considering parameters like the succession of occurrence, maximum flux, FWHMduration, or occurrence of impulses (e.g. see Figs. 2 and 7) we find that very similar smooth components were produced by sources of both types.

It was even more difficult to identify in the HXR images the impulses detected in the HXR light curves. Their duration was usually too short to accumulate enough counts for image synthesis. Therefore, only the strongest impulses offered a possibility to resolve the location in the HXR images where they came from. The images that were obtained during such impulses did not show any additional emission sources in comparison with images that were obtained before and after the impulse. Thus, we conclude that the impulses and the smooth component were emitted from the same sources. In flares with multiple smooth components, the impulses were emitted mainly from the source which was the strongest during the time of the impulse (Fig. 7).