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

1. Introduction

The Ca II K line has been widely used in observations and studies of solar flares. Since it is a collision-dominated line, its radiative temperature, in particular in the line wing, can well reflect the local kinetic temperature of the atmosphere where a line feature originates. There are some semi-empirical models of solar flares based on the Ca II K line (e.g., Machado et al. 1978; Gan & Fang 1987; Ding et al. 1994). Fang et al. (1992) used the line asymmetries of Ca II K to investigate the possible mass motions in lower atmospheric layers.

Recently, Fang et al. (1995) reported an unusual behavior of the Ca II K line in a solar white-light flare (WLF) occurring near the disk center (N12 E02) on 1974 October 11: the intensity of K₁ reached nearly half of the continuum intensity at the flare maximum ( 10⁶ ergs cm^-2 s^{- 1} ster^-1 Å^-1); the intensity of K₃, however, was only weakly increased in comparison. Semi-empirical models, within the general scope of flare models, have been shown unable to reproduce such spectral features (Yin et al. 1995), since the K₁ intensity corresponds to a radiative temperature as high as K (Fang et al. 1995).

It is well known that the K₁ feature is formed in the temperature minimum region (TMR). Consequently we propose an atmospheric model with an extremely hot TMR that can reproduce the high K₁ intensity. It is nearly impossible to produce such a TMR by canonical energy transport mechanisms in solar flares, where the flare energy is initially released in the corona and then transported downwards through various ways (e.g., particle beam bombardment, heat conduction, and soft X-ray irradiation). Instead, an in situ energy source appears to be required.

The purpose of this paper is to compute an atmospheric model for this WLF and to discuss the energy requirement in the lower atmosphere. A possible heating mechanism by high energy particles, injected from a lower layer, is in particular discussed.

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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