CHROMOSPHERIC EVIDENCE FOR MAGNETIC RECONNECTION

We study the decay phase of an M2.6 flare, observed with ground based instruments at NSO/Sac Peak and with the cluster of instruments onboar d Yohkoh. The whole set of chromospheric and coronal data gives a pict ure consistent with the classical Kopp-Pneuman model of two-ribbon fla res. We clearly witness new episodes of coronal energy release, most p robably due to magnetic reconnection, during the decaying phase of the flare. Within the newly created chromospheric ribbons, we identify se veral small kernels, footpoints of magnetic loops involved in the flar e, where energy is probably deposited by a conduction front from coron al sources. The presence of a source (T-e = 22 MK, and EM = 5. x 10(48 ) cm(-3)) thermally emitting in the 14-23 keV energy band confirms thi s hypothesis. A new, interesting observational result is given by the measure of chromospheric downflows in different parts of the flaring k ernels. For all the cases examined, we find a stronger downflow at the outer edge of the flaring structure. The flews have amplitude of tens of km s(-1), over regions only a few arcsec across, and seem to decre ase when the flaring kernels slow their motion on the solar surface an d fade away. We believe that these downflows directly map the outer bo undary magnetic field lines of the reconnecting loops, as predicted in recent reconnection models that take into account explicity the effec ts of heat conduction. The flows represent the chromospheric counterpa rt of coronal features observed in soft X-ray such as cusp-like struct ures and the temperature stratification in flaring loop systems. The o bserved amplitude of these chromospheric flows could be an important c onstraint for quantitative modelling of coronal reconnection mechanism s and their effects on the lower solar atmosphere.