Coordinated observations of solar activity phenomena. II. The velocity field pattern in an elementary flare.

Details Coordinated observations of solar activity phenomena. II. The velocity field pattern in an elementary flare. Coordinated observations of solar activity phenomena. II. The velocity field pattern in an elementary flare.

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...306..625c&link_type=abstract

Astronomy and Astrophysics, v.306, p.625

Astronomy and Astrophysics

Astrophysics

14

Sun: Activity, Chromosphere, Flares

Scientific paper

We present the line-of-sight velocity pattern determined from spectra obtained before and during a small flare, whose emission properties have been studied in Cauzzi et al. (1995, Paper I). The flare hard X-ray (HXR) emission, in the range 25-100keV, consists of five separate and short spikes (lasting 1-7s), which suggests that this flare is a sequence of separate elementary bursts. 40 seconds before the occurrence of any HXR emission the flare kernel is already bright in Halpha_+1.5A and shows a typical chromospheric flare spectrum with Balmer lines in emission up to H_13_. Also, few seconds before the impulsive 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 Hdelta_) and from metallic lines (Si I 3905, Fe I multiplets 4 and 5). These motions together with a simultaneous strong emission suggest that the early chromospheric modifications, which may be considered as signatures of flare precursor, are due to some in situ mechanisms. 6 seconds after the peak time of the first HXR spike we determine a downward velocity, ranging from 1km/s for the metallic lines up to 20km/s for Hdelta_ and CaII K lines, for all the points of the slit intersecting the Halpha_+1.5A kernel. According to the known dynamic flare models, the measured velocity can be explained either by direct heating of the chromosphere from non-thermal electrons with a low energy cutoff of 10keV, or by a thermal conduction front from the hot corona. The downward velocity continues to increase (roughly by a factor of 2) for 10s 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 changes over the Halpha_+1.5 A kernel and two small regions (=~3" in size), can be distinguished. In the first one a downward velocity of about 15km/s is derived for Hdelta_ and CaII K lines simultaneously to a velocity =~1km/s from the metallic lines. In the second area, the velocities derived from Hdelta_ and CaII K lines are upward directed, with values up to -40km/s, while the velocity derived from the metallic lines is still downward directed, with values =~1km/s. The sudden onset of an upward motion could be related to the development of a surge well visible after the flare; probably only the higher chromospheric layers supply material to the surge.

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