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Astron. Astrophys. 328, 371-380 (1997)

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4. Reconnecting loops and a thermal source

What kind of scenario can explain the observations reported in the previous section? Kurokawa et al. (1988) observed different bright kernels, seen in the H [FORMULA]  blue wing on the opposite polarity regions of a two-ribbon flare, that attained their maximum intensity simultaneously within 1 s. This brightening was also simultaneous, within the same temporal resolution, to the maximum of the HXR curve. They argued that the chromospheric kernels are the footpoints of a flaring loop system, heated by non thermal electrons with a time delay of no more than 1 s between them. A similar explanation can be invoked in our case. The temporal coincidence in the maximum emission (15:45:45 UT) of A1 and A2 supports the hypothesis that they are the footpoints of a flaring loop system, heated with a time delay of max. 2.5 s (our time resolution). Even if we have SXT images only at 15:46:20 UT, the presence of a loop system connecting A1 and A2 is evident in Fig. 3-c. We cannot yet draw conclusions about the energy transport mechanism from coronal to chromospheric levels, since we lack HXR light curves at the maximum time.

Evidence suggests that B1 and A2 are the chromospheric footpoints of another loop undergoing a new heating process, as proved by the new brightenings in the H [FORMULA]  wings and in He D3 (Zirin, 1988; Cauzzi et al., 1995). The temporal coincidence between the maximum emission of A2 and B1 at chromospheric level and the maximum emission of the coronal plasma in different energy bands, from 6.7 keV (Fe XXV) up to 23 keV, implies that the heating involves the loop at coronal levels as well. The apparent spatial coincidence of the chromospheric region A2 and the Lo-HXT source (Fig. 3-c), suggests that the HXR emission is mainly located in one of the loop footpoints, i.e. that it is due to non-thermal electrons impacting on the chromosphere (thick-target model, see Brown, 1971; Hudson, 1972; Emslie, 1978, 1983; Sakao, 1994 and references therein).

On the other hand, the similarity of the X-ray light curves (Fe XXV and Lo-HXT, see Sect. 3.1) may be due to the fact that we are watching the emission of the same plasma, i.e. that the plasma emitting in the Fe XXV line could be the same that thermally emits at 14-23 keV. To check this possibility, we can estimate what would be the photons flux in the center of the Lo-HXT channel, produced by the plasma emitting in the Fe XXV line. The temperature Te and the emission measure EM of such a plasma (Te =22 MK, EM=5. [FORMULA] cm-3 at the maximum time) were estimated from the spectral analysis using a program in the standard Yohkoh software package.

Following Mewe et al. (1986), we compute the photon number emissivity of the considered volume, at photon energy E per unit energy interval (phot s-1 keV-1)

[EQUATION]

using the appropriate Gaunt factor G [FORMULA] = 1.18 and the center of the Lo-HXT energy band (20 keV) as the photon energy E (Kosugi, 1993). Taking into account the area of the collimators and the efficiency of the detector we obtain a photon flux of the same order of magnitude as the one measured with Lo-HXT and with the same temporal behaviour. In Fig. 5 we show the computed and the measured photon flux; the good agreement between the two curves confirms that the hot plasma detected in the BCS Fe XXV channel emits a sufficient number of high energy photons to be detected with Lo-HXT.

[FIGURE] Fig. 5. (Thin line) Photon flux measured in the 18.9 keV band of the Lo-HXT channel. (Thick line) Photon flux computed at 20 keV using the Te and EM values derived from the spectral analysis of the BCS Fe XXV channel

The thermal origin of the HXR emission implies that the emitting plasma is located in the uppermost portion of the magnetic loop. This plasma is probably heated by a reconnection process that locally converts free magnetic energy into thermal and kinetic energy. Part of this thermal energy is transported down to the chromosphere by a conduction front and heats the chromospheric footpoints A2 and B1. We can then argue that the apparent spatial coincidence between A2 and the HXT source is only due to projection effects. If we consider the spatial resolution of HXT ([FORMULA]), the position of the loop on the solar disk, and its small size ([FORMULA] radius), we find that the projected location of a coronal loop-top HXT source could well coincide with A2.

We cannot say with certainty which heating mechanism is responsible for the brightening of the kernel pair A1-A2 (and of kernel C1), since we lack direct coronal data. However, if the pair B1-A2, for which we see both H [FORMULA]  and He D3 emission, is heated by conduction, it seems plausible that also the other kernels, for which we don't see any He D3 emission at all, are heated by conduction as well.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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