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Astron. Astrophys. 337, 294-298 (1998)

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3. Discussion and conclusion

We have computed the non-thermal Ly[FORMULA], Ly[FORMULA] and H[FORMULA] line emission produced by charge exchange in the case of propagation in the low chromosphere of energetic protons having a small pitch angle around a horizontal magnetic field. The magnetic field is supposed to be perpendicular to the line of sight. Compared to the quiet-Sun spectra, not only the line intensities increase significantly, but also the lines become very broad, especially for H[FORMULA]. This scenario can be used to explain the very broad H[FORMULA] line wings of EBs. If we take into account the non-thermal excitation and ionization and also the heating of the atmosphere caused by the energetic particles (see Ding et al. 1997), then the characteristics of the EB spectra can be at least qualitatively understood. Moreover, due to the high density in the low chromosphere, protons lose their energy in a short distance. For instance, when [FORMULA]= 1 [FORMULA] 1015 cm-3, 1 MeV protons lose their energy through collisions with ambient particles in a distance of about 4 km. In that case the size of the EBS is defined by the size of the reconnection region. It should also be mentioned that the results of our computation practically do not depend on the model of the atmosphere, which changes only the absorption near the line center, provided the hydrogen density and the ionization degree are kept constant.

In summary:

1. Proton-hydrogen charge exchange is a viable mechanism to explain the broad H[FORMULA] line and the nearly symmetrical wings observed in the spectra of EBs by protons accelerated in the lower chromosphere and propagating along a horizontal magnetic field perpendicular to the line of sight.

2. If the spectral line emission of EBs is produced by a proton beam located in the lower chromosphere, then the intensities of Ly[FORMULA] and Ly[FORMULA] line wings should increase by 2-3 orders of magnitude above that of the quiet-Sun intensity, providing a possible diagnostic tool for the mechanism of EBs. In order to get such intensities, the beam must have a section of about 10 km2. The associated energy carried by the beam over 10 mn, for a energy flux [FORMULA] = 5 [FORMULA] 1011 erg cm-2 s-1 would be of 3 1025 erg. That is four order of magnitude less than a small flare.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998