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

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1. Introduction

Ellerman bombs (EBs) or moustaches are known as fine structures with a typical 10 min lifetime and a size of [FORMULA] 1 arc sec characterized by Balmer line profiles with broad wings (about 10Å[FORMULA]half width in H[FORMULA]) and deep central absorption (e.g. Severny 1956, 1959; Engvold & Maltby 1968; Bruzek 1972; Kitai 1983; Rust & Kell 1992). Several mechanisms to explain the broad emission profiles of EBs have been proposed. Engvold & Maltby (1968) discussed two mechanisms: One is random macroscopic motions, the other is scattering due to electrons in a heated atmosphere. Severny (1968) suggested incoherent scattering in an expanding opaque layer. Canfield & Athay (1974) showed the possibility to explain the observed broad H[FORMULA] by assuming it was emitted in a heated and condensed atmosphere. Kitai (1983) made non-LTE computations and indicated that a heated ([FORMULA]T=1500 K) and condensed ([FORMULA]/[FORMULA]=5) layer in the lower chromosphere could produce a broad H[FORMULA] profile of EB. All mechanisms proposed so far to explain the broad profiles are based on thermal models.

However, linear polarization of hydrogen line emission in EBs was reported by many authors (Severny & Khokhlova 1958; Babin & Koval 1986, 1987; Firstova 1986). Firstova (1986) suggested that the excitation of hydrogen atoms by a flux of energetic electrons or by heat conduction may be responsible for the observed polarization. Recently, Firstova et al. (1997) observed that H[FORMULA] line in EBs was linearly polarized in the tangential direction, which could be interpreted as resulting from the bombardment of the solar atmosphere by beams of very energetic particles moving vertically.

Ding, Hénoux and Fang (1997) explored recently the possibility that the H[FORMULA] line profiles of EBs could be due to the effect of energetic particles bombarding the solar atmosphere, i.e. moving vertically. Their non-LTE computations, with non-thermal excitation and ionization included, showed that the characteristics of EB H[FORMULA] line profiles can be qualitatively reproduced in two cases: (1) high energy particles ([FORMULA] 60 keV electrons or [FORMULA] 3 MeV protons) being injected high in the solar atmosphere; (2) less energetic particles with a lower injection site (in middle chromosphere or deeper). However, except near the solar limb, the computed intensity at H[FORMULA] far line wings does not seem to be large enough to explain the observations.

On the other hand, several authors indicated that, by proton-hydrogen charge exchange, proton beam bombardment could produce Doppler shifted emission in Balmer lines and hence could enhance hydrogen line wings (Orral & Zirker 1976; Canfield & Cheng 1985). Using more recent atomic data and refined atmospheric models, Fang et al.(1995) confirmed their results and indicated that this effect will be obvious only at the beginning of solar flares. Recently, Zhao et al.(1997) (ZFH) computed the hydrogen line profiles caused by an oblique incident proton beam through proton-hydrogen charge exchange. They showed that the asymmetry and the intensity of the non-thermal emission profiles strongly depend on the beam incident pitch angle [FORMULA] and on the angle [FORMULA] between the directions of the magnetic field and of the line of sight: when [FORMULA], blue shifted emission is present, and the line even becomes symmetrical when [FORMULA].

All computations of H[FORMULA] line profiles based on charge exchange emission, given so far, indicate that for energetic protons accelerated in the corona and moving vertically around a vertical magnetic field, the H[FORMULA] line non-thermal emission is too small to be detectable. In fact EBs are not seen in H[FORMULA] line center, so they must originate from the chromosphere. Therefore, in this paper we propose proton-hydrogen charge exchange, due to a proton beam moving around a horizontal magnetic field in the chromosphere, to be at the origin of the EBs H[FORMULA] line profiles. Sect. 2 gives computational results, followed by a discussion and by conclusions in Sect. 3.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998