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

1. Introduction

Eruptive flares, a term that encompasses several classifications used in the past ("two-ribbon", "long-decay" or "dynamic" flares), are one of the most interesting activity phenomena on the Sun. While their morphological manifestations vary over a wide range, from eruptions of quiescent filaments (disparition brusque) with no visible flare in H , to powerful two-ribbon flares, common characteristics are given by a global magnetic field disruption and processes of energy release long lasting after the end of the impulsive phase. The magnetic reconnection model invoked to interpret these flares postulates that the magnetic field lines, open by some eruptive phenomenon, reconnect to a lower energy state. This reconnection process is very fast in the beginning of the flare and slows down in later phases (Kopp and Pneuman, 1976). The separation of the flare-ribbons at chromospheric level, and the rising of the system of loops in the corona, are considered signatures of the magnetic reconnection processes. During the last few years, the high-resolution images obtained from the Hard X-ray Telescope (HXT) and Soft X-ray Telescope (SXT) on Yohkoh have shown features consistent with the hypothesis of a reconnection site in the corona. In particular, a hard X-ray source located above the soft X-ray loops (Sakao et al., 1992; Masuda et al., 1994) and the cusp-like loop structure, suggestive of a reconnection site (Tsuneta et al. 1992; Tsuneta, 1996), support the idea that magnetic reconnection occurs above the loop top. Aschwanden et al. (1996) measured the electron time-of-flight distances for 5 flares presenting a Hard X-ray (HXR) double footpoint source in addition to a HXR source in the loop cusp, and found that the heights of the HXR source are consistent with the electron time-of-flight distance to the footpoints. This provides further evidence to the idea that particle acceleration occurs in the cusp region above the flare loop.

During the impulsive phase, when the reconnection rate rapidly increases, the loop top shows a high temperature region (Tsuneta, 1996), and it is plausible to assume that the chromosphere might be heated by non thermal-electron impact (thick target model) and/or by thermal conduction from this region. The response of the chromosphere to both energy deposition mechanisms is a strong chromospheric evaporation together with a chromospheric condensation moving downward (Fisher, 1989; Gan et al., 1991). The blue-shifted emission component of X-ray spectral lines observed in several flares (Antonucci et al., 1982) is considered a signature of the chromospheric evaporation and the red-shifted emission of chromospheric lines (Ichimoto & Kurokawa, 1984; Canfield et al., 1990; Falchi et al., 1992) a signature of the condensation motion.

In the case of a two ribbon flare, it is generally assumed that the outer edges of the chromospheric ribbons are the footpoints of newly reconnected hot coronal loops, whereas the inner edges are the roots of cooling loops that later might become visible in H  (Moore et al. 1980, Svestka 1989). In a partial revision of earlier reconnection models, Forbes & Acton (1996) found that, if the magnetic fields are sufficiently strong, the reconnecting magnetic field lines map in the chromosphere only to a thin region () of downward moving chromospheric plasma at the outer edge of the ribbons. Hence, the line-of-sight velocity in different regions of the chromospheric ribbons during an eruptive flare may be a powerful diagnostic of the processes taking place at coronal levels, in particular of magnetic reconnection. An earlier report of peculiar effects on the outer border of a two-ribbon flare was given by Svestka et al. (1980), that discussed a set of multi-slit observations. They briefly reported of small and very short-lived red-shifts in the H  line profiles along the outer edges of the ribbons in an eruptive flare, but did not specify the spatial dimensions of the red-shifts, nor their amplitude. To our knowledge, other systematic spectral observations of two-ribbon flares are available in the literature only for the late phase, when post-flare loops are already formed (Schmieder et al., 1987; Gu et al., 1992).

We report in this paper about flare observations obtained during a coordinated campaign between Yohkoh and ground based instruments, mainly the facilities of National Solar Observatory (both Kitt Peak and Sac Peak sites). In particular, we will describe an eruptive, two-ribbon flare (GOES class M2.6), observed on February 4, 1995. Qiu et al. (1997, Paper I) studied the pre-flare and impulsive phase of this event, characterized by a filament eruption and the subsequent formation of two bright ribbons at chromospheric level. In this paper we study the decay-phase after the maximum emission both in H  and soft X-ray (SXR). We will concentrate on the study of new chromospheric ribbons, formed simultaneously with new episodes of energy release at coronal level, in order to look for evidence of chromospheric signatures of the reconnection processes. The data are briefly illustrated in Sect. 2, as well as some of the flare characteristics. The flare evolution is described in Sect. 3 both at chromospheric and at coronal levels, while a possible scenario is discussed in Sect. 4. The motions observed in the ribbons are reported and discussed in Sect. 5, and conclusions are given in Sect. 6.

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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