COORDINATED OBSERVATIONS OF SOLAR-ACTIVITY PHENOMENA .2. THE VELOCITY-FIELD PATTERN IN AN ELEMENTARY FLARE

We present the line-of-sight velocity pattern determined from spectra obtained before and during a small flare, whose emission properties ha ve been studied in Cauzzi et al. (1995, Paper I). The flare hard X-ray (HXR) emission, in the range 25 - 100 keV, consists of five separate and short spikes (lasting 1 - 7 s), which suggests that this flare is a sequence of separate elementary bursts. 40 seconds before the occurr ence of any HXR emission the flare kernel is already bright in H-alpha + 1.5 Angstrom and shows a typical chromospheric flare spectrum with Balmer lines in emission up to H-13. Also, few seconds before the impu lsive phase of the flare (as marked by the occurrence of the first HXR spike) an upward motion is determined from lines originating in high chromospheric layers (CaII K and H-delta) and from metallic lines (Si I 3905, Fe I multiplets 4 and 5). These motions together with a simult aneous strong emission suggest that the early chromospheric modificati ons, which may be considered as signatures of flare precursor, are due to some in situ mechanisms. 6 seconds after the peak time of the firs t HXR spike we determine a downward velocity, ranging from 1 km s(-1) for the metallic lines up to 20 km s(-1) for H-delta and CaII K lines, for all the points of the slit intersecting the H alpha + 1.5 Angstro m kernel. According to the known dynamic flare models, the measured ve locity can be explained either by direct heating of the chromosphere f rom non-thermal electrons with a low energy cutoff of 10 keV, or by a thermal conduction front from the hot corona. The downward velocity co ntinues to increase (roughly by a factor of 2) for 10 s after the end of the first HXR spike; none of the existing models predicts a similar behavior. After the fourth HXR spike the direction of the velocity ch anges over the H-alpha + 1.5 Angstrom kernel and two small regions (si milar or equal to 3'' in size), can be distinguished. In the first one a downward velocity of about 15 km s(-1) is derived for H-delta and C aII K lines simultaneously to a velocity similar or equal to 1 km s(-) 1 from the metallic lines. In the second area, the velocities derived from H delta and CaII K lines are upward directed, with values up to - 40 km s(-1), while the velocity derived from the metallic lines is sti ll downward directed, with values similar or equal to 1 km s(-1). The sudden onset of an upward motion could be related to the development o f a surge well visible after the flare; probably only the higher chrom ospheric layers supply material to the surge.