Extending the concept of separatrices to QSLs for magnetic reconnection

Magnetic reconnection is usually thought to be linked to the presence of magnetic null points and to be accompanied by the transport of magnetic field lines across separatrices, the set of field lines where the field-line linkage is discontinuous. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed “quasi-separatrix layers” (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. Magnetic reconnection is expected to occur preferentially at QSLs in 3-D magnetic configurations.This paper surveys the evolution of the QSL concept from the beginning to its recent status. The theory was successfully tested with multi-wavelength observations of solar flares. This validates the reconnection scenario as the main physical process at the origin of flares. The confrontation of observations with the state-of-the-art theory gives us also hints how to further develop our understanding of 3-D magnetic reconnection.