Quasi-separatrix layers in solar flares. II. Observed magnetic configurations.

We show that the location of Hα or OV flare brightenings is related to the properties of the field-line linkage of the underlying magnetic region. The coronal magnetic field is extrapolated from the observed photospheric field assuming a linear force-free field configuration in order to determine the regions of rapid change in field-line linkage, called "quasi-separatrix layers" or QSLs. They are open layers that behave physically like separatrices: breakdown of ideal magnetohydrodynamics and release of free magnetic-energy may occur at these locations when their thickness is small enough. A feature common to all the flaring regions studied is found to be the presence of QSLs where Hα flare kernels are observed. The brightenings are along restricted regions of very thin QSLs; an upper bound of their thickness is 1Mm but it is several order of magnitude smaller in most of the cases. These places coincide in general with zones where the longitudinal field component is greater than 100G. These results allow us to constrain present models of solar flares and localise where a break-down of ideal MHD can occur. The studied flares are found to be fed in general by only one electric current loop, but they imply the interaction of two magnetic bipoles. The extrapolated coronal field lines involved in the process have their photospheric footpoints located at both sides of QSLs, as expected in recent 3D magnetic reconnection models.