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Astron. Astrophys. 328, 371-380

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Chromospheric evidence for magnetic reconnection

A. Falchi1, J. Qiu1*, and G. Cauzzi2

1Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
2Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy

Received 18 March 1997 / Accepted 5 August 1997


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 onboard Yohkoh. The whole set of chromospheric and coronal data gives a picture consistent with the classical Kopp-Pneuman model of two-ribbon flares. We clearly witness new episodes of coronal energy release, most probably due to magnetic reconnection, during the decaying phase of the flare. Within the newly created chromospheric ribbons, we identify several small kernels, footpoints of magnetic loops involved in the flare, where energy is probably deposited by a conduction front from coronal sources. The presence of a source (Te=22 MK, and EM=5.$\times 10^{48}$ cm-3) thermally emitting in the 14-23 keV energy band confirms this hypothesis.

A new, interesting observational result is given by the measure of chromospheric downflows in different parts of the flaring kernels. For all the cases examined, we find a stronger downflow at the outer edge of the flaring structure. The flows have amplitude of tens of km s-1, over regions only a few arcsec across, and seem to decrease when the flaring kernels slow their motion on the solar surface and fade away. We believe that these downflows directly map the outer boundary magnetic field lines of the reconnecting loops, as predicted in recent reconnection models that take into account explicity the effects of heat conduction. The flows represent the chromospheric counterpart of coronal features observed in soft X-ray such as cusp-like structures and the temperature stratification in flaring loop systems. The observed amplitude of these chromospheric flows could be an important constraint for quantitative modelling of coronal reconnection mechanisms and their effects on the lower solar atmosphere.

Key words: Sun: activity - chromosphere - corona - flares

*Astronomy Dept., Nanjing University, China

© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998