Quasi-Separatrix Layers and Three-Dimensional Magnetic Reconnection: Theory and Observations of Solar Flares (Invited Review)

Over the past three decades, models of solar flares and eruptions based on quasi-separatrix layers (QSLs) have made several important, observationally verified predictions regarding how the magnetic reconnection happens in 3D. Thus, they have become the best available theory of how and where solar flares and eruptions happen. We review the properties of QSLs, the close correspondence between QSL traces in the lower atmosphere and flare ribbons, together with their association to electric current enhancements, both modelled and observed ones. Furthermore, we review the slipping and slip-running nature of the magnetic reconnection in QSLs, and the associated apparent footpoint motions of the reconnecting structures, both modelled and observed. In addition, the purely 3D reconnection geometries involving the erupting magnetic flux rope are reviewed as well, along with the observational evidence for these processes. Finally, we discuss the indications that dynamics within the QSLs could play a role in heating the solar corona.