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Astron. Astrophys. 351, 368-372 (1999)

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4. Conclusions

The white-light flare on 1974 October 11 showed an unusual behavior since at flare maximum Ca II K line reached an intensity of K1 as high as half of the continuum intensity (Fang et al. 1995). Ordinary flare atmospheric models cannot reproduce such a spectral feature. A model having an extremely hot TMR, with a minimum temperature as high as [FORMULA] K, can explain the unusual Ca II K line but encounters the difficulty in meeting the energy deposit requirement in the lower atmosphere. Pure canonical heating models that transport energy from the corona to lower layers can hardly account for the production of this hot TMR. Therefore, we assume instead an in situ energy source. We further investigate the possible role of a particle beam injected from the TMR. The results show that a beam of hecta-keV electrons (or MeV protons) can sufficiently heat the TMR and lead to the formation of the very hot TMR provided that the energy flux of the beam is large enough.

We note that the present study does not rule out other possibilities that produce a hot TMR. In fact, it is very likely that other factors, including even some canonical heating mechanisms, may work together in this event. Theoretical studies of the flare processes in the lower atmosphere, such as magnetic reconnection, particle acceleration, and so on, can help to check what heating sources are plausible.

On the other hand, diagnostics of energetic particles rely on observations of hard X-ray emission or [FORMULA]-ray line emission. In particular, images at these wavelengths (say, that for hard X-rays) can be used to determine the height of particle acceleration. In the above computations, we have adopted a very large energy flux for the particle beam; however, the volume within which the particles are accelerated is assumed to be small, thus making the observed hard X-ray ([FORMULA]-ray) emission flux not unrealistically large. Moreover, it is also possible that some of the particles propagating upwards finally escape into the space. For an electron beam with [FORMULA] and [FORMULA] keV at [FORMULA] g cm-2, the particle number flux is attenuated to only less than 1% of the initial value when the beam reaches the corona. Thus, the particle flux which could be detected in interplanetary space may still be within the range of values for ordinary solar flares.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999